Recycling Solar PV Modules Literature Review

From Appropedia
Jump to navigation Jump to search

Background of Recycling Solar PV[edit | edit source]

Overall Evaluations of Current State-of-the-art[edit | edit source]

P. Dias et al., “Comprehensive recycling of silicon photovoltaic modules incorporating organic solvent delamination – technical, environmental and economic analyses,” Resources, Conservation and Recycling, vol. 165, p. 105241, Feb. 2021, doi: 10.1016/j.resconrec.2020.105241.

Abstract: Photovoltaic (PV) panel manufacturing is increasing worldwide, which subsequently increases the amount of waste PV. This study proposes to recycle waste PV using organic solvent delamination followed by downstream thermal and leaching procedures. Firstly, experimental data is obtained using small commercial modules by replicating a recycling route taken from the literature. Based on the experimental results, life cycle cost analysis (LCCA) and life cycle assessment (LCA) are applied to evaluate the experimental and optimized industry scale processes. Results show that the main profitable recycling avenues are for aluminum frame and junction box removal; and that downstream processes can separate and recover all the remaining materials, but not profitably. The laboratory and high-throughput-optimized processes, considering the median costs and revenues, have a net cost of 29.00 and 3.30 USD per module, respectively. The complete recovery of materials using the proposed method is unlikely to be profitable and this may only be achievable where labor is not expensive. Alternatively, the complete recycling of waste PV could be made economically viable by reducing process time, increasing automation and/or providing financial subsidies. The environmental analysis, however, shows that the optimized process modelled here has a positive net environmental impact. The results are also compared against the cost/environmental impact of landfilling such waste. In summary, the proposed recycling route is capable of completely recovering the main materials in waste PV (aluminum frames, junction box, silver, copper tabbing, silicon, backsheet and unbroken glass) and can have a positive environmental impact, but it is not economically profitable.

  • Aluminum frame and junction box most profitable
  • "Positive net environmental impact"
  • 60 to 78 million tonnes of waste by 2050
  • Thin film modules have toxic waste, c-Si has lead and silver toxic
  • Limited resources... Si is now on the critical raw material list for EU
  • Would be good to use what we already have
  • Backsheet made of polyvinyl flouride (PVF)
  • Encapsulant typically EVA
  • Lead-tin coated Cu is tabbing
  • Soda-lime low-iron tempered glass
  • Si doped with phosphorous or boron
  • Anti-reflective coating... made of what?
  • Metal contacts are silver
  • Bottom is coated with Al
  • Thermal followed by chemical etching
  • FRELP has automated system for breaking down frame and cables, then heat and cut with high-frequency knife
  • PV in laminate is incinerated, ash is sieved then leached
  • Or stuff is just ground up with electronic waste
  • Electro-hydraulic fragmentation with sieving also used
  • Blade rotor crushing, thermal treatment or hammer crushing, shredding, electrostatic separation
  • Careful about solvent emissions
  • Recover undamaged cells with organic solvent ***
  • EVA is partially cross-linked and not, so in some solvents cross-linked fraction swells, the rest dissolves
  • Bad gases from this process?
  • Toluene seems to be best solvent but volatile - go to Kang paper for details on HOW - also could be reused
  • Downstream process involved thermal treatment, manually removing tabs (use XRF on tabs), leaching in nitric acid at room T for 2 hrs "under magnetic agitation"
  • Their total immersion in toluene time averaged 99 hrs, glass still intact but EVA decomposed, Si is not undamaged
  • Recycling done at large scale is better than landfilling

Md. S. Chowdhury et al., “An overview of solar photovoltaic panels’ end-of-life material recycling,” Energy Strategy Reviews, vol. 27, p. 100431, Jan. 2020, doi: 10.1016/j.esr.2019.100431.

Abstract: End-of-life (EOL) solar panels may become a source of hazardous waste although there are enormous benefits globally from the growth in solar power generation. Global installed PV capacity reached around 400 GW at the end of 2017 and is expected to rise further to 4500 GW by 2050. Considering an average panel lifetime of 25 years, the worldwide solar PV waste is anticipated to reach between 4%-14% of total generation capacity by 2030 and rise to over 80% (around 78 million tonnes) by 2050. Therefore, the disposal of PV panels will become a pertinent environmental issue in the next decades. Eventually, there will be great scopes to carefully investigate on the disposal and recycling of PV panels EOL. The EU has pioneered PV electronic waste regulations including PV-specific collection, recovery and recycling targets. The EU Waste of Electrical and Electronic Equipment (WEEE) Directive entails all producers supplying PV panels to the EU market to finance the costs of collecting and recycling EOL PV panels in Europe. Lessons can be learned from the involvement of the EU in forming its regulatory framework to assist other countries develop locally apposite approaches. This review focused on the current status of solar panel waste recycling, recycling technology, environmental protection, waste management, recycling policies and the economic aspects of recycling. It also provided recommendations for future improvements in technology and policy making. At present, PV recycling management in many countries envisages to extend the duties of the manufacturers of PV materials to encompass their eventual disposal or reuse. However, further improvements in the economic viability, practicality, high recovery rate and environmental performance of the PV industry with respect to recycling its products are indispensable.

  • CO2 emissions from Si PV negligable
  • 99.1 GWh solar PV was grid connected in 2017, Germany was first to connect to grid in 2007
  • Europe is actually working on a plan for waste
  • c-Si has 80% market shares as of 2020
  • Japan, China, and CA have no plans
  • Toxic waste in landfill could contaminate drinking water
  • Causes of module failure: poor design and defect in manufacturing, issues with electrical equipment and grounding, degradation of AR and EVA, incoherency with cracked cells, repeated load cycles, temp change affect contact in junction boxes, glass breakage, frames, cell interconnections, 40% failure from microscopic cracks and failures
  • Frame can go through secondary metallurgy
  • What is in the waste? Off gas?
  • High temp tube furnace, gas at 2L/h, heat to 500 C at 450 C/h, held for 1 hr
  • 4.3 and Table 1 summarizes most of the current methods tried
  • Policy not developed for encouraging recycling bc not enough EOL modules to make it worth it

R. Deng, N. L. Chang, Z. Ouyang, and C. M. Chong, “A techno-economic review of silicon photovoltaic module recycling,” Renewable and Sustainable Energy Reviews, vol. 109, pp. 532–550, Jul. 2019, doi: 10.1016/j.rser.2019.04.020.

Abstract: Even with a long lifetime of 25–30 years of green energy production, end-of-life treatment of solar photovoltaic modules can negatively impact the environment if not handled properly. This is particularly urgent when the use of photovoltaics has grown at an unprecedented rate, generating clean energy all over the world. Therefore, it is essential to develop commercially viable end-of-life recycling technologies to guarantee a sustainable future for the photovoltaic technology. Silicon photovoltaic modules, the most popular photovoltaic technology, have been shown to be economically unattractive for recycling - the materials are mixed and difficult to separate, and have low value, so that the cost of recycling is hardly recovered. In this paper, we review the state-of-art recycling technology and associate it with a quantitative economic assessment to breakdown the cost structure and better understand the presented economic barrier. The techno-economic review allows us to identify essential framework and technology changes required to overcome the current barrier to implementing commercial-scale recycling. (i) The authority may impose price signal to impress direct landfill of end-of-life modules while proactively establish an effective collection network. (ii) The local recyclers may aim at value recovery as a step beyond mass recovery, especially targeting at recovery of intact silicon wafers and silver to guarantee the recycling revenue. Meanwhile, efforts should be put on reducing the recycling processing cost. (iii) Photovoltaic module manufacturers may take end-of-life responsibilities and up-design the product to facilitate end-of-life recycling, which includes features for simple disassembly, recycling, and reducing or eliminating the use of toxic components.

  • Most recycling processes mix materials so it is hard to separate, lowering their value after recycling
  • Offers 3 main solutions: policy to increase the cost to landfill waste PV, recycling aims for value recovery instead of just mass recovery, have manufacturers design PV modules to ease recycling at EOL
  • Precious/scarce materials include Ag, Ga, In, Ge, Cd, Te
  • Other materials include Al, Cu, glass, Si
  • Pay back time could be as low as 1.6 years
  • Boron or phosphorous doped Si, current electrodes are Ag, Al, or Cu, Ar coating is Si3N4
  • Cells connected with Cu ribbon, encapsulated with EVA, front is glass, back is polymer
  • Dowcycling is extracting contaminated material
  • Upcycling is extracting pure/valuable material
  • Modules are shredded by glass recyclers since 75 weight % is glass
  • Al frame, junction box, and Cu cables manually removed, the rest is shredded
  • Al and glass recycled via manual/mechanical separation
  • Glass was recovered at 91 weight % after a tripe shredding
  • Cyrogenic process has been attempted for delamination - interfacial bonds are weakened by this - still yields powders
  • High-frequency knife cutting at higher temp yielded 98 weight % glass recovered - I think the rest of the module was preserved - potential for organic solvent
  • Thermal decomp of EVA in inert atmosphere at 500 C (backfoil peeled off beforehand)
  • When pyrolyzed, EVA becomes acetic acid, propane, propene, ethane, methane..... not ideal....apparently can be bruned off under oxygen environment - this combustion can balance out the energy used to heat the furnace
  • Wafers are much thinner than they used to be - difficult to recover without cracking
  • For chemical delamination, glass separated by soaking in HNO3 for a day (Bruton source)
  • Swelling of EVA may require additional pyrolysis
  • Flourine backfoil peeled off at elevated temp, scraped off, or combusted in specific incinerator (toxic byproducts)
  • Leaching is used for contaminated Si powder, typically HNO3 - powder cannot be used because of its purity
  • Etching uses more chemicals than leaching
  • AR and emitter grinded after leaching reduces chemicals even more
  • Table 2 for etching processes
  • Thermal recycling of Si wafers provides 70% energy reduction in Si wafer production
  • Environmental impact of downcycling or upcycling is better than landfill
  • Focus must be on flourine backfoils
  • Not enough waste to make it economically viable - once we're at 19000 tonne/year
  • Revenue factors are material market price, material weight, material purity, generated and collected waste (being generalized in a lot of LCA)
  • Al frames are across the board worth recycling but any other material is costs more than it is worth
  • Must recover high purity Si (probably thermal recycling is best) and Ag
  • If recycling could be faster it'd be cheaper

E. Klugmann-Radziemska and A. Kuczyńska-Łażewska, “The use of recycled semiconductor material in crystalline silicon photovoltaic modules production - A life cycle assessment of environmental impacts,” Solar Energy Materials and Solar Cells, vol. 205, p. 110259, Feb. 2020, doi: 10.1016/j.solmat.2019.110259.

Abstract: To offset the negative impact of photovoltaic modules on the environment, it is necessary to introduce a long-term strategy that includes a complete lifecycle assessment of all system components from the production phase through installation and operation to disposal. Recycling of waste products and worn-out systems is an important element of this strategy. As the conclusions from the previous studies have shown, thermal treatment provides an efficient first step in the recycling process, while chemical treatment was more advantageous in the second step. This study aims to assess the environmental impact of recovering and recycling the valuable semiconductor silicon wafer material from photovoltaic solar cells. A comparison was made between producing new solar cells with or without recycled silicon material. The analysis of the photovoltaic cell life cycle scenario including material recycling presented in this article was performed using SimaPro software and data combined and extended from different LCI databases. The idea is that the use of recycled materials, which were energy-consuming in the primary production stage, allows to meaningly reduce the energy input in the secondary life cycle. All stages of the silicon cell life cycle contribute to the Global Warming Potential (GWP) and greenhouse gas emissions reductions through the use of recycled silicon material represented 42%. The total environmental impact of photovoltaic production can be reduced by as much as 58%, mainly through reduced energy consumption in the production process of high purity crystalline silicon.

  • Reusing materials that were expensive in first life cycle can decrease energy usage in second life cycle
  • Energy payback time for crystalline Si modules is 3.5-5 yrs, or 2.5 yrs for in desert
  • Reasons to recycle: landfill space for other waste, energy conservation, avoiding more raw material extraction, less emissions
  • Lots of energy to purify Si, Cl could be emitted which is toxic
  • Removal must occur in this order: front metal coating, bottom metal coating, AR coating, and n-p junction
  • How a wafer becomes a cell: texturization, emitter formation, parasitic junction removal, AR coating, front and back contacts
  • Claims monocrystalline Si cells are easier for second life cycle
  • EVA delamination takes a lot of energy, but recovering a whole cell can reduce new cell production energ consumption by 70%
  • If wafer isn't thick enough after process still viable as powder
  • Cells in this study were 270-300 um thick
  • Table 2 outlines production of a module in 2006
  • Ag recovered by electrolysis - ideal to reduce amount of Ag used in general
  • Figure 5 is a great diagram of recycling routes
  • LCA shows the environmental impact of reusing cells for production of new modules is two times better than making new ones

L. Frisson et al., “Recent improvements in industrial PV module recycling,” Proceedings of the 16th European Photovoltaic Solar Energy Conference, vol. 1, Apr. 2000.

