Efficient dye-sensitized solar cells based on carbon-doped TiO₂ hollow spheres and nanoparticles

Different structures of TiO₂ photoelectrodes are fabricated with various arrangement modes of the layers. TiO₂ nanoparticles, synthesized by stabilizing agent free non-hydrolytic sol–gel method, are employed as the under layer, whereas carbon-doped TiO₂ hollow spheres, prepared by hydrothermally grown carbon template, are used as the scattering layer of solar cells. The nanoparticles (22 nm) have anatase structure, while 300–700 nm hollow spheres show mixtures of anatase and rutile phases. X-ray photoelectron spectroscopy confirms that carbon is doped into TiO₂ hollow spheres, resulting in a decrease in band gap energy in the range 2.96–3.13 eV compared with 3.04 eV band gap energy for the nanoparticles. The diffuse reflectance spectroscopy measurements show that the hollow spheres exhibit higher scattering property than the nanoparticles. The single layer solar cell made of nanoparticles and carbon-doped TiO₂ hollow spheres shows the highest efficiency of 7.21 and 7.02 %, respectively. Further enhancement of photovoltaic performance is obtained by light scattering mechanism aided by fabrication of double layer solar cells due to higher incident photon-to-current conversion and less recombination. The double layer solar cell made of nanoparticles as the under layer and hollow spheres (containing 86 % anatase–14 % rutile) as the scattering layer shows the highest efficiency of 8.55 %.