Understanding the Role of Organic Hole Transport Layers on Pinhole Blocking and Performance Improvement in Sb2Se3 Solar Cells

Abstract

Sb₂Se₃ is an emerging semiconductor which has shown promise for low-cost photovoltaic applications. After successive record-efficiencies using a range of device structures, spiro-OMeTAD has emerged as the default hole transport material (HTM), however, the function of HTM layers remains poorly understood. Here, thin-film Sb₂Se₃ solar cells are fabricated with which three organic HTM layers - namely P3HT, PCDTBT, and spiro-OMeTAD are investigated. By comparing these against one another, and to a reference device, their role in the device stack are clarified. These organic HTM layers are found to serve a dual purpose, increasing both the average and peak efficiency by simultaneously blocking pinholes and improving the band alignment at the back contact, with marginal differences in performance between the different HTMs. This produced a champion device of 7.44% using P3HT, resulting from an improvement in all performance parameters. A more complex processing route, run-to-run variability, and lower overall device performance compared to the other organics challenge the assumption that spiro-OMeTAD is the optimal HTM for Sb₂Se₃ devices. A Schottky barrier at the Au-Sb₂Se₃ contact despite the deep work function of gold implies Fermi level pinning due to a defective interface, which each of the organic HTMs are equally capable of alleviating.