Infrared study of the MoO3 doping efficiency in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP)

► We investigated p-type doping of CBP with MoO3 using FTIR-spectroscopy. ► New vibrational features appear upon doping due to the formation of CBP cations. ► The formation of interface states leads to a broad electronic excitation. ► 11% of the admixed Mo3O9 trimers generate one CBP cation (deposition at 300 K). ► Agglomeration of MoO3 can be suppressed by cooling.