Open-Circuit Voltage Losses in Selenium-Substituted Organic Photovoltaic Devices from Increased Density of Charge-Transfer States

High Resolution Image Download MS PowerPoint Slide Using an analysis based on Marcus theory, we characterize losses in open-circuit voltage ( V OC ) due to changes in charge-transfer state energy, electronic coupling, and spatial density of charge-transfer states in a series of polymer/fullerene solar cells. We use a series of indacenodithiophene polymers and their selenium-substituted analogs as electron donor materials and fullerenes as the acceptors. By combining device measurements and spectroscopic studies (including subgap photocurrent, electroluminescence, and, importantly, time-resolved photoluminescence of the charge-transfer state) we are able to isolate the values for electronic coupling and the density of charge-transfer states ( N CT ), rather than the more commonly measured product of these values. We find values for N CT that are surprisingly large (∼4.5 × 10 21 –6.2 × 10 22 cm –3 ), and we find that a significant increase in N CT upon selenium substitution in donor polymers correlates with lower V OC for bulk heterojunction photovoltaic devices. The increase in N CT upon selenium substitution is also consistent with nanoscale morphological characterization. Using transmission electron microscopy, selected area electron diffraction, and grazing incidence wide-angle X-ray scattering, we find evidence of more intermixed polymer and fullerene domains in the selenophene blends, which have higher densities of polymer/fullerene interfacial charge-transfer states. Our results provide an important step toward understanding the spatial nature of charge-transfer states and their effect on the open-circuit voltage of polymer/fullerene solar cells.