Dissociation processes of singlet and triplet excitons in organic photovoltaic cells

The dissociation processes of singlet and triplet excitons are studied based on fluorescent aluminum (III) 8-hydroxyquinoline (Alq3) and phosphorescent fac-tris-(2-phenylpyridine) iridium [Ir(ppy)3] molecules. We find that phosphorescent Ir(ppy)3 shows a more efficient photovoltaic response as compared to fluorescent Alq3. The short-circuit photocurrent action spectra and magnetic-field-dependent photocurrents reveal that the triplet excitons dissociate directly into free charge carriers at the metal-electrode interface while the singlet excitons experience bulk dissociation through polaron-pair states. This interface dissociation of triplet excitons forms a mechanism for phosphorescent organic materials to yield efficient photovoltaic responses.