On the Interplay between CT and Singlet Exciton Emission in Organic Solar Cells with Small Driving Force and Its Impact on Voltage Loss

Abstract The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S₁) determines the recombination pathway and the resulting open circuit voltage (V_OC) of organic solar cells. By combining a well‐aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (E_CT) is varied by 130 meV while leaving the S₁ energy of the polymer () unaffected. It is found that the polymer exciton dominates the radiative properties of the blend when approaches , while the V_OC remains limited by the non‐radiative decay of the CT state. It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non‐radiative voltage loss because it lowers the radiative V_OC limit (V_OC,rad), but not because it is more emissive. The analysis further suggests that electronic coupling between the CT state and the S₁ will not improve the V_OC, but rather reduce the V_OC,rad. It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons.