Electronic structures at organic heterojunctions of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB)-based organic light emitting diodes

► The 'n-on-p' organic-organic heterojunctions were elucidated by photoemission (PES). ► Band bending in NPB induced by the non-polar n-type molecules was verified by PES. ► Giant surface potential moves the electrostatic potential of the polar overlayers. ► GSP shifted the whole PES features of the overlayers proportional to their thickness. ► Actual energy offsets of the heterojunctions are deduced after correction of BB & GSP.