Solution-processed Cu₂ZnSnS₄ nanocrystal solar cells: Efficient stripping of surface insulating layers using alkylating agents

Solution-processed photovoltaic (PV) devices based on semiconductor nanocrystals (NCs) such as Cu₂ZnSnS₄ (CZTS) and CuInS ₂ (CIS) are attracting much attention for use in next-generation solar cells. However, the performance of NC-based devices is hindered by insulating surface-capping ligands that limit transfer/transport of charged carriers. Here, to remove surface-capping ligands (long-chain fatty amines) from NCs, we use the strong alkylating agent methyl iodide, which converts primary amines to quaternary amines that have low coordinating affinity to the NC surface. X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy analyses confirm the successful removal of capping ligands from the CZTS surface after treatment with methyl iodide without changing the crystal structure of CZTS. CZTS and CIS NC-based devices treated with methyl iodide exhibit a reproducible PV response under simulated sunlight. The developed route can potentially enhance the performance of NC-based devices used in a broad range of applications.