Loss mechanisms in fullerene-based low-donor content organic solar cells

Low-weight-percent donor content fullerene-based organic solar cells have been reported to have good efficiencies although the exact reason for their performance is still a matter of discussion. We use a solution processable hole-transporting poly(dendrimer) with minimal absorption in the visible region to study the blend-ratio dependency of energy-loss mechanisms in fullerene low-donor content organic solar cells in which [6,6]-phenyl-C ₇₁ -butyric acid methyl ester is used as the primary absorber. The photocurrent losses for each of the steps that govern the device performance have been assessed, with the optimized device performance achieved when the donor was at a concentration of 6 wt %. The 6 wt % donor device had balanced charge transport, and transient absorption spectra revealed that although the low-donor content devices suffered from a low-exciton-quenching rate, the dissociation efficiency of the resulting charge-transfer states was almost unity. That is, essentially all of the formed charge-transfer states led to charge-separated states. A gradual increase in the open-circuit voltage was observed as the donor ratio decreased from 50 to 6 wt %. Internal quantum-efficiency measurements indicated efficient formation of charge-separated states from the intermediate charge-transfer states and high charge-collection efficiency at low-donor content resulting in an overall higher external quantum efficiency despite the lower interfacial area. Overall, these measurements highlight the competing effects of exciton dissociation (favored by the high surface area associated with large donor content) and charge-transfer state dissociation, which we observe to be more efficient when the donor content is low.