Recycling Solar PV Modules Literature Review/Recycling to Recover Si Cell

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