Publication date: 17th July 2025
Photon interconversion is a promising approach to increase the power conversion efficiency of photovoltaic devices. Downconversion of high energy photons can alleviate thermalization losses, while upconversion converts sub-bandgap photons to photons with a usable energy.
Here, we will focus on the upconversion process. To comply with energy conservation laws, the energy of two low energy photons is combined to form a single higher energy photon. For upconversion to be relevant for solar energy conversion, it must be efficient at low incident powers. In triplet-triplet annihilation the energy is stored in long-lived spin-triplet states, which enables efficient upconversion at solar-relevant powers. Since the direct optical generation of triplet states is forbidden per selection rules, sensitizers are required to generate triplet states with high yields. Currently, triplet sensitizers span a broad range of material classes including metal-organic complexes, semiconductor nanomaterials, and bulk perovskite films.
I will present the current understanding of triplet generation at the bulk lead halide perovskite/organic interface and discuss the role of molecular aggregation and intermolecular coupling on the energy landscape underlying the upconversion process. To date, the ‘gold standard’ for solid-state NIR-to-visible upconversion (UC) is rubrene, which is in large part due to intermolecular interactions influencing the optical properties of the organic molecule – the triplet annihilator. Unfortunately, the triplet energy (T1 = 1.14 eV) of rubrene is energetically not well matched with the 1.55 eV bandgap of the typically utilized formamidinium-rich perovskites, leading to a large inherent energy loss during triplet generation. To increase the achievable apparent anti-Stokes shift obtained during the upconversion process, annihilators with a higher triplet energy are desired. [1],[2]
We acknowledge funding by the National Science Foundation under Grant Nos. DMR-2237977 and DMR-2517590, the Camille and Henry Dreyfus Foundation (TC-23-050) and the Alfred P. Sloan Foundation.
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