Oxygen doping of TiO2/poly(phenylene–vinylene) bilayer solar cells

The change in electronic properties of poly[2-methoxy-5-(3,7dimethyloctyloxy), paraphenylene–vinylene] (MDMO–PPV) in oxygen-saturated ${\mathrm{SnO}}_2:{\mathrm{F/TiO}}_2/\mathrm{MDMO–PPV/Hg}$ cells caused by optical irradiation is the subject of the present study. During preparation, the samples are exposed to air with exclusion of light, or in a glovebox in the presence of light. When these samples are subjected to a well-controlled light dose in ambient atmosphere, their current–voltage behavior changes. If open-circuit conditions prevail during irradiation, the dark forward and reverse currents increase. However, when the cells are shorted during irradiation, in ambient atmosphere, virtually no change in the $\mathrm{I–V}$ response occurs. The changes in the $\mathrm{I–V}$ response are reversible and are not related to photodegradation. These findings can be accounted for by the occurrence of oxygen doping. When oxygen is present it can interact with electrons in the lowest occupied molecular orbital (LUMO) band of the polymer to form a charge transfer complex. Due to this electron transfer, holes are generated in the MDMO–PPV highest occupied molecular orbital band. This type of oxygen doping takes place upon illumination under open-circuit conditions, since in this case LUMO-band electrons are not transferred to ${\mathrm{TiO}}_2.$ When cells are short circuited, the photogenerated electrons are injected into the conduction band of ${\mathrm{TiO}}_2$ and quickly escape from the interface. In this case the electron transfer to oxygen is inhibited. To evaluate the oxygen-doping effect quantitatively, capacitance–voltage scans have been recorded. From these scans the increase of the acceptor density due to the light dose is determined. It is found that soon after the onset of irradiation the acceptor density becomes so high that tunneling of electrons through the barrier at the ${\mathrm{TiO}}_2/\mathrm{MDMO–PPV}$ interface becomes possible.