Morphological Tuning of the Energetics in Singlet Fission Organic Solar Cells

Effective singlet fission solar cells require both fast and efficient singlet fission as well as favorable energetics for harvesting the resulting triplet excitons. Notable progress has been made to engineer materials with rapid and efficient singlet fission, but the ability to control the energetics of these solar cells remains a challenge. Here, it is demonstrated that the interfacial charge transfer state energy of a rubrene/C-60 solar cell can be modified dramatically by the morphology of its constituent films. The effect is so pronounced that a crystalline system is able to dissociate and collect triplets generated through singlet fission whereas an as-deposited amorphous system is not. Furthermore, a novel technique for studying the behavior of this class of devices using external quantum efficiency (EQE) measurements in the presence of a background light is described. When this method is applied to rubrene/C-60 solar cells, it is shown that triplet-triplet annihilation makes significant contributions to photocurrent in the amorphous device-enhancing EQE by over 12% at relatively low intensities of background light (4 mW cm(-2))-while detracting from photocurrent in the crystalline device. Finally, the conclusions on how the material system is affected by its morphology are strengthened by time-resolved photoluminescence experiments.