Rear-earth ions doped mesoporous TiO2 nanostructures and the caffeine booster in MAPbl3 perovskite material

Abstract

Titanium dioxide (TiO2) nanostructures were synthesized by sol-gel method and co polymers polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) and pluronic F127 were used as pore forming agents. The samples were then calcined at 550 °C for 4 hours in a pre-heated muffle furnace. The synthesized TiO2 showed porous morphology with spherical nanoparticles. Nitrogen adsorption-desorption isotherm studies revealed that the samples were mesoporous in nature. BET further indicated that the specific surface areas of TiO2@F127, TiO2@PVP and TiO2@PEG were ~ 69.82, 37.80 and 57.08 m2 /g, respectively. The pore size was found to be ~13.01, 10.10 and 8.53 nm For TiO2@F127, TiO2@PVP, and TiO2@PEG. Mesoporous TiO2 (mp-TiO2) was then doped with rare-earth ions (Ho3+, Er3+ and Tm3+) and incorporated into methylammonium lead iodide (MAPbI3). Thereafter, photon absorption and electrical conductivity were evaluated. The photon absorbance showed a slight redshift in the 400–550 nm range with tiny absorption bands appearing at ~645–653 nm for TiO2:Ho3+:MAPbI3, ~684–693 nm for TiO2:Er3+:MAPbI3 and ~684– 793 nm for TiO2:Tm3+:MAPbI3. Current – Voltage (I-V) characteristics showed an improved electrical conductivity and low sheet resistance from TiO2:Er3+:MAPbI3 and TiO2:Tm3+:MAPbI3 samples, implying that these samples are suitable as active photon absorption layers in the application of perovskite solar cells. The thermal stability and crystallinity of MAPbI3 were investigated by introducing different concentrations of 1,3,7-trimethylxanthine (caffeine) (2%, 4%, 6% and 8%) into the system. TGA analysis revealed that MAPbI3 was stable at lower temperatures (0-91 oC) then upon adding caffeine into MAPbI3 the thermal stability was improved in higher temperature range (196-242 oC). XRD analysis showed that the samples exhibited intense diffraction peaks at ~2θ = 19.66o and 39.81 o , respectively assigned to the (112) and (224) planes of tetragonal MAPbI3 structure which indicates that the desired MAPbI3 film was formed. MAPbI3@Caffeine (8%) and MAPbI3@Caffeine (6%) exhibited high PL quenching which indicates their high charge carrier extraction efficacy of which can be attributed to the higher crystallinity and fewer defect states of the film compared to MAPbI3@Caffeine (2%) and MAPbI3@Caffeine (4%) samples.