Magnetic resonance spectroscopy of organic photovoltaic cells

Organic photovoltaics (OPV) have the potential advantages of low-cost, flexibility and high throughput production. However, at present their efficiency is lower than other thin film technologies and they are susceptible to degradation which limits cell lifetimes. Magnetic resonance spectroscopy is a powerful technique to study the key processes involved in the operation of OPV cells. In this thesis a range of electron paramagnetic resonance (EPR) methods are used to investigate the processes which influence cell efficiencies. The understanding of degradation pathways and how they influence cell performance is important if OPV cells are to reach commercialisation. The efficiency of PTB7:PC₇₁BM cells is severely reduced when exposed to ambient atmosphere during processing. Current-voltage analysis was combined with EDMR spectroscopy to investigate the source of this performance loss. This investigation revealed that exposure of PTB7:PC₇₁BM films to the solvent additive DIO and ambient atmosphere leads to electron trap formation on the PC₇₁BM which acts as a recombination centre. Using time resolved EPR spectroscopy the variation of charge separation across blends of the DTS family of small molecule electron donors with PC₆₁BM is investigated. Charge separation is found to be slowest in the [1,2,5]thiadiazolo[3,4-c]pyridine blend. This slower separation is accompanied by a higher population of triplet excitons formed by back electron transfer. This finding demonstrates that back electron transfer is a loss mechanism in these molecular systems when charge separation is slow. The EPR signatures of negative polarons on two high efficiency non-fullerene acceptors, ITIC and IDTBR, are identified using multifrequency light induced EPR spectroscopy. The polaron signatures of ITIC and IDTBR were found to overlap with polarons on P3HT at all three microwave frequencies. Using multifrequency simulations the negative polaron signatures and g-tensors of ITIC and IDTBR were determined for the first time