Charge-Transfer State Dissociation Efficiency Can Limit Free Charge Generation in Low-Offset Organic Solar Cells

We investigate the charge-generation processes limitingthe performanceof low-offset organic bulk-heterojunction solar cells by studyinga series of newly synthesized PBDB-T-derivative donor polymers whoseionisation energy (IE) is tuned via functional group (difluorinationor cyanation) and backbone (thiophene or selenophene bridge) modifications.When blended with the acceptor Y6, the series present heterojunctiondonor-acceptor IE offsets (& UDelta;E (IE)) ranging from 0.22 to 0.59 eV. As expected, small & UDelta;E (IE) decrease nonradiative voltage losses but severely suppressesphotocurrent generation. We explore the origin of this reduced charge-generationefficiency at low & UDelta;E (IE) through acombination of opto-electronic and spectroscopic measurements andmolecular and device-level modeling. We find that, in addition tothe expected decrease in local exciton dissociation efficiency, reducing & UDelta;E (IE) also strongly reduces the charge transfer(CT) state dissociation efficiency, demonstrating that poor CT-statedissociation can limit the performance of low-offset heterojunctionsolar cells.