  • Pyrolysis with microwave heating causes cell cracks
  • Tri-ethylene glycol dissolution at 220-290 C also failed
  • Hot nitric acid may be worth trying but lots of acid required
  • Do I need to do my own TGA for EVA?
  • EVA decomp in air 450 C, decomp in N2 480 C - heating in N2 avoids exothermic reaction of EVA decomp, no swelling?
  • They heated modules in SiO2, sort of like the compressive force, with even T, mixing, and constant contact
  • Is this an expensive process?
  • Heating at an angle/in a "netting envelope" may provide better recovery
  • In this study, cells heated to 450 C for 20 min
  • Lifetime mapping with MFCA to verify quality maintained
  • HF, H2SO4, nitric acid process used
  • IV characteristics preserved
  • Etching in 20% NaOH is bad for interruptions at grain boundaries

Y. R. Smith and P. Bogust, “Review of Solar Silicon Recycling,” in Energy Technology 2018, Cham, 2018, pp. 463–470, doi: 10.1007/978-3-319-72362-4_42.

Abstract: Photovoltaic (PV) modules are becoming an ever increasingly larger part of our energy portfolio. As more and more PV modules are installed and come on-line, management of end-of-life (EOL) modules becomes an important issue. Currently, management of overburden EOL PV modules is not an issue, but is anticipated to be by 2030. Recovery and recycling of valuable metals in PV modules presents several environmental and economic advantages. In this brief review, we will describe processes for refurbishing and recycling of PV silicon. These processes involve some combination of mechanical, thermal, and chemical processing, all of which all have their oPV) modules have become wn respective challenges. Also, projections of PV module material streams are also highlighted.

  • Amorphous Si not really used any more, but it still will need to be recycled!!
  • 40% of Si ingot gets cut away when sliced - this loss as well as energy necessary to produce new wafers makes reusing already wafered Si appealing (Sources 16, 20, 23)
  • Can we used PVF backing for anything?

What's in a Solar PV Module?[edit | edit source]

  • Modules I will be working with
  • Multicrystalline Si
  • Glass is 3.2 mm thick, high transmission, tempered, does contain a ARC
  • EVA encapsulant
  • Backsheet described as "white"

R. D. McConnell and A. Hansen, “NCPV FY 1998 Annual Report,” Annual Report, p. 572, 1999.

  • Naugard P is a typical antioxidant, Lupersol 101 and Cyasorb UV-531 are typical luminecents used in encapsulant
  • Other materials commonly used include cerium oxide-containing glass, Tefzel
  • They acheived 83% gel content (cross linking) as opposed to the typical 80% - good or bad?

Encapsulant (EVA)[edit | edit source]

E. Klampaftis and B.S. Richards, "Improvement in multi-crystalline silicon solar cell efficiency via addition of luminescent material to EVA encapsulation layer," Progress in Photovoltaics: Research and Applications, vol. 19, April 2011, doi: 10.1002/pip.1019. Abstract: This paper shows that an enhancement in the performance of commercial screen‐printed multi‐crystalline silicon photovoltaic modules can be achieved via luminescent down‐shifting of the incident light. Using fluorescent organic dyes dissolved in the pre‐existing poly‐ethylene vinyl acetate (EVA) encapsulation layer, an increase in the short‐wavelength external quantum efficiency of over 10% absolute, which results in 0.18% absolute higher efficiency, is reported. This approach offers the opportunity for prompt application at production scale, since it neither requires any modification to the module manufacturing process nor does it add to the cost of power generation. Copyright © 2010 John Wiley & Sons, Ltd.

  • Luminescent down-shifting(LDS) of incident light by having some species in the encapsulant
  • Generally all module EQE suffer in shorter wavelength range
  • 300-400 nm have high energy content so interact a lot with front layers of module
  • Reasons for EQE suffering at short wavelength: "parasitic absorption and reflection" from SiNx ARC, high doping means fast recomb rates, front glass reflects, encapsulant absorbs
  • LDS can help with above reasons besides the front glass reflection
  • So the species absorbs short wavelength then emits at a longer length so it can reach the PV
  • Used Lumogen-F Violet 570 dye as species to increase EVA absorption
  • PLQY and transmission confirm homogeneity of EVA
  • Used already made PV, sandwich of borosilicate glass and EVA on both sides of PV, vac laminated
  • High dye concentration emitted photons at 400-570 nm
  • Species didn't affect IQE, no affect on electronic properties
  • Species did increase EQE in 300-400 nm range

J. Pern, “Module Encapsulation Materials, Processing and Testing,” p. 33., Dec. 2008.

  • Encapsulation contributes to 20-30 year life of PV module
  • Typical encapsulants: EVA, TPU, PVB, Silicones, Silicone/PU hybrid, Ionomer, UV-curable resin
  • Typical edge sealants: Polybutyl and Silicones for Al framed crystalline Si, Desiccant-type for thin film CdTe and CIGS, PIB-type
  • Typical superstrate: Glass (low-iron, tempered, plain or textured, UV-filtering/Ce, SiO2 ARC) or Flouropolymer (Tefzel, Tedlar, THV220)
  • Typical substrate/backfoil: Polymer Multi-laminates (Tedlar based, PET or PEN based - Protekt, Teijin Teonex, BaSO4 filled PET) or Glass
  • Will I need to dissolve backfoil?
  • Find out what kind of sealing I'm working with
  • Contains more details about encapsulation degradation

US Granted Patent No. 6093757

  • May be necessary to know how old module is - cross linking in EVA is dependent on time/degree of photo-oxidative degradation, and amount of cross linking present is the "gel" that can't easily be dissolved
  • Chromophores often produced in curing process, they (with UV) are an intitiant of the degradation of EVA via hydroproxidation
  • Aim for a fast cure, often causes bubbling which affects absorption
  • What makes a good encapsulant: photothermal and photochemical stability, not many chromophores made in curing, fast cure rate, ideal amount of cross linking, little bubbling, protection for antioxidant against moisture and thermal decomp

N. S. Allen, M. Edge, M. Rodriguez, C. M. Liauw, and E. Fontan, “Aspects of the thermal oxidation, yellowing and stabilisation of ethylene vinyl acetate copolymer,” Polymer Degradation and Stability, vol. 71, no. 1, pp. 1–14, Jan. 2000, doi: 10.1016/S0141-3910(00)00111-7.Abstract: The thermal oxidation of ethylene-vinyl acetate copolymer [EVA-17 and 28% w/w VA (vinyl acetate) units] has been examined by thermo-gravimetric and hydroperoxide analysis, FTIR (Fourier transform infra-red), fluorescence spectroscopy and yellowness index. Thermal analysis indicates the initial loss of acetic acid followed by oxidation and breakdown of the main chain. The degradation rate is greater in an oxygen atmosphere as is the formation of coloured products. FTIR spectroscopic analysis of the oxidised EVA shows evidence for de-acetylation followed by the concurrent formation of hydroxyl/hydroperoxide species, ketone groups, α, β-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. Hydroperoxide evolution follows typical auto-oxidation kinetics forming ketonic species. In severely oxidised EVA, evidence is given for the subsequent formation of anhydride groups. The initial fluorescence excitation and emission spectra of EVA is not unlike that reported for polyolefins confirming the presence of low levels of unsaturated carbonyl species. There are, however, significant differences in a long wavelength component in the fluorescence emission indicating the presence of other active chromophores. These long wavelength emitting components grow in intensity and shift to longer wavelengths with ageing time. However, unlike studies on PVC these emission spectra are limited due to the vinyl polyconjugation lengths and tend to be consistent with the formation of specific degraded units, possibly polyunsaturated carbonyl species of a limited length confined to the EVA blocks. During oxidation of EVA the original unsaturated carbonyl species remain as distinct emitting chromophores. This suggests that the growth and decay of these chromphores is virtually constant indicating that they could be an integral part of the EVA polymer that are responsible for inducing degradation. Degradation is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. The EVA degradation is therefore, different from that of PVC where in the latter case polyconjugated vinyl groups are evident through conjugated absorption bands in the UV spectrum. In the case of degraded EVA no such bands are observed. Also, degraded coloured EVA is not bleached by treatment with bromine, maleic anhydride or peracetic acid. Primary phenolic antioxidants exhibit variable activity in inhibiting the yellowing of EVA while combinations with phosphites and hindered piperidine stabilisers display powerful synergism confirming the importance of hydroperoxides as precursors. Thermal oxidation was also was displayed through the inhibition of lactone, carboxylic acid and alkene groups illustrating the fact that oxidation and yellowing are synonymous reactions.

  • EVA degrades because acetic acid forms then main chains break, so ethylene and vinyl acetate don't interact - does acetic acid form because of UV?
  • EVA hydroperoxidates so ketone and unsaturated ketone groups form

Chitra, D. Sah, K. Lodhi, C. Kant, P. Saini, and S. Kumar, “Structural composition and thermal stability of extracted EVA from silicon solar modules waste,” Solar Energy, vol. 211, pp. 74–81, Nov. 2020, doi: 10.1016/j.solener.2020.09.039.Abstract: Ethylene-vinyl acetate (EVA), a copolymer of ethylene and vinyl acetate, is widely used as an encapsulant in the silicon solar module to bind the different layers together and protecting the solar cells from over stressing, cracking, and environmental effects. In this work, EVA has been recovered successfully from the used silicon solar module by thermal treatment at 170 °C temperature and the application of mechanical force. The established process is completely environment-friendly, as the EVA layer was recovered without any degradation and emission of any gas. The presence of extracted EVA and its chemical composition was confirmed from FTIR and EDAX measurements. It was observed from Thermogravimetry (TGA) and Differential thermogravimetry (DTG) that thermal degradation of EVA was a two-step process, and also the rate of reaction was fast in an air environment as compared to nitrogen environment. The extracted EVA is thermally stable until 215 °C in the air environment. From Differential scanning calorimetry (DSC) analysis, two endothermic peaks were observed at temperature 37 °C and 55 °C, which may be due to beginning of melting of vinyl acetate and ethylene crystallites respectively in air and nitrogen environment. From UV–visible spectroscopy, it was found that above 500 nm, the extracted EVA is transparent. After examined through the various characterization, it has been observed that extracted EVA shows quite similar properties as that of commercially available EVA. Therefore, the recovered EVA may be used in the encapsulation of solar modules and other applications in packaging and textile industries.

  • They mechanically removed Al frame, the rest was sheared with diamond wheel cutter rotating
  • Tedlar removed from module at 130 C
  • The rest was heated to 170 C, then EVA could be scraped off - no yellowing or residual gas - what about swelling or cracking of SI?
  • Concluded the EVA has same properties as before used as encapsulant, so can be reused for solar modules if processed at 170 C

K. V. den Broeck, N. V. Hoornick, J. V. Hoeymissen, R. de Boer, A. Giesen, and D. Wilms, “Sustainable treatment of HF wastewaters from semiconductor industry with a fluidized bed reactor,” IEEE Transactions on Semiconductor Manufacturing, vol. 16, no. 3, pp. 423–428, Aug. 2003, doi: 10.1109/TSM.2003.815624.

Abstract: Within the semiconductor industry, large volumes of hydrogen fluoride (HF) containing wastewaters need to be treated. This paper describes a technique that makes it possible to treat HF wastewater with virtually no waste production. A method based on crystallization of CaF/sub 2/ on sand particles in a fluidized bed reactor, with trade name Crystalactor, was assessed for the first time with real wastewater originating from a semiconductor prototyping line. Several process parameters were investigated such as influent fluoride concentration, calcium reagent excess, pH, single versus multiple calcium reagent dosing, etc. It can be concluded that the performance of the reactor is greatly dependent on the initial concentrations of calcium and fluoride at the bottom of the reactor (referred to as local saturation). The original hardware design was changed from a single reagent dosing to a multiple reagent dosing to prevent too high local supersaturation values of fluoride and calcium and thus obtain higher CaF/sub 2/ crystallization efficiencies. Fluoride loads up to 7 kg/m/sup 2//spl middot/h were still possible with multiple reagent dosing, compared to a maximum treatable load of 3.5 kg/m/sup 2//spl middot/h for single reagent dose. Additionally, an alternative post-treatment process based on replacement of calcite by fluorite in a granular calcite column is demonstrated.

