Dissociation Processes of Singlet and Triplet Excitons in Organic Photovoltaic Cells

The dissociation processes of singlet and triplet excitons were studied based on single-layer photovoltaic cells using: fluorescent Aluminum (III) 8-hydroxyquinoline (Alq$_{3})$ and phosphorescent \textit{fac }tris (2-phenylpyridine) iridium (Ir(ppy)$_{3})$ molecules. We found that triplet exciton dissociation leads to a more efficient photovoltaic response as compared to singlet excitons. The short-circuit photocurrent action spectra suggest that the triplet excitons dissociate mainly at the metallic electrode interface while the singlet excitons exhibit bulk dissociation. This interface dissociation of triplet excitons forms a mechanism for phosphorescent organic materials to show efficient photovoltaic responses. Therefore, control of singlet-to-triplet exciton ratio presents a new pathway to enhance photovoltaic response from organic materials.