Molecular engineering of highly efficient dopant-free spiro-type hole transporting materials for perovskite solar cells

Abstract

Up to now, the most efficient perovskite solar cells (PSCs) typically utilize Spiro-OMeTAD as hole transporting materials (HTMs). The unique “spiro” structure offers appropriate energy levels for hole transfer and high thermal stability with suppressed aggregation. However, the pristine Spiro-OMeTAD requires additional oxidizing dopants to work efficiently due to its low hole mobility. To retain the advantages of spiral structure and overcome its shortcomings, we demonstrate the design of three dopant-free HTMs with spiral structure by molecular engineering, in which three groups with different conjugated lengths, namely benzene, naphthalene and anthracene, are inserted between spiral core and electron donor. These designed molecules, Y-1~Y-3, are initially identified with quantum chemical calculations based on the mother molecule X59 and then are obtained by easy synthetic routes. Our studies show that the intramolecular charge transfer (ICT) states are formed in the designed molecules due to the introduction of conjugated groups, which produces a self-doping effect without the need to add any external dopant. The best-performing PSCs using the dopant-free Y-1 as HTM achieves a champion power conversion efficiency (PCE) of 16.29% under one sun illumination, which is higher than that of devices with X59 as dopant-free HTMs (14.64%). The present work provides an effective strategy for designing, synthesizing of highly efficient and stable dopant-free HTMs.