The primary block to achieving higher efficiencies in bulk-heterojunction organic solar cells (BHJ-OSCs) is energy losses through non-radiative pathways due to the decay of a charge-transfer state (CTS) to the ground state via energy transfer to vibrational modes. It has previously been suggested that the open circuit voltage (V_oc) in OSCs is largely determined by the energy of the donor-acceptor CT state^[1], and thus a large number of recent studies have focussed on increasing the energy of CT states by minimizing the energy offset between the donor and acceptor. This relationship can be rationalized by understanding that a higher overlap of the vibrational modes of the CT and ground states increases the rate of non-radiative recombination. 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^[2].

Here, we systematically investigate the effect that the CT state properties have on the voltage losses of BHJ-OSCs by using a series of increasingly fluorinated PBDB-T donors, in conjunction with a variety of different fullerene and non-fullerene acceptors^[3]. Firstly, we show that by downshifting both the HOMO and LUMO through fluorination of the donor, the energy of the donor-acceptor CTS can be effectively moved closer to the first excited state. Secondly, by performing a detailed voltage loss analysis of the various blends we find that the non-radiative voltage losses are not reduced systematically for higher CT state energies; instead, for systems where the CT state and first excited state energies are very close in energy the non-radiative voltage losses are reduced significantly and can reach as low as 0.22V while maintaining a high PCE over 9%.  We suggest that this reduced non-radiative voltage loss is due to the hybridization of the CT state with the first excited state, as has been previously suggested in other systems^[4]. Using a model to quantify the non-radiative voltage loss, we simulate the latter behaviour through an increase in both the CT state energy and the oscillator strength of the CT state to ground state transition. We propose that the increase of the oscillator strength may be due to the effect of hybridization that allows the CT state to borrow oscillator strength from the first excited state through the intensity borrowing mechanism. From the results of our model we show that in order to increase non-radiative voltage losses further, material combinations with strong electronic coupling between CT and first excited states as well as high excited state to ground state oscillator strengths should be used. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.