The surface modification of SnO2 electron transporting layer (ETL) is investigated for the performance and stability improvement of perovskite solar cells by adding citric acid (SnO2-citric) and graphene quantum dots (SnO2-GQDs) in SnO2 ETL precursors for carboxyl group and GQDs, respectively. As a result, the hydrophobic surface of SnO2-GQDs is obtained by adding GQDs to the SnO2 precursor, forming SnO2-GQDs ETL. In addition, the hydrophobic surface of SnO2 ETL is significant in increasing grain size crystallinity of perovskite obtained by this modification process. Also, the charge trap density and recombination carriers between ETL and perovskite interface were advanced. Thus, the surface modification of SnO2-GQDs ETL can enhance the power conversion efficiency (PCE) of PSCs at the highest value of 17.81% from 15.52% with a decrease of the hysteresis effect. The enhancement is due to the combining effect of carboxyl groups and GQDs interface engineering between ETL and perovskites layers. Also, the stability of perovskite solar cells without encapsulation for 28 days is achieved. These suggest that the combining effect can be used in surface modification and for efficiency enhancement of PSCs.