Overcoming the Absorption Bottleneck for Solid-State Infrared-to-Visible Upconversion

Upconversion (UC) of low-energy photons to higher-energy photons has enabled exciting advances in applications such as 3D printing, bioimaging, and more. In particular, UC of near infrared photons into visible photons has been identified as a process which can enhance photovoltaic, night vision, and anti-counterfeiting technologies. Triplet-triplet annihilation UC is particularly attractive for these applications due to its low upconversion thresholds and broadband, tunable absorption. However, current state-of-the-art near-infrared-to-visible triplet-triplet annihilation solid-state UC devices made of PbS quantum dots and rubrene are limited by (1) low absorption of near infrared photons, (2) low energy transfer rates, and (3) highly parasitic back transfer processes, leading to low external quantum efficiencies unsuitable for wide application. Here, we propose a device architecture that allows for strongly absorbing PbS films with improved efficiencies. We use 5-tetracene carboxylic acid as an interlayer to improve Dexter energy transfer to rubrene and alleviate parasitic back transfer leading to an improvement by a factor of 5 compared to control devices. Finally, we demonstrate that these devices allow for visible upconversion anti-counterfeiting with an incoherent light source at modest intensities, highlighting their potential for UC-facilitated technologies.