Effect of porous Si and an etch-back process on the performance of a selective emitter solar cell

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Abstract

This paper analyzes the effects of a wet chemical etch-back process and the porous Si that is formed during the etch-back process on the optical and electrical performances of a selective emitter solar cell. In this paper, we assert that the sheet resistance of the emitter after the etch-back process could be controlled by monitoring the reflectivity of the porous Si at 500 nm. The scanning electron microscope (SEM) photographs of the porous Si reveal that the thickness of the porous Si is not entirely homogenous, gradually decreasing from the top of the pyramid to the valley. The etch-back process results in pyramid morphology variations that increase the reflectivity of the texture. After removing the porous Si, the minority carrier lifetime of the emitter increases significantly. Because of this effect, the conversion efficiency of the SE solar cell is 0.27% higher than that of the regular homogenous emitter solar cells. Although this approach increases the texture reflectivity, the etch-back process greatly improves the internal quantum efficiency in the short wavelength range, which in turn increases the short-circuit current density by a large amount. The calculated Jsc shows that the peak gain of the short-circuit current density occurs at approximately 400 nm.

Highlights

► The emitter sheet resistance keeps linear relationship with reflectivity of PS at 500 nm. ► The thickness of PS gradually goes down from the top of the pyramid to the valley. ► The morphology variation of the pyramid results in the increase of surface reflectivity. ► Etch-back process improves internal quantum efficiency in short wavelength range. ► The calculated Jsc shows that the peak of Jsc gain occurs at about 400 nm.

Introduction

A selective emitter (SE) solar cell can improve the short wavelength quantum efficiency and lower the contract resistance, which can effectively improve the open-circuit voltage (Voc) and the short-circuit current (Isc). Recently, SE solar cells have successfully been commercially mass produced. There are many techniques that solar cell producers can utilize to manufacture SE solar cells [1], [2], [3], [4], [5]; however, the wet chemical etch-back method stands out from other methods because of its cost-effectiveness. The SUNRISE Company optimizes SE cells with etch-back instruments provided by SCHMID and boosts the efficiency of SE cells to 19.2% [6]. The basic procedure of the wet chemical etch-back method begins with uniform texturization and heavy diffusion. The area on the emitter that will be the contact is then masked with an acid resist material. Subsequently, the silicon wafer is etched back using the low-concentration HF/HNO3 system, which is followed by unaltered standard PECVD-SiNx coating, screen printing and co-firing after removal of the mask. A characteristic of this method is that a layer of porous Si (PS) is formed when the heavily doped silicon wafer is etched back in the HF/HNO3 solution. Schwartz and Robbins [7] thoroughly explain the corrosion mechanism of the HF/HNO3 system on crystalline Si, which is an autocatalytic reaction. Starostina [8] believes that the root cause of this generation of PS lies in the emergence of cavities that are the result of the oxidation asymmetry of HNO3. The existence of these cavities endows the resultant HF with a certain selectivity in the dissolution reaction of the oxidation layer. Cavities emerge in large numbers if the concentration of the HNO3 in the corrosive liquid is below a certain threshold, making this selectivity more obvious, and these cavities eventually form PS. Book et al. [9] find that the etch-back process affected the morphology of the texture of the silicon wafer as well as the short-circuit current density of the SE solar cell.

The most essential step of the etch-back method is to control the emitter sheet resistance, which is the factor that is most directly related to the generation of PS. Formation of a layer of PS can etch off surface emitters with high phosphorus concentrations and can lower the surface recombination velocity. The color and the thickness of PS are closely related; thus, the post-etch-back sheet resistance can be determined by monitoring the color of the PS. Nevertheless, it is very difficult to accurately measure the value of the sheet resistance based on the PS color. Because PS is an intermediate that arises during the course of SE solar cell manufacturing when the wet chemical etching method is used, monitoring of the sheet resistance can be performed more accurately by measuring the changes of the porous silicon’s nature and optical performance. The PS etch-back process and its subsequent treatments have a significant effect on the performance of the SE solar cell. However, reports that detail methods of accurately monitoring the sheet resistance by measuring the changes in the optical properties of PS and describe the effect of the PS microstructure on the optical and electrical performance of solar cells are rare. Based on the measurement of and research on PS reflectivity, this paper asserts that the emitter sheet resistance could be monitored by measuring the change in the reflectivity at 500 nm and discusses the effects of the nature of the PS and the etch-back process on the performance of the resulting solar cells.

Section snippets

Experimental

After cleaning and texturization, a 125×125 mm2 p-type monocrystalline silicon wafer was heavily doped through a one-step diffusion process. The sheet resistance on the surface was approximately 30–40 Ω/□. A layer of PS was formed during the subsequent etch-back process using a HF/HNO3 solution. The reflectivity of the PS was measured using a spectrophotometer, and the sheet resistance of the Si wafer was determined after removing the PS with a 1% NaOH solution for 10 s at 25 °C. The morphology of

Relationship between the reflectivity of PS and the sheet resistance

The PS that formed on the surface of the silicon wafer had a loose, light-trapping structure. As the thickness varied, the PS exhibited diverse colors. As the etching time increased, the color of the PS gradually deepened, transitioning from grey to light yellow, yellow, gold, dark yellow, blue, red, and so forth.

We maintained a concentration ratio of 1:5 between HF and HNO3 by diluting with deionized water and observed the changes in the sheet resistance after etching back with solutions of

Conclusion

The HF/HNO3 wet chemical etch-back process is a significant method of manufacturing SE solar cells. The formation of PS during the etch-back process is the most crucial step. The sheet resistance after the etch-back process can be controlled by monitoring the reflectivity of the PS at 500 nm because of the essentially linear relationship between PS reflectivity and sheet resistance. The PS on the top of the pyramid was observed to be thicker than that in the valley, and the valley of the pyramid

Acknowledgements

This work was supported by the National Natural Science Foundation (contract number 61176055) and a grant from the Science and Technology Project (2011A080804009) of Guangdong Province, China.

References (11)

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