Crystal structure, optical and electrical characteristics of rutile $\hbox {TiO}_{2}$ nanocrystallite-based photoanodes doped with $\hbox {GeO}_{2}$

Abstract

The effect of germanium dioxide, $\hbox {GeO}_{2}$ doping on dye-sensitized solar cell (DSSC) $\hbox {TiO}_{2}$ nanocrystallite photoanodes with composition ($\hbox {TiO}_{2}$–$(\hbox {GeO}_{2})_{x}$: $0\le x \le 0.3\hbox { wt}\%$) has been studied. The pure sample and $\hbox {GeO}_{2}$-doped samples have been synthesized by a conventional solid-state reaction method and analysed by means of X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and ultraviolet–visible spectroscopy techniques. The photovoltaic characteristics of the prepared samples have been studied by employing J–V measurements. The pattern of XRD depicted that the dominating phase in the sample with $x=0$ is a rutile tetragonal phase with the $P4_{2}/{ mnm}$ space group. An increase in $\hbox {GeO}_{2}$ concentration leads to an appearance and rise of another hexagonal phase structure of $\upalpha$-$\hbox {GeO}_{2}$ with the $P3_{2}21$ space group. Data obtained from the UV–visible spectroscopy measurements reflect that the optical energy gap ($E^{\mathrm{optical}}$) increases with increasing $\hbox {GeO}_{2}$ content, while the optical refractive index decreases. J–V photovoltaic characteristics confirm that the DSSCs doped with low-concentration doping, $x=0.05$ and 0.1 of $\hbox {GeO}_{2}$ have higher values for conversion efficiency ($\eta$), fill factor and short circuit current density ($J_{\mathrm{sc}}$) compared with samples with high-concentration doping ($x=0.2$ and 0.3) of $\hbox {GeO}_{2}$. The present results showed that $\hbox {TiO}_{2}$–$(\hbox {GeO}_{2})_{x}$ ($x=0.05$ and 0.1 wt%) films are potential candidates for optical filter materials and optoelectrical and photo-conversion energy devices.