The design of novel electron accepting materials for photovoltaic applications
Creators: name: Wadsworth, Andrew
Publisher: name: Imperial College London
Year: 2018

description: Significant progress has been made in the field of organic photovoltaics in the last few years, with efficiencies of over 11-12 % now commonplace. A large amount of this success can be apportioned to the development of low bandgap, high-performance donor polymers. However, the most substantial breakthroughs are the recent emergence of strategically designed nonfullerene electron acceptors. For over two decades the use of fullerene derivatives as the electron acceptor in organic photovoltaics has been prevalent. Despite a number of favourable properties, the potential of organic solar cells has been somewhat limited by the shortcomings associated with the fullerene acceptors. Early nonfullerene acceptors have paved the way towards greater efficiencies and stabilities. In this thesis, a number of novel nonfullerene acceptors have been synthesized and characterized, culminating in the fabrication of organic solar cells, in order to evaluate whether they were viable alternatives to the well-established fullerenes. First, new acceptor-donor-acceptor nonfullerene small molecules were designed, based on the previously reported rhodanine-based FBR, in which the electron-donating core was extended. These new acceptors displayed an increased aggregation tendency, improving the phase separation when blended with a donor polymer, such that an optimal blend morphology could be attained. Also, the most successful small molecules in this series (O- and EH-IDTBR) possessed much narrower bandgaps, allowing improved photon absorption. Binary solar cells with poly(3-hexylthiophene) (P3HT), were then fabricated, achieving a maximum efficiency of 6.4% in devices; this is still the highest P3HT:nonfullerene binary reported to date. These promising new electron acceptors were then included in ternary solar cells, to improve the voltage that devices were able to achieve and reduce recombination of free charges, which resulted in the achievement of up to 7.6% efficiencies, the highest efficiency achieved in any single-junction P3HT-based organic solar cells. The second section of this thesis focuses on structural modification to the promising new acceptor, O-IDTBR. Substitution of the carbon bridgeheads for germanium was used as a strategy to reduce the synthetic complexity of the acceptor’s synthesis and it was hoped that it would induce greater crystallinity. Unexpectedly, the inclusion of germanium appeared to reduce the crystallization tendency of the acceptor. Next, substitution of the peripheral rhodanine groups’ alkyl chains were carried out to study the impact on crystallinity and aggregation tendency of varying steric bulk at the π-stacking periphery of the acceptor. The replacement of the ethyl chains with the smaller methyl groups greatly improved the crystallinity, however this led to excessive aggregation and therefore poor device performance. The use of the bulkier phenyl unit on the rhodanines was expected to reduce the crystallinity, however it led to a small improvement in order, and the phenyl analogue was able to match the efficiencies of O-IDTBR. The final part of this thesis considers important factors for the commercial viability of processing organic photovoltaics. A high performing (11%) system was successfully processed from a relatively benign non-halogenated solvent, showing improved reproducibility and morphological stability compared to its chlorinated solvent processed counterpart. Finally, the impact of batch-to-batch variations in polymer molecular weight were investigated using the IDTBR acceptors. It was found that O-IDTBR devices displays a sensitivity to P3HT molecular weight, however efficiencies approaching 7% are possible through careful selection of the P3HT batch. EH-IDTBR devices do not display a molecular weight dependence, although they are unable to match the maximum efficiencies of O-IDTBR.