Super-diffusion of Excitation Energy

demonstrated within red-shifted conjugated copolymers
 

The practical applications of solution-processable semiconductor polymers are limited by the speed and distance of travel of excitation motion along with the excitation loss during transport. Simultaneously achieving ultrafast, long-range and low-loss excitation energy transfer from the photo-receptor location to a functionally active site is pivotal for the cost-effective semiconducting polymers esp. for their applications in the emerging fields including photobiocatalysis, biosensing, medical therapy, etc., besides the more conventional OLEDs, thin-film optoelectronics such as photovoltaics and (flexible) transistors. Within this recent PNAS paper, Dr. Yuping Shi and colleagues in the Landfester Department here at MPI-P, University of California, Berkeley, and Princeton University, report the occurrence of long-sought “super-diffusion of excitations” in some remarkable red-shifted (e.g., strong deep-red to near-infrared light absorption enabled by much elongated conjugation in planar polymer backbones) conjugated copolymers, via the first implementation of state-of-the-art phase-cycled pump-probe measurement in polymer solid deposits.

The authors demonstrate record-high exciton diffusion constants of >0.5 cm²/s and ultralong diffusion lengths in the facilely fabricated large-area continuous thin films of the copolymer, which literally far exceeds the film thickness. These desirable semiconducting properties are ascribed to entering into a brand new transport regime that is fundamentally limited by exciton-exciton annihilation (EEA), in which the generated excitons are allowed to encounter and separate several times in the lifetime (thus, enabling a rather low EEA probability per encounter and much lower energy losses); this new mechanism is found beyond the classical diffusion-limited transport regime with 100% EEA rate on the first contact of travelling excitons due to strong electronic interactions. Remarkably, they find that the unwanted energy losses arising from many-body interactions are greatly suppressed in the device-relevant thin film as compared to isolated polymer chains in the solution phase.

In general, a rich range of device fabrication and performance benefit from large diffusion lengths and this newly discovered annihilation-limited transport mode. In the case of the diffusion length exceeding the thickness of an organic semiconducting layer, the mainstream bulk-heterojunction fabrication may not be necessary, potentially allowing for building the simple double-layer architecture of organic photovoltaic devices. When the diffusion length is larger than the critical dimension of catalytic or sensing nanostructures, it is expected to mitigate or prevent the formation of “dead volume” where the excitations are trapped inside within lifetime and therefore hardly arrive at the outer active sites; in this way, the reaction activity and overall power efficiency can be upgraded simultaneously toward contributing to building a greener and healthier society.