A facile impregnation synthesis of Ni-doped TiO2 nanomaterials for dye-sensitized solar cells

Abstract

This study reports a facile impregnation method for synthesizing Ni-doped TiO2 nanomaterials using P25-TiO2 as a starting material. The as prepared nanomaterials were subjected to structural and optical characterizations and subsequently employed in photovoltaic studies. X-ray diffraction (XRD) and Raman studies confirmed that Ni doping did not alter the anatase and rutile contents of P25-TiO2. Also, the presence of the constituent dopants and their ionic states were confirmed by Energy-Dispersive X-ray (EDX) and X-ray photoelectron (XPS) spectroscopies. Topographic Atomic Force Microscopic (AFM) images illustrated that Ni doping had increased the surface roughness of the TiO2. Optical characterization by UV-Visible spectroscopy revealed that the Ni doping had caused red shift in light absorption due to reduced TiO2 bandgap and improved the dye adsorption on TiO2 films. Then, the photocurrent–photovoltage property of the fabricated devices was investigated and the optimized 0.10 wt% Ni-doped TiO2 photoanode based device exhibited pronounced power conversion efficiency (PCE) of 6.29% under air mass (AM) 1.5 conditions (100 mWcm-2, 1 sun). Improved charge transport properties were also observed by the electrochemical impedance spectroscopic (EIS) study for the device with optimized Ni-doped TiO2 compared to the control device.metal oxides or dopants [3]. However, its overall performance in the photovoltaic application is still limited due to poor electron mobility of nano porous TiO2 [4] and limited solar spectral response of the dye in the visible region [5, 6]. In the literature, various attempts have been reported to overcome these limitations by placing a compact metal oxide blocking layer on the Transparent Conducting Oxide (TCO), coating the TiO2 film with a thin layer of a wide band gap semiconductor, forming composites with TiO2 electrode, modifying the morphology of nanostructured semiconductors using self-assembled monolayer [7] / insulator [8] and doping/co-doping the TiO2 with other elements [9]. Many studies have reported the promising effect of dopants on the TiO2 based DSSCs. Among the effective dopants, transition metals (Zn [5, 10], Ag [11], Nb [12], W [13], Cu [14], etc.) are found to be superior to non-transition elements as they improve light absorption and electrical conductivity by incorporating new impurity energy levels within the TiO2 band gap or modifying the conduction band or valence band of the TiO2 [15, 16]. Among the transition elements, it is noted that high electrical conductivity of Nickel (Ni) prevents loss of electrons during electron transfer and thus increases the current density and efficiency of DSSCs when used as a dopant [17]. Also, Ni doping on TiO2 improves visible light harvest through a red shift in the wavelength of absorption of the solar radiation [18]. Many studies on the influence of Ni-doped TiO2 on the performance of DSSCs are reported in the literature. Power conversion efficiencies (PCEs) of 2.86%, 3.60% and 4.04% were demonstrated for DSSCs fabricated with Ni-doped TiO2 photoanode, where the doped nanomaterial was synthesized by sol-gel method [19–21]. In a separate study, Malik and his group has reported that hydrothermally synthesized Ni-doped TiO2 based device increases the photovoltaic current and hence improves the device efficiency up to 6.72% [22]. A common feature in all the above doping methods is the treatment of Ni dopant with a titanium precursor. Alternatively, doping could be achieved by treating the Ni dopant with premade TiO2. Moreover, the method of doping also plays a crucial role in altering the properties of the TiO2 which in turn strongly influences the corresponding DSSC performance. So far, the treatment of Ni dopant with the premade TiO2 (P25-TiO2) nanomaterial and its influence on the device performance have not been investigated. Hence, an attempt was made to synthesize the Ni-doped TiO2 nanomaterial by a facile wet impregnation method using commercially available P25- TiO2 and NiCl2.6H2O. This is a very simple approach which involves introduction of the dopant atoms directly into the pre-made TiO2 lattice in a liquid solvent and subsequent removal of the solvent at high temperature.