Boosting the Photoelectrochemical Activity of TiO₂-Au Films by Harvesting Infrared Photons with a Li-Codoped NaYF₄:Yb,Er Upconversion Layer

Conventional solar-harnessing devices have limited ability to capture photons in the infrared (IR) region, necessitating the introduction of additional methods to minimize these light harvesting losses. In this study, we first investigated the effects of Li element codoping on the upconversion (UC) properties of NaYF₄:Yb,Er nanoparticles (NPs) prepared by a thermal decomposition method. It was shown that UC quantum yield (QY) of optimal Li-codoped NaYF₄:Yb,Er NPs was enhanced to ∼4.54 times, resulting in more intense UC emission. Next, we investigated the ability of UC NPs combined with the plasmonic properties of gold NPs to convert near-infrared (NIR) light into visible light, thereby increasing the absorption range and photoelectrochemical (PEC) activity efficiency of TiO₂ thin films. A chronoamperometry study revealed that UC NPs can improve the IR light harvesting ability of TiO₂-Au thin films, resulting in a photocurrent density enhancement of around 21.1 and 36.3% for NaYF₄:Yb,Er NPs and Li-codoped NaYF₄:Yb,Er NPs, respectively. Our findings underscore the potential of UC materials with improved optical properties in expanding light absorption of solar-harnessing devices in the IR range.