The role of triplet excitons in enhancing polymer solar cell efficiency: a photo-induced absorption study

TL;DR

This study demonstrates that incorporating heavy metal atoms into polymer backbones creates triplet excitons that, through interfacial dissociation, significantly improve the efficiency of polymer solar cells, as evidenced by photo-induced absorption spectroscopy.

Contribution

It provides experimental evidence that triplet excitons in heavy-metal-doped polymers enhance photovoltaic efficiency via interfacial dissociation mechanisms.

Findings

Triplet character in CTC states correlates with higher PV efficiency.

Absorption peak at 1.65 eV indicates formation of polaron-pairs.

Heavy metal atoms induce triplet excitons that improve charge separation.

Abstract

Inclusion of heavy metal atoms in a polymer backbone allows transitions between the singlet and triplet manifolds. Interfacial dissociation of triplet excitons constitutes a viable mechanism for enhancing photovoltaic (PV) efficiencies in polymer heterojunction-based solar cells. The PV efficiency from polymer solar cells utilizing a ladder-type poly (para-phenylene) polymer (PhLPPP) with trace quantity of Pd atoms and a fullerene derivative (PCBM) is much higher than its counterpart (MeLPPP) with no Pd atom. Evidence is presented for the formation of a weak ground-state charge-transfer complex (CTC) in the blended films of the polymer and PCBM, using photo-induced absorption (PIA) spectroscopy. The CTC state in MeLPPP:PCBM has a singlet character to it, resulting in a radiative recombination. In contrast, the CTC states in PhLPPP:PCBM are more localized with a triplet character. An absorption peak at 1.65 eV is observed in PhLPPP:PCBM blend in the PIA, which may be converted to weakly-bound polaron-pairs, contributing to the enhancement of PV efficiency.