Thionation Enhances the Performance of Polymeric Dopant-Free Hole-Transporting Materials for Perovskite Solar Cells

To date, the most efficient perovskite solar cells (PSCs) require hole-transporting materials (HTMs) that are doped with hygroscopic additives to improve their performance. Unfortunately, such dopants negatively impact the overall PSCs stability and add cost and complexity to the device fabrication. Hence, there is a need to investigate new strategies to boost the typically modest performance of dopant-free HTMs for efficient and stable PSCs. Thionation is a simple and single-step approach to enhance the carrier-transport capability of organic semiconductors, yet still completely unexplored in the context of HTMs for PSCs. In this work, a novel polymeric semiconductor, P1, based on a diketopyrrolopyrrole (DPP) moiety, is proposed as a dopant-free HTM. Its modest performance in PSCs (power conversion efficiency (PCE) = 7.1%) is significantly enhanced upon thionation of the DPP moiety. The resulting dithioketopyrrolopyrrole-based HTM, P2, leads to PSCs with nearly 40% performance improvement (PCE = 9.7%) compared to devices based on the nonthionated HTM (P1). Furthermore, thionation also remarkably boosts the shelf-storage and thermal stability with respect to traditional 2,2 ',7,7 '-tetrakis(N,N-di-p-methoxyphenylamine)-9,9 '-spirobifluorene-based PSCs. This work provides useful insights to further design effective dopant-free HTMs employing the straightforward one-step thionation strategy for efficient and stable PSCs.