Underlayer engineering of grain strain toward efficient and stable tin perovskite solar cells

Lead-free tin perovskite solar cells (TPSCs) have gained prominence as a promising green photovoltaic technology. However, the rapid crystallization of tin perovskites leads to residual strain within the film, generating a large number of deep-level defects, which severely restrict the enhancement of power conversion efficiency (PCE) and lifetime of TPSCs. Here, we have developed an underlayer engineering strategy to release the residual compressive strain of tin perovskite films through the design of long-chain alkylamines as crystallization buffer molecules, which enhanced the photovoltaic performance and stability of TPSCs. Through tuning the backbone length of the alkylamines to modify the interface between the perovskite and hole transporting layer (HTL), octadecanammonium iodide (ODAI) was demonstrated to be the most effective to produce high-crystallinity and strain-free tin perovskite films. Moreover, the released residual strain can reduce the trap state density and suppress the nonradiative recombination in tin perovskite thin films. The resulting device with an ODAI underlayer achieved a champion PCE of 13.82% and a voltage of 0.818 V, respectively, maintained over 92% and 88% of initial PCE under continuous one sunlight illumination and 65 degrees heating for 1000 hours.