Detailed Analysis of Shallow and Heavily-Doped Emitters for Al-BSF Bifacial Solar Cells

Abstract

A selective emitter structure is a promising approach to improve the cell efficiency of industrial type silicon solar cells by minimizing the losses at the front surface and in the emitter. Selective emitters can be produced by numerous processing sequences, resulting in different doping profiles. This paper focuses on the analysis of emitter formation for bifacial solar cell application. In this research, liquid phosphorus oxy-trichloride (POCl3) has been used as a diffusion source for emitter formation. The diffusion temperature was varied from 800 to 900 °C in order to determine an optimum diffusion profile. In this study, the mask-free diffusion process forms diffused emitter on both side of Si wafer. In order to determine the emitter characteristics, the sheet resistance of Si wafer after POCl3 diffusion process was measured using a four-point probe. Based on the sheet resistance value of ~47 ohm/sq, the emitter has been classified as heavily-doped emitter. The performance analysis using surface photovoltage (SPV) and spectral response presents a diffusion length of 2.19 μm. The POCl3-diffusion and screen printed Al-BSF led to bifacial solar cells with a front surface efficiency of 12.8 % and back surface efficiency of 5.08 %.