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

Near infrared-to-visible upconverting NaYF4: Yb3+, Er3+@TiO2 composites are engineered by controlled titanium dioxide coating of citrate-modified beta-NaYF4:Yb3+, Er3+ sub-microcrystals of similar to 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 beta to alpha 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 beta-NaYF4:Yb3+, Er-3 crystals with inadequate TiO2 thickness results in a decrease in photovoltaic performance due to increased charge recombination. By ensuring the beta-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.