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

Abstract

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]) C 61 (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: excitondissociation 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 excitondissociation, exciton-charge reaction, and recombination of dissociated charge carriers in organic bulk-heterojunction solar cells.