Sawing methods used to cut crystalline silicon and the influence of sawing and cleaning processes on texture
Chapter 7 Solar cell results
7.3 KOH-PVA texture for advanced solar cell processes
To texture (100) p-type Si-wafers, an etch solution which consists of 6 liters of deionized (DI) water, KOH and VPA was used. A temperature of 100oC and an etching time of 30 min were used. With this etch solution Cz-Si (200 µm thick) and FZ-Si (230 µm thick) wafers with a resistivity of 1-3 Ωcm and 1 Ωcm, respectively, were textured. The etching process took place in a glass beaker heated on a hotplate.
Silicon wafers textured with the KOH-VPA solution were processed into solar cells. Textured Cz-Si-wafers were processed into solar cells using the standard industrial screen-printing method and an advanced industrial method (selective emitter). Textured FZ-Si-wafers were processed into solar cells using an advanced photolithography-based process. While the texturing was performed within this work,
cell processing was carried out by Johannes Junge [6: Junge 2012].
The screen-printing method for processing solar cells was already explained in chapter 2 (2.5).
The processing scheme of the selective emitter process is very similar to the screen-printing processing scheme. There are only two differences between these two cell processes. The first is that the selective emitter process starts with a stronger emitter diffusion which produces an emitter with a sheet resistivity of 30 Ω/□. The second difference consists in the formation of a selective emitter. This is carried out as follows: after POCl3 diffusion an acid resistive mask is selectively screen printed on the emitter, which protects it from further acid etching. Then by using an acidic solution (HF, HNO3, H2O), the emitter is lightly etched until it reaches a sheet resistivity of 50 Ω/□ [7: Haverkamp 2008]. After that, the printed mask is removed.
Front contact fingers are printed on regions with high phosphorous doping, i.e., regions that were not etched back.
The photolithography-based process [6: Junge 2012] (see Fig. 7.2) starts with the cutting of the FZ-Si-wafers to a size of 5x5 cm2 to fit the requirements of the photolithographic equipment available at the University of Konstanz. After that, the wafers are textured as explained above. The POCl3 diffusion process is carried out to form an emitter with a sheet resistivity of 80-100 Ω/□. Subsequently, the wafers receive a PECVD SiNx:H layer as an anti-reflection coating. After that, a firing step is carried out in a conventional belt furnace. Then the front side is masked with a hot melt ink, and the emitter on the rear side is removed in a polishing etch consisting of HF, HNO3 and CH3COOH. After this, a dielectric rear-side passivation layer of aluminum oxide (Al2O3) is applied by atomic layer deposition, and an optional SiNx:H layer is deposited to protect the very thin passivation layer. Afterwards, the front contacts are defined by photolithography and evaporation of Ti, Pd and Ag.
Aluminum is evaporated on the rear side. Then the rear contact is established using a laser-fired contact (LFC) process. The front contacts are thickened by silver plating.
Finally, four solar cells (2x2 cm2) are cut with a dicing saw. After preliminary characterization, a microwave-induced remote hydrogen plasma (MIRHP) step is implemented to enhance hydrogen passivation, improve the rear-surface passivation, and sinter the front contacts. After the IV characterization of all solar cells, the best cells additionally receive a second anti-reflection coating (DARC) by means of thermally evaporated magnesium fluoride (MgF2).
Fig. 7.2: Process flow chart detailing the photolithography-based process featuring the Al2O3
rear side (with an optional SiNx:H rear side capping layer).
In table 7.3 the IV data of the processed solar cells are shown.
Table 7.3: I-V results of the processed solar cells. Textured Cz-Si-wafers with an area of 12.5x12.5 cm2 are processed into solar cells via the standard screen-printing method (averaged over 8 cells) and by the selective-emitter method (averaged over 10 cells). Textured FZ-Si-wafers with a size of 5x5 cm2 are processed into four 2x2 cm2 solar cells using the advanced cell process method (best cell).
