Role of Hybrid Charge Transfer States in the Charge Generation at ZnMgO/P3HT Heterojunctions

Hybrid charge transfer states (HCTS) have been shown to form at interfaces between ZnMgO and P3HT. Combined analysis of the electronic structure of ZnMgO/P3HT interfaces and the corresponding photovoltaic performance, also as a function of temperature, reveals that the HCTS are constituted of an electron in the metal oxide's conduction band and a hole in the polymer's highest occupied molecular orbital. Their formation is thus an intrinsic feature of hybrid interfaces. Radiative recombination of HCTS is observed in the infrared spectral range, of the electroluminescence spectrum. Band gap engineering by varying the Mg content in ZnMgO allows tuning of the HCTS energy in order to conduct a systematic study of charge carrier generation and recombination. The results show that recombination of the HCTS effectively competes with its dissociation and limits the performance of hybrid photovoltaic cells. Evidently, a single charge separating interface does not suffice to ensure efficient photocarrier generation. Further measures have to be taken to funnel the charges away from the interface in order to prevent their recombination. The results of this study provide guidance for optimization of metal oxide/organic hybrid interfaces.