Optimization of antireflection coating thickness for textured polycrystalline silicon solar cells — an experimental study

Solar Cells, 28 (1990) 253

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OPTIMIZATION OF ANTIREFLECTION COATING THICKNESS FOR TEXTURED POLYCRYSTALLINE SILICON SOLAR CELLS -AN EXPERIMENTAL STUDY B. L. SOPORI

Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 (U.S.A.) (Received and accepted June 14, 1989)

Summary Experimental results showing dependence of short-circuit current density J~ on the thickness of an antireflection (AR) film are given for two types of textured polycrystalline silicon solar cells. Results which identify changes in Jsc caused by encapsulation are also given for different AR film thicknesses. These results can be interpreted in terms of the changes in the optical coupling associated with the changes in the AR film thickness and as a result of encapsulation. Typical variations in optical reflectance and the spectral responses of the cell, as a result of changes in the AR film thickness, are presented to illustrate changes in the optical coupling.

1. Introduction Surface texturing has become an important approach for improving the coupling of a broad-band input (solar) spectrum to silicon solar cells. An added advantage of texturing is that it gives rise to an increase in the effective optical absorption by increasing the path length of light transmitted into the cell. The latter is a result of two different mechanisms: (1) scattering at the textured surface, which alters the direction of the light transmitted into the cell compared with that of the incoming light; (2) trapping of some light within the cell, if an appropriate backside configuration is used. There is a high degree of interest in studying the optical characteristics of textured surfaces because of these advantages. Already a great deal of the previously published work pertains to solar cells fabricated on (100) oriented, textured single-crystal substrates [1 - 3]. It has been shown that a ray optics approach can be used effectively to study reflectiontransmission characteristics of such surfaces and for the design of antireflection (AR) coatings. Now it is also well recognized that texturing can be a very effective means of reducing reflectance of polycrystalline (poly) silicon solar cells [4]. Furthermore, as in textured single-crystal cells, texturing of polycrystalline silicon alleviates tolerances in the parameters of the AR film and can pro0379-6787/89/$3.50

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duce a broad-band low-reflectance surface with a single-layer coating. This is a particularly important result as polycrystalline silicon is now being extensively used for solar cells. However, unlike textured single crystals, the optics of textured polycrystalline silicon cells is difficult to handle theoretically using a ray optics approach. This difficulty arises because the shape of the texture for each grain orientation is different, which results in a multitude of different optical paths over the entire cell. There are perhaps other theoretical approaches that can be developed to predict adequately the performance of textured polycrystalline cells [5]. However, in the absence of a suitable theory, experimental studies can produce valuable information to serve as a basis for the cell design and also to provide data to confirm the appropriateness of a theoretical approach. An area of major interest for polycrystalline silicon cells is the design of AR coatings which optimizes cell performance. Such a design involves selection of a suitable material with an appropriate refractive index and film thickness. However, it is recognized that unlike the case for a quarterwave film on a planar surface, the refractive index of a single-layer AR film is not a critical parameter for textured polycrystalline material. For example, a thin film of SiO2 (which has a refractive index of about 1.45) on a textured silicon surface can produce excellent AR characteristics and has been used in high-efficiency cells [6]. The main design consideration which remains is the choice of film thickness. This paper describes results of an experimental study on the effect of varying the thickness of an AR coating (Si3N4) on the performance of textured polycrystalline silicon solar cells. The performances of the cells are measured in terms of the short-circuit current density Jsc, as the primary effect of change in the optical characteristics is manifested in Jsc. This effect is studied for both unencapsulated and encapsulated configurations.

2. Experimental Solar cells were fabricated on polycrystalline silicon substrates obtained from two vendors: Wacker Chemtronics and Crystal Systems. The substrates of each group were from the same castings to minimize wafer-to-wafer variations. Both types of substrates were p type (boron doped) with resistivities of about 3 ~ cm for Wacker Chemtronics and about 1 ~2 cm for Crystal Systems materials. The substrates were textured in a KOH-based solution and solar cells were fabricated using the following processes: PH3 diffusions were done at 850 °C to form N+/p junctions, typically about 0.4 #m deep. The AR coating consisted of a chemically vapor-deposited Si3N4 film (on 50 - 100 A SiO2) on the textured cell. The metallization was done by electroless deposition of nickel followed by a solder dip. Two approaches were used to measure the dependence of the cell parameters on the AR coating (Si3N4) thickness. In one case, completed cells were etched in buffered HF to thin the Si3N4 layer. Several lots of

