Conduction band electronic states of ultrathin thiophene-phenylene co-oligomer and substituted biphenyl films on the surface of layer-by-layer grown ZnO

The results of studying the electronic states of the conduction band and interface potential barrier during the formation of ultrathin films of thiophene-phenylene co-oligomer CH$_3$-phenylene-thiophene-thiophene-phenylene-CH$_3$(CH$_3$-PTTP-CH$_3$) on the surface of ZnO and films of biphenyl tetracarboxylic dianhydride (BPDA) on the ZnO surface are presented. A 100 nm thick ZnO layer was prepared by atomic layer deposition (ALD). Organic CH$_3$-PTTP-CH$_3$ films and BPDA films up to 8 nm thick were formed by thermal vacuum deposition. During film deposition, the electronic characteristics of the surface were studied using total current spectroscopy (TCS) in the energy range from 5 eV to 20 eV above $E_{\mathrm{F}}$. In this energy range, the structure of the maxima of the unoccupied electronic states of CH$_3$-PTTP-CH$_3$ and BPDA films was determined. As a result of the CH$_3$-PTTP-CH$_3$ film deposition, a decrease in the work function to 4.0 eV was found, compared with the value of the work function of 4.2 eV measured from the ALD ZnO-substrate. This corresponds to the transfer of a negative charge from the CH$_3$-PTTP-CH$_3$ film to the substrate. The charge transfer at the interface between the BPDA film and the ALD ZnO-substrate occurs in the opposite direction, since a 4.7 eV increase of the work function was registered during the formation of this interface. The CH$_3$-PTTP-CH$_3$ and BPDA films studied and the layer-by-layer grown ZnO film represent a continuous coating on sufficiently large surface areas of the order of 10 $\mu$m $\times$ 10 $\mu$m. The roughness of the ZnO surface does not exceed 4 nm, and the surface roughness of CH$_3$-PTTP-CH$_3$ and BPDA films was 10–15 nm.