Experimental and Theoretical Understanding of The Y6:Rubrene Upconverting Bilayer

Photon upconversion via triplet-triplet annihilation (TTA) is a promising approach to overcome the detailed balance limit of single band-gap photovoltaics. However, low triplet densities in the bulk often limit TTA efficiency. Recently, Izawa et al. [1] demonstrated a new bilayer upconversion architecture comprising a non-fullerene acceptor (Y6) and rubrene. Photoexcitation of Y6 is believed to produce charge-separated states at the interface, which then recombine with normal spin statistics (avoiding the energetic cost of inter-system crossing), generating a high density of triplets in the rubrene which then fuse into higher-energy emissive singlets. Despite this promising 2021 result, details of the upconversion mechanism and dynamics in this system remain largely unexplored.<br/><br/>We study this system with complementary experimental and theoretical techniques.<br/><br/>Performing optical and electron paramagnetic resonance (EPR) spectroscopy helps us understand the nature of the photo excited states and how their population changes with time. EPR spectroscopy directly detects the spin of the photo excited species.<br/>Interpretation of this data is greatly helped by simulation of the non-adiabatic dynamics. This multi scale modelling approach starts with molecular dynamics (to get a representative interface) and then quantum chemistry (to understand and parameterise the eigenstates) and then a path-integral approach to the non adiabatic dynamics (to simulate the reduced density matrix as a function of time). The path integral approach to non adiabatic dynamics offers a principled and consistent hierarchy of simulation methods from fully quantum to semi-classical, ensuring trust in the results.<br/><br/>Our motivation for developing such a detailed understanding of the Izawa and Hiramoto Y6:Rubrene bilayer system is to then be able to make suggestions for synthesis for materials that have a higher upconversion efficiency and therefore can be used to engineer high efficiency photovoltaic systems. These upconversion based system would use large band-gap solar cells, making use of earth abundant chalcogenide and oxide semiconductors, and non donor-acceptor organic photovoltaics.<br/><br/>[1] Izawa, S., Hiramoto, M. (2021). Efficient solid-state photon upconversion enabled by triplet formation at an organic semiconductor interface. Nature Photonics, 15(12), 895–900. https://doi.org/10.1038/s41566-021-00904-w