Facile control of intra- and inter-particle porosity in template-free synthesis of size-controlled nanoporous titanium dioxides beads for efficient organic-inorganic heterojunction solar cells

In thin film solid-state heterojunction solar cells (HSCs), titanium-dioxide (TiO₂) electrodes need to be optimized to have large specific surface area, controllable pore sizes, and superior light scattering properties. In this study, we synthesize hierarchical nanoporous TiO₂ beads with sub-micron diameters by a template-free, fast, and low-temperature synthetic scheme to satisfy the aforementioned requirements for HSCs. These nanoporous TiO₂ beads are composed of numerous TiO₂ nano crystallites that provide mesopores, and the inter-particle distances of size-controlled TiO₂ beads can provide additional controllable macropores. We report the first successful application of TiO₂ bead films (SP250, SP450) with controllable hierarchical nanostructure to be sensitized with Sb₂S₃ for all-solid-state heterojunction solar cells (Sb-HSCs). The Sb-HSCs made using the controlled TiO₂ beads as photoanodes exhibit a superior light-to electricity conversion efficiency of 4.8%, yielding more than 15% enhancement in comparison with that (3.6%) of commercial TiO₂ nanoparticle (NP40) electrodes. The well-tailored photoanode with high surface area, fewer grain boundaries, multi-scale pore structure, and enhanced optical scattering results in much better infiltration of hole-conducting materials, decreased recombination with increased electron lifetime, and enhanced light scattering, which result in the enhanced photovoltaic properties.