Structure–property relationships of diketopyrrolopyrrole- and thienoacene-based A–D–A type hole transport materials for efficient perovskite solar cells†
Hole transporting materials (HTMs) with high charge carrier mobilities and receptive functionalities have gained significant attention recently due to their direct effect on boosting the efficiency and lifetime of perovskite solar cells (PVSCs). Herein, two new simple small molecular diketopyrrolopyrrole-based HTMs with different central π-bridges (thieno[3,2-b]thiophene (DPP-TT), and dithieno[3,2-b : 2′,3′-d] thiophene (DPP-DTT)) were synthesized via a facile molecular engineering approach. Both small molecules exhibited strong absorption between 500 and 750 nm and an optical bandgap of ∼1.48 and 1.52 eV; however, their crystalline natures were significantly different, resulting in marked differences in their coating ability and PVSC performances. Thus, optimized DPP-TT-based PVSC devices with a lithium (bis(trifluoromethanesulfonyl)imide) dopant demonstrated a maximum PCE of 15.57% with superior energy level matching and an optimal trade-off between aggregation and processability, resulting in effective perovskite layer passivation via high surface coverage and suppression of charge recombination. In comparison, DPP-DTT had a slightly lower PCE of 14.49%, attributed to its poor film quality caused by its high aggregation tendency. Additionally, these HTMs delivered superior stability owing to superior hydrophobic characteristics. These findings provide valuable insight into the design of new HTMs with varying central π-bridge, which is a simple and effective approach.