Facile Exciton Diffusion in Fused Ring Electron Acceptor Films

Sreelakshmi Chandrabose^a^,^b, Kai Chen^a^,^b, Alex J. Barker^c, Joshua J. Sutton^a^,^d, Shyamal Prasad^a^,^b, Jingshuai Zhu^e, Keith C. Gordon^a^,^d, Zenqi Xie^f, Xiaowei Zhan^e, Justin M. Hodgkiss^a^,^b

^aThe MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand

^bSchool of Chemical and Physical Sciences,Victoria University of Wellington, New Zealand, New Zealand

^cCenter for NanoScience and Technology, Italian Institute of Technology, Via Pascoli 70/3, 20133 Milano, Italy

^dDepartment of Chemistry, University of Otago, New Zealand

^ePeking University, Department of Material Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, China, Beijing, China, 100871, China

^fInstitute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, China

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)

Roma, Italy, 2020 May 12th - 14th

Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo

Oral, Alex J. Barker, presentation 174
DOI: https://doi.org/10.29363/nanoge.hopv.2020.174
Publication date: 6th February 2020

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells, however, this morphology compromises charge collection. Here, we reveal facile exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells.

Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 ×10^-2 cm² / s – substantially exceeding typical organic semiconductors, and consistent with the 20-50 nm domain sizes in optimized blends. Enhanced 3-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, and low disorder – all enhancing long-range resonant energy transfer.

Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Long exciton diffusion lengths also adds tolerance to morphology variation, and further enhancements may even obviate the need for the bulk heterojunction morphology.

Acknowledgements:

JMH and KC acknowledge support from the Marsden Fund and a Rutherford Discovery Fellowship to JMH. XZ acknowledges support from NSFC (No. 21734001 and 51761165023).

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