2, Ecole Polytechnique Fédérale de Lausanne, Neuchâtel, , Switzerland
3, Centre Suisse d'Electronique et de Microtechnique, Neuchâtel, , Switzerland
4, Princeton University, Princeton, New Jersey, United States
Recently in the organic photovoltaics community, there has been increased attention to the role of local morphology on the energetics and behavior of organic donor/acceptor charge transfer (CT) states. Different molecular orientations, local dielectric environments, and degrees of exciton delocalization at the donor/acceptor interface have been shown to significantly impact the CT state energy. This morphological sensitivity of the CT state energy is of particular importance in singlet fission and upconversion solar cells, where a specific alignment of the CT state with respect to the triplet state of the singlet fission or upconversion material is a necessary condition to the function of each device. In a singlet fission solar cell, the CT state must be lower in energy than the triplet state of the fission material in order to take advantage of the multiple exciton generation process.
Pentacene/C60 is one of the most widely studied singlet fission donor/acceptor systems, and in the literature, it is usually assumed that its donor/acceptor interface is triplet dissociating. In our study, we use sensitive external quantum efficiency and photothermal deflection spectroscopy methods to investigate the CT state spectrum of pentacene/C60 planar and bulk heterojunction solar cells under different blend morphologies and find that the interface is triplet dissociating for most, but not all, of the device configurations. We find that the CT state energy is raised in pentacene:C60 blends with low pentacene content, and if raised sufficiently, the interface presents an energetic barrier for pentacene triplet dissociation. We also investigate the impact of a poly(3-hexylthiophene) (P3HT) underlayer on the CT state spectra, and find that the CT state energy remains lower than that of the pentacene triplet for all of the studied blends. P3HT is often used as a triplet blocking layer in pentacene/C60 solar cells, but based on these results, the underlayer may also have the added benefit of promoting a blend morphology that induces a low energy CT state that is always able to dissociate pentacene triplets. Our spectroscopic studies are complemented by atomic force microscopy imaging, which is used to correlate the patterns in the CT state energy spectrum to the nanoscale morphology of different pentacene/C60 films. The results of our study demonstrate the morphological sensitivity of donor/acceptor CT states and highlight the need for careful CT state characterization in singlet fission solar cells.