Sol-gel synthesis and characterization of Ho3+ doped TiO2 nanoparticles: Evaluation of absorption efficiency and electrical conductivity for possible application in perovskite solar cells

Abstract

Titanium dioxide (TiO2) and holmium (1.2 mol% Ho3+) doped TiO2 nanoparticles were synthesized via sol-gel method using polyvinylpyrrolidone (PVP) co-polymer as a pore forming agent. The nanoparticles were dispersed in a solvent and drop-cast on a glass substrate to make films. The films were then calcined at 550 °C for 4 h in a pre-heated muffle furnace. Methylammonium lead iodide (MAPbI3) was incorporated with TiO2 and TiO2:Ho3+ nanoparticles. Therefore, the samples TiO2, TiO2:Ho3+, TiO2:MAPbI3 and TiO2:Ho3+:MAPbI3 were obtained and their properties as an electron transporting and photon absorption layers, respectively, were evaluated for possible application in perovskite solar cells. XRD results revealed a rutile phase which was found to be consistent with the RAMAN analysis. FESEM showed spherical nanoparticles with irregular pore size for TiO2 and TiO2:Ho3+ samples, whereas MAPbI3 nanocrystals seemed to have filled-up the pores in TiO2:MAPbI3 and TiO2:Ho3+:MAPbI3 samples. UV–Vis results showed an expected UV absorption of TiO2 with the absorption band at ∼366 nm and upon doping with 1.2 mol% of Ho3+ ions, an improved absorption and a shift towards the visible region was observed. Small absorption bands at ∼453, 541 and 645 nm were detected, which are resulting from the Ho3+ transitions. When MAPbI3 was incorporated with TiO2:Ho3+, absorption in the visible range remained improved with the bands at ∼541 and 645 becoming intense. TiO2:MAPbI3 sample showed a further absorption improvement with a red-shift of the absorption band. The band gap energy was estimated and found to be ∼3.00, 2.35, 2.32 and 2.34 eV for TiO2, TiO2:Ho3+, TiO2:MAPbI3 and TiO2:Ho3+:MAPbI3, respectively. The red-shift and the reduction of the bandgap values is a good indication that these materials will absorb as much photons in the visible and near-infrared regions. The current-voltage (I–V) analysis revealed that the sample TiO2:MAPbI3 have higher current flow, implying lower resistance in the material. The PL results indicated low electron-hole recombination as the intensity was considerably decreased upon doping with Ho3+ and incorporating MAPbI3.