Deciphering the effect of replacing thiophene with selenophene in diketopyrrolopyrrole (DPP)-based low cost hole transport materials on the performance of perovskite solar cells

Abstract

Attributed to the capability of hole-transporting materials (HTMs) in facilitating commercialization of perovskite solar cells (PVSCs) with high-performance, superior device reproducibility, and long-term stability, exploration of new low-cost HTMs has become more likely inevitable for their future growth. In this contribution, two new diketopyrrolopyrrole (DPP)-based small molecules, namely, DPP-Th and DPP-Se, with different chalcogenophenes (sulfur (S) or selenium (Se)) as a central donor unit and two DPP units as terminal acceptor cores were synthesized and tested as HTMs in PVSCs. The impact of the variation of chalcogenophenes on the crystallinity, optical, and electrochemical properties of HTMs and subsequently on the photovoltaic properties in PVSCs was systematically investigated. Endowed by the insertion of selenophene, DPP-Se demonstrated bathochromic absorption spectra, slightly higher highest occupied molecular orbital energy level, higher crystallinity with decreased π–π stacking, and improved hole mobility (μ_h), hence favoring efficient charge transport in PVSCs than its DPP-Th counterpart. Consequently, optimized planar PVSCs based on MAPbI₃ with Li-TFSI as dopant and DPP-Th and DPP-Se as HTM achieved a remarkable power conversion efficiency (PCE) of 14.69% and 16.83%, respectively. Moreover, these PVSCs with pristine DPP-Se and DPP-Th as HTMs displayed superior stability under ambient temperature and thermal stress conditions and lower synthetic cost, contrasting with the reference spiro-OMeTAD studied in parallel. In conclusion, these results reveal the effectiveness of these design tactics for developing low cost, easily scalable, efficient organic small molecule HTMs for PVSC applications.