In this work, we report the benefits of incorporating phenethylammonium cation (PEA+) into (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15 perovskite for the first time. After adding small amounts of PEA cation (<10%), the perovskite film morphology is changed but, most importantly, grain boundaries are passivated. This is supported by Kelvin Probe Force Microscopy (KPFM). The passivation results in the increase in photoluminescence intensity and carrier lifetimes of test structures and open-circuit voltages (VOC) of the devices as long as the addition of PEA+ is ≤4.5%. The presence of higher-band-gap quasi-2D PEA incorporated perovskite is responsible for the grain boundary passivation, and the quasi-2D perovskites are also found to be concentrated near the TiO2 layer, revealed by PL spectroscopy. Results of moisture exposure tests show that PEA+ incorporation is effective in slowing down the degradation of unencapsulated devices compared to the control devices without PEA+. These findings provide insights into the operation of perovskite solar cells when large cations are incorporated.