Recycling Solar PV Modules Literature Review/Processing of Recovered Si Cell

Processing of Recovered Si Cell[edit | edit source]

Encapsulation

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

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)

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.

Abstract:

• 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

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: https://doi.org/10.1002/pip.975.Abstract: 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