Role of Oxygen Adsorption in Nanocrystalline ZnO Interfacial Layers for Polymer-Fullerene Bulk Heterojunction Solar Cells

Colloidal zinc oxide (ZnO) nanoparticles are frequently used in the field of organic photovoltaics for the realization of solution-producible, electron-selective ID interfacial layers. Despite the widespread use, there is still lack of detailed investigations regarding the impact of structural properties of the ZnO particles, like the particle size and shape on the device performance. In the present work, ZnO nanoparticles with varying surface-area-to-volume ratio were synthesized and implemented into indium tin oxide free polymer-fullerene bulk heterojunction solar cells featuring a gas-permeable top electrode. By comparison of the electrical characteristics before and after encapsulation from the ambient atmosphere, it was found that the internal surface area of the ZnO layer plays a crucial role under conditions where oxygen can penetrate into the solar cells. The adsorption of oxygen species at the nanopartide surface is believed to cause band bending and electron depletion next to the surface. Both effects result in the formation of a barrier for electron injection and extraction at the ZnO/bulk heterojunction interface and were more pronounced in the case of small ZnO nanocrystals with a high surface-area-to-volume ratio. Different transport-related phenomena in the presence of oxygen are discussed in detail, i.e., increasing ohmic losses, expressed in terms of series resistance, as well as the occurrence of space-charge-limited currents, related to the accumulation of charges in the polymer-fullerene blend. Since absorption of UV light can cause desorption of adsorbed oxygen species, the electrical properties depend also on the illumination conditions. With the help of systematic investigations of the current versus voltage characteristics of solar cells under different air exposure and illumination conditions as well as studies of the photoconductivity of pure ZnO nanoparticle layers, we gain detailed insight into the role of the ZnO nanopartide surface for the functionality of the organic solar cells.