The dissociation processes of singlet and triplet excitons are studied based on fluorescent aluminum (III) 8-hydroxyquinoline (Alq 3) 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 Alq 3. 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.
Bibliographical noteFunding Information:
This research was supported by AFOSR (FA9550-06-10070), the NSF (ECS-SGER-0521474 and ECS-SGER-0551914), and the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory under Contract No. DE-AC05-00OR22725.