Crystalline fraction and doping concentration effect on heterojunction solar cells n-doped μc-Si: H back surface field layer

The back surface field (BSF) plays a vital role for high efficiency in the Heterojunction Intrinsic Thin (HIT) film solar cell. This paper investigated the effect of crystalline volume fraction (X_c) and 1% hydrogen diluted phosphine (PH₃) gas doping concentration of the n-type μc-Si:H back surface file (BSF) layer. Initially, the thickness of the n-type μc-Si:H BSF layer was optimized. With increase in X_c from 6% to 59%, the open circuit voltage (V_oc) increased from 573 mV to 696 mV, and the fill factor (FF) also increased from 59% to 71%. In the long wavelengths region (≥ 950 nm), the QE of the solar cells decreased over the optimized X_c of the n-doped micro BSF layer, due to the defects of a film. In the second part of this paper, the effect of high conductivity n-type μc-Si:H BSF layer with optimized thickness on the performance of HIT solar cells was investigated, by doping gas ratio variation. Even though X_c decreased, conductivity was increased, with increasing PH₃ doping concentration. Under the optimized condition, a n-μc-Si:H BSF layer has a dark conductivity of 2.59 S/cm, activation energy of 0.0519 eV, and X_c of 52%. The conversion efficiency of 18.9% was achieved with a V_oc of 706 mV, fill factor of 72%, and short circuit current density of 37.1 mW·cm^-2.