Doping of organic semiconductors is a frequently used technique in order to increase the conductivity of hole and electron transport layers (see, for example, Ref. 1). In a recent publication, Wu et al. investigated p-type doping of the organic hole transport material 4,4′-bis(N-carbazolyl)−1,1′-biphenyl (CBP) with the transition metal oxide MoO3 by using UV-Vis-NIR and Fourier-transform IR spectroscopy.2 They show that in the IR spectra of CBP:MoO3 (83 wt. %), certain vibrational modes decrease in intensity regarding the spectrum of a pure layer of CBP. They take this intensity decrease as an indication for the formation of charge transfer complexes (CTCs) between CBP and MoO3.
In the following, we will show that (i) in the IR spectra of intentionally not doped CBP presented by Wu et al.,2 several vibrational modes are occurring that are not evoked by CBP and that only the absorption bands that decrease in intensity in the spectra with higher doping concentrations are evoked by CBP. Therefore, the spectral changes reported by Wu et al. certainly are due a lower concentration of CBP-molecules in the doped layers and thus do not indicate the formation of CTCs. We will also explain that (ii) the IR spectra of CBP:MoO3 presented by Wu et al. do not show spectral features of MoO3. Hence, these layers certainly do not contain MoO3. Our detailed comments:
Wu et al. show an IR spectrum of CBP that reveals four strong absorption bands in the range between 1068 cm−1 and 1228 cm−1.2 They assigned these features to C–H deformation vibrations of CBP but did not motivate their assignment. As in the layer with a doping concentration of 83 wt. %, less than one third of the amount of CBP-molecules is present compared to the undoped CBP layer, one would expect a clear decrease in intensity for these four absorption bands in the spectra of the doped layers if they are due to CBP vibrations. But, in Ref. 2, only one of the four absorption bands (at 1228 cm−1) decreases in intensity for higher doping concentrations, while the intensity of the remaining three strong absorption bands does not change significantly. So only the absorption band at 1228 cm−1 is clearly related to CBP vibrations and the three other strong absorption bands are not from CBP but probably from some contamination of the samples or absorptions in the beam path of the spectrometer. Such assignment is in agreement to the IR spectra that we present in Ref. 3 that is a combined study using in situ IR spectroscopy and density functional theory (DFT) calculations. As we found out, the IR spectrum of CBP shows only rather weak absorption lines between 850 cm−1 and 1300 cm−1, except for only one stronger band at 1230 cm−1. The position and the relative intensity of the absorption bands at 750 cm−1, 825 cm−1, and in the range between 1347 cm−1 and 1600 cm−1 in Ref. 2 are within experimental errors in accordance to the spectra of CBP that we show in Ref. 3. In the publication of Wu et al., these absorption bands show lower intensities for the spectra of the doped layers. Wu et al. take this intensity decrease as an indication for a charge transfer between CBP and MoO3. However, all the absorption bands in the IR spectra of Wu et al. that we would assign to CBP in accord to our spectra3 decrease in intensity for higher doping concentrations. This fact simply indicates the decreasing concentration of CBP molecules. Hence, this spectral change that Wu et al. report cannot indicate the formation of CTCs.
In the publication of Anwar et al.4 and also in our publication,3 it is shown that the IR spectrum of amorphous MoO3 shows one broad absorption band below 1000 cm−1 and a shoulder at about 1000 cm−1 that is related to the stretching vibration of the terminal oxygen. One can expect that these vibrational signatures of MoO3 show up in the spectra of CBP:MoO3, at least for doping concentrations as high as 83 wt. %. But no additional absorption bands appear in the spectra of the doped layers measured by Wu et al., which leaves some doubts on their MoO3 doping.
The experimental spectra presented by Wu et al. are thus not suitable to draw conclusions on MoO3 doping effects in CBP.
Financial support by BMBF via MESOMERIE Project (FKZ 13N10724) is gratefully acknowledged.