Triplet-triplet annihilation upconversion: Evaluating the 50-fold efficiency leap in selenium films

A newly synthesised selenium-integrated heptamethine cyanine photosensitiser claims to achieve an upconversion quantum yield of 1.0% under 830 nm excitation. Historically, the field of triplet-triplet annihilation upconversion has struggled to bridge the gap between near-infrared (NIR) absorption and efficient visible emission without relying on toxic heavy metals. Previous attempts using organic small molecules frequently stalled due to poor energy transfer rates and rapid decay, rendering them largely ineffective for robust applications.

The core of this advancement rests on the specific interaction between the new photosensitiser (Cy2) and the annihilator, rubrene. By employing spin coating to create a single-layer film, the team produced a material that absorbs low-energy light at 830 nm and emits higher-energy light at 557 nm. While a 1.0% yield may appear modest in absolute terms, it represents a 50-fold improvement over the previously reported IR806/rubrene pair. This sharp increase suggests that the structural modification of the cyanine skeleton has successfully addressed some of the fundamental losses inherent in earlier designs.

Selenium integration vs iodine substitution

To understand the efficiency gains, one must contrast the new selenium-integrated architecture (Cy2) with the older iodine-substituted variants (Cy1). In previous iterations, the heavy-atom effect of iodine was intended to facilitate the transition of electrons to the triplet state. However, the rates were often insufficient to outcompete non-radiative decay pathways. The photophysical characterisation in this study indicates that replacing iodine with selenium increases the intersystem crossing rate to 3.22 × 10⁸ s^-1. This is a 3.5-fold acceleration compared to Cy1. Furthermore, the triplet excited state quantum yield in the selenium model reached 23%, a 2.6-fold rise over the iodine counterpart. Where the iodine model was sluggish, the selenium integration forces the molecule to populate the triplet state more aggressively, providing the rubrene annihilator with a more substantial energy reservoir.

Implications for Triplet-triplet annihilation upconversion

Despite the relative success, scepticism regarding commercial viability is warranted. The theoretical maximum efficiency for this upconversion process is 50%; achieving 1.0% means the vast majority of photon energy is still lost. The study characterises the film as having "excellent transmittance," yet for solar energy harvesting—a primary target for this technology—every fraction of a percent in efficiency is vital. The jump from 0.02% to 1.0% is statistically massive but practically early-stage. The findings demonstrate that metal-free systems can operate at longer wavelengths than previously thought possible, extending the utility of organic photonics. However, until yields approach double digits, these films may remain restricted to niche biophotonic applications rather than broad-scale energy solutions.