Integration of upconverting β-NaYF4:Yb3+,Er3+@TiO2 composites as light harvesting layers in dye-sensitized solar cells

Abstract

Near infrared-to-visible upconverting NaYF₄: Yb³⁺,Er³⁺@TiO₂ composites are engineered by controlled titanium dioxide coating of citrate-modified β-NaYF₄:Yb³⁺,Er³⁺ sub-microcrystals of ∼300 nm in size and employed as light harvesting layers in dye-sensitized solar cell fabrication. The NIR-to-vis upconversion response of the as-prepared composites is enhanced by a 2 orders of magnitude increase in the upconversion luminescence as a result of annealing effects induced by the thermal processing steps involved in DSSC fabrication. In addition, the TiO₂ coating suppresses the β to α phase transition in NaYF₄ thereby maximizing the upconversion response. The as-prepared upconversion crystals as well as 2 types of composite materials are integrated as internal light-harvesting layers in dye-sensitized solar cells (DSSCs). The insertion of the as-prepared or TiO₂ coated β-NaYF₄:Yb³⁺,Er³ crystals with inadequate TiO₂ thickness results in a decrease in photovoltaic performance due to increased charge recombination. By ensuring the β-NaYF₄:Yb³⁺,Er³⁺ crystals are fully coated in TiO₂ (“core@shell” configuration), their integration into the DSSC is optimized, resulting in 16% relative increase in power-conversion efficiency over the control devices without the NaYF₄:Yb³⁺,Er³⁺@TiO₂ composite internal layer. This increase, as determined by EIS and IPCE under 1 sun illumination, is attributed to the recovery of adequate charge recombination resistance and light harvesting enhancement via scattering.