Ho-Sub Kim

Recent progress in dye-sensitized solar cells for improving efficiency: TiO 2 nanotube arrays in active layer

Dye-sensitized solar cells (DSSCs) have been widely studied due to several advantages, such as low cost-to-performance ratio, low cost of fabrication, functionality at wide angles and low intensities of incident light, mechanical robustness, and low weight. This paper summarizes the recent progress in DSSC technology for improving efficiency, focusing on the active layer in the photoanode, with a part of the DSSC consisting of dyes and a TiO2 film layer. In particular, this review highlights a huge pool of studies that report improvements in the efficiency of DSSCs using TiO2 nanotubes, which exhibit better electron transport. Finally, this paper suggests opportunities for future research.

Ag Nanoparticle–Functionalized Open-Ended Freestanding TiO2 Nanotube Arrays with a Scattering Layer for Improved Energy Conversion Efficiency in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) were fabricated using open-ended freestanding TiO2 nanotube arrays functionalized with Ag nanoparticles (NPs) in the channel to create a plasmonic effect, and then coated with large TiO2 NPs to create a scattering effect in order to improve energy conversion efficiency. Compared to closed-ended freestanding TiO2 nanotube array–based DSSCs without Ag or large TiO2 NPs, the energy conversion efficiency of closed-ended DSSCs improved by 9.21% (actual efficiency, from 5.86% to 6.40%) with Ag NPs, 6.48% (actual efficiency, from 5.86% to 6.24%) with TiO2 NPs, and 14.50% (actual efficiency, from 5.86% to 6.71%) with both Ag NPs and TiO2 NPs. By introducing Ag NPs and/or large TiO2 NPs to open-ended freestanding TiO2 nanotube array–based DSSCs, the energy conversion efficiency was improved by 9.15% (actual efficiency, from 6.12% to 6.68%) with Ag NPs and 8.17% (actual efficiency, from 6.12% to 6.62%) with TiO2 NPs, and by 15.20% (actual efficiency, from 6.12% to 7.05%) with both Ag NPs and TiO2 NPs. Moreover, compared to closed-ended freestanding TiO2 nanotube arrays, the energy conversion efficiency of open-ended freestanding TiO2 nanotube arrays increased from 6.71% to 7.05%. We demonstrate that each component—Ag NPs, TiO2 NPs, and open-ended freestanding TiO2 nanotube arrays—enhanced the energy conversion efficiency, and the use of a combination of all components in DSSCs resulted in the highest energy conversion efficiency.

Carbon-doped freestanding TiO2 nanotube arrays in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated with both closed- and open-ended freestanding TiO2 nanotube arrays that were doped with carbon via chemical vapor deposition to improve their electron transport properties. The energy conversion efficiencies of DSSCs increased from 5.07% to 6.21% after doping with a small amount of carbon, i.e., an enhancement of 22.4%. This suggests that the π–π conjugation introduced by carbon doping improved the efficiency of electron transport. However, energy conversion efficiencies of DSSCs with a large amount of carbon doping on the TiO2 nanotube arrays decreased from 5.07% to 2.87%, with a lower short-circuit current, open circuit voltage, and fill factor because of the reduced level of dye adsorption on the TiO2 nanotube arrays.

Multi-Shaped Ag Nanoparticles in the Plasmonic Layer of Dye-Sensitized Solar Cells for Increased Power Conversion Efficiency

The use of dye-sensitized solar cells (DSSCs) is widespread owing to their high power conversion efficiency (PCE) and low cost of manufacturing. We prepared multi-shaped Ag nanoparticles (NPs) and introduced them into DSSCs to further enhance their PCE. The maximum absorption wavelength of the multi-shaped Ag NPs is 420 nm, including the shoulder with a full width at half maximum (FWHM) of 121 nm. This is a broad absorption wavelength compared to spherical Ag NPs, which have a maximum absorption wavelength of 400 nm without the shoulder of 61 nm FWHM. Therefore, when multi-shaped Ag NPs with a broader plasmon-enhanced absorption were coated on a mesoporous TiO2 layer on a layer-by-layer structure in DSSCs, the PCE increased from 8.44% to 10.22%, equivalent to an improvement of 21.09% compared to DSSCs without a plasmonic layer. To confirm the plasmon-enhanced effect on the composite film structure in DSSCs, the PCE of DSSCs based on the composite film structure with multi-shaped Ag NPs increased from 8.58% to 10.34%, equivalent to an improvement of 20.51% compared to DSSCs without a plasmonic layer. This concept can be applied to perovskite solar cells, hybrid solar cells, and other solar cells devices.

Increasing the surface area of TiO2 nanotube membranes by filling the channels with onion type carbon materials and TiCl4 for dye-sensitized solar cells

We have significantly enhanced the power conversion efficiency of dye-sensitized solar cells based on TiO2 nanotube membranes by increasing the inner surface area of the channels. Onion type carbon materials (OTCMs) were produced by injecting ethylene gas continuously into the Ar plasma of a thermal plasma system. The average size was about 20 nm. OTCMs were functionalized by treatment with sulfuric and nitric acids. Functionalized OTCMs were chemisorbed on the inner surface of the channels by spontaneous diffusion. To maximize filling, we vacuum-filled functionalized OTCMs and TiCl4 into the channels of TiO2 nanotube membranes in sequence, and then sintered at 500 °C. When being sintered, TiCl4 transformed to TiO2, while the carbon materials burnt. By filling OTCMs and TiCl4 in sequence and sintering, the inner surface became very rough, and the amount of dye adsorbed on the photoanode was increased about 50%. The electron lifetime and effective carrier diffusion length were increased by filling with functionalized OTCMs and TiCl4 in sequence. The power conversion efficiency of the DSSC based on TiO2 nanotube membranes was improved from 6.99 to 9.33%, a 33.5% enhancement. The efficiency enhancement is mainly due to an increase in the total available surface area for dye adsorption.

Tunable Phosphorescent Emission through Energy Transfer within Multilayer Thin Films Based on a Carbazole-Based Host and Ir(III)-Complex Guest System

A new method to tune both phosphorescence emission intensity and spectroscopic colors based on the alternatively structured host/guest multilayer thin films prepared by the spin-assisted LbL deposition is presented. Their emission characteristics are demonstrated with pairs of positively charged Ir-PEI complexes as guests and negatively charged CBZ-PAA as hosts. The phosphorescent emission of Ir-PEI complexes is enhanced by the energy transfer from the host to the guest and, additionally, this energy transfer can be finely tuned by the insertion of spacer layers between the phosphorescent donor and acceptor layers to vary the emission intensity as well as to render different emission colors.