Bulk Heterojunction for Improved Solid-State Upconversion with Non-Fullerene Acceptors

Solid-state upconversion, a process whereby two lower energy photons are converted to one higher energy photon, is promising for a variety of applications, including photovoltaics, bioimaging, and night vision. For upconversion to be useful in these and other applications, a high external quantum efficiency (EQE) is required to generate sufficient quantities of upconverted light. Typical solid-state upconversion systems, based on intersystem crossing (ISC) in a sensitizer and subsequent triplet-triplet annihilation (TTA) in an annihilator, use dilute mixtures or very thin films of sensitizer, which results in minimal absorption of the low energy light to be upconverted, and hence low EQEs. Recent work from Izawa and Hiramoto [1] uses non-fullerene acceptors (NFAs) as sensitizers, which are paired with a donor-annihilator in a bilayer structure to generate triplets at an interface without the need for ISC or triplet diffusion in the sensitizer - significant loss mechanisms in solid-state upconversion systems. With this method, they achieve record near infrared (NIR)-to-visible EQEs of up to 2.3%. Our work builds upon this concept, inspired by organic photovoltaic (OPV) architectures, to create a bulk heterojunction (BHJ) upconversion film where the sensitizer and annihilator are combined in one solution-processed layer. This results in superior EQEs than their bilayer counterparts, due to the increased surface area between the two materials resulting in improved triplet generation. The benefits and applicability of this optimized BHJ upconversion system are then demonstrated with a NIR-to-visible imaging system.<br/><b>References:</b><br/>[1] Izawa, S., Hiramoto, M. Efficient solid-state photon upconversion enabled by triplet formation at an organic semiconductor interface. <i>Nat. Photon.</i> <b>15, </b>895–900 (2021). https://doi.org/10.1038/s41566-021-00904-w