Charge generation in morphology-controlled organic photovoltaic materials

© 2017 Dr. Kyra Noelle SchwarzOrganic photovoltaics have attracted significant interest due to their potential low cost of production and fabrication as thin films on flexible substrates. The efficiency of organic photovoltaic devices is intimately linked to the nanoscale donor-acceptor morphology of the photoactive layer. However, the successful scale-up of devices and further improvements in efficiency will rely on active layer optimisation and the rational design of new materials. These advances hinge on the understanding and control of the complex effects of active layer morphology on charge photogeneration. Strategies to gain greater control and tunability over bulk heterojunction nanomorphologies are increasingly being employed to optimise device efficiencies, using a wide range of methods. Recent developments in the mechanistic understanding of charge transfer have revealed its interconnection with morphology of donor and acceptor components. Despite recent advances, charge generation in specific morphological strategies and newly developed high performing materials is not yet fully understood. In this thesis, alternative approaches to morphological control are investigated, and their consequences for charge photogeneration are explored spectroscopically. Charge photogeneration in organic photovoltaic materials occurs on ultrafast timescales, and requires time-resolved experiments, such as high sensitivity transient absorption spectroscopy, that can resolve the relevant processes. Using these techniques, the examined morphological approaches include: solvent vapour annealing, composite nanoparticles and block copolymers. Solvent vapour annealing is an ambient temperature process that is used post-deposition to subtly alter bulk heterojunction morphology. This method results in a four percentage point efficiency increase for blends comprising the recently developed electron donor, benzodithiophene terthiophene rhodanine (BTR). An important loss mechanism, the formation of BTR triplet excitons through sub-nanosecond bimolecular recombination, was identified in untreated blend films, and was significantly suppressed after solvent vapour annealing. This indicates that morphology and spin share a closely interconnected role in mediating triplet loss channels and hence, influence device efficiency. Preparations of composite semiconducting nanoparticles provide a strategy for nanoscale morphological control and permit the use of polar, non-toxic solvents suitable for large- scale printing. Poly(3-hexyl thiophene): phenyl-C61-butyric acid methyl ester (P3HT: PCBM) composite nanoparticles were prepared using two different methods, with contrasting outcomes for internal morphology, P3HT free exciton bandwidth and charge generation. Notably, the mini-emulsion nanoparticle fabrication process resulted in aggregates exhibiting charge generation dynamics similar to thermally annealed bulk heterojunction films of these materials, despite the lack of any elevated temperature processing. Block copolymers that synthetically link donor and acceptor units within their molecular structure have the potential to simultaneously control morphological assembly and interfacial properties. The characteristics of the intramolecular interface were examined in a triblock copolymer comprising a (poly(9,9-diocetylfluorene-co-bis-N,N’- (4-methylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine)) (PFM) electron donor and a (poly[(9,9-di-n- octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)) (F8BT) electron acceptor, forming PFM-F8BT-PFM. Donor excitation exhibited minor interfacial interactions, apart from energy transfer which was observed in non-polar environments. In contrast, excitation of the F8BT acceptor resulted in delayed fluorescence behaviour mediated by a charge transfer state at the interface of the blocks. Collectively, this thesis presents a spectroscopic insight to the consequences of different morphological strategies on charge generation in organic photovoltaic materials. A greater understanding of charge generation in these systems will lead to improved and informed morphological and molecular design choices, with the goal of aiding the future advances for efficient, processable organic photovoltaic materials