Effect of nitration of TiO2 thin film window layer on the performance of TiO2/In (OH) iSj/Pb (OH) xSy composite solar cell

Abstract

In this work, nitrogen-doped and undoped titanium dioxide (TiOz) thin films deposited at different substrate temperatures were structurally and optically characterized. The effect of substrate temperature on the optical properties of the films was also investigated with the optimum substrate temperature of 200ﾰC. The film thicknesses were determined using a surface profiler. An extremely thin absorber (ETA) TiOz/In(OH)$/Pb(OH)xSy solar cell was fabricated. The microstructure of the films were analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (ED X). TiOz thin films of thickness of 400 nm were deposited and the films crystallized upon annealing at 400ﾰC in air.

TiOz thin films were doped with nitrogen (Nj) gas and the presence of nitrogen was confirmed in the films by EDX spectrum analysis. The optical properties such as refractive index (n), energy band gap (Eg) and Urbach energy (Eu) were determined from spectrophotometric measurements of reflectance and transmittance for both undoped and nitrogen doped TiOz thin films. Undoped TiOz thin films had an energy band gap of 3.25 eV while the doped films had 2.90 eV. The Urbach energy increased from 1.00 eV for undoped films to 1.04 eV for the Nj-doped films. The reduction in energy band gap and increase in Urbach energy was attributed to the introduction of Nz impurity states on the bands (conduction band and or valence). It was established that the refractive index increased with substrate deposition temperature.

The effect of doping titanium dioxide window layer on the efficiency of the ETA TiOz/In(OH)$/Pb(OH)xSy solar cell was investigated using conventional current-voltage (I-V) technique. The photovoltaic conversion efficiency (I]) was 1.06% for the solar cell with undoped films and 1.32% for the solar cell with doped films. The increase in photovoltaic conversion efficiency on doping was attributed to decrease in band gap due to nitrogen doping. Light soaking effect on the nitrogen-doped cell was studied and it was established that: open circuit voltage (Voc) reduced from 0.165 V to 0.120 V, short circuit current (Jsc) decreased from 8.5 mAlcm2 to 7.7 ma/cnr' and photovoltaic conversion efficiency decreased from 0.71 % to 0.36% when the soaking time was increased from 15 minutes to 60 minutes.