Modification of the ultrathin N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) insertion layer on the electronic structures of the CH3NH3PbI3 (MAPbI3)/MoO3 interfaces is investigated using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). It is found that when an ultrathin NPB insertion layer of 16 Å is inserted between MAPbI3 and MoO3, the chemical reaction between the latter two can be effectively suppressed, and a favorable energy-level alignment is achieved. The valence band ma...
