How To Compare Texturisation To Etching On Solar Cells

Transcription

1 COMPARISON OF ACIDIC AND ALKALINE TEXTURED MULTICRYSTALLINE SOLAR CELLS IN A SOLAR PANEL PRODUCTION T. Geipel, S. Pingel, J. Dittrich, Y. Zemen, G. Kropke, M. Wittner, J. Berghold SOLON SE, Am Studio 16, Berlin, Germany torsten.geipel@solon.com, Tel: ABSTRACT: Most crystalline silicon solar modules manufactured today contain textured multicrystalline solar cells which usually have higher cell efficiencies when measured without encapsulation compared to textured solar cells. However an optimization of the cell efficiency under STC does not necessarily result in efficiency advantages when encapsulated in panels. The aim of this work is to compare the two texturisations with regard to the final power output in the panel and in the field. Therefore, 3 and textured cells coming from the same wafer batch and utilizing same production parameters are processed to solar panels. A small outdoor field is equipped with both texturisations. The output STC-power is contrasted with the input STC-power. Moreover, energy yield, reliability as well as breakage rates are compared. Keywords: Texturisation, Multi-Crystalline, PV Module 1 INTRODUCTION reduce other influences as much as possible. The efficiency of solar cells is determined according to standard test conditions (STC). Presently, the aim of the cell manufacturers is an optimization of the optical and electrical properties of the cells with respect to STCmeasurements. However, STC-optimizations do not necessarily lead to increased output after encapsulation within solar panels. The main question in this paper is, if texturisation of multicrystalline wafers has any benefits over texturisation for a solar panel production regarding output in the panel, energy production in the field, longterm reliability and breakage. Therefore we designed a mass production experiment and let one of our main cell suppliers fabricate 3 cells of each texturisation - and - using the same batch of wafers, the same production line and process parameters. In a similar manner we then built up panels from these cells using the same machines and parameters, monitored breakage rates and compared the power output with each other. Afterwards a small test site for each type was installed using equal inverters. The test sites were monitored for several weeks and their energy yield was compared. In parallel, some panels were exposed to reliability tests. Cell Manufacturer Panel Manufacturer Alkaline Texturing Same Batch of Wafers Emitter diffusion Edge Isolation SiNx deposition Metallization Acidic Texturing Back Surface Field and Contact Firing Electrical Characterization Cell Interconnection Layup Lamination Framing / Junction Box Installment Electrical Characterization 2 CELL AND PANEL PROCESSING 2.1 Overview An overview of the complete processing sequence of the solar cell and panel production can be seen in Figure 1. The cell manufacturer processed a total quantity of around 6 multicrystalline wafers with a size of 156mm x 156mm and a thickness of 2µm coming from the same batch. The wafers were split up in two groups one to be textured and the other to be textured. The cell supplier declared that no optimization of the etching bath was made since it is not a standard product and would have caused extended downtimes of the production. The remaining process steps for the solar cell production as well as the panel production at Solon were kept constant for both groups to Figure 1) Overview of the cell and panel processing steps The panels consist of 6 in series connected cells. We used lightly structured solar glass, standard EVA and white backsheet foil. Acidic and work orders were processed alternatingly to average out any possible process instabilities. An reference panel for both types was used for the final electrical characterization. 2.2 Texturisation Multicrystalline wafers for industrial solar cell production contain a surface layer of saw damage that is removed using an etch (also called saw damage etch) at the beginning of the solar cell production process [11]. The etch is done using a solution of NaOH

2 or KOH and results in an optically flat and highly reflective wafer surface. Because of the anisotropic nature of the etching process it is not effective for creating a uniform texturisation over differently oriented grains. The total overall reflectance remains in the order of 3 to 36% [1],[2],[8]. An alternative method was presented by IMEC using an etching (also called: isotexturing) based on a mixture of HF, HNO 3 and organic additives to create a uniform texturisation on multicrystalline wafers [1]. It was later optimized without organic additives for an easier implementation in an industrial production [2]. The method is a defect etch and requires the surface damage to be present on the wafer. During the etch 5-1µm are removed from the surface and structures in form of small, rounded etch pits are obtained. The diameter of the etch pits is up to 1µm and they are uniformly and randomly distributed over differently oriented grains. The technique leads to a lower total wafer reflectance between 22 to 27% [1],[2],[8]. After antireflective coating