Material / Cell process jsc
(mA/cm2) Voc
(mV) FF (%)
η
(%) Cz / Screen printing 35.5 628 79.0 17.6 Cz / Selective emitter 36.5 637 78.3 18.2 FZ / Photolithography 39.3 660 77.6 20.0
Comparing the results of both kinds of industrially processed solar cells, we observe an absolute gain in solar cell efficiency of 0.6% for solar cells processed using the selective emitter process. This increase is mainly caused by the higher short-circuit current j and open-circuit voltage V of these cells. The increase in j and V is
attributable to the better blue response on etched-back regions (thinner dead layer and less Auger recombination) and the resulting better surface passivation.
Although the reflection values are almost the same for the silicon wafers textured with the KOH-PVA solution, the solar cells processed on the FZ-Si-wafer using the advanced process reach the highest jsc (39.3 mA/cm2) [8: Ximello 2011]., which is near the theoretical value of jsc (42.5 mA/cm2) estimated for the technologically achievable AM1.5G efficiency limit of Si solar cells [9: Aberle 1995].. This result demonstrates that the texture on FZ-Si-wafers displays characteristics appropriate to develop solar cells with efficiencies closer to the technological limit.
Also, this photolithographic cell process allows for the definition of very narrow front metal contact fingers, which in combination with a low-doped emitter explains the higher value of the short-circuit current jsc achieved on this solar cell. Furthermore, it shows the importance of the high quality passivation layer of Al2O3 on the rear side, which results in a high value of the open-circuit voltage Voc. Moreover, this cell process shows very encouraging results due to its low thermal budget.
7.4 References
1. D.H. Neuhaus et al., Industrial silicon wafer solar cells, Advances in OptoElectronics, 2007, Article ID 24521 (2007).
2. A. Dastgheib-Shirazi et al., Selective emitter for industrial solar cell production: a wet chemical approach using a single diffusion process, Proc. 23th EU PVSEC, 1197 (2008).
3. J. Junge et al., Evaluating the efficiency limits of low cost mc Si materials using advanced solar cell process, Proc. 25th EU PVSEC, 1722 (2010).
4. N. Ximello et al., A new KOH-etch solution to produce a random pyramid texture on monocrystalline silicon at elevated process temperatures and shortened process time, Proc. 24th EU PVSEC, 1958 (2009).
5. N. Ximello et al., Influence of pyramid size of chemically textured monocrystalline silicon wafers on the characteristics of industrial solar cells, Proc. 25th EU PVSEC, 1761 (2010).
6. J. Junge, High efficiency process development for defect-rich silicon wafer materials, Dissertation (2012).
7. H. Haverkamp et al., Minimizing the electrical losses on the front side:
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8. N. Ximello et al., Up to 20% efficient solar cell on monocrystalline silicon wafers by using a KOH-high boiling alcohol (HBA) texturing solution, Proc. 26th EU PVSEC, 849 (2011).
9. A.G. Aberle et al., Limiting loss mechanism in 23% efficient silicon solar cells, J. Appl. Phys. 77, 3491 (1995).
Chapter 8 Conclusions
This dissertation presents research on various different etch solutions which can be used to produce a pyramidal texture on solar cells. A new alkaline etch solution which employs a long chain alcohol (polyvinyl alcohol, PVA) has been found.
Comparing the new potassium hydroxide (KOH)-PVA solution with the standard KOH- isopropyl alcohol (IPA) etch solution, the new solution was found to have the following advantages:
1. – It is less sensitive to the surface characteristics of as-cut silicon wafers.
Therefore, it can be applied to many different assortments of as-cut silicon wafers without the necessity of pre-treatment cleaning.
2. – The KOH-PVA texture shows a 1% lower weighted reflection in the wavelength range between 400 and 1000 nm, i.e., the light-trapping properties are slightly enhanced.
3. – The KOH-PVA texture average pyramid size was approx. 4 µm, which is about half the pyramid size of the KOH-IPA texture (8 µm). Although we observed low reflection values for small pyramids, the SUNRAYS simulation program finds similar light-trapping properties with both kinds of texture. This discrepancy is due to the small pyramids and step like structures on pyramids with lengths smaller than 1 µm which produce diffraction phenomena and therefore better light trapping properties. The diffraction phenomena can be simulated if the wave equation is solved. Light trapping properties on textured silicon surfaces are better explained by using the theory related to solve the wave equation, because it also can give information of the random nature of the texture, and it also considers a pyramidal texture with pyramid sizes between 1-4 µm.
