Direct molecular quantification of electronic disorder in N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine on Au(111)

Organic light-emitting diodes are important in display applications, but thin films used in these devices often exhibit complex and highly disordered structures. We have studied the adsorption of a typical hole transport material used in such devices, N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD), on the Au(111) surface. Scanning tunneling microscopy images reveal the appearance of different conformations in the first monolayer with submolecular resolution. Scanning tunneling spectra identify the highest occupied molecular orbital on several different adsorption structures. We directly compare the statistical distribution of this orbital energy between an ordered monolayer structure and a disordered bilayer structure of α-NPD on Au(111). The disordered structure exhibits a very broad distribution that is consistent with inferences from prior organic device studies and that we propose arises from minor conformational variations.