The microstructure evolution during MoS_2 films growth and its influence on the MoS_2 optical-electrical properties in MoS_2/p-Si heteroj unction solar cells

摘要

The microstructure of MoS_2 films is crucial important for its photoelectrical properties, which has significant impact on its application in the field of photovoltaic devices. In this paper, a set of MoS_2 thin films with varied thickness was prepared by magnetron sputtering. The influence of the MoS_2 microstructure on its optical and electrical properties and, especially, the MoS_2/p-Si solar cells photovoltaic performance was investigated in terms of thin films growth. The results reveal that, as the thickness increases, the microstructure of the MoS_2 films evolves from amorphous to microcrystalline with the in-plane E_(1/2g) and out-of-plane A_(1g) Raman modes enhancing and the parallel (002) growth orientation increasing. The films electrical conductivity significandy improves owing to the enhanced MoS_2 crystallinity. When the thickness is 10 nm, the small ordered MoS_2 crystal clusters gradient distribute from the surface to the bulk in the films, contributing to the stress release in the amorphous precursor. Simultaneously, the optical band gap of the MoS_2 films shows a maximum of 1.3 eV, due to the less band tail states in the films at the initial phase transition. The optimized optical-electrical properties of the MoS_2 films lead to the reducing defects recombination and the enhanced carrier transportation at MoS_2/p-Si interface, which promotes the rectification behavior of the pn-junction up to 10~2. Besides, the significant improvement of the photo-response in MoS_2/p-Si heterojunction is obtained, especially in short wavelength range. Correspondingly, the photovoltaic characteristics (V_(oc), J_(sc) and FF) of the MoS_2/p-Si solar cells increase, and thus an enhancement of the cells conversion efficiency is achieved with 10-nm MoS_2 layer.