Photoemission measurements for N,N'-bis-(1-naphthyl)-N,N'-diphenyl-1, 1'-biphenyl-4, 4'-diamine (NPB) overlayer on Ag (111) are reported. The growth of NPB overlayer from submonolayer on the single-crystal surface of Ag (111), ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS) measurements at each growth step allowed a better determination of band bending and interface dipole related change in work function. The XPS measurements show a weak interaction between NPB and Ag (111) and the C 1s core level shift provides a measurement of the band bending occurring with the NPB overlayer. UPS measurements show a decrease in work function with increasing thickness of NPB overlayer. The dipole contribution to the work function as a function of NPB thickness is deduced by subtracting the band bending from the change in work function. The dipole contribution has a minimum at a NPB thickness of about a monolayer. The decrease of work function is attributed to both the band bending occurring within NPB overlayer and dipole layer formed due to surface polarization. Additionally, the top of the highest occupied molecular orbital of NPB is located at about 1.7meV below Fermi level.