Improvement of photovoltaic response using triplet excitons

We report an enhancement of photovoltaic response by dispersing phosphorescent dye fac tris (2-phenylpyridine) iridium (Ir(ppy)₃) in organic solar cells of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV) doped with surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6] C₆₁ (PCBM). It is known that photoexcitation generates both singlet and triplet states through intersystem crossing caused by hyperfine or spinorbital coupling. Due to long diffusion length the triplet excitons can migrate from their generation sites to the interfaces of donor-acceptor interaction and directly dissociate into charge carriers. We found, based on the studies of magnetic field-dependent photocurrent, that the dispersed Ir(ppy)₃ molecules increase the spin-orbital coupling strength and triplet density in the MEH-PPV matrix due to the penetration of MEH-PPV π electrons into the large field of orbital dipoles of the Ir(ppy)₃. Especially, the triplet excitons facilitate the direct dissociation into charge carriers at the donor-acceptor interacting interfaces in the composite of MEH-PPV and PCBM, and consequently improve the photovoltaic response in organic solar cells.