  Colloidal zinc oxide (ZnO) nanoparticles are frequently used in the field of
organic photovoltaics for the realization of solution-producible,
electron-selective interfacial layers. Despite of the widespread use, there is
a lack of detailed investigations regarding the impact of structural properties
of the particles on the device performance. In this work, ZnO nanoparticles
with varying surface-area-to-volume ratio were synthesized and implemented into
polymer-fullerene bulk heterojunction solar cells with a gas-permeable top
electrode. By comparing the electrical characteristics before and after
encapsulation, it was found that the internal surface area of the ZnO layer
plays a crucial role under conditions where oxygen can penetrate the solar
cells. The adsorption of oxygen species at the nanoparticle surface causes 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 case of small ZnO
nanocrystals (high surface-area-to-volume ratio). Different transport-related
phenomena in the presence of oxygen are discussed in detail, i.e., Ohmic
losses, expressed in terms of series resistance, as well as the occurrence of
space-charge-limited currents, related to charge accumulation 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 nanoparticle surface for the functionality of the
organic solar cells.
