Stabilizing effects of zinc(II)-benzene-1,3,5-tricarboxylate metal organic frameworks on the performance of TiO2 photoanodes for use in dye-sensitized solar cells

Abstract This work reports on the effect of zinc(II)-benzene-1,3,5-tricarboxylate metal organic frameworks (Zn-BTC MOFs) towards enhancing the power conversion efficiency of titanium dioxide (TiO2) based dye-sensitized solar cells (DSSCs). Zn-BTC MOFs and TiO2 nanoparticles were synthesized via a modified hydrothermal method. This was followed by the addition of an extrinsic mixture of TiO2 powder and MOFs to form TiO2-Zn-BTC MOF(2%), TiO2-Zn-BTC MOF(4%) and TiO2-Zn-BTC MOF(6%) nanocomposites with varying concentration. The structure, morphology, optical properties and thermal stability of MOF powders were subsequently evaluated. XRD analysis demonstrated that the incorporation of MOFs into TiO2 decreases the crystallinity of TiO2 due to the poor crystalline structure of MOFs. The presence of the Ti-C bond and the induced redshift of the E1g peak suggesting the migration of electrons between TiO2 and MOFs was confirmed by FTIR and Raman spectroscopy data, respectively. Microscopic studies showed the formation of irregularly shaped nanoparticles with a size range of 5 – 40 nm. Additionally, the BET surface area improved with an increase in Zn-BTC loadings, leading to the uniform dispersion of TiO2 nanoparticles in the nanocomposites. The optical properties of the nanocomposites were enhanced by the incorporation of Zn-BTC MOFs resulting to low band gap values. The TiO2-Zn-BTC MOF(2%) exhibited a lower band gap of 2.9 eV in comparison to TiO2-Zn-BTC MOF(4%) with a band gap of 3.1 eV and TiO2-Zn-BTC MOF(6%) which also had a band gap of 3.1 eV. Hence, TiO2-Zn-BTC MOF(2%) exhibited reduced recombination rates of the electron-hole pairs and increased photoactivity as confirmed by the photoluminescence (PL) data, as well as reduced charge-transfer kinetics shown by electrochemical impedance results. The I-V results showed that TiO2-Zn-BTC MOF(2%) has a strong ability to act as an effective electron transporter which significantly enhanced the photoefficiency of TiO2 thus leading to better photoanodes for DSSCs.