255 cells were etched for different times to obtain cells of different nitride thicknesses. In some cases, the same cells were etched several times to produce progressively thinner Si3N4 layers. After each etch step the nitride thickness, I - V characteristics and the spectral response of the cells were measured. This procedure allowed accumulation of data for different nitride thicknesses on the same cell, thereby eliminating the ambiguity of sample-tosample variations and serving as a guideline to verify other data. The etchback of nitride from the metallized cells was initially found to be a very sensitive process which also resulted in etching of some metal, with a resultant shunting. However, with appropriate conditions the etching was reproducible w i t h o u t any electrical or physical damage to the cell. Nevertheless this approach is very tedious and time-consuming. Consequently, only a few samples could be analyzed in this way. In the second approach, different lots of wafers were coated with nitride of different thickness and the cell fabrication was then completed. The cells of different nitride thicknesses were encapsulated as 5 in × 5 in -subassemblies using Sunadex glass, polyvinyl-butyral (PVB) as p o t t a n t and white tedlar as back-skin. The characteristics of encapsulated cells were also measured. It should be pointed out that to minimize the failure rate through the entire cell fabrication and encapsulation processes, and during extensive testing at various stages of fabrication, a sturdy metallization had to be used. This led to a large shadow fraction of 14%. The measurements of the cell characteristics and the spectral response were done in conventional manner. The nitride thickness of each cell was measured using one or both of the following techniques: (a)grains of (111) orientation were identified and the thickness of the nitride was measured on these grains using a laser ellipsometer; (b) a special reflectometer (described in ref. 7) was used to measure the average thickness of nitride over the entire wafer. Reflectance measurements were performed on some samples to identify the changes in the cell response due to changes in reflectance which, in turn, are associated with different nitride thicknesses. These measurements were made on a Cary 17 spectrometer using an integrating sphere. The reflectance measurements performed on the encapsulated samples required active water cooling of the samples. This need arose because of added absorption of the samples as well as added thermal insulation because of encapsulation.

3. Results The results described in this paper relate to the changes in the optical coupling caused by variation in the thickness of AR coating. The experimental data of J~ as a function of AR film thickness are given for cells with and w i t h o u t encapsulation. In addition, some details of the spectral responses and the wavelength dependence of reflectance are presented which can provide a considerable insight into the optical processes.

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Fig. 1. Spectral responses of a cell for various nitride thicknesses produced by step etching. Figure 1 shows typical changes in the spectral response of a cell through various steps of etching the nitride film. The cell was fabricated on a Wacker Chemtronics substrate and the measurements of the a.c. spectral response were made w i t h o u t light bias. Curve A in Fig. 1 corresponds to the initial (pre-etch) state of the cell. The cell parameters (open-circuit current, Voe and Jsc) and the nitride thickness t corresponding to curve A are 552 mV, 31.7 mA cm -2 and 870 A respectively. Curves B, C, D and E are the spectral responses measured after successive etching steps. The cell parameters and the nitride thickness after each etch step are: B -- 554 mV, 31.7 mA cm -1, 804 A; C -- 556 mV, 31.4 mA cm -2, 671 A; D -- 559 mV, 31.1 mA cm -2, 561 A; E -- 560 mV, 30.2 mA cm -2, 475 A. It is seen t h a t J ~ decreases with each reduction in the nitride thickness. From Fig. 1 it is seen that a reduction in the nitride thickness improves the short-wavelength response but reduces the long-wavelength response. This behavior is in agreement with the reflectance variation of the cell as a function of wavelength. Figure 2 O.5

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Fig. 3. S p e c t r a l r e s p o n s e s o f cells d e p o s i t e d w i t h d i f f e r e n t t h i c k n e s s e s o f t h e n i t r i d e film.

