Achieving efficient passivating carrier-selective contacts (PCSCs) plays a critical role in high-performance photovoltaic devices. However, it is still challenging to achieve both an efficient carrier selectivity and high-level passivation in a sole interlayer due to the thickness dependence of contact resistivity and passivation quality. Herein, a light-promoted adsorption method is demonstrated to establish high-density Lewis base polyethylenimine (PEI) monolayers as promising PCSCs. The promoted adsorption is attributed to the enhanced electrostatic interaction between PEI and semiconductor induced by the photo-generated carriers. The derived angstrom-scale PEI monolayer is demonstrated to simultaneously provide a low-resistance electrical contact for electrons, a high-level field-effect passivation to semiconductor surface and an enhanced interfacial dipole formation at contact interface. By implementing this light-promoted adsorbed PEI as a single-layered PCSC for n-type silicon solar cell, an efficiency of 19.5% with an open-circuit voltage of 0.641 V and a high fill factor of 80.7% is achieved, which is one of the best results for devices with solution-processed electron-selective contacts. This work not only demonstrates a generic method to develop efficient PCSCs for solar cells but also provides a convenient strategy for the deposition of highly uniform, dense, and ultra-thin coatings for diverse applications.