Toward Efficient Organic Photovoltaics: From Formulation and Novel Materials to Device Architectures

Abstract

Organic photovoltaic (OPV) offer unique advantages such as molecular engineering, flexibility and roll-to-roll manufacturing that can significantly reduce production costs. However, their efficiencies still lag behind inorganic PVs. It has been demonstrated that morphology control and recombination mechanisms play a key role in the efficiency. In this way, methods to enhance the performance by enabling morphology tuneability to further optimise OPVs are needed. To this end, this thesis introduces three novel approaches that can be utilised to improve the efficiency in OPVs.
In the first part of this work, a formulation approach is investigated by examining OPVs in which the morphology is optimised by ternary blends. Particularly, donor polymers P3HT, PTB7 and fullerene acceptors PC71BM and ICBA were systematically characterised. It is shown that different combinations of ternary blends outperformed binary controls. Results demonstrated that depending on the donor material used, the molecular intermixing of the constituents was different, yielding varied impacts on the performance and thus determining whether ternary blends offer a benefit or not.
The second approach reviewed consisted in the study of charge transport in novel materials, specifically non-fullerenes (NFA), hybrid fullerenes and fullerenes. For the first time, electron mobility was probed, which can be a limiting factor of the efficiency. It was demonstrated that NFA realised higher electron mobilities (closer to benchmark PCBM) in contrast to hybrid fullerenes, which yielded low mobilities. It was shown that the synthetic approach utilised to fabricate these compounds and its resulting morphology had a profound impact in their capability to mediate electron transport.
For the third method, device architectures were simulated in the form of multi junction OPVs termed as tandems. In such devices, a current matching between photoactive layers is generally required to maximise the performance. Through advanced modelling, it was shown that devices with unbalanced current matching are dominated by recombination losses of charges in the layer doing most of the absorption. It was demonstrated that changing the transport properties of the layer performing least of the absorption could assist alleviating these losses. It is also shown that different illumination conditions can further reduce the performance in tandem OPVs and that one can mitigate these reductions by suitable materials selection.