Spectroscopic studies of the charge transfer state and device performance of polymer:fullerene photovoltaic blends

In organic solar cells, the understanding of loss mechanisms, especially the energetic losses driven by the offset at type-II heterojunction and recombination, are crucial to improve the device performance. The best-performing organic solar cells are polymer:fullerene blends, and despite an abundance of donor materials, phenyl-C61/71-butyric acid methyl ester (PCBM) remains the most-used acceptor material. In this thesis, we use fullerene multiadducts as new acceptor materials that allow us to study energetic losses in polymer:fullerene blends by tuning the offset at the heterojunction. In addition, we analyse their performance in blends with high-performance polymer donors. The first chapter of results addresses design rules for fullerene multiadducts and energetic disorder. By adding multiple sidechains to the fullerene cage, the LUMO level can be raised by up to 400 meV compared to PCBM, which allows increased open circuit voltages. Fullerene multiadducts, however, are a mixture of different isomers with increased packing and energetic disorder and show reduced electron transport. Using Differential Pulse Voltammetry measurements, we quantify the amount of energetic disorder present in a variety of fullerene multiadducts. In the second results chapter, the fullerene multiadducts are employed in photovoltaic devices with poly(3-hexylthiophene) (P3HT) and other donor polymers. While most fullerene multiadducts perform reasonably well with P3HT as donor, their performance in blends with other donor polymers is usually much lower when compared to blends with PCBM as acceptor. We find that for many polymer:fullerene blends with multiadducts, the offset of the organic heterojunction is too small to allow efficient charge generation, especially for donor polymers optimised for PCBM. Even if the offset in the blend is sufficiently high, the lower electron mobility of the fullerene multiadducts is likely to reduce device performance, only donors featuring high hole mobility and high crystallinity show reasonable performance. Energetic losses in organic solar cells and limits of the charge transfer (CT) state energy are studied in the third results chapter of the thesis. We establish that electroluminescence (EL) from the CT state originates from transport levels in the density of states and that the spectrum shifts very little with increased injection currents. This allows us to use the EL emission peak as a proxy for the energy of the CT state. By employing indenofullerene multiadducts in blends with various polymers, we consistently find an additional loss pathway via polymer or fullerene exciton formation. If the energy of the CT state approaches the smaller optical bandgap of either component in the blends (Eopt,min), photocurrent and fill factor are likely to be reduced by increased recombination. We find this reduced performance in a number of blends, which allows us to empirically determine an limit of the open circuit voltage for efficient solar cell relative to Eopt,min for these systems