  • What to do with HF throughout solar PV process
  • Lots of HF used in production and recycling of semiconductor PV
  • Most of the time Ca is added in some way to form CaF2, Al is also good absorbent because bonds to F are higher but it is more costly
  • So they use reactor to grow CaF2 crystals on surface of sand grains

Backsheet (PVF)[edit | edit source]

C. Farrell et al., “Assessment of the energy recovery potential of waste Photovoltaic (PV) modules,” Scientific Reports, vol. 9, no. 1, Art. no. 1, Mar. 2019, doi: 10.1038/s41598-019-41762-5.

Abstract: Global exponential increase in levels of Photovoltaic (PV) module waste is an increasing concern. The purpose of this study is to investigate if there is energy value in the polymers contained within first-generation crystalline silicon (c-Si) PV modules to help contribute positively to recycling rates and the circular economy. One such thermochemical conversion method that appeals to this application is pyrolysis. As c-Si PV modules are made up of glass, metal, semiconductor and polymer layers; pyrolysis has potential not to promote chemical oxidation of any of these layers to help aid delamination and subsequently, recovery. Herein, we analysed both used polymers taken from a deconstructed used PV module and virgin-grade polymers prior to manufacture to determine if any properties or thermal behaviours had changed. The calorific values of the used and virgin-grade Ethylene vinyl acetate (EVA) encapsulant were found to be high, unchanged and comparable to that of biodiesel at 39.51 and 39.87 MJ.Kg−1, respectively. This result signifies that there is energy value within used modules. As such, this study has assessed the pyrolysis behaviour of PV cells and has indicated the energy recovery potential within the used polymers found in c-Si PV modules.

  • Refer to for polymer ID
  • In order of mass, glass, anodized Al frame, two layers EVA, junction box, backsheet (usually either TPT or TPE)
  • Alternatives to Tedlar (flourine based polymer) are polyester, e-layers of ethylene copolymers, polyamides, or blends with PMMA
  • International Energy Agency claims chemical/thermal methods superior to mechanical methods of delamination
  • This study goes into how by-products of EVA decomp could be used for energy
  • Figure 1 is good reference for FT-IR
  • EVA decomp between 310-390 C, second decomp step between 410-510 C
  • Higher decomp T with lower ramp rate
  • Want to extract heat/energy from polymers
  • Page 7 breaks down EVA decomp process
  • Cut samples with water jet cutting
  • Samples heated for 10 min at 190 C, little cuts on the side to make peeling the backsheet off (watch for cooling) - some came off well, some left white residue on EVA
  • Page 10 might come in handy for FPV

K. J. Geretschläger, G. M. Wallner, and J. Fischer, “Structure and basic properties of photovoltaic module backsheet films,” Solar Energy Materials and Solar Cells, vol. 144, pp. 451–456, Jan. 2016, doi: 10.1016/j.solmat.2015.09.060. Abstract: In this paper commercially relevant backsheets are characterized as to their material and laminate structure and basic optical and mechanical properties. Various multilayer backsheet materials were selected and analysed by optical microscopy, Raman spectroscopy, infrared spectroscopy (FTIR-ATR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), ultra violet visible near infrared spectroscopy (UV/VIS/NIR) and monotonic tensile testing. By Raman and infrared spectroscopy and DSC the polymeric materials of the various layers were identified. Furthermore it was shown for the thermoplastic ethylene vinylacetate copolymer (EVA) layers that three different vinylacetate (VA) contents were used. By TGA a content of inorganic fillers and pigments from about 6%m to approximately 20%m was detected. The hemispherical solar reflectance of the white pigmented backsheets was ranging from 0.695 to 0.838. The highest Young׳s modulus was found in polyvinylfluoride based multilayers while the lowest values were detected for polyamide based structures.

  • EVA substitutes inner layer of PVF, other outer layers include PVDF, ECTFE and THV
  • ID polymers with Raman confocal microscope and FTIR spectrometer with ATR (attenuated total reflexion)
  • Raman and infrared detected flouropolymers, and only Raman detected TiO2 and short glass fibers
  • Figure 5 and 6 for FT-IR expectations
  • Other engineering thermoplastics used in solar PV are PET, EVA, PA, and PP

Anti-reflective Coating[edit | edit source]

R. Li, J. Di, Z. Yong, B. Sun, and Q. Li, “Polymethylmethacrylate coating on aligned carbon nanotube–silicon solar cells for performance improvement,” J. Mater. Chem. A, vol. 2, no. 12, pp. 4140–4143, Feb. 2014, doi: 10.1039/C3TA14625A.

Abstract: Polymethylmethacrylate (PMMA) coating has been spin-coated onto aligned carbon nanotube–silicon (CNT–Si) solar cells and the efficiency increased from 7.1% to 11.5%, and was further increased to 13.1% when doped with nitric acid (HNO3) under air mass (AM 1.5) conditions. The antireflection of PMMA coating and the decreased resistance at the CNT–Si interface during PMMA drying process together contributed to the performance improvement.

  • PMMA is antireflective
  • Carbon nanotube(CNT) -Si interface has lower resistance
  • Surface texturing with pyramid, nanocone, nanowire, and random for trapping light
  • Deposition of TiO2, Si3N4, Ta2O5 thin films for trapping light
  • Ag paste and liquid state GaIn eutectic used on Si substrate
  • PMMA thickness increase, wavelength of lowest reflectance increases
  • CNT-Si interface increases when CNT pushed into film by "shrink force" from PMMA coating and solvent evap

Recycling to Recover Si Cell[edit | edit source]

E. Klugmann-Radziemska and P. Ostrowski, “Chemical treatment of crystalline silicon solar cells as a method of recovering pure silicon from photovoltaic modules,” Renewable Energy, vol. 35, no. 8, pp. 1751–1759, Aug. 2010, doi: 10.1016/j.renene.2009.11.031.

Abstract: Photovoltaic technology is used worldwide to provide reliable and cost-effective electricity for industrial, commercial, residential and community applications. The average lifetime of PV modules can be expected to be more than 25 years. The disposal of PV systems will become a problem in view of the continually increasing production of PV modules. These can be recycled for about the same cost as their disposal. Photovoltaic modules in crystalline silicon solar cells are made from the following elements, in order of mass: glass, aluminium frame, EVA copolymer transparent hermetising layer, photovoltaic cells, installation box, Tedlar® protective foil and assembly bolts. From an economic point of view, taking into account the price and supply level, pure silicon, which can be recycled from PV cells, is the most valuable construction material used. Recovering pure silicon from damaged or end-of-life PV modules can lead to economic and environmental benefits. Because of the high quality requirement for the recovered silicon, chemical processing is the most important stage of the recycling process. The chemical treatment conditions need to be precisely adjusted in order to achieve the required purity level of the recovered silicon. For PV systems based on crystalline silicon, a series of etching processes was carried out as follows: etching of electric connectors, anti-reflective coating and n-p junction. The chemistry of etching solutions was individually adjusted for the different silicon cell types. Efforts were made to formulate a universal composition for the etching solution. The principal task at this point was to optimise the etching temperature, time and alkali concentration in such a way that only as much silicon was removed as necessary.

  • Claims modules can be recycled for same cost as disposal
  • Materials involved: glass, aluminum frame, EVA, PV cell, installation box (?), Tedlar backfoil, assembly bolts
  • Thermal separation is best economically and ecologically
  • AR and n-p dissolves with acid or basic solutions: HF, H2SiF6, HNO3, CH3COOH, solution ratios are described in section 3
  • AR coatings typically Ta2O5, TiO2, SiO, SiO2, Si3N4, Al2O3, ITO (I think this is most common), MgF2
  • Explains how AR works
  • Should know how thick different layers are
  • Stirring during etching helps get uniform dissolution
  • Ag recovered from left over solution with electrolysis
  • Low concentration solution between 60-80 C seemed optimal for Al recovery
  • HF/CH3COOH/HNO3 (1:2:5) works well for AR and n-p layers
  • Etching took off about 20 um at each T

T. Doi, I. Tsuda, H. Unagida, A. Murata, K. Sakuta, and K. Kurokawa, “Experimental study on PV module recycling with organic solvent method,” Solar Energy Materials and Solar Cells, vol. 67, no. 1, pp. 397–403, Mar. 2001, doi: 10.1016/S0927-0248(00)00308-1.

Abstract: We propose an organic solvent method to recover silicon cells from conventional crystalline silicon PV modules. From dissolution tests of EVA by various kinds of organic solvents, it was found that trichloroethylene could dissolve cross-linked EVA sample kept at 80°C. Applying this method to “one cell” module (125×125mm), it was found that mechanical pressure is important to suppress swelling of the EVA. After immersing the module in trichloroethylene at 80°C for 10 days, we have successfully recovered the silicon cell without any damage.

  • Good to just have to produce modules, not cells, by reusing cells
  • EVA is very moisture resistant, low cost, melting point of non-cross-linked is 76 C, 70% cross-linked when heated at 150 C for 10 min (which it has been in lamination process)
  • EVA degrades via UV exposure, thermal stress on interconnecting wires
  • Tempered glass, EVA, flourinated plastic film
  • Electrode metals diffuse into Si at high T
  • Swelling of EVA causes cells to crack
  • Trichloroethylene can dissolve EVA at 80 C for 10 days
  • Mechanical stress used to mitigate EVA swelling
  • Mechanism: EVA becomes fluid with heatin, non-cross-linked EVA dissolves and flows out, cross-linked EVA swells (a force occurs perpendicular to cells, cause crack)
  • They also tried o-dichlorobenzene at 120 for a week with and without pressure, no cracking - Is o-DCB volatile?

S. Kang, S. Yoo, J. Lee, B. Boo, and H. Ryu, “Experimental investigations for recycling of silicon and glass from waste photovoltaic modules,” Renewable Energy, vol. 47, pp. 152–159, Nov. 2012, doi: 10.1016/j.renene.2012.04.030.

Abstract: This paper reports a new procedure for the recovery of resources from waste photovoltaic modules. The tempered glass was recovered using organic solvents. The metal impurities were removed by applying a chemical etching solution on the surface of the PV cell. We offer a much more efficient approach for recycling PV cells than the conventional method. The highest yield of silicon recovered was 86% when the PV cell was placed in the chemical etching solution for 20 min, along with the surfactant, which accounted for 20% of the total solution's weight at room temperature. This investigation showed that a high yield of pure silicon with purity of 99.999% could be obtained. The recovered pure silicon from waste PV modules would be contributed to the solution of several problems such as the supply of silicon, manufacturing costs, and end-of-life management of PV modules.

  • Most EOL modules are buried
  • Lamination at 150 C for 20 minutes under vac
  • 60% Si recovered in recycling processes in 2012
  • Nitric acid and thermal decomposition are main avenues - entire module must go into furnace
  • Their only success was with THF, but still had to thermally decompose EVA, I think this is toxic....
  • Then they etched cracked Si cell, not really what I'm aiming for, we want uncracked
  • They did manage to recover 86% of the Si

J.-K. Lee et al., “Photovoltaic performance of c-Si wafer reclaimed from end-of-life solar cell using various mixing ratios of HF and HNO3,” Solar Energy Materials and Solar Cells, vol. 160, pp. 301–306, Feb. 2017, doi: 10.1016/j.solmat.2016.10.034.

Abstract: This study presents the re-fabrication of a crystalline silicon (c-Si) solar cell using a Si wafer reclaimed from the solar cell of an end-of-life (EoL) module, and an evaluation of its performance. A 6-in. commercial solar cell was used in the etching process by wet chemical process in order to investigate the optimal mixing ratio of a mixture of HNO3 and HF. The silicon nitride (SiNx) and aluminum (Al) back contact on both sides of the solar cell were not completely removed at a high ratio of aqueous HNO3, and the precipitation of Ag particles on the surface of Si wafer were deposited at a high ratio of aqueous HF in a mixed acid solution. The optimum etching condition for the recovery of the c-Si wafer was applied to the EoL module, which consisted of 4″ solar cells. The photovoltaic (PV) performance of the re-fabricated 4″ solar cell was measured by conventional solar cell processing, which shows the best results reported so far. The higher boron (B) concentration and reflectance of the re-fabricated solar cell reduced cell efficiency by 0.6% compared with the commercial 6″ solar cell. However, it has sufficient potential for use in the PV industry.

