Synthesis of Cobalt-Doped TiO₂ Based on Metal-Organic Frameworks as an Effective Electron Transport Material in Perovskite Solar Cells

In this study, Co-doped TiO₂ was prepared successfully using a solvothermal method with trimesic acid (H₃BTC) as an organic framework to form the Co-doped Ti metal-organic framework (Co-doped Ti-MOF). By thermally decomposing the Co-doped Ti-MOF in air, the framework template was removed, and porous Co-doped TiO₂ was obtained. The crystal structure of the material was analyzed using X-ray diffraction. The morphology was examined using scanning electron microscopy (SEM) and focused ion beam SEM. The large specific surface area was determined to be 135.95 m² g^-1 using Brunauer-Emmett-Teller theory. Fourier transform infrared spectroscopy verified the presence of Ti-O-Ti and Co-O vibrations in the as-prepared sample. Furthermore, the results of UV-vis spectroscopy showed that doping with Co remarkably improved the absorption ability of Ti-MOF toward the visible-light region with a band gap energy of 2.38 eV (λ = 502 nm). Steady-state photoluminescence and electrochemical impedance spectroscopy were conducted to illustrate the improvement of electron transfer in the doped material further. The optimum power conversion efficiency of solar cells using 1 wt % Co-doped TiO₂ as an electron transport layer was found to be 15.75%, while that of solar cells using commercial dyesol TiO₂ is only 14.42%.