the difference in reflectance between both types usually diminishes but due to a better utilization of the UV and infrared light still a gain of around 1 ma/cm² in current density compared to cells can be achieved [1]. Acidic texturisation leads to significantly higher cell efficiencies under STC when measured without encapsulation. The final cell has a uniformly matt visual appearance as shown in Figure 2b. a) b) Figure 2) Visual appearance of the different texturisations on multicrystalline cells Texturisation has a large influence on the mechanical stability of the cells. The etching time and consequently the etching depth is a major process parameter influencing the mechanical stability. It was reported that texture leads to a strong increase in stability compared to the as-cut wafer due to the removal of the saw damage. However extended etching times can lead to the generation of steps between the grains that reduce the stability [3]. Acidic texturing is more difficult to control because of the composition change of the etching bath due to the ongoing consumption of the two main components to water and its exothermic nature requiring effective cooling. A temperature-rise can drastically increase the etching rate [9]. With regard to mechanical stability texturing seems to be more dependent on the type and quantity of saw damage initially present on the wafer compared to etching [5]. Finally, the composition and lifetime of the etching bath plays a role on the mechanical stability. It was found in [1] that solutions with high HF content lead to the formation of deep groves along grain boundaries that can severely worsen the mechanical properties. Based on this discussion we expect the etching to be relatively uncritical concerning mechanical stability compared to etching. 3 ELECTRICAL CHARACTERIZATION 3.1 Cells In total we received 312 cells and 316 cells in a distribution as shown Figure 3. As expected texturing yields higher cell classes than texturing. On average we received a cell STCpower of 3.78Wp for and 3.88Wp for texturing. However, it becomes apparent from Figure 3 that etching, although not optimized for this experiment, yields a narrower distribution of cell classes which provides better control of the outgoing panels. quantity ,7 3,75 3,8 3,85 power (W) Figure 3) Distribution of delivered cells 3,89 3,94 TABLE I shows average I-V parameters of the cell class 3.85Wp for both texturisations. The short circuit current, Isc for cells is on average 3.1% higher which is explained by the uniformly textured surface and its lower reflection. Unexpectedly, we found the fill factor, FF of cells to be regularly around 1% in absolute higher than for cells and the series resistance around 1% lower. This effect could be the result of a lower resistive contact of the metallization pastes on the macroscopic rough but microscopic flat wafers compared to the overall roughly structured surface. Moreover the gap in Voc of 6mV can be traced back to a better surface passivation of the textured cells. TABLE I) I-V curve parameters of cell class 3.85Wp for both texturisation types 1 Voc [mv] % Isc [A] % Vmpp [mv] % Impp [A] % Pmax [W] % FF [%] % 3,99

3 3.2 Panels After panel production we obtained the distribution of panel power classes shown in Figure 4. In TABLE II the average input cell STC-power is contrasted with the average output panel STC-power. It becomes obvious from TABLE II that both texturisations yield almost identical panel outputs although on average the cells had 2.6% relative higher efficiencies. quantity power (W) 228 Figure 4) Distribution of the outgoing panels 23 TABLE II) Ingoing cell STC-power compared to outgoing panel STC-power 1 input cell STCpower [Wp] % output panel STC-power [Wp] % The averagely achieved panel power for the different cell classes is shown in Figure 5. The horizontal lines mark an idealized panel power by multiplying the rated cell power with the number of cells in the panel. It is clear that panels made from textured cells lie much closer to the ideal output than panels from textured cells. We obtain an encapsulation loss of -.7% for the cells and -2.8% for the cells relative to the ideal value. panel power (W) ideal ,7 3,75 3,8 3,85 3,89 3,94 3,99 cell power (W) Figure 5) Average panel power achieved for the different cell classes The main reason for this difference lies in a stronger increase of the current in the range of 2.5% for cells in the panels while cells only increase about.5%. An increase in current after encapsulation is the result of a better matching of the optical interfaces (SiN- EVA, EVA-glass, glass-air) and a backscattering effect of the white backsheet in the gaps between the cells. Simulations in [1] have shown that encapsulation losses increase with increasing degree of texturisation. Optical gains are usually compensated by resistive losses due to the interconnection of the cells. We found those resistive losses to be equal for both texturisations. 