  • Immersed in HNO3 and HF for 6 min, ratios varied, reaction got up to 100 C and bubble/stirring occurred, rinsed in deionized water
  • Recovering unbroken Si cells they used Lee et al. patent
  • Not enough HF to get rid of AR coating and Al electrode, so N and Al still on wafer, the Ag did get removed by HNO3
  • Wafer thickness affects its performance but efficiency not affected beyond 200 um
  • QE is ratio of carriers of the cell to photons with a specific energy that is incident on cell - recomb decreases this
  • EQE factors in reflection and transmission
  • Large pyramid texture provide lower reflectivity
  • Recovered Si cell reflects more due to acid leaching, not so great for low wavelengths
  • Recovered sell has more surface recomb and carrier collection in emitter region, so IQE is lower
  • Open circuit voltage is directly related to minority carrier lifetime
  • High Boron concentration means lower minority carrier lifetime... caused by small pyramid texture on recovered cell

M. F. Azeumo, C. Germana, N. M. Ippolito, M. Franco, P. Luigi, and S. Settimio, “Photovoltaic module recycling, a physical and a chemical recovery process,” Solar Energy Materials and Solar Cells, vol. 193, pp. 314–319, May 2019, doi: 10.1016/j.solmat.2019.01.035.Abstract: End-of-life photovoltaic modules can be hazardous wastes if they contain hazardous materials. The main problem arising from this type of waste is the presence of environmentally toxic substances and the poor biodegradability of the waste, which occupies great volumes when landfilled. For these reasons, photovoltaic modules have to be treated before landfilling as required by the legislation. The subject of this paper is the polycrystalline silicon type photovoltaic modules. They were treated with a physical and a chemical process. The physical process was aimed at the recovery of glass, metals, and the polyvinyl fluoride film. The modules were initially shredded with a knife mill and then processed with heavy medium separation, milling, and sieving. The glass (76%) and 100% of the metals were recovered respectively, at a grade of about 100% and 67%. Finally, a flow sheet of the physical process was proposed. The chemical process was aimed at identifying the best conditions which allow the dissolution of the EVA (ethylene vinyl acetate), that is the polymer that attaches the three layers that make up the module, namely the glass, the polycrystalline silicon, and the polyvinyl fluoride support. The experimental factors investigated were: type of solvent, thermal pretreatment, treatment time, temperature, and ultrasound. The best conditions to completely dissolve EVA in less than 60 min were the use of toluene as a solvent at 60 °C combined with the use of ultrasound at 200 W, while the pretreatment at 200 °C appeared to be useless.

  • Raw material extraction will go up at some point
  • They've talked about keeping waste modules in cement
  • Pyrolysis actually are bad for environment due to emissions and very energy consuming
  • They used a circular saw to cut up panels once frames were dismantled, then a rock saw to get smaller samples
  • Milled the samples, then mixed into a solution of water, sodium chloride, and sodium polythungstate, then float and sick are collected and sieved
  • Tried the samples in water, toluene (99.8% purity), xylene, 2,4-trimethylpentane, n-heptane, and N,N-dimethylformamide with pre-treatment at 200 C, at boiling point of solvent for 120 min with ultrasonic bath - this gave them the best solvent to then try different pre-treatment, hold times, temp, w/ or w/o ultrasound
  • Calculated degree of detachment via weight (excluding polyvinyl flouride backfoil)
  • 60 C with ultrasound had similar degree of detachment to 100 C w/o ultrasound, at times longer than 50 min - which requires less energy? They decided 60 C w/ ultrasound was better
  • They conclude concentration of solvent was more impactful than ultrasonic bath

F. Pagnanelli et al., “Solvent versus thermal treatment for glass recovery from end of life photovoltaic panels: Environmental and economic assessment,” Journal of Environmental Management, vol. 248, p. 109313, Oct. 2019, doi: 10.1016/j.jenvman.2019.109313.Abstract: End of life photovoltaic panels of different technologies (poly crystalline Si, amorphous Si, and CdTe) were treated mechanically in pilot scale by single shaft shredder minimizing the production of fine fractions below 0.4 mm (<18% weight). Grounded material was sieved giving: an intermediate fraction (0.4–1 mm) of directly recoverable glass (18% weight); a coarse fraction (which should be further treated for encapsulant removal), and fine fractions of low-value glass (18%), which can be treated by leaching for the removal of metal impurities. Encapsulant removal from coarse fraction was successfully performed by solvent treatment using cyclohexane at 50 °C for 1 h giving high-grade glass (52% weight), which can be reused for panel production. Experimental results of solvent treatment were compared with those from thermal treatment by economic analysis and Life Cycle Assessment, denoting in both cases the advantages of solvent treatment in recovering high-value glass.

  • Crushing with single shaft shredder, undersieve of 20 mm
  • 4 fractions of different courseness
  • Biggest pieces treated in cyclohexane without magnetic stirring, between 40-60 C - cyclohexane recovered by separating solids and liquids, allowing it to be reused
  • Middle sized fractions characterized by mineralization in nitric acid, hydrochloric acid, and H2O2
  • Fine fraction was leached in sulfuric acid
  • No Si recovered
  • Maybe look into cyclhexane at least for EVA detachment?
  • "Reducing sample size decreases the time needed for the solvent to penetrate the polymeric matrix, favoring the kinetics of detachment"

US Granted Patent 9455367 B2

  • For recovering cells there are three methods
  • Method 1: heating module, remove insulating encapsulant, oxidize in heat for metallic oxide to form with metal coating on cell, collect ribbon from cell once separated via oxide layer
  • Method 2: Forming a crack on the glass, forming a pattern on second encapsulant, heat glass and second encapsulant
  • Method 3: Form a pattern on second encapsulant, heat glass and second encapsulant

Y. Kim and J. Lee, “Dissolution of ethylene vinyl acetate in crystalline silicon PV modules using ultrasonic irradiation and organic solvent,” Solar Energy Materials and Solar Cells, vol. 98, pp. 317–322, Mar. 2012, doi: 10.1016/j.solmat.2011.11.022.Abstract: Using probe-type ultrasonic irradiation, the dissolution of ethylene vinyl acetate (EVA) in photovoltaic (PV) modules was investigated in various organic solvents, including O-dichlorobenzene (O-DCB), trichloroethylene (TCE), benzene, and toluene. The experiments were carried out at different solvent concentrations, temperatures, ultrasonic powers, and irradiation times. In the presence of 450W of ultrasonic radiation, EVA in PV modules was completely dissolved in 3M toluene at 70°C; however, the PV cell was damaged due to the swelling of EVA. At an irradiation power of 900W, the dissolution ratio was greater than that obtained at a power of 450W, and the effects of ultrasonic power were confirmed at 70°C. In TCE and benzene, a decrease in the dissolution of EVA was observed as the temperature increased from 55 to 70°C due to the occurrence of pyrolysis and pyrolytic reactions, which were attributed to the low boiling point and ultrasonic degradation of the solvent, respectively. Except when O-DCB was used, cracks were observed in the PV cell, and the complete dissolution of EVA was attained. Thus, O-DCB is the most effective solvent for recovering PV cells via ultrasonic irradiation.

  • Study involved ultrasonic testing
  • They put organic solvent in a container that could be heated and in path of ultrasonic waves
  • They tried toluene, TCE, o-DCB, and benzene, diluted with ethyl alcohol
  • Used 25, 55, and 70 C, 5-60 min, ultrasonic 450 and 900 W, solvent concentrations 1 and 3 M
  • Measured dissolution by area of EVA dissolved
  • With benzene 3 M at 70 C for 1 hr, 450 W they got 5 % not dissolved
  • With toluene 3 M at 70 C for 1 hr, 450 W they got 100 % dissolved but some cracking, probably due to high concentration.... conclude ultrasound could be enhanced
  • At 900 W, EVA didn't really dissolve at low T
  • Dissolution rate needs to be higher than swelling
  • Studied dissolution rate with fixing concentration and ultrasound, varying time and T
  • Got flawless cell with o-DCB 3 M at 70 C for 1 hr, 900 W
  • Toluene at 3 M 70 C for 1 hr, 450 W and 900 W got cracking
  • What is o-DCB boiling point? They conclude benzene wasn't successful at higher T because its boiling T is 80 C

J. Shin, J. Park, and N. Park, “A method to recycle silicon wafer from end-of-life photovoltaic module and solar panels by using recycled silicon wafers,” Solar Energy Materials and Solar Cells, vol. 162, pp. 1–6, Apr. 2017, doi: 10.1016/j.solmat.2016.12.038.

Abstract: This paper details an innovative recycling process to recover silicon (Si) wafer from solar panels. Using these recycled wafers, we fabricated Pb-free solar panels. The first step to recover Si wafer is to dissolve silver (Ag) and aluminium (Al) via nitric acid (HNO3) and potassium hydroxide (KOH), respectively. The next step is to remove anti-reflection coating (ARC) and emitter on the surface by using an etching paste which contains phosphoric acid (H3PO4). Wafers onto which the etching paste was applied were heated for 2min at 320, 340, 360, 380, and 400°C. The recycled wafers showed properties with the thickness of over 180µm, resistivity of 0.5–4Ωcm, which are almost identical to those of commercial virgin wafers. Furthermore, the solar cells manufactured with the recycled wafers showed an efficiency equivalent to that of the virgin cells. Pb-free solar panels were fabricated with the solar cells by using 60Sn-38Bi-2Ag solder to assemble the solar panels. Thermal cycling test based on the standard IEC 61215 were performed on the solar panels in order to confirm their stability.

  • Used p-type multicrystalline Si, pulled from modules that were delaminated by heat
  • Etched with nitric acid to dissolve Ag, KOH to dissolve Al
  • Etching paste from Solartech for SiNx layer, contains phosphorous acid , then annealed at different T for 2 min, followed by KOH dip
  • Their recycled wafers were more than 180 um thick (just shouldn't be lower than 170 um for reprocessing)
  • They conclude resistivity is not affeced by annealing T
  • Got consistent carrier lifetime values with commercial wafers
  • Emitter layer was gotten rid of successfully because negligable amount of phosphorous on front of wafer

J. Park and N. Park, “Wet etching processes for recycling crystalline silicon solar cells from end-of-life photovoltaic modules,” RSC Adv., vol. 4, no. 66, pp. 34823–34829, 2014, doi: 10.1039/C4RA03895A.Abstract: Chemical wafer recovering processes fabricate virgin-like c-Si wafers from degraded c-Si solar cells. The ideal approach for disposing of end-of-life photovoltaic (PV) modules is recycling. Since it is expected that more than 50 000 t of PV modules will be worn out in 2015, the recycling approach has received significant attention in the last few years. In order to recover Si wafers from degraded solar cells, metal electrodes, anti-reflection coatings, emitter layers, and p–n junctions have to be removed from the cells. In this study, we employed two different chemical etching processes to recover Si wafers from degraded Si solar cells. Each etching process consisted of two steps: (1) first etching carried out using a nitric acid (HNO3) and hydrofluoric acid (HF) mixture and potassium hydroxide (KOH), (2) second etching carried out using phosphoric acid (H3PO4) and a HNO3 and HF mixture. The first etching process resulted in deep grooves, 36 μm on average, on the front of recycled wafers that rendered the process unsuitable for wafers to be used in solar cell production. Such grooves occurred due to different etching rates of Ag electrodes and silicon nitride (SiNx). On the other hands, the second etching process did not result in such grooves and produced a recovered Si wafer with a uniform and smooth surface. The recycled wafers obtained by the second etching process showed properties almost identical to those of commercial virgin wafers: thickness, 173 μm; minimum and maximum resistivity, 1.6 and 10 Ω cm, respectively; and average carrier lifetime, 1.785 μs. In addition, P and Al atoms were not detected in the recycled wafers by secondary ion mass spectroscopy.

  • Initial thickness of Si was 200 um, 30 um Al electrode on backside
  • Al frame dismantled, EVA burned off
  • Tried two different etching: HNO3 plus HF followed by KOH, or H3PO4 followed by HF plus HNO3
  • Second one worked really well, emitter layer, p-nn junction, AL, and back surface field gone - the first one left grooves because HNO3 plus HF etches Ag faster than the SiNx so wafer would need to be polished down 40 um, leaving it at 130 um thick
  • Used secondary ion mass spectroscopy (SIMS) to measure doping concentration - don't want to detect P or Al
  • I should probably estimate how much Si will be etched
  • Table 2 contains commercial virgin wafer characteristics - thickness 200 +-10 um, resistivity between 1-10 ohm cm, carrier lifetime between 1-3 us
  • Also surface was very smooth, like commercial unused wafer

M. Tammaro, J. Rimauro, V. Fiandra, and A. Salluzzo, “Thermal treatment of waste photovoltaic module for recovery and recycling: Experimental assessment of the presence of metals in the gas emissions and in the ashes,” Renewable Energy, vol. 81, pp. 103–112, Sep. 2015, doi: 10.1016/j.renene.2015.03.014.

