A tin-based perovskite solar cell (PVSC) includes a surface treatment layer containing a functionalized fullerene derivative having a structure of Formula I below, in which R is a monocyclic or polycyclic aromatic ring including one or two nitrogen atoms. A process of preparing a tin-based PVSC includes the steps of: providing a substrate; coating a hole transport layer onto the substrate; and preparing a perovskite layer including the step of coating a perovskite precursor solution containing a tin-based perovskite composition onto the hole transport layer, in which the tin-based perovskite composition has the empirical formula: CsxEDAyFA1-x-2ySnI3, where x is from about 0 to about 0.12; y is from about 0.01 to about 0.03; FA is formamidinium; and EDA is ethylenediamine.