Solution-processed bulk heterojunction solar cells based on interpenetrating CdS nanowires and carbon nanotubes

Incorporation of a bulk heterojunction is an effective strategy to enhance charge separation and carrier transport in solar cells, and has been adopted in polymeric and colloidal nanoparticle solar cells to improve energy conversion efficiency. Here, we report bulk heterojunction solar cells based on one-dimensional structures, fabricated by mixing CdS nanowires (CdS NWs) and single-walled carbon nanotubes (CNTs) to form a composite film with mutually interpenetrating networks through a simple solution-filtration process. Within the composite, the CNT network boosts charge separation by extracting holes generated from CdS NWs and also forms the transport path for carrier collection by the external electrode. At an optimized CNT loading of about 5 wt.%, the CdS NW/CNT bulk heterojunction solar cells showed three orders of magnitude increase in photocurrent and cell efficiency compared to a cell with the same materials arranged in a stacked layer configuration with a plain heterojunction. External quantum efficiency and photoluminescence studies revealed the efficient charge transfer process from photoexcited CdS NWs to CNTs in the mixed form. Our results indicate that the bulk heterojunction structure strategy can be extended to semiconductor NWs and CNTs and can greatly improve solar cell performance.