Improvement of photovoltaic response based on enhancement of spin-orbital coupling and triplet states in organic solar cells

This article reports an improvement of photovoltaic response by dispersing phosphorescent Ir(ppy)3 molecules in an organic solar cell of poly[2-methoxy-5-(2′-ethylhexyloxy)-1 4-phenylenevinylene] (MEH-PPV) blended with surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C61 (PCBM). The magnetic field–dependent photocurrent indicates that the dispersed Ir(ppy)3 molecules increase the spin-orbital coupling strength with the consequence of changing the singlet and triplet ratios through intersystem crossing due to the penetration of the delocalized π electrons of MEH-PPV into the large orbital magnetic field of Ir(ppy)3 dopants. The tuning of singlet and triplet exciton ratios can lead to an enhancement of photovoltaic response due to their different contributions to the two different photocurrent generation channels: exciton dissociation and exciton-charge reaction in organic materials. In addition, the photoluminescence temperature dependence reveals that the dispersed Ir(ppy)3 reduces the recombination of dissociated charge carriers in the PCBM doped MEH-PPV. As a result, adjusting singlet and triplet ratios by introducing heavy-metal complex Ir(ppy)3 provides a mechanism to improve the photovoltaic response through controlling exciton dissociation, exciton-charge reaction, and recombination of dissociated charge carriers in organic bulk-heterojunction solar cells.