Engineering the Assembly Freedom of Donor-Acceptor Type Self-Assembled Monolayers Toward Efficient and Stable Flexible Perovskite Photovoltaics.

Flexible perovskite solar cells (f-PSCs) are promising contenders for portable and wearable photovoltaics, yet developing f-PSCs that simultaneously achieve high power conversion efficiency (PCE) and superior operational stability, including mechanical robustness, remains challenging. Herein, we propose a strategy of modulating the assembly freedom of donor-acceptor type self-assembled monolayers (SAMs) to address this issue. The newly designed 2FMPA-BT-PPA (PPA) SAMs exhibit higher assembly freedom on flexible ITO substrates compared to the previously reported 2FMPA-BT-BA (BA) SAMs, delivering two key benefits: first, they enable the formation of a higher-quality monolayer via improved conformational adaptability and strengthened π-π stacking, which facilitates efficient carrier transport. Second, the SAMs' conformational adaptability and uniformly tilted orientations can effectively dissipate interfacial strain under external mechanical loads, thereby enhancing the mechanical robustness of flexible devices. These synergies yield f-PSCs with a champion PCE of 25.3% (26.3% for rigid device), alongside exceptional operational stability. Crucially, PPA-based devices retain 98% initial PCE after 10 000 multidirectional bending cycles (3 mm radius) with no observable structural damage, outperforming BA-based devices and all reported SAMs-based f-PSCs. This work offers donor-acceptor SAMs design experiences for efficient, robust f-PSCs, revealing assembly freedom's key role in interfacial carrier extraction and mechanical robustness.