Improving CNT-Si solar cells by metal chloride-to-oxide transformation

Transitional metal oxides (TMOs) are important functional materials in silicon-based and thin-film optoelectronics. Here, TMOs are applied in carbon nanotube (CNT)-Si solar cells by spin-coating solutions of metal chlorides that undergo favorable transformation in ambient conditions. An unconventional change in solar cell behavior is observed after coating two particular chlorides (MoCl5 and WCl6, respectively), characterized by an initial severe degradation followed by gradual recovery and then well surpassing the original performance. Detailed analysis reveals that the formation of corresponding oxides (MoO3 and WO3) enables two primary functions on both CNTs (p-type doping) and Si (inducing inversion layer), leading to significant improvement in open-circuit voltage and fill factor, with power conversion efficiencies up to 13.0% (MoO3) and 13.4% (WO3). Further combining with other chlorides to increase the short-circuit current, ultimate cells efficiencies achieve >16% with over 90% retention after 24 h, which are among the highest stable efficiencies reported for CNT-Si solar cells. The transformation of functional layers as demonstrated here has profound influence on the device characteristics, and represents a potential strategy in low-cost manufacturing of next-generation high efficiency photovoltaics.