Suppression of Sn2+/Sn4+ oxidation in tin-based perovskite solar cells with graphene-tin quantum dots composites in active layer

Tin (Sn) halide perovskite has been a promising candidate in lead-free perovskite solar cells (PSCs), but its chemical instability attributed to Sn2+/Sn4+ oxidation reduces device performance and stability. To address this problem, we propose a new approach, i.e. fabrication of mesoporous n-i-p Sn-based PSCs with the photoactive composite made of mixed-organic-cation Sn halide perovskite and reduced graphene oxide (rGO) sheets anchored with Sn quantum dots (i.e., FA0.8MA0.2SnI3/rGO-Sn QDs). The rGO-Sn QDs in active layer not only suppress the destructive Sn2+/Sn4+oxidation and recombination but also accelerate charge transport, improve charge-carrier lifetime and reduce trap state density. Compared to the power conversion efficiency (PCE) of Sn-PSC without Sn QDs, the composite-basted champion device showed 55% increase in efficiency, attributed to strong suppression of Sn2+/Sn4+oxidation and recombination. Furthermore, the champion device showed remarkable reproducibility and stability improvement, representing an essential step for the practical use of low-cost and lead-free PSCs.