  • Estimate wieght % of each component for samples
  • This study heats modules for 30 min at 600 C (12.8 C/min) - they got solid course grained residue(after sieving was Si, glass, and metal electrode), PV cell, and glass
  • PVF decomp at 450 C, EVA decomp at 350 C
  • Refer to Figure 6 for expected metal comp with PVF vs glass backing
  • Metal in fumes related to how the metal is present in module originally
  • Maybe should use EDS to chracterize samples before experimenting/after glass and EVA separation
  • Enough Ag present to be worth it
  • Ti present probably because of TiO2 of ARC
  • Cr and Pb present in off gasing, dangerous
  • Ashes have toxic metals but also valuable metals

T. Wang, J. Hsiao, and C. Du, “Recycling of materials from silicon base solar cell module,” in 2012 38th IEEE Photovoltaic Specialists Conference, Jun. 2012, pp. 002355–002358, doi: 10.1109/PVSC.2012.6318071.

Abstract: As the growing of photovoltaic (PV) industry, the environmental problems become a new consideration. Therefore, we propose a thermal method to recover materials, such as silicon, glass, and metal from conventional crystalline silicon modules. Two steps heating were used in the thermal treatment process in this study. During the thermal process, the EVA could be burned out and the whole glass plate could be obtained without breaking. The recycle glass could be directly used again as the module component when the temperature was well controlled. The recycle yield of silicon was 62% and the purity of obtained silicon material was 8N after cleaning by chemical solution treatment. The copper could be recovered in further acid treatment. The recycle yield of copper was 85%. The results show that the recycling of materials from silicon based solar module is promising.

  • Tedlar backsheet removed by heating module at 330 C for 30 min
  • EVA and Tedlar burned out at 400 C for 120 min - bad gases from Tedlar?
  • This study's cells broke - most likely too high of T

W.-H. Huang, W. J. Shin, L. Wang, W.-C. Sun, and M. Tao, “Strategy and technology to recycle wafer-silicon solar modules,” Solar Energy, vol. 144, pp. 22–31, Mar. 2017, doi: 10.1016/j.solener.2017.01.001.

Abstract: A major obstacle to sustainable solar technologies is end-of-life solar modules. In this paper, a recycling process is proposed for wafer-Si modules. It is a three-step process to break down Si modules and recover various materials, leaving behind almost nothing for landfill. Two new technologies are demonstrated to enable the proposed recycling process. One is sequential electrowinning which allows multiple metals to be recovered one by one from Si modules, Ag, Pb, Sn and Cu. The other is sheet resistance monitoring which maximizes the amount of solar-grade Si recovered from Si modules. The purity of the recovered metals is above 99% and the recovered Si meets the specifications for solar-grade Si. The recovered Si and metals are new feedstocks to the solar industry and generate $11–12.10/module in revenue. This revenue enables a profitable recycling business for Si modules without any government support. The chemicals for recycling are carefully selected to minimize their environmental impact. A network for collecting end-of-life solar modules is proposed based on the current distribution network for solar modules to contain the collection cost. As a result, the proposed recycling process for wafer-Si modules is technically, environmentally and financially sustainable.

  • HNO3 and HF used to avoid over-etching
  • Using HNO3 for recovering metals
  • Using HF to eliminate SiNx (ARC)
  • Using NaOH to eliminate eimitter and back surface field
  • NaOH with HNO3 make NaNO3 which is neutral/good fertilizer
  • HF from polymer burn off (PVF burning, off gas with water makes HF), can we use it in the next step of etching the SiNx?
  • Need scrubbers to contain flourine and NO and NO2 off gases into water
  • Did not discuss their method for burning polymer off

EVA dissolution[edit | edit source]

J. Park, W. Kim, N. Cho, H. Lee, and N. Park, “An eco-friendly method for reclaimed silicon wafers from a photovoltaic module: from separation to cell fabrication,” Green Chem., vol. 18, no. 6, pp. 1706–1714, Mar. 2016, doi: 10.1039/C5GC01819F.

Abstract: A sustainable method for reclaiming silicon (Si) wafers from an end-of-life photovoltaic module is examined in this paper. A thermal process was employed to remove ethylene vinyl acetate and the back-sheet. We found that a ramp-up rate of 15 °C min−1 and an annealing temperature of 480 °C enabled recovery of the undamaged wafer from the module. An ecofriendly process to remove impurities from the cell surface was developed. We also developed an etching process that precludes the use of hydrofluoric (HF) acid. The method for removing impurities consists of three steps: (1) recovery of the silver (Ag) electrode using nitric acid (HNO3); (2) mechanical removal of the anti-reflecting coating, emitter layer, and p–n junction simultaneously; and (3) removal of the aluminum (Al) electrode using potassium hydroxide (KOH). The reclaimed wafers showed properties that are almost identical to those of commercial virgin wafers: 180 μm average thickness; 0.5 and 3.7 Ω cm minimum and maximum resistivities, respectively; and 1.69 μs average carrier lifetime. In addition, cells fabricated with the reclaimed wafers showed an efficiency equivalent to that of the initial cells.

  • Poly crystalline prices going down due to demand, but recent increases have caused module production to increase (we should reuse cells if we can)
  • Need a method to recover pure cells without toxic chemicals
  • This study tries methods without HF (typically used to remove ARC), reduced nitric and phosphorous acid, reduced monetary and energy cost for production of cells
  • Maybe try to know the conversion efficiency of module before EOL
  • Employed a fixture to apply compressive stress during thermal decomp of EVA - metal plate on top with grooves for gases to escape - how heavy?
  • Three steps: 60% nitric acid at room T to remove Ag, 20 rpm grinding on SiC powder to remove ARC, emitter, and p-n junction, 45% KOH at 80 C dip to remove grind damage and Al on back of cell
  • Characterization methods: thickness via digital indicator, resistivity via four-point probe, surface impurities via SEM EDS, P and Al via secondary-ion mass spectroscopy, carrier lifetime via microwave detection of photo-conductance decay, NO SE
  • Really good results with compressive force at 480 C (15 C/min) - ramp rate is important bc of gel content
  • By-products of EVA decomp is propane, propene, ethane, butane, hexene-1, butene-1; can be safely disposed of with elctrostatis precipitator or fabric filter (Tammaro source)
  • EVA and back-sheet start to decompose at 260 C
  • Nitric acid good for no cracking(due to Ag being raised above cell surface) and Ag recovery - can produce toxic gas, just conduct under hood
  • KOH is for Al contact, back surface field, grinding damage - can process leftover solution to recover Al
  • Resistivity of commercial virgin wafers is 0.5-3 ohm cm (without passivation), they got 0.87-2.34
  • No P or Al left behind in any of their samples
  • Carrier lifetime of commercial virgin wafers is 0.5-3 us (without passivation), they got 0.87-2.34 us

Etching after polymer decomposition[edit | edit source]

M. Lippold, S. Patzig-Klein, and E. Kroke, “HF-HNO3-H2SO4/H2O Mixtures for Etching Multicrystalline Silicon Surfaces: Formation of NO2+, Reaction Rates and Surface Morphologies,” Zeitschrift für Naturforschung B, vol. 66, no. 2, pp. 155–163, Feb. 2011, doi: 10.1515/znb-2011-0208. Abstract:The reaction behavior of HF-HNO 3 -H 2 O etching mixtures, which are frequently used for texturing silicon surfaces, is significantly influenced by the addition of sulfuric acid. For high concentrations of sulfuric acid, nitronium ions NO 2 + ions have been detected by means of 14 N NMR spectroscopy, and results of Raman spectroscopic investigation have allowed the quantification of the nitronium ions. Maximum etching rates of 4000 - 5000 nm s −1 are reached for HF (40 %)-HNO 3 (65%)-H 2 SO 4 (97%) mixtures with w (40%-HF)/w (65%-HNO 3 ) ratios of 2 to 4 and w (97%-H 2 SO 4 )&lt;0.3. For higher concentrations of sulfuric acid, H 2 SO 4 can be considered as a diluent. In order to investigate the influence of the sulfuric acid at constant HF and HNO 3 quantities, fuming HNO 3 (100 %) was used and the water in the mixtures successively replaced by H 2 SO 4 . A sudden increase of etching rates was found for sulfuric acid concentrations around 6 mol L −1 correlating with the characteristic color of the etching solutions. Decreased reaction rates at &gt; 7 molL −1 H 2 SO 4 are attributed to high solution viscosities and the formation of fluorosulfuric acid. Generally, in HF-HNO 3 -H 2 SO 4 /H 2 O etching mixtures a reduced dissociation of nitric acid, the formation of nitronium ions, the solubility of neutral nitrogen intermediates (e. g. NO 2 , N 2 O 3 ), as well as other effects influence the attack of silicon surfaces. The structure of etched silicon surfaces was investigated by means of scanning electron (SEM) and laser scanning microscopy (LSM). The morphologies are influenced most significantly by the relative amounts of sulfuric acid. Unexpectedly, in nitronium ion-containing mixtures rough surfaces with pore-like etching pits are generated. Graphical Abstract HF-HNO 3 -H 2 SO 4 /H 2 O Mixtures for Etching Multicrystalline Silicon Surfaces: Formation of NO 2 + , Reaction Rates and Surface Morphologies

  • When etching Si with HNO3, nitrous oxide, hexaflourocilicic acid, water, H2, NO2, N2O, NH4 form, so we should add something(H2SO4) else to lessen products
  • Si dissolves much more once H2SO4 concentration exceeds 6 mol/L
  • Accomodate this high dissolution rate by increasing solution viscosity and form more flourosulfuric acid
  • With H2SO4 rich solutions, pore-like pits form on surface

Recycling via Crushing/Grinding[edit | edit source]

P. Zhao et al., “A novel and efficient method for resources recycling in waste photovoltaic panels: High voltage pulse crushing,” Journal of Cleaner Production, vol. 257, p. 120442, Jun. 2020, doi: 10.1016/j.jclepro.2020.120442.

Abstract: Photovoltaic power generation technology has developed rapidly in the past decade due to its clean and efficient characteristics. However, with the development of photovoltaic power generation technology, a large number of waste photovoltaic panels are generated, but there is no clean and effective method for resources recycling in waste photovoltaic panels. High-voltage pulsing tends to cause fractures at interfaces of materials with different dielectric constants, which has a satisfactory recovery effect on layered materials like photovoltaic panels In this paper, high voltage pulse crushing is used to dissociate and enrich waste photovoltaic panels, the experimental results show that there are differences in the selectivity of different components during high-voltage pulse crushing (selectivity: Ag > Si > glass). This makes high voltage pulse crushing have good enrichment effect on photovoltaic panels. Most of the high-value elements are enriched to lower grain size, the glass purity of 0.5∼4 mm grain size can be directly recycled while it reaches over 98%. High-voltage pulse crushing has the most obvious enrichment effect on silver, Selectivity increases with the decrease of field strength and pulse number. The enrichment rate of silver in the lower field strength and pulse can reach 3.08, and the recovery rate is 54.07%. As the field strength and pulse number increase, the enrichment rate of silver drops to 1.67 but the recovery rate increases to 89.41%. The silver recovery can be effectively improved by adjusting the electrode gap. High-voltage pulse crushing can effectively enrich and recover the silver in the waste photovoltaic panels, providing convenience for subsequent sorting.

  • Materials of different dieletric constants fracture at interface during high-voltage pulsing, so it is used for PV modules, traditionally used for mineral processing and recycling circuit boards
  • 9.6 million tonnes of waste PV by 2050
  • Against organic solvent, thermochemistry, chemical etching, and mechanical recycling methods: poor dissociation, high cost, reagant harm, pollution, high energy consumption
  • High-voltage pulsing could allow for enrichment
  • Heating in muffle furnace 650 C for 1 hr pyrolyzes EVA and Tedlar backfoil... what about bad byproducts or EVA swelling?
  • High-voltage pulse applied to solid submersed in water, electric field intensity in the solid breaks down but in water does not so a channel of discharge occurs and expands which causes a pressure wave which breaks down the solid
  • Increase voltage, decrease in average particle size that is produced due to more crushing of initially crushed solid, good for amount recovered
  • Electrode gap increase, decrease in electric field intensity so less crushing happens
  • More Ag and Si recovered with higher voltage
  • Most product dissociate at higher number of pulses
  • Ag gets crushed first, then Si, then glass
  • Easier to dissociate between materials with more different dielectric constants
  • Ag gets enriched, no Si recovery

M. F. Azeumo, C. Germana, N. M. Ippolito, M. Franco, P. Luigi, and S. Settimio, “Photovoltaic module recycling, a physical and a chemical recovery process,” Solar Energy Materials and Solar Cells, vol. 193, pp. 314–319, May 2019, doi: 10.1016/j.solmat.2019.01.035.Abstract: End-of-life photovoltaic modules can be hazardous wastes if they contain hazardous materials. The main problem arising from this type of waste is the presence of environmentally toxic substances and the poor biodegradability of the waste, which occupies great volumes when landfilled. For these reasons, photovoltaic modules have to be treated before landfilling as required by the legislation. The subject of this paper is the polycrystalline silicon type photovoltaic modules. They were treated with a physical and a chemical process. The physical process was aimed at the recovery of glass, metals, and the polyvinyl fluoride film. The modules were initially shredded with a knife mill and then processed with heavy medium separation, milling, and sieving. The glass (76%) and 100% of the metals were recovered respectively, at a grade of about 100% and 67%. Finally, a flow sheet of the physical process was proposed. The chemical process was aimed at identifying the best conditions which allow the dissolution of the EVA (ethylene vinyl acetate), that is the polymer that attaches the three layers that make up the module, namely the glass, the polycrystalline silicon, and the polyvinyl fluoride support. The experimental factors investigated were: type of solvent, thermal pretreatment, treatment time, temperature, and ultrasound. The best conditions to completely dissolve EVA in less than 60 min were the use of toluene as a solvent at 60 °C combined with the use of ultrasound at 200 W, while the pretreatment at 200 °C appeared to be useless.

