Random-Texturing of Phosphorus-Doped Layers for Multi-Crystalline Si Solar Cells by Plasmaless Dry Etching

We investigated a texturing process for crystalline Si solar cells by dry etching with chlorine trifluoride (ClF3) gas without plasma excitation. Recently our research group demonstrated improved electrical characteristics of single-crystalline Si solar cells textured by dry etching of the phosphorus-doped layers. In this report, we attempted to improve the electrical properties of multi-crystalline Si solar cells by modifying the experimental procedure and optimizing the process conditions. The reflectance of the treated surfaces was around 10% at 600 nm without an anti-reflection film. We demonstrated the characteristics of multi-crystalline solar cells by random-texturing by plasmaless etching. This is the first report to prove the validity of plasmaless dry texturing for multi-crystalline Si solar cells.


Introduction
A current goal in research for semiconductor solar cells is to develop a low-cost method for fabricating cells with high conversion efficiency.One of the methods for improving efficiency is to reduce the surface reflection loss of the incident light.Texturing is one of the methods for reducing surface reflection loss, and will be important for light trapping especially in future thin crystalline Si solar cells because of the relatively low absorption coefficient of Si.
(100) oriented single-crystalline Si substrates are mainly textured into pyramid-like structures with an alkaline solution by using the dependence of the etch rate on crystalline direction (Campbell & Green, 1987).However, an alkaline solution is not effective for texturing multi-crystalline Si.Random-textured structures created by wet etching with mixed acid solutions, HF/HNO 3 /CH 3 COOH (Nishimoto et al., 1999;Hauser et al., 2004;Nievendick et al., 2012) or HF/HNO 3 /H 2 SO 4 (Watanabe et al., 2013) for multi-crystalline Si have been investigated, but they are insufficient for obtaining a low reflectance.Moreover, the wet process will be difficult for thin substrates with low mechanical strength in the future.
Reactive-ion etching (RIE) using plasma is known to form textured structures on multi-crystalline Si surfaces at specific etching conditions.Inomata et al. (1997) reported around 17% efficiency for multi-crystalline Si solar cells textured by RIE with a Cl 2 gas, but the detailed experimental condition was not given.Various pieces of research indicated low reflectance below 4% textured by RIE with SF 6 /O 2 (Murias et al., 2012) and NF 3 /Ar (Cecchetto et al., 2013) gas mixtures, but the characteristics of the solar cells have not been referred.Recently Park et al. (2013) reported around 16.8% efficiency for multi-crystalline Si solar cells with cone and pyramid-like textured structures formed by RIE with SF 6 /O 2 .However, the RIE method often has disadvantages, such as plasma damage to the substrates (Yoo et al., 2008), contamination (Tucci et al., 2006), and high throughput mass production of solar cells.A different, low-cost, and damageless texturing method is considered to be required.
Our research group has intensively investigated a procedure for texturing crystalline Si by dry etching without plasma.The ClF 3 gas can etch silicon even at room temperature without plasma (Ibottoson et al., 1984).In addition, plasmaless dry etching is an isotopic reaction, and hardly damages substrates (Saito & Kosuge, 2007).
We can easily obtain a low reflectance near 10% at wavelengths between 300 -800 nm for Si surfaces randomly textured by ClF 3 .We fabricated solar cells by phosphorus thermal diffusion onto the textured surfaces.However, the increase of efficiency of the random-textured solar cells was below 10%.This value was less than the increase of the absorbed light in the cells, around 35%.The insufficient improvement was considered to be due to submicron structures caused by the dry texturing (Kohata & Saito, 2010).
Recently, we etched dry-textured surfaces with an acid solution to form appropriately-textured structures.Improved electrical characteristics were obtained for solar cells with dry-textured and wet-etched surfaces (Sanda & Saito, 2013).However, the two step process will not be preferred because of cost increase in practical fabrication.
In the related previous report (Kohata & Saito, 2010), we attempted to enhance the performance of dry textured solar cells by modifying the experimental procedure.Phosphorus-doped emitter layers were slightly etched with a ClF 3 gas to texture the cell surfaces.The textured surfaces had a reflectance of about 17.8% at a wavelength of 600 nm.We fabricated single-crystalline Si solar cells with a textured doped layer, and their efficiency was sufficiently improved in comparison with that of the mirrored cell.
In this study, the emitter etch-back texturing process was applied to the fabrication of multi-crystalline silicon solar cells in order to prove the validity of plasmaless texturing for multi-crystalline substrates.We attempted to improve the electrical performances by modifying the fabrication step and optimizing the experimental conditions.

