Efficiency enhancement in hybrid P3HT/silicon nanocrystal solar cells

Hybrid organic-inorganic solar cells from poly(3-hexylthiophene) (P3HT) and freestanding silicon nanocrystals (Si-ncs) combine the advantages of siliconbased photovoltaics with the cost-efficient solution processing technique. At present, the microwave-plasma synthesis of Si-ncs that allows for a future upscaling to industrial demands is at the expense of the Si-nc surface quality and the number of charge-trapping defects. Here, we present an enhancement of the solar cell performance by identifying the major factors which are limiting the device efficiency. With the help of low-cost post-growth treatments of the Si-ncs and the optimization of various device parameters, P3HT:Si-ncs bulk heterojunction solar cells with an efficiency up to 1.1% are achieved. In particular, etching of the Si-ncs with hydrofluoric acid to remove the surface oxide shells and surface defects has a strong impact on the solar cell performance. An intermediate Si weight ratio of around 60% is found to lead to the highest current densities. For Si-ncs with very small diameters, an additional enhancement of the open circuit voltage was observed. Moreover, we show that the structural order of P3HT has a strong influence on the efficiency, which can be explained by an improved charge carrier separation at the P3HT/Si-ncs interface in combination with an enhanced charge transport in the P3HT phase.