High efficiency CNT-Si heterojunction solar cells by dry gas doping

Carbon nanotubes (CNTs) have been integrated with silicon to construct CNT-Si heterojunction solar cells; doping CNTs is one of the promising techniques to improve the solar cell performance. Here, we propose a gas-doping method by exposing as-fabricated CNT-Si solar cells to ozone for a short time. We find that a 5-min ozone treatment does not destroy the CNT structure but produces a significant impact on the cell characteristics (output voltage and fill factor), increasing the cell efficiency from 5.29% to 12.70% without degradation in 24 h. Ozone treatment donates holes to CNTs; this p-doping effect increases the work function of CNTs and subsequently enhances the built-in potential and Schottky barrier at the heterojunction for more efficient charge separation. Moreover, the ozone-treated CNT-Si cell could combine other established techniques such as colloidal antireflection and acid doping, to achieve an instantaneous high efficiency of >17% under standard illumination conditions (air mass 1.5, 100 mW/cm2). Compared to many reported doping methods relying on liquid or hazardous chemicals, our dry gas-doping strategy is very simple, highly effective, environmentally benign, and compatible to low-cost large-area solar cell manufacturing.