Double-layer dye-sensitized solar cells based on Zn-doped TiO2 transparent and light scattering layers: Improving electron injection and light scattering effect

A new strategy for enhancing the efficiency of TiO2 dye-sensitized solar cells (DSSCs) by design of a new double-layer film doped with Zn ions, with various morphologies and phase compositions, is reported. X-ray photoelectron spectroscopy (XPS) revealed that Zn2+ (in the range 0.25-0.1 at.%) was successfully incorporated into the TiO2 lattice without forming secondary phases. X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) analyses showed that the synthesized nanoparticles had nanometer grains with different phase compositions and average crystallite sizes in the range of 8-48 nm, depending upon Zn atomic percentage. UV-vis absorption verified that Zn introduction enhanced the visible light absorption of TiO2 nanoparticles by shifting the absorption onset to visible light region. Furthermore, the band gap energy of nanoparticles was decreased with an increase in dopant concentration due to reduction of particle size (e.g., 3.17 eV for TiO2 and 2.83 eV for 0.5 at.% Zn-doped TiO2). It was found that, 0.5 at.% Zn-doped TiO2 DSSC in the form of a double-layer film composed of Zn-doped TiO2 nanoparticles, as the under-layer, and mixtures of Zn-doped TiO2 nano- and micro-particles, as the over-layer, (i.e., ZT5/NM5 solar cell) had the highest power conversion efficiency of 6.58%, short current density of 16.02 mA/cm2 and open circuit voltage of 680 mV. This can be related to achievement of a balance among the electron injection, light scattering effect and dye sensitization parameters. Optimization of light scattering effect of photoanode electrode led to improve the photovoltaic performance of ZT5/NM5 double-layer solar cell and was demonstrated by diffuse reflectance spectroscopy (DRS). The following well-incorporated study would present an intellectual development in the fabrication of low-cost DSSCs with high power conversion efficiency.