Abstract
Organic photovoltaics (OPV) is a promising technology to account for the increasing demand for energy in form of electricity. Whereas the last decades have seen tremendous progress in the field witnessed by the steady increase in efficiency of OPV devices, we still lack proper understanding of fundamental aspects of light-energy conversion, demanding for systematic investigation on a fundamental level. A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. Illuminating conjugated polymers not only leads to excited states, but sheds light on some of the most important aspects of device efficiency in organic electronics as well. The interplay between electronic structure, morphology, flexibility, and local ordering, while at the heart of structure—function relationship of organic electronic materials, is still barely understood. (Time-resolved) electron paramagnetic resonance (EPR) spectroscopy is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior spectral resolution compared to optical methods. This article aims at giving a non-specialist audience an overview of what EPR spectroscopy and particularly its time-resolved variant (TREPR) can contribute to unraveling aspects of structure–function relationship in organic semiconductors.
Highlights
Using organic photovoltaics (OPV) to account for the increasing demand for energy in form of electricity becomes more and more important (Darling and You, 2013; Yeh and Yeh, 2013; Wang et al, 2016; Haque et al, 2018; Xue et al, 2018)
Electron paramagnetic resonance (EPR) spectroscopy is perfectly suited to address these issues on a molecular scale, as most species formed in the course of charge generation and charge separation in organic solar cells are inherently paramagnetic
The big advantage of EPR spectroscopy over more conventional, optical spectroscopy is its molecular resolution due to its inherent sensitivity to the local environment of the electron spin used as a probe for the electronic structure of the molecule
Summary
Using organic photovoltaics (OPV) to account for the increasing demand for energy in form of electricity becomes more and more important (Darling and You, 2013; Yeh and Yeh, 2013; Wang et al, 2016; Haque et al, 2018; Xue et al, 2018). The big advantage of EPR spectroscopy over more conventional, optical spectroscopy is its molecular resolution due to its inherent sensitivity to the local environment of the electron spin used as a probe for the electronic structure of the molecule. Whereas the focus of the author’s research is on applying TREPR spectroscopy to short-lived excited states, predominantly triplet excitons, EPR spectroscopy can contribute even further to a more thorough understanding of organic electronic materials (Niklas and Poluektov, 2017). In a series of showcase studies, TREPR spectroscopy of triplet excitons is demonstrated to reveal information on different aspects of the all-important structure—function relationship of organic semiconductors, such as solution and film morphology, conformational flexibility, local ordering, triplet routes, and electronic structure. For aspects of charge separation and charge transport and the relevant contributions of EPR spectroscopy that are not covered here, the reader is referred to the literature (Kraffert and Behrends, 2017; Niklas and Poluektov, 2017)
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