  • Raw material extraction will go up at some point
  • They've talked about keeping waste modules in cement
  • Pyrolysis actually are bad for environment due to emissions and very energy consuming
  • They used a circular saw to cut up panels once frames were dismantled, then a rock saw to get smaller samples
  • Milled the samples, then mixed into a solution of water, sodium chloride, and sodium polythungstate, then float and sick are collected and sieved
  • Tried the samples in water, toluene (99.8% purity), xylene, 2,4-trimethylpentane, n-heptane, and N,N-dimethylformamide with pre-treatment at 200 C, at boiling point of solvent for 120 min with ultrasonic bath - this gave them the best solvent to then try different pre-treatment, hold times, temp, w/ or w/o ultrasound
  • Calculated degree of detachment via weight (excluding polyvinyl flouride backfoil)
  • 60 C with ultrasound had similar degree of detachment to 100 C w/o ultrasound, at times longer than 50 min - which requires less energy? They decided 60 C w/ ultrasound was better
  • They conclude concentration of solvent was more impactful than ultrasonic bath

F. C. S. M. Padoan, P. Altimari, and F. Pagnanelli, “Recycling of end of life photovoltaic panels: A chemical prospective on process development,” Solar Energy, vol. 177, pp. 746–761, Jan. 2019, doi: 10.1016/j.solener.2018.12.003.Abstract: The application of photovoltaics has been rapidly increasing over the past two decades driven by the idea that it could provide a fundamental contribution to the transition from traditional fossil fuels to renewable energy based economies. However, long-term sustainability of photovoltaics will be largely dependent on the effectiveness of the process solutions that will be adopted to recycle the unprecedented volume of end-of-life panels expected to be generated in the near future. Recycling is indispensible to avoid the loss of the valuable materials employed to produce the photovoltaic panels and, at the same time, prevent that harmful elements, including, for example, heavy metals, could be dispersed into the environment through improper disposal practices. In this article, the process solutions proposed over the past two decades to recycle photovoltaic panels are critically reviewed. Main objective is to provide the basis for the identification of the recycling solutions that can effectively sustain the continuous increase of the photovoltaic market. In order to assess the requirements that should be satisfied by the recycling processes, the legislation currently in force to regulate the management of end-of-life photovoltaic panels is reviewed, and the evolution of the PV market over the past two decades is analysed. Based on this analysis, forecasts are derived for the flux of end-of-life panels that will be generated over the coming four decades. A technical survey of the previously proposed recycling processes is successively performed by including, in addition to the analysis of the research studies published in scientific articles, a detailed review of the patented recycling processes. Indications are given to which may be the most promising processes in terms of their economic sustainability and environmental impact.

  • Reduced use of metal in PV modules means recycling them is less valuable
  • Polycrystalline Si PV cells are least expensive to produce, in the market right now
  • Concentrator PV have trackers to move with the sun - added cost
  • Dye-sensitized cells also expand the variety of modules that will eventually become waste
  • Year of manufacture is relevant for recycling and LCA
  • 2036 will see spike in PV waste because 2008-2011 had a spike in PV production
  • Best glass recovery from two blade rotors crushing followed by hammer crushing and thermal treatment
  • Al and Ag recovered by aluminum chloride solution, yielding poly-aluminum-hydroxide-chloride

Recycling Thin Film PV Modules[edit | edit source]

W. Berger, F.-G. Simon, K. Weimann, and E. A. Alsema, “A novel approach for the recycling of thin film photovoltaic modules,” Resources, Conservation and Recycling, vol. 54, no. 10, pp. 711–718, Aug. 2010, doi: 10.1016/j.resconrec.2009.12.001.

Abstract: A sustainable recycling of photovoltaic (PV) thin film modules gains in importance due to the considerable growing of the PV market and the increasing scarcity of the resources for semiconductor materials. The paper presents the development of two strategies for thin film PV recycling based on (wet) mechanical processing for broken modules, and combined thermal and mechanical methods for end-of-life modules. The feasibility of the processing steps was demonstrated in laboratory scale as well as in semi-technical scale using the example of CdTe and CIS modules. Pre-concentrated valuables In and Te from wet mechanical processing can be purified to the appropriate grade for the production of new modules. An advantage of the wet mechanical processing in comparison to the conventional procedure might be the usage of no or a small amount of chemicals during the several steps. Some measures are necessary in order to increase the efficiency of the wet mechanical processing regarding the improvement of the valuable yield and the related enrichment of the semiconductor material. The investigation of the environmental impacts of both recycling strategies indicates that the strategy, which includes wet mechanical separation, has clear advantages in comparison to the thermal treatment or disposal on landfills.

  • Estimated life span of thin film modules is 25-35 years
  • At EOL hazardous materials may affect environment, humans
  • The processes explored in this study limit or eliminate chemicals/reagents

Processing of Recovered Si Cell[edit | edit source]

Encapsulation[edit | edit source]

B. V.G. Mohan, J. Mayandi, J. M. Pearce, K. Muniasamy, and V. Veerapandy, “Demonstration of a simple encapsulation technique for prototype silicon solar cells,” Materials Letters, vol. 274, p. 128028, Sep. 2020, doi: 10.1016/j.matlet.2020.128028. Abstract: The impact of encapsulation on solar photovoltaic (PV) modules includes insulation and protection, which alters the device performance as a function of wavelength of incoming light. Most lab-scale PV research ignores these features, but with a promising rise in front surface spectral conversion mechanisms, methods of optical enhancement and biomimetic layers makes this oversight unacceptable. To enable encapsulation of lab-scale PV, this study evaluates a simple encapsulation method. Multi-crystalline silicon (mc-Si) wafers were encapsulated using a pouch laminator and compared with a (poly)-methyl methacrylate (PMMA) front coated cell and an unencapsulated control cell. The cell’s diffuse reflectance with the encapsulant exhibits better photon absorption in the UV region, which is verified from improved external quantum efficiency. Despite the loss of a small percentage of visible photons, the electrical performances of the encapsulated cells were not affected. On the other hand, the PMMA coated cells showed an outstanding photon to electron conversion, but did not result in effective charge collection. The results show that a low-cost pouch laminate at the lab scale is an adequate method for encapsulating solar cells without overly degrading performance. In addition, for short lifetime small-scale PV applications, this method represents a means of distributed PV manufacturing.

  • Polymer thin films with luminescent for encapsulant
  • Typically PMMA or EVA
  • Also spectral converter
  • PMMA also Anti-Reflection Coating
  • Polyethylene terephthalate: ethylene vinyl acetate used for lamination
  • They got lower reflectance above 320 nm, no UC reflectance 250-320 nm
  • Bare cell absorbs no photons beyond 300 nm
  • Diode's series resistance goes up when dielectric material coats surface
  • More resistance slows down photocurrent between diode cells
  • Lamination is better than surface coating

Texturing[edit | edit source]

K. Chen et al., “MACE nano-texture process applicable for both single- and multi-crystalline diamond-wire sawn Si solar cells,” Solar Energy Materials and Solar Cells, vol. 191, pp. 1–8, Mar. 2019, doi: 10.1016/j.solmat.2018.10.015. Abstract: The photovoltaic (PV) industry requires efficient cutting of large single and multi-crystalline (sc- and mc-) silicon (Si) wafers. Historically multi-wire slurry sawing (MWSS) dominated, but the higher productivity of diamond-wire-sawing (DWS) holds promise for decreasing PV costs in the future. While surface texturing of DWS wafers is more complicated than of MWSS wafers, especially in mc-Si wafers, nanotexturing has been shown to overcome this challenge. While the benefit of nanotexturing is thus clearer in mc-Si, a universal nano-texture process that also works on sc-Si would simplify and reduce the investments costs of PV production-lines. In this paper, such a nano-texture process is developed using a metal-assisted chemical etch (MACE) technique. Step-by-step characterization of surface structure and reflectance of the MACE process is used after: 1) wafering, 2) standard acidic texturing etch, 3) silver nanoparticles deposition, and 4) MACE nanotexturing for both sc and mc-Si. The results show that the same MACE process works effectively for both sc-Si and mc-Si wafers. Finally, the nano-textured wafers are processed into PV cells in an industrial process line with conversion efficiencies of 19.4% and 18.7%, for sc-Si and mc-Si solar cells, respectively.

  • MWSS is multi-wire slurry sawing, typical procedure for wafering Si
  • Method of wafering affects surface texture
  • Alkali for MWSS single crystal Si, acid for MWSS multi crystal Si
  • Why surface texturing necessary?
  • DWS is diamond wire sawing - much faster than MWSS, less Si waste, avoids thick layers of damage
  • We want less damage from saw (wafer quality)
  • We want more damage from saw (texturing)
  • Multi crystal wafers need acid texturing process, only works well with deep damage which you get from MWSS not DWS
  • How to texture multi crystal that's been DWS?
  • Introduce black Si nano structure (also improves light absorption)
  • MACE is wet metal-assisted chemical etching - first deposit catalyst(Au, Pt, Ag, Cu) via sputtering, electrochemical deposition evaporation, electro-less displacement
  • Nano-pore formation then polishing in acid then Ag removed
  • Get a inverted pyramid structure when alkali process used after DWS, helps with absorption
  • MACE after DWS lowers reflectance but increases surface area so more surface recombination
  • Second polishing step aimed to reduce surface area
  • They got less saw marks with polishing step than acidic texturing, wafer is flatter, because more Ag deposits around crystal defects of DWS marks
  • Wider diameter and shallower depth for nano-texture is good for reflectivity
  • Surface morphology indicates shape of emitter layer
  • Their single crystal IQE was a little lower than other work, their multi crystal IQE was equal to other work

U. Gangopadhyay, S. K. Dhungel, P. K. Basu, S. K. Dutta, H. Saha, and J. Yi, “Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication,” Solar Energy Materials and Solar Cells, vol. 91, no. 4, pp. 285–289, Feb. 2007, doi: 10.1016/j.solmat.2006.08.011.Abstract: Alkali etchant cannot produce uniformly textured surface to generate satisfactory open circuit voltage as well as the efficiency of the multi-crystalline silicon (mc-Si) solar cell due to the unavoidable grain boundary delineation with higher steps formed between successive grains of different orientations during alkali etching of mc-Si. Acid textured surface formed by using chemicals with HNO3–HF–CH3COOH combination generally helps to improve the open circuit voltage but always gives lower short circuit current due to high reflectivity. Texturing mc-Si surface without grain boundary delineation is the present key issue of mc-Si research. We report the isotropic texturing with HF–HNO3–H2O solution as an easy and reliable process for mc-Si texturing. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation and also lowers the reflectivity of etched surface. The study of surface morphology and reflectivity of different mc-Si etched surfaces has been discussed in this paper. Using our best chemical recipe, we are able to fabricate mc-Si solar cell of ∼14% conversion efficiency with PECVD AR coating of silicon nitride film. The isotropic texturing approach can be instrumental to achieve high efficiency in mass production using relatively low-cost silicon wafers as starting material with the proper optimization of the fabrication steps.

