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

Materials for Renewable and Sustainable Energy (2025) 14:28 https://doi.org/10.1007/s40243-025-00304-y ORIGINAL PAPER An experimental and computational investigation of Thulium doped TiO2 as n-type material for potential application in bulk heterojunction organic solar cells Dieketseng Tsotetsi1 · David O. Idisi2 · Nicholas Rono2 Mokhotjwa Dhlamini1 · Edson L. Meyer2 · Evans M. Benecha3 · Pontsho Mbule1 · Received: 7 December 2024 / Accepted: 14 March 2025 / Published online: 28 March 2025 © The Author(s) 2025 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%, Voc of 1.54 V, Jsc 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. Keywords n-type material · TiO2 doped Tm · Organic solar cell · Density of states · Charge transfers Introduction David O. Idisi Nicholas Rono 1 Department of Physics, CSET, University of South Africa, Johannesburg 1710, South Africa 2 Institute of Technology, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa 3 Center for Augmented Intelligence and Data Science (CAIDS), University of South Africa, Johannesburg 1709, South Africa Organic solar cells (OSCs) are becoming more and more recognized as a viable substitute for conventional inorganic solar cells because of their distinct benefits, which include low production costs, flexibility, and lightweight. The fundamental idea behind OSCs is that photons are absorbed by an organic substance, which then separates electron-hole pairs and accumulates charges across electrodes to convert light into electricity. Typically, OSCs are made up of two different kinds of organic materials called electron acceptors (A) and electron donors (D), which come together to form a heterojunction. At the interface where the D and A layers meet, electrons are transported from the D to the A layer. A 13 28 Page 2 of 12 variety of variables, including the materials’ charge transport characteristics, exciton diffusion length, and absorption spectra, affect the efficiency of OSCs. Over the past few decades, there has been a major advancement in the creation of OSCs, as evidenced by the outstanding achievement of increasing the efficiency of OSCs from less than 1% to over 18% in the laboratory [1–7]. However, there are certain inherent limits to the bulk heterojunction (BHJ) layer fabrication method that prevent further advancement and technological deployment: (i) The morphology evolution of the BHJ photoactive layer during the donor (D) and acceptor (A) mixture in laboratory spin coating optimization is complex kinetically, requiring a careful balancing of treatment conditions, such as D/A ratio, processing solvent and additive, thermal and/or solvent annealing. For the sake of the best possible charge separation and charge transport, achieving a balance between phase purity and separation remains quite difficult. In the current trial-and-error optimization, the vertical component distributions are still uncertain, much alone precise. (ii) The phase separation, which is directly tied to the acceptor and donor’s miscibility and solubility in the blend solution and so restricts the choice of processing solvents, has a significant impact on the BHJ performances [8–10]. Several approaches have been used to boost OSC performance, including creating new donor and acceptor fragments of the photoactive blend, managing the photoactive layer’s morphology, using interfacial layers, and creating and implementing novel device structures. Therefore, a consistent growth in the efficiency and stability of OSC modules is often highlighted even in the context of the limited demonstration of real large-scale installations of these modules. To increase the efficiency and stability of the BHJ OSCs, interfacial layers, particularly the hole transport layer (HTL) and the electron transport layer (ETL), are frequently added between the anode-photoactive and cathode-photoactive interfaces, respectively [11]. ETL materials tend to include oxides, such as titanium dioxide (TiO2) or zinc oxide. When employed as a “optical spacer,” a transparent thin layer of this kind can also enhance light absorption in the photoactive layer [12]. The high absorption coefficients and better photoconductivity of metal/metal oxides relative to organic semiconductors are predicted to make solar cells using metal or metal oxide nanoparticles an effective solution to the absorption problem in OSCs. As a result, the OBHJ’s photon harvesting may be improved by using the hybrid technique [13]. TiO2 is nontoxic and cheap, and it demonstrates highphotocatalytic activity, good chemical stability, photocorrosion resistance, and excellent biocompatibility. Various TiO2 nanostructures have been produced successfully by researchers thus far, and they have been used for 13 Materials for Renewable and Sustainable Energy (2025) 14:28 environmental cleanup. Furthermore, TiO2 has a number of disadvantages, which limits its applicability in solar cell application in its pristine form. Due to its wide-energy band and the rapid recombination of electron-hole pairs, TiO2 can only absorb about 5% of the solar spectrum, which significantly limits i (...truncated)