Unveiling Growth and Photovoltaic Principles in Density-Controllable TiO₂ Nanorod Arrays for Efficient Solar Cells

Titanium dioxide (TiO₂) nanorod arrays (TiO₂-NA) are widely used in optoelectronic devices. Controlling the number density (N_D) of nanorods without altering their dimensional features in TiO₂-NA is of great importance to the tailored performance of the optoelectronic devices, which unfortunately remains challenging up to now. Here, a facile strategy is developed to control the N_D without changing the TiO₂ nanorod sizes in the rutile TiO₂-NAs hydrothermally grown on an anatase TiO₂ film on a large scale. Moreover, N_D-controllable TiO₂-NAs are applied to CuInS₂ solar cells, achieving a champion efficiency of 10.44% for solution-processed CuInS₂ solar cells. It is found that the hydrolysis time (t_H) in preparing the anatase TiO₂ film provides good control over N_D in TiO₂-NA as the result of t_H-governed nanoparticle size in the anatase TiO₂ film. A gel-chain-limited crystallization model for t_H-governed anatase TiO₂ nanoparticle size, an orientation-competing-epitaxial nucleation/growth model for the out-of-plane growth of single-crystalline rutile TiO₂ nanorod on polycrystalline anatase TiO₂ film, and a volume-surface-density model for the N_D-governed photocurrent generation in nanoarray-based solar cells are proposed.