  • Alkali etchant delineates grain boundary so don't get uniform texturing - provides pyramidic texture
  • Isotropic etching with acid is preferred - provides meso and macro porous structures, lowers reflectivity - HF/HNO3/H2O (14:1:5) 2 minutes or HF/HNO3 (98:2) for 2 and 5 minutes
  • Less pores of high depth and small diameter seemed to provide reduction in reflectivity (produced by double acid etching) - pores on surface of Si reduce refractive index, and index depends on porosit and depth, also higher roughness an reduce reflectivity by increasing scattering
  • Also smaller grain size compared to commercial Si increase scattering for lower wavelengths
  • Deep small pores lead to "non-homogeneous impurity distribution" which leads to recomb of charge carriers, also diode leakage current observed for this texture
  • Double acid treatment results in higher short circuit current and more absorption
  • Surface texture matters for the short circuit current which effects cell efficiency

K. Xiong, S. Lu, D. Jiang, J. Dong, and H. Yang, “Effective recombination velocity of textured surfaces,” Appl. Phys. Lett., vol. 96, no. 19, p. 193107, May 2010, doi: 10.1063/1.3396078.Abstract: Surface texturization is an effective way to enhance the absorption of light for optoelectronic devices but it also aggravates the surface recombination by enlarging the surface area. In order to evaluate the influence of texture structures on the surface recombination, an effective surface recombination velocity is defined which is assumed to have an equivalent recombination effect on a flat surface. Based on numerical and analytical calculation, the dependences of effective surface recombination on the pattern geometry, the surface recombination velocity, and the diffusion length are analyzed.

  • Effective recomb velocity helps figure out how texture affects surface recomb
  • This study assumes a uniform doping profile
  • "Effective recomb velocity is proportional to total surface area and surface recomb velocity" when there is a uniform excess carrier distribution
  • Diffusion length and surface recomb velocity affect excess carrier density at surface so essentially the effective surface recomb velocity

Passivation (ALD)[edit | edit source]

G. von Gastrow et al., “Analysis of the Atomic Layer Deposited Al2O3 field-effect passivation in black silicon,” Solar Energy Materials and Solar Cells, vol. 142, pp. 29–33, Nov. 2015, doi: 10.1016/j.solmat.2015.05.027.


  • Study used (100) Si wafer
  • Reactive ion etching to get b-Si: etched in sulfuric acid/O2 plasma for 7 min at -120 C, 10 mTorr
  • Followed by HF dip and ALD 20 nm alumina, then annealed at 425 C for 30 min, N2 atmos
  • Charge at ALD interface measured with COCOS (corona charge)
  • What exactly is effective surface recomb velocity
  • Platinum sputtered over sample for contrast in characterization
  • Achieved a highly conformal layer
  • Characterize surface passivation quality with interface defect density and total dielectric charge density
  • Damage is likely with REI - loss of crystallinity
  • Crystallographic structure of Si remained, so REI not too damaging
  • Good passivation = high carrier lifetime (could be from enhanced field-effect, not from nanostructure or surface area)
  • For n-type Si, negative surface charge repels the majority carriers from surface to generate the depletion region, if charge large enough the n-type becomes p-type at the surface
  • Alumina provides high negative charge density - it is fixed, adds to total electric field, therefore enhanced field-effect
  • High charge from high surface area
  • Low charge density: depletion region follows shape of nanostructure (cones in this case)
  • High charge density: depletion region gets deeper into Si so effective recomb is lower, resembles planar Si, good passivation

P. Repo et al., “Effective Passivation of Black Silicon Surfaces by Atomic Layer Deposition,” IEEE Journal of Photovoltaics, vol. 3, no. 1, pp. 90–94, Jan. 2013, doi: 10.1109/JPHOTOV.2012.2210031.Abstract: The poor charge-carrier transport properties attributed to nanostructured surfaces have been so far more detrimental for final device operation than the gain obtained from the reduced reflectance. Here, we demonstrate results that simultaneously show a huge improvement in the light absorption and in the surface passivation by applying atomic layer coating on highly absorbing silicon nanostructures. The results advance the development of photovoltaic applications, including high-efficiency solar cells or any devices, that require high-sensitivity light response.

  • Nanotexturing helps with absorption, passivation helps with adverse affects(increased surface recomb) of nanotexture
  • Coating for passivation also aids in absorption
  • REI advantages: fast, inexpensive, no mask layers (Sainiemi et al.), rate independent of crystalline planes
  • Alumina is good for passivating p-type emitters in n-type Si
  • Advantages of ALD: conformality, pinhole free
  • Study used p-type (100) Si, magnetic, also used sulfuric acid/O2 to etch (same process at Gastrow et al.)
  • For ALD, O3 is used for its higher reactivity (better film)
  • See (Sainiemi et al. "Suspended nanstructured alumina membranes") for their ALD conformality
  • ALD doesn't change structure at all, but thermal oxidation does, this is where reflectivity may increase/change
  • In both cases (planar and b-Si) b-Si has lower minority lifetime but for low resistivity samples the difference in lifetimes for planar and b-Si is within 1 ms
  • Want some initial surface roughness for ALD
  • Low excess carrier density(due to high negative charge of alumina) prevents minority carriers from getting to surface
  • Max surface recomb velocity when there's a depletion of majority carriers (or the surface has an electric field to attract minority) but still a high majority carrier concentration at surface
  • Without a critical level of charge density at surface, surface recomb determined by density of interface states(chemical passivation) (Hoex et al.)
  • Too much above critical level of charge density at surface can mean the electric field lowers recomb velocity

P. Saint-Cast et al., “Very low surface recombination velocity of boron doped emitter passivated with plasma-enhanced chemical-vapor-deposited AlOx layers,” Thin Solid Films, vol. 522, pp. 336–339, Nov. 2012, doi: 10.1016/j.tsf.2012.08.050.Abstract:

  • Efficiency of Si solar cells limited in recomb in p-type cell (Boron doped)
  • Must use n-type, but this must be passivated
  • Since ALD is slow, people study thin layers
  • Study on plasma-enhanced CVD (alternative to ALD)
  • Wafer gets electrolytic etching first, then some get thermally grown SiO2, plasma-assisted ALD Al2O3, or PECVD Al2O3
  • They used a lifetime tester?
  • For planar Si, they concluded that ALD and PECVD yield same results for B-doped
  • Higher value of J0e for SiO2 means it doesn't passivate B as well
  • Annealing may increase field-effect, giving better passivation?
  • High number of dopants limits recomb bc carriers can't get to surface, when there's a high surface recomb velocity
  • For FLAT Si: low emitter(B in this case) saturation current with Al2O3 layer - good quality

T. Pasanen, V. Vähänissi, N. Theut, and H. Savin, “Surface passivation of black silicon phosphorus emitters with atomic layer deposited SiO2/Al2O3 stacks,” Energy Procedia, vol. 124, pp. 307–312, Sep. 2017, doi: 10.1016/j.egypro.2017.09.304.Abstract: Black silicon (b-Si) is a promising surface structure for solar cells due to its low reflectance and excellent light trapping properties. While atomic layer deposited (ALD) Al2O3 has been shown to passivate efficiently lightly-doped b-Si surfaces and boron emitters, the negative fixed charge characteristic of Al2O3 thin films makes it unfavorable for the passivation of more commonly used n+ emitters. This work studies the potential of ALD SiO2/Al2O3 stacks for the passivation of b-Si phosphorus emitters fabricated by an industrially viable POCl3 gas phase diffusion process. The stacks have positive charge density (Qtot = 5.5·1011 cm-2) combined with high quality interface (Dit = 2.0·1011 cm-2eV-1) which is favorable for such heavily-doped n-type surfaces. Indeed, a clear improvement in emitter saturation current density, J0e, is achieved with the stacks compared to bare Al2O3 in both b-Si and planar emitters. However, although the positive charge density in the case of black silicon is even higher (Qtot = 2.0·1012 cm-2), the measured J0e is limited by the recombination in the emitter due to heavy doping of the nanostructures. The results thus imply that in order to obtain lower saturation current density on b-Si, careful optimization of the black silicon emitter profile is needed.

  • Al2O3 thin films not ideal for passivating common n-type emitters (fixed negative charge)
  • Emitter sat current suffers when nanostructures have high doping because emitters have high recomb - doping profile must be optimized
  • b-Si passivation depends a lot on field effect
  • Thin film must be conformal with fixed positive charge
  • Study uses 22 nm ALD Al2O3 and stack of 6.5 nm plasma-assisted ALD SiO2 with 30 nm thermal ALD Al2O3 (Used Si with O2 plasma, and TMA with H2O for ALD precursors), 200 C
  • Followed by annealing at 400 C for 30 min, N2 atmosphere
  • Emitter saturation current density is telling of passivation quality, as well as total oxide charge density(ideally high) and interface defect density
  • They also look at corona charge test, how it changes emitter sat current density
  • Doping is too high in Al2O3 sample so emitter sat current isn't good, the film can't invert the cell from n-type to p-type
  • Emitter sat current density improves more with lower doping because phosphorous is more active and Auger recomb is not so dominant
  • Corona charge did not do much to stacked samples
  • Short ALD purge and pulses can be okay for flat Si, not b-Si becuase high aspect ratio (Elam et al.)
  • For b-Si though, emitter sat current density should be able to be better with a better ALD process (different purge/pulse times)

L. Aarik, T. Arroval, H. Mändar, R. Rammula, and J. Aarik, “Influence of oxygen precursors on atomic layer deposition of HfO2 and hafnium-titanium oxide films: Comparison of O3- and H2O-based processes,” Applied Surface Science, vol. 530, p. 147229, Nov. 2020, doi: 10.1016/j.apsusc.2020.147229.Abstract: Atomic layer deposition (ALD) of HfO2 and hafnium-titanium oxide (HTO) in O3- and H2O-based processes by using a flow-type reactor was studied. Growth per cycle (GPC) recorded for the HfCl4-O3 process at substrate temperatures of 225–600 °C was 0.05–0.13 nm. At temperatures exceeding 300 °C, the O3-based process yielded films with lower GPC and marked thickness gradients, but with lower chlorine contamination levels than the HfCl4-H2O process did. In the HTO films grown from HfCl4, TiCl4 and O3, the thickness gradients decreased with increasing TiO2 content to values that were smaller than those of the films deposited from HfCl4, TiCl4 and H2O. The O3-based ALD of HTO resulted in lower chlorine concentration and higher GPC in the films with Hf/(Hf + Ti) atomic ratios of 0–0.8 and 0.3–0.8, respectively. Independently of the oxygen precursor used, the as-grown HTO films contained anatase at Hf/(Hf + Ti) values of 0–0.16, monoclinic phase with inclusions of cubic, tetragonal or orthorhombic phase at Hf/(Hf + Ti) values of 0.71–1.00, and predominantly amorphous phase at intermediate Hf/(Hf + Ti) values. Differently form the O3-based process, the H2O-based one allowed growth of monoclinic phase with well-developed preferential orientation in the films with Hf/(Hf + Ti) atomic ratios of 0.88–1.00.

  • Starting to mix (or stack) materials for better permittivity and refractive index - like mixing HfO2 and TiO2
  • ALD gives more control over composition of these mixed films
  • Are binary films with two materials and ternary with three?
  • They utilized QCM to characterize growth - growth rate at each step in cycle should be accessible
  • Used Si (100), carrier gas flow at 240 sccm, chamber pressure at 200-250 Pa (lower than we keep ours), 5-2-5-5 s was their cycle for HfCl4-purge-ozone-purge
  • XRF used for mass thickness and elemental comp
  • SE used for thickness and refractive indices
  • Glancing angle XRD used for phase comp (angle fixd at 0.420 +- 0.002), also XRD used for thickness and density of films
  • Amount adsorbed is meant to prevent more from adsorbing to surfaces, this dependent on precursor pressure of gas - this can affect thickness gradient in flow-type ALD
  • If we know the concentration of surface species related to the adsorption saturation, we can estimate the amount of precursor is that gaseous and its reactivity to surface
  • Thickness gradient also affected by gaseous reaction products, not controllable via amount of precursor introduced in system
  • Should determine growth delay - how many cycles is the minimum for film to start forming?
  • Monoclinic HfO2 more present in thicker films whereas cubic, tetragonal, and orthorhombic phases more in thinner films

D. M. Hausmann, E. Kim, J. Becker, and R. G. Gordon, “Atomic Layer Deposition of Hafnium and Zirconium Oxides Using Metal Amide Precursors,” Chem. Mater., vol. 14, no. 10, pp. 4350–4358, Oct. 2002, doi: 10.1021/cm020357x.Abstract: Atomic layer deposition (ALD) of smooth and highly conformal films of hafnium and zirconium oxides was studied using six metal alkylamide precursors for hafnium and zirconium. Water was used as an oxygen source during these experiments. As deposited, these films exhibited a smooth surface with a measured roughness equivalent to that of the substrate on which they were deposited. These films also exhibited a very high degree of conformality:  100% step coverage on holes with aspect ratios greater than 35. The films were completely uniform in thickness and composition over the length of the deposition reactor. The films were free of detectable impurities and had the expected (2:1) oxygen-to-metal ratio. Films were deposited at substrate temperatures from 50 to 500 °C from precursors that were vaporized at temperatures from 40 to 140 °C. The precursors were found to be highly reactive with hydroxylated surfaces. Their vapor pressures were measured over a wide temperature range. Deposition reactor design and ALD cycle design using these precursors are discussed.

