Trap-Assisted Transition Energy Levels of SrF₂:Pr³⁺−Yb³⁺ Nanophosphor in TiO₂ Photoanode for Luminescence Tuning in Dye-Sensitized Photovoltaic Cells

Luminescent nanophosphors as spectral converters offer immense potential for dye-sensitized photovoltaics (DSPV) to harvest a wide range of the solar spectrum. Herein, a novel structural design of DSPV using a downconversion (dc) nanophosphor layer in the TiO₂ photoanode for both indoor (ambient) and outdoor applications is demonstrated. Cubic SrF₂:Pr³⁺−Yb³⁺ nanoparticles are synthesized by a template-free hydrothermal technique. The dc nanophosphor absorbs photons of the blue region, leading to emission of a broad luminescence band (green and red), which is well matched with N719-dye absorption. The mixed-valence state of Pr ions (Pr³⁺ and Pr⁴⁺) leads to trap-assisted transition levels, which result in a broad visible emission. For the first time, a unique Pr³⁺−Yb³⁺ codoped dc system yielding tuned and intensified luminescence by effective crossrelaxation (CR) with a back energy transfer (BET) mechanism is designed and efficient working of the dc nanophosphor-layered DSPVs under both outdoor 1 sun (AM 1.5 G) and indoor light (Warm-3200 K; Day-5000 K) conditions is demonstrated. Improved efficiency of 9.07% is attained in dc-dye-sensitized solar cells (DSSC) compared with a control-DSSC (8.39%) at 1 sun intensity. Under indoor low-light conditions (1000 lux), the dc-DSPV achieves high power conversion efficiencies (PCEs) of 14.85 and 15.9%, respectively. This approach results in a 63.44% increment in output power density for dc-DSPV compared with the control-DSPV under LED 3200 K irradiation. These findings suggest that this configuration of dc-layered DSPV can provide a new strategy for future indoor electronic operations under ambient light conditions.