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1/9 - Faculty Candidate Seminar (physical): Sean Roberts, USC

Sean Roberts
University of Southern California
Host: Haw Yang

Can Triplet Excitons Play a Role in Plastic Photovoltaics?

Organic photovoltaics (OPVs) are a promising class of devices that combine the processing advantages of plastics with the electrical properties of semiconductors. The ability to process these devices at low temperature through a variety of solution or vapor phase deposition techniques onto light, flexible substrates provide OPVs with significant commercial potential. Light absorption by an OPV creates a molecular excited state, or exciton, which must diffuse to an interface between an electron donor and an acceptor to generate charge carriers. However, the short lifetime of singlet excitons (~ns) limits their diffusion length to ~5-10 nm, forcing OPV design to focus on reducing the distance over which excitons need to travel within the device. One strategy to improve exciton diffusion and ease constraints on device design is to exchange singlet excitons for triplets that have long excited state lifetimes (μs – ms).

To understand how triplet excitons impact OPV performance, we have investigated the excited state dynamics of a model OPV system that uses 5,12-diphenyltetracene (DPT) doped with platinum tetraphenylbenzoporphyrin (PtTPBP) as the primary light absorber. Photo-excitation of PtTPBP creates a singlet excitation that rapidly intersystem crosses to a triplet that transfers to the DPT host matrix. We find that despite their low energy, these triplets dissociate at a C60 interface to produce photocurrent. Surprisingly, excitation of the amorphous DPT host also produces triplets through singlet fission, a process wherein a singlet state divides its energy to form two lower energy triplets.

Singlet fission is an intriguing phenomena since it can be used to design inexpensive devices that mimic the behavior of high performance multi-junction solar cells. In the PtTPBP doped film, we find that fission accounts for ~60% of the triplet production following DPT excitation. Our results demonstrate that triplets can positively impact OPV performance and suggest pathways for the design of improved triplet-based devices.