Solution processed thin films of Nb-doped TiO2 nanoparticles as hole blocking layer for organic solar cells

• We synthesize Nb-doped TiO2 nanoparticles by sol–gel method with up to 6 mol% doping.
• We disperse nanoparticles in alcohols without aggregation.
• Particle size decreases by high Nb doping.
• Uniform thin films of Nb-doped TiO2 are obtained by spin coating.
• Thin films used as hole blocking layer in organic solar cell enhance cell efficiency.

In this study, Nb-doped TiO2 nanoparticles were synthesized by sol–gel method with controlled doping ratios up to 6 mol%. Catalyzed by p-toluene sulfonic acid, alkoxide precursors of Ti and Ni were stabilized by acetylacetone ligand, and converted to the organically modified nanoparticles which were well dispersible in alcohols. Transmission electron microscopy and X-ray diffraction measurements revealed that TiO2 nanoparticle possesses anatase morphologies, and the doping ratios were confirmed by X-ray photoelectron spectroscopy. Uniform thin films of Nb-doped TiO2 were obtained by spin coating dilute alcohol dispersions of Nb-doped nanoparticles. Ellipsometric characterizations were carried out for the as-coated film and the sintered film as well to characterize the indices of refraction, and atomic force microscopy showed subnanometer-scale roughness of Nb-doped films. Owing to the improved roughness via doping, Nb-doped TiO2 thin films exhibited enhanced solar cell efficiencies when used as hole-blocking layer of organic hetero-junction solar cell.