Control of π-π Stacking of Dithienopyrrole-Based, Hole-Transporting Materials via Lateral Substituents for High-Efficiency Perovskite Solar Cells

Dissecting the relationship of lateral substituents in organic semiconductors with molecular packing motif, charge transfer integral, and thin film morphology is of paramount importance to enhancing the mobility of hole-transporting layers and photovoltaic performance of emerging perovskite solar cells (PSCs). In this work, two dimethyltriphenylamino-substituted dithieno[3,2-b:2′,3′-d]pyrroles are synthesized for the hole-transporting layer in triple-cation lead halide PSCs. X-ray crystallographic analysis of organic single crystals and theoretical modeling on the microscopic hole transport paths have disclosed that with respect to AZ1 possessing one n-propyl lateral substituent, the AZ2 hole transporter with 4-methoxyphenyl is characteristic of a closer intermolecular packing owing to a less short-axis slipped cofacial π-stacking configuration, and a larger averaged domain of molecule aggregates, which jointly contribute to a higher thin film hole mobility. The AZ2-based PSC exhibits an excellent power conversion efficiency of 19.4%, which is even higher than that (19.1%) of the control cell based on the state of the art hole-transporter spiro-OMeTAD. It seems that the weak interaction between 4-methoxyphenyl and lead ions at the perovskite boundaries can improve hole extraction kinetics and can passivate trap states at the perovskite surface, which brings forth a negligible hysteresis during the reverse and forward potential scans, along with high carrier mobility.