Toughened self-assembled monolayers for durable perovskite solar cells

Hole-selective self-assembled monolayers (SAMs)^1,2 have played a key role in driving the certified power conversion efficiency (PCE) of inverted perovskite solar cells^3-5 to 26.7% (ref. ⁶). However, their instability often compromises the operational performance of devices, strongly hindering their practical applications^7,8. Here we employ a cross-linkable co-SAM to enhance the conformational stability of hole-selective SAMs against external stresses, while suppressing the formation of defects and voids in SAM during self-assembly. The azide-containing SAM can be thermally activated to form a cross-linked and densely assembled co-SAM with a thermally stable conformation and preferred orientation. This effectively minimizes substrate surface exposure caused by wiggling of loose SAMs under thermal stress, preventing perovskite decomposition. This enables a certified PCE of 26.92% to be achieved for the best-performing cell, which also possesses excellent thermal stability with negligible decay under maximum-power-point tracking at 85 °C for 1,000 h. It also retains >98% of initial PCE after 700 repetitive thermal cycles between -40 °C and 85 °C, representing the state of the art of the field. This work offers an in-depth understanding of SAM degradation mechanisms to guide the design of a more robust buried interface for SAM-based devices adopting high-roughness substrates to realize highly efficient and durable perovskite solar cells.