An experimental and computational investigation of Thulium doped TiO2 as n-type material for potential application in bulk heterojunction organic solar cells

Abstract

Solar energy harvesting and conversion has attracted a lot of scientific interest because solar energy is believed to be clean and sustainable. In this study, we report the synthesis of porous TiO₂ by sol-gel method and later doped with Thulium rare earth ions (Tm³⁺) for potential application in organic solar cells as electron transport layers (ETL). Additionally, density functional theory (DFT) calculation was performed with CASTEP computational suite to explore further the optoelectronic and charge transfer mechanisms in the Tm(III)-doped TiO₂ nanomaterials. Thereafter, the experimental material’s band gap values were extracted and used in the numerical simulation of the designed organic solar cell with a general configuration of FTO/TiO₂/PBDB-T/ITIC/Cu₂O/Ag, via SCAPS-1D numerical simulator. The experimental results showed a steady reduction in the band gap of TiO₂ with increased Tm³⁺ doping. The electrical conductivity properties showed an enhanced feature when TiO₂ was doped with Tm³⁺ nanoparticles. The calculated band gap from the density functional theory study shows a similar decreasing band gap trend with that of the experimental data, suggesting the transport properties from DFT are sufficient to describe the experimental data. The electronic transfer behaviour is analogous to metal-metal and metal-oxides transport features, which can be attributed to Ti – Tm and Tm – O – Ti hybridizations, as indicated in the orbital state alignment. The best performing modelled device with Tm(III)-doped TiO₂ (1.0 mol%) as ETL attained a PCE of 21.83%, V_oc of 1.54 V, J_sc of 31.87 mA cm^− 2 and FF of 44.44% which was attributed to better charge transfer characteristics and effective band alignment between the ETL and absorber, thus, better efficiency. The study proposes that Tm(III)-doped TiO₂ can act as a suitable n-type material that can propel the realisation of high-performance OSCs for commercialization in the future.