4. – The recovery of at least one-third of the PVA can be accomplished almost directly. The controlled cooling of the etch solution to room temperature allows PVA to set into a thin film, which can then be easily skimmed off the solution.
5. – The cost of the KOH-PVA solution is lower, as only one-tenth of the alcohol (30 grams in 6 liters of solution) is used, compared with the amount of alcohol used in the standard KOH-IPA solution (300 ml in 6 liters of solution).
All the above results were found in the laboratory using a glass beaker (10 liter volume).
Although the advantages of the KOH-PVA solution above mentioned, some disadvantages were also found:
1. – After the texturing process some of the alcohol remains on the surface of the textured wafer, as was found by lifetime measurements and by observing that after the texturing process the textured wafers were hydrophilic.
2. – Therefore, an extra cleaning step should be implemented to obtain textured silicon wafers without alcohol residues. The well-known RCA and IMEC cleaning processes could be employed here.
3. – The KOH-PVA solution requires an anti-foaming agent in order to control the foam produced during the etching process.
4. – The enlargement of the etch solution process to industrial scale depends on the properties of the material used to construct the container used to hold the solution.
Despite the disadvantages noted above, solar cells with the KOH-PVA texture (12.5x12.5 cm2) were processed, and in some cases the results with solar cells processed with KOH-IPA texture were compared.
By using the screen-printing method, solar cells achieved an efficiency of 17.7%
for the KOH-IPA texture, whereas for the KOH-PVA texture, an efficiency of 17.8%
was achieved. This small gain could be attributed to the 1% lower absolute reflection values. At this point it is important to note that both kinds of solar cells were processed together, i.e. no extra cleaning step was performed on the KOH-PVA texture. The same assortment of as-cut silicon wafers was used.
For another assortment of as-cut silicon wafers (12.5x12.5 cm2), the KOH-PVA textures were again processed into solar cells by means of both the screen-printing method and the selective-emitter method. Here again, the solar cells were processed together. In this case, the screen-printed solar cells achieved a solar efficiency of 17.6%, whereas the selective-emitter solar cells achieved an efficiency of 18.2%.
Again, after the texturing process, no extra cleaning step was performed.
A solar cell (2x2 cm2) was processed by means of the photolithography-based method; here the solar cell efficiency was 20.0%.
Thus, despite the disadvantages observed on the KOH-PVA texture, the solar cells produced results similar to those found with the standard KOH-IPA texture.
However, the implementation of the KOH-PVA solution in industrial production will require much more work, i.e., the disadvantages of the KOH-PVA solution must be overcome.
In this study was also introduced for the first time a closed etching bath for texturing silicon wafers was also introduced. The closed etching bath was developed by the Lotus wet chemical company.
By using a KOH-IPA solution at 80oC (below the boiling point of IPA, 82.4oC) in the closed etching bath, the new etching method allows the periodic application of pressures below atmospheric pressure (for example: 0.6 the value of the atmospheric pressure) during the texturing.
The low pressure inside the closed bath also means a decrease of the boiling point of IPA (for example: down to 70oC). And because the etching process is taking place at 80oC, IPA starts to evaporate for very short periods of time when the pressure is decreased, thereby removing hydrogen bubbles and monosilicic acid particles from the surfaces of silicon wafers.
The sudden detachment of hydrogen bubbles and monosilicic acid particles permits the application of fresh etch solution to the surfaces of the wafers. Thereby, the etching time in a closed etching bath was considerably reduced by using a vacuum process. This requires only half the time needed with the standard KOH-IPA solution. The characteristics of the pyramidal texture produced with a vacuum process are similar to those found with the KOH-PVA texture, i.e. small pyramids and lower reflection values.
The closed etching bath also allowed the recovery of the evaporated IPA by means of a cooling system installed above the closed etching bath.
Despite the advantages of the closed etching bath for the KOH-IPA solution, the experiments with the KOH-PVA solution did not decrease etching times. The problem here was that the vacuum pump did not lower the pressure inside the bath at an etching temperature of 100oC. Here the employment of a more powerful pump is necessary. In this case, the recovery of water vapor was possible by means of the cooling system.
References
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