Magnetic resonance spectroscopy of organic photovoltaic cells
Abstract
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.
Type
Thesis, PhD Doctor of Philosophy
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