The performance of organic photovoltaics has seen drastic improvements over the last few decades, with power conversion efficiencies now approaching 20% [1]. Due to advantages over other types of photovoltaic technologies, organic photovoltaics are particularly promising for building-integrated photovoltaics and for in-door photovoltaics as self-sustaining power source for Internet of Things (IoT) devices. Investigation of the paramagnetic species at the basis of the conversion of solar energy to electricity by Electron Spin Resonance (ESR) spectroscopy can help provide new insights into the energy conversion process and the molecular requirements for high conversion efficiencies.
We are using ESR spectroscopy to characterise the nature and dynamics of paramagnetic states generated following photoexcitation in a series of state-of-the-art donor:acceptor blends for organic photovoltaics. Transient ESR measurements reveal spin-correlated pairs of charges on donor and acceptor molecules with an emissive and absorptive spin polarisation pattern characteristic of a singlet precursor at short times after photoexcitation for all investigated blends. At long times after photoexcitation, the spectra are dominated by long-lived fully absorptive signals of the charges on donor and acceptor molecules observed both in transient and pulse EPR measurements. By complementing ESR with Electrically Detected Magnetic Resonance (EDMR) on fully assembled devices, relevance of the charged donor and acceptor species characterised by ESR to the current-generating process in organic photovoltaics is demonstrated.