We will discuss about our recent first-principles calculations on lattice defects at interfaces in perovskite solar cells (PSCs). In this presentation, we will show computational results on some kind of defect systems including point defects and impurities. The main focus in the discussion will be on defects near the surface of the metal halide perovskite layer. The calculation shows that, when the surface with underlying lattice defects is fabricated with appropriate molecules, defect levels are modulated and become inactive for carrier trapping. Some factors pertinet to the level modulation are clarified through detailed analyses on the computational results. The present computational analysis suggests importance of surface modification to enhance the energy conversion efficiency of the PSCs.