For highly stable and efficient perovskite solar cells, functional nanocomposites based on metal oxides, graphene and perovskite were developed. Compared to a reference cell, the composites-based devices showed an efficiency over 20 %. More importantly, the composites-based PSCs without encapsulation exhibited remarkable thermal- and photo-stability and long-term stability with retaining 95-99 % of the initial values of photovoltaic parameters with sustaining over 300 days under ambient conditions. However, most PSCs contain harmful lead (Pb). Recently, Sn-based PSCs have received much attention as a promising lead-free alternative, but Sn2+ is rapidly oxidized to Sn4+ in oxygen and moisture, causing a serious problem of poor device performance and stability. To address this, we proposed a novel approach, i.e. fabrication of the Sn-based PSCs with composites made of mixed-organic-cation Sn halide perovskite and graphene-Sn quantum dots. The composite-based champion device showed 55% efficiency improvement and significant reproducibility and stability improvement.