Soft conjugation extension strategy of self-assembled molecules for achieving efficient and mechanically stable flexible perovskite solar cells

Abstract

Flexible perovskite solar cells (f-PSCs) hold immense potential for wearable and portable applications but face critical challenges in terms of efficiency and mechanical durability. Herein, we propose a soft conjugation extension strategy for designing self-assembled molecules (SAMs) to simultaneously address these issues. Interestingly, by developing a series of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) derivatives using this strategy, we show that (2-(3,6-bis(2-phenylthiophen-5-yl)-9H-carbazol-9-yl)ethyl)phosphonic acid (PhT-2PACz) offers strong interactions at “all-side” interfaces, including ITO/SAMs and SAMs/perovskite interfaces, and improves electrical and mechanical contacts. Specifically, PhT-2PACz exhibits a superior self-assembly quality on ITO due to enhanced intermolecular interactions brought about by the soft conjugation moiety. Meanwhile, PhT-2PACz actively bonds to the perovskite at the buried interface. Furthermore, PhT-2PACz improves the crystallinity and flexibility of perovskite films. These synergies yield f-PSCs with a champion power conversion efficiency (PCE) of 24.75% (26.02% for rigid device) and exceptional operational stability (T80 > 1000 hours), surpassing widely used 2PACz-based devices. Crucially, PhT-2PACz devices retain 97% of their initial PCE after 4000 multidirectional bending cycles (radius: 4 mm) with ignorable structural damage, while 2PACz devices degrade catastrophically after 1400 cycles with adverse structural damage and electrical failures. Mechanical tests performed under harsher conditions show that our devices show the best mechanical durability among SAM-based f-PSCs. This work contributes to the design of SAMs for simultaneously enhancing electronic performance, operational stability, and mechanical durability of f-PSCs, advancing their commercial viability.