Beyond Langevin Recombination: How Equilibrium Between Free Carriers and Charge Transfer States Determines the OpenCircuit Voltage of Organic Solar Cells

However, they have low open-circuit voltages and typically cannot be made optically thick while maintaining high fi ll factors. [ 4,5 ] For comparison, the best silicon solar cell has a bandgap of 1.1 eV and an open-circuit voltage of 0.71 V, corresponding to a difference between the bandgap and qV oc of only 0.40 eV. [ 1 ] In contrast, one of the best performing organic solar cells, PTB7:PC 71 BM, has an optical gap of 1.65 eV and an open-circuit voltage of 0.76 V, a difference of 0.89 eV. [ 6 ] The lower qV oc of organic solar cells relative to their optical gaps directly translates into lower power conversion effi ciencies. [ 7 ]