Justin Hodgkiss, MacDiarmid Institute
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 rapid exciton diffusion in films of fused-ring non-fullerene acceptors that, when blended with a donor produce high-performing 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.[1] Enhanced 3-dimensional diffusion accounted for computationally [2] and 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, high chromophore density, and low disorder – all enhancing long-range resonant energy transfer.
Moreover, we show that crystalline non-fullerene acceptors are also amenable to high throughput computational screening of exciton diffusion. 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. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology [3].
[1] S. Chandrabose, et al. High Exciton Diffusion Coefficients in Fused Ring Electron Acceptor Films J. Am. Chem. Soc., 2019, 141, 6922-6929. https://doi.org/10.1021/jacs.8b12982 [2] P. A. Hume & J. M. Hodgkiss Long-range exciton diffusion in a non-fullerene acceptor: approaching the incoherent limit J. Mat. Chem. C. 2021, 9, 1419-1428. 10.1039/D0TC05697A [3] S. Y. Park, et al Photophysical pathways in efficient bilayer organic solar cells: The importance of interlayer energy transfer Nano Energy 2021, 84, 105924. https://doi.org/10.1016/j.nanoen.2021.105924About the presenter
Prof Justin Hodgkiss uses ultrafast optical spectroscopy at the Victoria University of Wellington in search of molecular electronic materials for new, low-cost printable electronics – primarily solar cells. His group invented transient grating ultrafast fluorescence spectroscopy and has used laser spectroscopy to develop a detailed understanding of the physics of photocurrent generation in printable solar cells. Prof Hodgkiss’s group also studies bio-templated assembly of organic semiconductors, recently developing a class of hybrid materials using natural peptides to encode the assembly of organic semiconductors into functional electronic devices.