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A significant challenge to achieving higher efficiencies in organic solar cells (OSCs) is increasing the open circuit voltage (Voc) relative to the absorption threshold. The reason why voltage losses in OSCs are higher than in other material systems has been attributed to the molecular heterojunction that is required to dissociate photogenerated excitons, which gives rise to electron-hole pairs bound across the donor-acceptor interface known as charge-transfer (CT) states. Whilst these CT states are crucial to charge separation, their decay to the ground state via energy transfer to vibrational modes is a significant non-radiative recombination pathway in the device, which has meant that non-radiative contributions to Voc losses are significantly larger in OSCs than in comparative solar cell technologies.
Given the importance of CT-states in the charge generation process of OSCs, an understanding of the factors that govern CT-state recombination is crucial in designing higher efficiency systems, especially given the recent emergence of non-fullerene based systems that have managed to decrease non-radiative voltage losses to below 0.2 eV without affecting charge-generation efficiency. It has previously been suggested that the open circuit voltage (Voc) in OSCs is largely determined by the energy of the donor-acceptor CT state, since a higher overlap of the vibrational modes of the CT and ground states increases the rate of non-radiative recombination in accordance with the energy gap law. However, more recent studies have found that increasing the CT state energy does not always result in a reduction in the non-radiative voltage losses, which suggests that other properties of the CT state to ground state transition appear to affect the trend.
Here, we present spectroscopic studies of CT-states to determine their impact on the Voc and the subsequent efficiency of OSCs. We present a model of recombination via CT-states in OSCs that shows how particular CT-state properties affect voltage losses, and specifically focus on the case where the energy difference between the exciton and the CT-state becomes very small, as has often been the case for many recently published high efficiency systems. In this case, hybridization between the CT and excited state may occur, leading to an increase in the radiative ability of the CT-state due to intensity borrowing. We show both experimentally and using our model that this leads to a decrease in non-radiative voltage losses, and further address the questions of what molecular parameters of the CT-state may be beneficial to further reduce voltage losses in low offset OSC blends.
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