3.3 Energy Yield We have shown that texturisation has significantly lower encapsulation losses. To identify differences in the energy production of two test fields were installed each consisting of 1.6kWp STC-power with all panels having almost identical Wp. This was achieved by choosing panels from cell class 3.8W for and 3.89W for (close to the average power of each type). The panels were installed with a 3 fixed tilt and an orientation of south in Berlin. Both test fields run with equal inverters and equipment. The yield data was obtained in a relatively short time frame during May and August 29. The specific energy yield (Y f ) over time is presented in Figure 6 and a table is accompanied for better quantification of the results. It can be seen that both test fields run comparably within the measurement errors. Moreover, Figure 7 shows the specific yield over different light intensities. No significant differences in the lowlight performance can be seen. Specific Yield (kwh/kwp) Figure 6) Specific yield vs time Time (d) Y f month 1 Y May -.2% June +.1% July -1.2% August -.7% -.5% Specific Yield (Wh/Wp) f Irradience [W/m²] Figure 7) Specific Yield vs irradience

4 4 RELIABILITY Usually, a panel manufacturer guarantees a minimum power reliability of 25 years. Extended environmental chamber testing such as thermal cycling (-4 C 85 C) and damp heat (85 C with 85% rel. humidity) on the basis of IEC are used to detect possible failure modes that could lead to a reduction of the predicted lifetime. These tests with double durations, which are standard at Solon, have been used to compare the panels of the differently texturized cells regarding their longterm reliability. In total four panels were exposed to reliability tests. Two panels that were mechanically stressed beforehand as described in [6] were treated with thermal cycling tests (TC) and two panels with damp heat tests (DH). The results of this testing can be seen in Figure 8 and Figure 9. The mechanically stressed panels did not show any significant power degradation after 4 thermal cycles. After damp heat testing both texturisations also perform comparably. We found the highest Pmaxdegradation of 1.2% of the panels after DH 2. After double the exposure time the power degradation of all panels is well within the maximum allowed 5% for a single test step according to the IEC qualification. The slightly stronger decrease of in the damp heat test is most likely the result of measurement uncertainties rather then degradation effects. Hence we conclude that both texturisations offer the same longterm reliability. power loss -,1% -,3% -,5% -,7% -,9% -1,1% -1,3% -1,5% initial TC 2 TC 4 Figure 8) Power loss after thermal cycling power loss -,1% -,3% -,5% -,7% -,9% -1,1% -1,3% -1,5% initial DH 5 DH 1 DH 2 Figure 9) Power loss after damp heat 5 BREAKAGE Cell breakage besides power output is one of the main economic factors in a solar panel production. As described earlier texturisation can have a large impact on the mechanical stability of the cells. We monitored the breakage rates during the production runs and found the average results shown in TABLE III. The information we obtained from the cell supplier is also included. The median and the quartiles, which are insensitive to outliers, are shown here instead of arithmetic mean and standard deviation. TABLE III) Breakage rates cell production < 1.75% [7] panel production (median) 1.39 %.89 % Q %.29 % Q % 1.19 % Although both texturisations show comparable breakage rates below critical values the rates and deviations for cells are slightly higher. To understand the differences we made additional thickness measurements on several sample cells using a manual thickness gauge. We found that the samples had an average thickness (including metallization) of 248µm with a standard deviation of ± 3µm whereas the cells were found to be only 243µm with a larger standard deviation of ± 7µm. We assume that this is the effect of an excessive etching caused by the unoptimized process that the cell manufacturer had to run to avoid downtimes in their production line. An unwanted step generation between the grains on the wafers weakening the stability could have already taken place. Although both breakage rates are in a comparable range it will be the aim of our further work to identify more clearly and based on optimized processes the differences in the mechanical stability of and textured cells as well as other cell types. 6 SUMMARY AND DISCUSSION TABLE IV) Positive and negative aspects found during the mass production experiment - lower encapsulation - higher cell losses efficiencies pro - narrower power - visual appearance distribution contra equal - lower cell efficiencies - energy yield - reliability - breakage - higher encapsulation losses Acidic and textured multicrystalline solar cells coming from the same wafer batch and production line have been compared in a solar panel production regarding power output, energy yield, reliability and breakage (see TABLE IV for a summary of the main points). Although the incoming STC-power for cells was significantly higher both types result in an identical STC-power in the panel. The encapsulation losses for cells were found to be 2% higher than for. Since both types also show comparable reliability, energy yield and breakage rates we conclude that the benefits of texturing is highly questionable regarding the whole value chain of photovoltaics.

