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Electrochemical doping produces clear changes in the vibrational spectra of organic semiconductors as we show here for the system molybdenum oxide (MoO₃) doped into the charge transport material 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). Based on density functional theory (DFT) calculations of vibrational spectra, the new spectral features can be attributed to the CBP cation that forms as a result of electron transfer from CBP to MoO₃. The intensity of the new vibrational lines is a direct measure for the probability of charge transfer. MoO₃ agglomerating within the CBP matrix limits the active interface area between the two species. The appearance of a broad electronic transition in the infrared range indicates a new electronic structure at the interface compared to the individual components. The intensity of this electronic excitation serves as a measure for the interface area indicating a linear increase with MoO₃ concentration. Deposition onto cooled substrates results in smaller agglomerates, and thus yields a higher efficiency.