Enhanced photon harvesting in dye-sensitized solar cells by doping TiO2 photoanode with NaYF4:Yb3+,Tm3+ microrods

An effective strategy to improve the efficiency of dye-sensitized solar cells (DSSCs) by infrared radiations (IR) and near infrared radiations (NIR) harvesting, using upconversion crystalline microrods NaYF4:Yb3+,Tm3+ is put forward. NaYF4:Yb3+,Tm3+ crystals were prepared with varying compositions of NaYF4, Yb3+ and Tm3+, and then in three different morphologies including small hexagonal microprisms, big hexagonal microprisms and hexagonal microrods for the investigation of the role of composition and morphology on upconversion efficiency. Thus, the crystals with efficient upconversion can be to screened out for doping with TiO2 photoanode of solar cells. Crystalline microrods, synthesized by hydrothermal method, show better upconversion as compared to microprisms prepared in two different sizes. On the other hand, these crystals also show downconversion. Addition of small amount of these microrods in TiO2 electrode of DSSCs increases light harvesting ability of photoanode TiO2 and consequently, photocurrent conversion efficiency is enhanced. Moreover these rare earth ions, being p-type dopant, raise the energy level of the TiO2 film and resultantly increase the photovoltage. Maximum power conversion efficiency 7.03% was achieved at an optimized proportion of NaYF4:Yb3+,Tm3+ in TiO2 (by weight). This 7.03% efficiency is 14.7% higher than the efficiency of pure TiO2 photoanode (6.13%). This improvement in efficiency can be credited to upconversion characteristics of doping material.