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.