5 Alkaline textured multicrystalline cells are a technologically equal alternative to textured cells. It is illusory to animate cell manufacturers to return and optimize texturisation since texturisation is established nowadays and seems to be one key technology for achieving highest efficiencies for industrial solar cells. However, cell manufacturers should optimize their products on the output in the final encapsulated panel, not primarily on STC-efficiency. In consequence, the panel manufacturers have responsibility to optimize with regard to energy production in the field being confronted with aspects such as longterm stability, lowlight performance as well as temperature dependence. It must also be our focus to lower resistive losses which seem to be a predominant loss mechanism when using high efficiency multicrystalline solar cells. The best solution to this challenge lies in close cooperation between panel manufacturer and cell supplier. [1] P. Grunow, S. Krauter, Modelling of the Encapsulation Factors for Photovoltaic Modules in Proc. of the 4 th WCPEC, Waikoloa, Hawaii, 26 [11] D. L. King, M. E. Buck, Experimental Optimization of an Anisotropic Etching Process for Random Texturization of silicon solar cells in Proc. of the 22 nd IEEE PVSC, Las Vegas, USA, 1991 REFERENCES [1] R. Einhaus, E. Vazsonyi, J. Szlufcik, J. Nijs, R. Mertens, Isotropic Texturing of Multicrystalline Silicon Wafers with Acidic Texturing Solutions in Proc. of the 26 th IEEE PVSC, Anaheim,1997 [2] A. Hauser, I. Melnyk, P. Fath, S. Narayanan, S. Roberts, T. M. Burton, A Simplified Process for Isotropic Texturing of MC-Si in Proc. of the 3 rd WC PEC, Osaka, 23 [3] A. Schneider, G. Bühler, F. Huster, K. Peter, P. Fath, Impact of Individual Process Steps on the Stability of Silicon Solar Cells Studied with a Simple Mechanical Stability Tester in Proc. of the PV in Europe from PV Technology to Energy Solutions Conference, Rome, Italy, 22 [4] A. Schneider, E. Rueland, A. Kraenzl, P. Fath, Mechanical and Electrical Characterization of Thin Multi-Crystalline Silicon Solar Cells in Proc. of the 19 th EPVSEC, Paris, France, 24 [5] G. Coletti, C. J. J. Tool, L. J. Geerligs, Quantifying Surface Damage by Measuring Mechanical Strength of Silicon Wafers in Proc. of the 2 th EPVSEC, Barcelona, Spain, 25 [6] S. Pingel, Y. Zemen, J. Berghold, O. Frank, T. Geipel, Mechanical Stability of Solar Cells within Solar Panels, this conference [7] Internal Project Documentation between Cell Supplier and Solon [8] D. H. Macdonald, A. Cuevas, M. J. Kerr, C. Samundsett, D. Ruby, S. Winderbaum, A. Leo, Texturing industrial multicrystalline silicon solar cells in Solar Energy 76 (24) [9] S. De Wolf, P. Choulat, E. Vazsonyi, R. Einhaus, E. Van Kerschaver, K. De Clercq, J. Szlufcik, Towards Industrial Application of Isotropic Texturing for Multi- Crystalline Silicon Solar Cells in Proc. of the 16 th EPVSEC, Glasgow, UK, 2