2D Ruddlesden-Popper perovskite solar cells (PSCs) with structure formulated as R2MAn-1PbnI3n+1 (R is the organic bulky cation) exhibit superior environmental stability, but their efficiency still lags behind that of their 3D counterpart, which is mainly limited by the insufficient light absorption and carrier transportation. Herein, we incorporated guanidinium (GA+) into the PEA at the "R"site to form mixed bulky cations and investigated the doping effect of GA+ on the photovoltaic performance of layered 2D perovskites. Our results show that incorporating GA+ into the "R"site of 2D PSCs exhibits a stronger and wider light absorption, faster charge carrier mobility, and better quality perovskite films. Consequently, both the short circuit current density (Jsc) and fill factor (FF) of the 2D PSCs were significantly improved. As a result, the energy conversion efficiency (PCE) of the (PEA, GA)2MA4Pb5I16 champion device achieved 15.27%, which is much higher than the 11.58% PCE of the single cation 2D PSCs. Also, the 2D PSCs with the mixed (PEA, GA) cations exhibit improved environmental stability. Our work provides important guidance to obtain efficient and stable 2D PSCs through bulky cation engineering and modulating the interlayer interaction in 2D perovskite. © 2020 Author(s).