shows changes in the reflectance characteristics of a textured poly sample for different nitride thickness. It is seen that a decrease in the nitride thickness shifts the reflectance minimum towards shorter wavelengths. Furthermore, a decrease in the film thickness is accompanied by a decrease in the reflectance at shorter wavelengths and an increase at longer wavelengths. Figures 1 and 2 demonstrate the existence of certain optical effects on textured surfaces that are well known for planar surfaces with thin film coatings. Figure 3 shows measured spectral responses for a group of cells fabricated on Wacker Chemtronics substrates on which different nitride thicknesses were deposited during fabrication. The cells correspond to substrates selected from the same ingot and adjacent to each other. The values of nitride thickness are also shown in Fig. 3. The changes in the response as a function of wavelength follow a behavior that can be predicted from the reflectance data of Fig. 2. F r o m Fig. 3 we can qualitatively infer t h a t as the nitride thickness is reduced from 1200 A the integrated cell response first improves and then goes through an o p t i m u m and decreases again. The Jse values, in mA cm -2, for curves 1 - 6 are 30.7, 31.7, 32.2, 31.7, 31.4 and 31.1, respectively. The m a x i m u m Jse value occurs for a nitride thickness of 840 A. Similar results were obtained for cells fabricated on the Crystal Systems substrates. The Jg¢ data corresponding to cells o f different nitride thickness, fabricated on Wacker Chemtronics substrates, are shown in Fig. 4. The filled circles represent an average value of each lot of unencapsulated cells of the same nominal thickness of nitride. Figure 5 shows similar data for cells using Crystal Systems substrates. The J,~ data for Wacker Chemtronics and Crystal Systems cells after encapsulation are shown in Figs. 4 and 5 by

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open circles. Solid and broken lines are drawn through unencapsulated and encapsulated cell data to represent a continuous dependence of J~ on the nitride thickness. From Figs. 4 and 5 it is seen that: (1) The unencapsulated cells, both Wacker Chemtronics and Crystal Systems type, show an optimum nitride thickness within a broad range around 800 - 850 A. (2) The peaks corresponding to maximum Js¢ after encapsulation occur in the same range of the nitride thickness as for unencapsulated cells. This is an important result as it shows that, in this particular case of cell

259 processes and substrate parameters, the cells optimized for air are also optimized for module operation. (3) For values of nitride thickness less than a b o u t 600 A the encapsulation causes an increase in the Jsc of both types of cells. For greater thicknesses the behavior of the t w o groups of cells is slightly different. The Wacker Chemtronics cells continue to show a slight increase in Jsc upon encapsulation. This feature is clearly different from the general belief that the performance of cells decreases upon encapsulation because of the optical losses associated with reflections at various interfaces and the bulk absorption of the module media. 4. D i s c u s s i o n

The results reported in the previous section pertain to changes in the optical coupling as a result of changes in the thickness of AR coating. It is seen from the values that initial rapid increase in the cell current with increase in nitride thickness occurs because of improved short-wavelength coupling. Consequently, this change depends strongly on the junction and near-surface quality of the cell. In lower ranges of the film thickness an improved cell response upon encapsulation results from improved coupling between p o t t a n t and silicon relative to the air and silicon coupling. The improved coupling can offset the losses due to reflection at the air-glass interface, absorption loss in glass and the absorption in the pottant. The improvement in the coupling from p o t t a n t to the cell is expected at all values of the film thickness; however, b e y o n d the o p t i m u m thickness this increase is small and may n o t be adequate to compensate for the losses. Furthermore, at higher values of film thickness the improvement in the cell response depends strongly on the minority carrier diffusion length and the quality of the surface texture. The quality of the surface texture is related, in part, to the orientation distribution of the substrate. This feature is also believed to contribute to the continued improvement in the current of Wacker Chemtronics cells, upon encapsulation, for values of nitride thickness b e y o n d the o p t i m u m range. Although the characteristics of the cells described here are specific to the selected fabrication technique and to the choice of silicon nitride as the AR film, it should be emphasized that substrate characteristics (such as resistivity, crystallography and minority carrier diffusion length) from the t w o vendors are quite different; however, the trends in optical coupling vs. film thickness share many general features. Thus, it is expected that these trends are valid over a range of cell parameters and for various other AR coating materials. References 1 M. A. Green and P. Campbell, Proc. 19th [EEE Photovoltaic Specialists' Conf., N e w Orleans, LA, 1987, IEEE, New York, 1987, 912, and references cited therein.

260 B. L. Sopori and R. A. Pryor, Sol. Cells, 8 (1983) 249. P. Illes, J. Vac. Sci. Technol., 14 (1977) 1101. B. L. Sopori, Sol. Cells, 25 (1988) 15. E. Yablonovitch and G. D. Cody, IEEE Trans. Electron Devices, 29 (1982) 300. R. R. King, R. A. Sinton and R. M. Swanson, Conf. Record, 20th IEEE Photovoltaic Specialists' Conf., Las Vegas, NV, September 1988, IEEE, New York, 1989, 538. 7 B. L. Sopori, Rev. Sci. Instrum., 59 (1988) 725.

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