Improved performance of ZnO based inverted organic photodetectors with morphological and interfacial modification

Abstract

In order to achieve organic photodetectors with high performance for health detection, ZnO based inverted organic photodetectors using PTB7-Th:PC₆₁BM as the active layer were fabricated with morphological and interfacial modification. Using chlorobenzene instead of 1,2-dichlorobenzene as the solvent, the active layer spin-coated on the pristine ZnO layer exhibited more uniform phase separation, which was favorable for charge transport and charge collection. The dark current density and external quantum efficiency could be effectively regulated by changing the active layer thickness, the annealing temperature and the applied bias. When ZnO was modified with ethylene glycol (EG), the valence band maximum of ZnO decreased and the potential barrier of hole injection from the indium tin oxide electrode increased, resulting in a lower dark current density of ZnO/EG based devices. The improved external quantum efficiency and reduced dark current density of ZnO/EG based organic photodetectors were attributed to the amount of trap reduction using the capacitance–frequency curve, charge transfer resistance increase using impedance analysis and charge doping density reduction using Mott–Schottky analysis. Compared to pristine ZnO based organic photodetectors, the dark current density of ZnO/EG based organic photodetectors was reduced to 7.85 × 10⁻⁹ A cm⁻² by almost two orders of magnitude. Eventually, inverted ZnO/EG based organic photodetectors achieved a high specific detectivity of up to 9.11 × 10¹² Jones and a fast response as low as 1 μs, and have great potential for health monitoring applications.