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 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 comparing 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 nanoparticle 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 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 nanoparticle surface for the functionality of the organic solar cells.

Soluble molecular semiconductors are a promising alternative to semiconducting polymers in the field of organic photovoltaics. Here, three custom-made symmetric 1,3-bis(N,N-alkylated-2,6-dihydroxy-anilino)squaraines containing systematic variations in their molecular structures are compared regarding their applicability as donor materials in bulk-heterojunction solar cells. The terminal substitution pattern of the squaraines is varied from cyclic over linear to branched including a stereogenic center. Single crystal structures are determined, and, in the case of chiral squaraine, unusual formation of stereoisomer co-crystals is revealed. The thin film absorbance spectra show characteristic signatures of H- and J-bands or hint at the formation of tautomers. The general feasibility of these model compounds for photovoltaic applications is studied by light-induced electron spin resonance spectroscopy. The impact of the different molecular substitution patterns on aggregation behavior and, consequently, their optoelectronic solid state properties including charge carrier mobility and finally the solar cell performance are investigated.

Activation energy for charge transport, carrier concentration and recombination rate are identified to strongly affect the device characteristics.

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage ( V OC ) is the one with most room for improvement. Existing models for the description of V OC assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe V OC of disordered organic devices. For two representative donor:acceptor blends, it is shown that V OC is actually 0.1–0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced V OC .