Correlation among Work Function Dynamics, Charge Transport, and Annealing Temperature of the ZnO Layer in Organic Photovoltaics

Zinc oxide (ZnO) is the dominantly used electron-transport layer (ETL) in inverted organic photovoltaic (OPV) devices. However, the devices typically show an S-shape in current-density characteristics and poor efficiency (without ultraviolet light activation) due to the abundant surface defects of ZnO. Understanding the ZnO ETL is still not sufficient for fabricating reproducible and reliable high-performance OPV devices. In this work, we report the correlation among work function (WF) dynamics, charge transport properties, and the annealing temperature (130–300 °C) of the ZnO ETL. Combining Kelvin probe measurement, atomic force microscopy measurements, X-ray diffraction analysis, X-ray photoelectron spectroscopy analysis, electrical conductance characterizations, and photovoltaic performance, it is demonstrated that increasing the annealing temperature would increase the crystallinity and reduce the adsorbed oxygen on the ZnO film surface. As the annealing temperature rises, the WF decrease (ΔE1) of ZnO under UV irradiation is less sensitive to oxygen adsorption. Within the same exposure time to air (210 s), the WF of higher-temperature processed ZnO recovered more closely to initial values. The electrical conductance of the ZnO layer was enhanced at higher annealing temperatures. OPV devices with ZnO ETL annealed at 180–240 °C showed higher performance and do not need light soaking activation under UV-free conditions. This work provides insights for ZnO-based OPVs applied in UV-free operational scenarios, such as UV filtered-off illuminations.