How to Choose an Interfacial Modifier for Organic Photovoltaics Using Simple Surface Energy Considerations.

Organic photovoltaics are typically composed of at least four different materials, including the donor and acceptor components of the bulk heterojunction, the interfacial layers at each electrode, the electrodes themselves, and solution additives that may persist in the final sandwich structure. The interplay of surface energies between these different layers is profoundly complex, as the deposition and annealing of one layer on top of another may be influenced by the surface energy of these interfaces. While the energy levels of one layer with respect to adjacent layers are important to facilitate charge separation and collection at the electrodes, the relative surface energies of the interfaces in contact with the multicomponent bulk heterojunction can be beneficial or disadvantageous, or be neutral, with respect to the performance of the OPV device. Because the bulk heterojunction is a mixture of donor and acceptor polymers and/or small molecules, the accumulation of one of the components on the underlying electrode interface can be driven by surface energy considerations. A donor- or acceptor-rich interface may affect charge carrier flow to the electrode, thus affecting the overall efficiency. Here, ITO/PEDOT:PSS electrodes in forward organic photovoltaic devices are treated with five different thin interfacial layers to change the relative surface energy of this electrode with respect to the adjacent bulk heterojunction. Contact angle measurements with four probe liquids enable calculation of the surface energies, and the results are compared with the performance of forward-biased organic photovoltaic devices. Time-of-flight secondary ion mass spectrometry results substantiate the predictions of gradients in the bulk heterojunction layers, and grazing-incidence wide-angle X-ray scattering measurements show the impact on the polymer crystallites. Thus, a simple algorithm based on surface energy considerations may inform which interfacial layer for a given bulk heterojunction in an organic photovoltaic device can be the most appropriate.