Effects of oxidation state on photovoltaic properties of reactively magnetron sputtered hole-selective WOx contacts in silicon heterojunction solar cells

Abstract

The stoichiometry value x of WOx, or its oxidation state, is crucial for improving performances of the hole-selective contact heterojunction silicon solar cell. However, it is challenging to tune the films’ oxidation state using the well-known evaporation method. In this study, a simulation was performed to analyze the effect of x on short-circuit current (Jsc) loss, attributed to the hole-selective contact in the device. Compared to the thickness of WOx layer, x has a more important role in minimizing Jsc loss. Based on the simulation, the WOx/c-Si heterojunction solar cells having hole-selective WOx contacts with tuned x to vary its oxidation state were fabricated using reactive magnetron sputtering. The relationships of the open-circuit voltage (Voc) and Jsc with respect to x were similar. The experimentally determined Jsc increased from 34.7 to 36.6 mA cm−2 when x was increased from 2.72 to 2.77; this result is consistent with the simulation. Nevertheless, fill factor (FF) reduced with the increase of x, owing to the reduced conductivity of WOx. Both oxidation state and film conductivity must be as high as possible to simultaneously achieve high Voc, Jsc, and FF. The lowest x yielded a solar cell efficiency of 13.3%.