Improving the performances of photovoltaic (PV) devices by suppressing nonradiative energy losses through surface defect passivation and enhancing the stability to the level of standard PV represents one critical challenge for perovskite solar cells. Here, reported are the advantages of introducing a tetrapropylammonium (TPA(+)) cation that combines two key functionalities, namely surface passivation of CH3NH3PbI3 nanocrystals through strong ionic interaction with the surface and bulk passivation via formation of a type I heterostructure that acts as a recombination barrier. As a result, nonencapsulated perovskite devices with only 2 mol% of TPA(+) achieve power conversion efficiencies over 18.5% with higher V-OC under air mass 1.5G conditions. The devices fabricated retain more than 85% of their initial performances for over 1500 h under ambient conditions (55% RH +/- 5%). Furthermore, devices with TPA(+) also exhibit excellent operational stability by retaining over 85% of the initial performance after 250 h at maximum power point under 1 sun illumination. The effect of incorporation of TPA(+) on the structural and optoelectronic properties is studied by X-ray diffraction, ultraviolet-visible absorption spectroscopy, ultraviolet photon-electron spectroscopy, time-resolved photoluminescence, and scanning electron microscopy imaging. Atomic-level passivation upon addition of TPA(+) is elucidated employing 2D solid-state NMR spectroscopy.