Solid-State Molecular Film Enables Sunlight-Driven Visible-to-UV Upconversion

Kyushu University researchers reported a solid-state film that converts visible light to UV via TTA-UC under daylight, achieving 1.9 % efficiency and a threshold intensity of ~1.2 mW cm?².

New Solid Material Turns Ordinary Sunlight Into High-Energy UV Light

Solid-state upconversion suffers from ?-electron overlap that quenches triplet excitons, whereas liquid systems need toxic solvents. DHI molecules bearing alkyl chains on sp³ carbons act as steric spacers, suppressing non-radiative quenching while preserving energy transfer.

New Solid Material Turns Ordinary Sunlight Into High-Energy UV Light

The study was led by Yoichi Sasaki and Nobuo Kimizuka of Kyushu University, together with the research team. Ir(ppy)3 was the triplet sensitizer; metal-free TADF sensitizers 4CzIPN and DABNA6 were also tested.

Harvesting UV Light from sunlight just got ‘solid’

Unsubstituted DHI shows ?F = 96 % in solution but only 10 % in crystal; iBu-DHI reaches ?F = 69–83 % in solid form. In solid films iBu-DHI/Ir(ppy)3 yields an absolute ?UC of 1.9 % after correction, with Ith as low as 1.2 mW cm?² (drop-cast). Triplet lifetimes reach 4.0 ms and the lower-limit TET efficiency for iBu-DHI is 71 %.

Nature

UV light drives air sterilization, 3D-printing resin curing and dental material hardening, yet natural UV is only ~6 % of solar irradiance. A daylight-operable upconverter could replace conventional UV lamps, cut energy use and enable low-power applications such as indoor air purifiers and solar-driven photocatalysis.

New Solid Material Turns Ordinary Sunlight Into High-Energy UV Light

Yoichi Sasaki said process “adds together the energy of two visible photons to produce one UV photon,” highlighting its novelty for ambient light; Kyushu University noted steric protection of ?-electron systems and announced a patent filing. Researchers reported the system operates without lasers and works in air-saturated conditions.

Harvesting UV Light from sunlight just got ‘solid’

The 1.9 % upconversion efficiency is modest compared with laser-driven benchmarks, and film performance varies with crystal size and grain boundaries. iBu-DHI fluorescence yields (69–83 %) and triplet lifetimes (1.4–7.8 ms) differ with side-chain length, showing the need for further optimization of film-forming conditions and donor molecules.

Nature

• “What we do here is ‘add together’ the energy from two visible light photons to make one ultraviolet photon. It’s a fascinating process called photo upconversion,” — Yoichi Sasaki, Associate Professor, Kyushu University

Interesting Engineering

• “When that happens, triplets easily fizzle out before they ever meet. Molecules must be close enough for energy to transfer but separated enough to prevent quenching of excitons.” — Yoichi Sasaki

New Solid Material Turns Ordinary Sunlight Into High-Energy UV Light

• “We handed the draft to Professor Kimizuka just 11 days before he left the lab, which for us felt like a heartfelt retirement gift,” — Yoichi Sasaki

New Solid Material Turns Ordinary Sunlight Into High-Energy UV Light

• “To our knowledge, this is the best value for the solid-state Vis-to-UV TTA-UC with I th lower than 10 mW cm–2 ever reported.” — Authors, *Nature Communications*

Nature

Future work will explore metal-free sensitizers, refine crystallisation protocols, and test prototype devices for solar-driven photocatalysis, indoor air purification and low-intensity 3D printing. Detailed studies of metal-free TTA-UC systems are planned.

Nature