  • HfO2 has dielectric constant at least 4 times higher than SiO2 - this is ideal for layering on PV cells
  • Alkylamides make less corrosive products than chloride precursors
  • Higher T in CVD results in higher crystallinity so you get rougher surface
  • Self-limiting and high reactivity are necessary characteristics for precursors to get high conformality at low T
  • They were using a flow type ALD
  • Heated the system so the precursor was at leat 1 Torr, with constant N2 gas flow at 0.25 Torr, 50 - 500 C so includes our goal T
  • Used at least 5 sec purge
  • Substrates were Si (100) dipped in 48% HF for 5 sec then put under UV lamp for 3 min (ozone) before ALD
  • Low-angle XRR (glancing incidence XRD?) used at 0.005 angle increments for 100 sec each angle, used for film thickness and density measurements
  • SEM FIB used for cross sectional analysis
  • QCM for monitering reactivity and vapor pressure, during experiment - so QCM probe used instead of substrate
  • Each dose of Hf precursor used an average of o.41 +- 0.04 umol, they found this agree with observed film density 2.45 Hf atoms/nm^2
  • At lamda=633 nm, HfO2 film had refractive index 2.05 +- 0.02
  • Bulk film density found to be 9.23 g/cm^3 which is 95% of bulk monoclinic HfO2
  • SE determined thickness 2-3 nm greater than thickness determined by XRR - may be due to HF and UV pre treatment because the surface would have been oxidized a bit
  • Purge time must be sufficient to remove unreacted gas-phase
  • They found that not enough purging increased film thickness but still uniform
  • Thickness gradient observed probably because of CVD reactions happening or "multilayer physisorption" (?)
  • "Water vapor required a longer purge time than the metal precursors at temp lower than 200 C"
  • At 100 C they found the min purge time for water was 300 sec, for metal amide about 100 sec
  • QCM measured increase in mass after pulse of metal amide, then decrease after pulse of water*
  • Do QCM study to find out if there is a min dose/exposure required
  • TDMAH saturated the QCM probe exactly at a dose of 5.6 mL whose vapor pressure at 70 C is 0.68 Torr - gathered by the QCM being saturated within seconds of introduction at pressure below 0.65 Torr but not after one min at pressure above 0.70 Torr
  • Concluded TDMAH melting point 30 C, temp at 0.1 Torr is 48 C, temp at 1 Torr is 75 C, decomp temp 90 C, enthalpy of vap 78 kJ/mol, entropy of vap 168 kJ/mol
  • Calculate sticking probability (AKA ratio between number of molecules that land on surface - determined by vapor pressure - and number that adsorb) per dose
  • Mechanism: metal amide is absorbed chemically on hydroxide-terminated surface (metal-nitrogen bond must break so metal bonds with oxygen, associated with the amide taking a proton from the surface hydroxyl), then water reacts with the amides of the surface and add a proton back to the surface hydroxyl. First two dialkylamides form, then they are replaced by 2 hydroxides*

J. Aarik, H. Mändar, M. Kirm, and L. Pung, “Optical characterization of HfO2 thin films grown by atomic layer deposition,” Thin Solid Films, vol. 466, no. 1, pp. 41–47, Nov. 2004, doi: 10.1016/j.tsf.2004.01.110.Abstract: Optical absorption and photoluminescence of amorphous and crystalline HfO2 thin films grown by atomic layer deposition from HfCl4 and H2O were studied. Band-gap energy of (5.55±0.03) eV was determined for monoclinic HfO2 with mean crystallite sizes of 30–40 nm as well as for amorphous HfO2. Excitation in the range of intrinsic absorption resulted in emission that had maximum intensity at 3.2 eV in the case of amorphous films and at 2.6 or 4.4 eV in the case of monoclinic films. The emission intensity of crystalline films exceeded that of amorphous films by an order of magnitude at all temperatures studied. The main luminescence band at 4.4 eV was tentatively assigned to the emission of self-trapped excitons while the emission at lower photon energies was attributed to defects and impurities. With the increase of temperature from 10 to 295 K, the low-energy edges of excitation spectra shifted towards lower energies by 0.1 eV in the case of amorphous films and by 0.15 eV in the case of crystalline films, indicating corresponding changes in the band-gap energies.

  • In this study, films were grown from HfCl4 and water precursors on a-Si and single crystal (111) Si dipped in HF and rinsed in deionized water then ultrasonic ethanol bath, pulse and purge times were 2 sec, substrate T very high (500-1200 K)
  • Crystallite size approximated with XRD, Voigt deconvolution, Scherrer equation
  • Peak broadening analysis done with LaB6 as standard material
  • See Swanepoel source for SE calculations
  • Samples at high T believed to have higher surface roughness, and they have less transmission
  • Band gap energies for both a- and crystalline-HfO2 were 5.55 ev
  • A-HfO2 band gap energy possibly affected by absorption edge/band tails

J. M. Khoshman and M. E. Kordesch, “Optical properties of a-HfO2 thin films,” Surface and Coatings Technology, vol. 201, no. 6, pp. 3530–3535, Dec. 2006, doi: 10.1016/j.surfcoat.2006.08.074.Abstract: Amorphous hafnium oxide (a-HfO2) thin films were grown on silicon and quartz substrates by RF reactive magnetron sputtering at temperature <52 °C. X-ray diffraction revealed that the thin films grown on the substrates are amorphous. The optical constants of a-HfO2 films were obtained by analysis of the measured ellipsometric spectra in the wavelength range 200–1400 nm, using the Cauchy–Urbach and Sellmeier models. Refractive indices and extinction coefficients of the films were determined to be in the range 1.86–2.15 and 0.07–2.6×10−5, respectively. The absorption coefficients, α, of a-HfO2 has been determined by spectroscopic ellipsometry and spectrophotometric methods over the energy range 0.88–6.2 eV. Analysis of α shows the bandgap energy of the films to be 5.68±0.09 eV. Measurement of the polarized optical properties reveals a high transmissivity (80%–97%) and low reflectivity (<15%) in the visible and near infrared regions at angles of incidence between 10° to 80°.

  • Put in the why HfO2
  • Layers of the film must be thin enough to avoid direct tunneling, film needs to be uniform
  • A-HfO2 can be used in flexible applications, like flexible solar modules
  • This study used Si (111) and quartz, with sputtering system at T less than 52 C
  • c-Si (111) really good for SE measurements
  • Is low T responsible for amorpous oxide?
  • For SE: 200-1400 nm at 10 nm steps, SE done from 70 to 75 degree AOI, R&T done from 20 to 80 degree AOI, also used to calculate bandgap energy
  • Applied Sellmeier model since k was nearly 0 for 300-11400 nm
  • Section 3.1 good for SE reference
  • n and k are strongly dispersed and decrease monotonically as wavelength increases
  • n found to be 1.85-1.95 for a-HfO2 films, k super small at wavelength above 350 nm (films transparent at these wavelengths)
  • Because k increases with decreasing wavelength, a-HfO2 films are absorbent in UV light
  • Absorption coefficient calculated with the Beer-Lambert law and SP data, or square law linear extrapolation with SE data (E alpha n)
  • n is function of photon energy, known by SE data
  • Amorphous different from crystalline because the crystalline momentum is undefined in amorphous...irregular atom positions
  • For R&T data, R for s-polarized and p-polarized light are increasing and decreasing respectively as AOI decreases
  • Lower band gap energy than crystalline HfO2
  • Average roughness of these films were 1.7-2.2 nm, pretty smooth
  • Less than 15% reflectivity in visible and near-infrared light

Characterizing New Cells[edit | edit source]

K. Bothe, R. Krain, R. Falster, and R. Sinton, “Determination of the bulk lifetime of bare multicrystalline silicon wafers,” Progress in Photovoltaics: Research and Applications, vol. 18, no. 3, pp. 204–208, 2010, doi: The determination of the bulk lifetime of bare multicrystalline silicon wafers without the need of surface passivation is a desirable goal. The implementation of an in-line carrier lifetime analysis is only of benefit if the measurements can be done on bare unprocessed wafers and if the measured effective lifetime is clearly related to the bulk lifetime of the wafer. In this work, we present a detailed experimental study demonstrating the relationship between the effective carrier lifetime of unpassivated wafers and their bulk carrier lifetime. Numerical modelling is used to describe this relationship for different surface conditions taking into account the impact of a saw damage layers with poor electronic quality. Our results show that a prediction of the bulk lifetime from measurements on bare wafers is possible. Based on these results we suggest a simple procedure to implement the analysis for in-line inspection. Copyright © 2010 John Wiley & Sons, Ltd.

  • As-cut multicrystalline Si gets max around 0.9 us effective carrier lifetime
  • Bare multicrystalline Si after KOH etch gets max around 1.1 us effective carrier lifetime
  • SiN passivated multicrystalline Si gets max around 33 us effective carrier lifetime (equal the bulk carrier lifetime)

CIGS[edit | edit source]

Y.-I. Kim, K.-B. Kim, and M. Kim, “Characterization of lattice parameters gradient of Cu(In1-xGax)Se2 absorbing layer in thin-film solar cell by glancing incidence X-ray diffraction technique,” Journal of Materials Science & Technology, vol. 51, pp. 193–201, Aug. 2020, doi: 10.1016/j.jmst.2020.04.004.

Abstract: In or Ga gradients in the Cu(In1-xGax)Se2 (CIGS) absorbing layer lead to change the lattice parameters of the absorbing layer, giving rise to the bandgap grading in the absorbing layer which is directly associated with the degree of absorbing ability of the CIGS solar cell. We tried to characterize the depth profile of the lattice parameters of the CIGS absorbing layer using a glancing incidence X-ray diffraction (GIXRD) technique, and then investigate the bandgap grading of the CIGS absorbing layer. When the glancing incident angle increased from 0.50 to 5.00°, the a and c lattice parameters of the CIGS absorbing layer gradually decreased from 5.7776(3) to 5.6905(2) Å, and 11.3917(3) to 11.2114(2) Å, respectively. The depth profile of the lattice parameters as a function of the incident angle was consistent with vertical variation in the compositionof In or Ga with depth in the absorbing layer. The variation of the lattice parameters was due to the difference between the ionic radius of In and Ga co-occupying at the same crystallographic site. According to the results of the depth profile of the refined parameters using GIXRD data, the bandgap of the CIGS absorber layer was graded over a range of 1.222–1.532 eV. This approach allows to determine the In or Ga gradients in the CIGS absorbing layer, and to nondestructively guess the bandgap depth profile through the refinement of the lattice parameters using GIXRD data on the assumption that the changes of the lattice parameters or unit-cell volume follow a good approximation to Vegard’s law.

  • Bandgap grading occurs when lattice parameter changes in absorbing layer, so device absorbs more
  • CIGS is chalcopyrite crystal structure, direct bandgap for solar PV, high absorption coefficient
  • Absorbing layer is inhomogeneous distribution of In and Ga, loss of performance here
  • Glancing incidence XRD used to profile Ga In as function of sample depth, nondestructive, controls penetrative depth (increases close to critical angle)
  • CIGS absorbing layer grown via co-evap at 400 C, CdS layer made by chemical bath deposition, ZnO and ITO layers made by radio frequency sputtering, metal grids made by electron-beam evap
  • Sample annealed to lower residual stress between layers
  • Bandgaps of crystalline materials depend on lattice parameters
  • CIGS bandgap is not constant
  • Interstitials occupying any two or both available sites in CIGS make diffraction pattern not look like calculated pattern
  • Difference between In and Ga cation's ionic radii changes interplanar spacing
  • Increasing glancing incidence increase overall intensities so Mo phase peaks can be seen
  • As incident angles shift to "high angle peaks" so shrinkage in lattice
  • Shift they saw in peaks with incident angle has to do with In or Ga atoms co-occupying a crystallographic site
  • Shrinkage in the lattice(or interplanar distance) suggests the "relative sit-occupancy ratio of Ga to In atoms" for co-occupying changes from surface to bottom region
  • So lattice shrinks with change in incident angle bc X rays are reaching different depths, and ^^ is changing with depth
  • Lattice parameter for this layer function of incident angle (and depth?)
  • In and Ga composition varies vertically... Ga content increases, In content decreases - the co occupying and ionic radii sizes contribute to this
  • Stoichiometry and electronic properties are functions of depth in CIGS absorbing layer
  • Good for absorbing layer to have graded band gap - carrier collection of photons with longer wavelength, less carrier recomb at heterojunction and back region