Experimental Details
P-type multi-crystalline Si substrates with a resistivity of 0.5 -1.5 cm and a thickness of 250m were used and were cut to a size of 2 cm × 3 cm.The substrates were etched with a hydrofluoric acid and nitric acid-based solution to remove slice-induced damaged layers and were cleaned with a hot ammonia and hydrogen peroxide-based solution.Schematic diagrams for the following fabrication process flow of solar cells are summarized and shown in Figs.1(a)-(i).After the chemical cleaning, a pn junction was formed by thermal diffusion of phosphorus as shown in Fig. 1(a).The aluminum films were prepared by evaporation as shown in Fig. 1(b), and the films were patterned to the front side electrodes as shown in Fig. 1(c).The detailed process conditions up to this step are described elsewhere (Kohata & Saito, 2010).The alumi annealed a

Solar cells the mirror patterning resistance
Optical re spectropho electron m illuminatio

Characteristics of Dry-Textured Cells
Dry textured and mirror-etched multi-crystalline Si solar cells were fabricated.The sheet resistance of the mirror-etched surface was near 50 /sq.in this experiment.As shown in Figure 5, the thickness of the n-type layer for the mirrored cell corresponds to that of the cell dry-textured for 5 -6 min.
Electrical characteristics for the solar cells illumination were measured under AM1.5 at 25 o C. The current density was obtained by dividing the current by the aperture area, 3.42cm 2 , of the solar cell.The cell performance parameters, obtained from the measured current vs. voltage characteristics, are summarized in Table 1.The short circuit current density, Jsc, and the conversion efficiency, , of the textured cell were improved up to about 35% and 38% compared with those of a mirror-etched cell, respectively.The cell performance was improved with the texturing time until 6 min, because the reflectance decreased and the sheet resistance approached an empirically appropriate value of about 50 /sq., as shown in Figure 5.In this study, the best performance was obtained for the cell textured for 6 min.The cell performance, especially Jsc, was degraded with the texturing time over 7min, probably because the remaining n-layer became partially too thin.
The absolute spectral responses of the textured cell (6 min) and the mirrored cell, which were obtained from the ratio of the photo-induced current with respect to the light intensity at each wavelength, are shown in Figure 6.The absolute spectral response of the textured cell is larger than that of the mirrored cell at wavelengths over 470 nm.The improved spectral response is mainly due to the increase of the absorbed light by the textured structure.Figure 7 shows the ratios of the spectral response and the absorbed light intensity of the textured cell with respect to those of the mirrored cell, where the absorbed light intensity is calculated from the reflectance shown in Figure 4.The values of the ratio of the spectral response almost agree with those of the absorbed light intensity at each wavelength over 650 nm.The improvement of the spectral response, therefore, corresponds to the increase of absorbed light in the long wavelength region.
However, the values for the spectral response largely deviate from those for the absorbed light intensity in the short wavelength region below 500 nm as shown in Figure 7.The degradation of the spectral response is considered to be due to the decrease of internal quantum efficiency (IQE).(1) (2)

J unction interface Current flow
Cross-sectional diagrams near the phosphorus-diffused layers textured by the dry process are illustrated in Figure 8 to explain the degradation of IQE in the short wavelength region.Most of the incident light with wavelengths larger than 500 nm enters into the region near the pn junction interface, and the photo-induced minority carriers (holes) can contribute to the current as shown in Figure 8(a).On the other hand, most of the incident light with wavelengths around 400 nm is estimated to be absorbed within an 80 nm depth from the surfaces because of large absorption coefficients, as shown in Figure 8(b).A large part of the photo-induced minority carriers near the surfaces will recombine and diminish likely due to surface states, the residual n+ region, and/or the long pathway to the junction interface.In the short wavelength region, the IQE of the textured cell is less than that of the mirrored cell, in which the n+ region is removed by the wet etching.Moreover, the reflectance of the textured surfaces is much less than that of the mirrored surface because of multi-reflection in the short wavelength region.These cause the deviation between the improvement of spectral response and the increase of absorbed light intensity by the texturing, as indicated in Figure 7.If the IQE is improved to that of the wet-etched cell, the increase of the short circuit current is estimated to be a few percent.A method for improving the low IQE in the short wavelength region is under investigation.We consider that surface passivation can improve the IQE.

Conclusion
In this study, we applied plasmaless dry texturing to doped layers of multi-crystalline Si solar cells.We modified the fabrication procedure and optimized the etching conditions to improve the cell performance.The reflection of the textured surface was decreased to about 10% at 600 nm.The efficiency of the fabricated cell with the textured surfaces was improved up to 38%, compared with that of the mirror-etched cell.It was experimentally demonstrated that our texturing method is effective for multi-crystalline Si solar cells.The next goal for this texturing procedure is to improve IQE in the short wavelength region to improve cell performance further.
Photo-resist film

Figure 1 .
Figure 1.Schematic diagrams of procedure fabricating textured and mirror-etched cells Note.(a) Phosphorus diffusion, (b) Al evaporation, (c) Al patterning, (d) Texturing of diffused layer, (e) Photo-resist patterning, (g) Isolation and Al evaporation onto rear side, (h) Mirror-etching of diffused layer, (i) Isolation and Al evaporation onto rear side.

Figure 5 .
Figure 5. Reflectance at 600nm and sheet resistance of phosphorus-doped multi-crystalline Si as function of dry etching time (ClF 3 partial pressure: 1.2 Pa)

Figure 6 .
Figure 6.Absolute spectral responses of textured cell (6 min) and mirrored cell, which were obtained from ratio of photo-induced current with respect to light intensity Photo-induced hole

Figure 8 .
Figure 8. Schematic cross-sectional images for models of light absorption and photo-induced carriers in a phosphorus-diffused surface of Si with finely-textured structures Note.Wavelengths of incident light are (a) below 450nm and (b) over 500nm.

Table 1 .
Dependence of cell performance parameters on texturing time