According to relative energy band positions between TiO₂ and visible-light-absorbing semiconductors, three different types of heterojunction were designed, and their visible-light photocatalytic efficiencies were analyzed. In Type-A heterojunction, the conduction band (CB) level of sensitizer is positioned at a more negative side than that of TiO₂, whereas in Type-B system its valence band (VB) level is more positive than that of TiO₂ and in Type-C system the sensitizer energy level is located between the CB and VB of TiO₂. In evolving CO₂ from gaseous 2-propanol (IP) under visible-light irradiation, the Type-B systems such as FeTiO₃/TiO₂, Ag₃PO₄/TiO₂, W₁₈O₄₉/TiO₂, and Sb-doped SnO₂ (ATO)/TiO₂ demonstrated noticeably higher photocatalytic efficiency than the Type-A such as CdS/TiO₂ and CdSe/TiO₂, while the Type-C such as NiTiO₃/TiO₂, CoTiO₃/TiO₂, and Fe₂O₃/TiO₂ did not show any appreciable improvement. Remarkably high visible-light photocatalytic activity of Type-B heterojunction structures could be explained by inter-semiconductor hole-transfer mechanism between the VB of the sensitizer and that of TiO₂. The evidence for the hole-transport between sensitizer and TiO₂ was also obtained by monitoring the hole-scavenging reactions with iodide (I⁻) and 1,4-terephthalic acid (TA).

Submicron-sized monodispersed TiO₂ spheres (SPs) with high porosity were synthesized by a controlled hydrolysis of titanium tetraisopropoxide (TTIP) and subsequent hydrothermal treatment at 230 °C. By adjusting the ratio of TTIP to water (the r-factor) in the hydrolysis reaction, the diameters of SPs were selectively controlled to 260, 350, 450, 560, 800, and 980 nm. The prepared SPs in the pure anatase phase were highly porous structures with crystallite sizes of ∼15 nm and surface areas of 101–121 m²g⁻¹ . The synthesized nanoporous SPs in different sizes were then applied as the light-scattering layer (LSL) of dye-sensitized solar cells (DSCs) for efficient utilization of solar spectrum, and the size-dependent light-scattering effects of those SPs were systematically investigated. The 450 nm sized SP (SP450) provided the highest light-scattering efficiency among those in the 260–800 nm range. Relatively higher efficiency is caused by the characteristic light-scattering effect based on its unique diameter and also by the photonic reflection effect originating from its size-uniformity and long-range ordering. As a result the photovoltaic conversion efficiency (η) of DSC was improved from 6.92 to 9.04% with introducing the nanoporous SP450 as LSL.

Three novel carbazole-based molecules have been synthesized and successfully applied as hole-transporting materials (HTMs) of CH₃NH₃PbI₃-based perovskite solar cells. In particular, the perovskite cell with SGT-405, having a three-arm-type structure, exhibited a remarkable photovoltaic conversion efficiency (PCE) of 14.79%.