Chang-Yeol Cho

Hierarchically porous TiO2 electrodes fabricated by dual templating methods for dye-sensitized solar cells.

IO N Dye-sensitized solar cells (DSSCs) are of great interest due to their projected cost-effectiveness, their relatively high photoelectric conversion effi ciency, and the unique advantages of transparent cells over conventional silicon photovoltaics. [ 1 , 2 ] In DSSCs, light is harvested by a TiO 2 layer. This layer performs three functions: it acts as a substrate on which the dye molecules adsorb, it transfers the photogenerated electrons, and it serves as a diffusion pathway for ions in the electrolyte solution. The engineering of TiO 2 electrodes with regard to characteristics, such as nanostructure, crystalline morphology, and surface properties, is therefore a crucial aspect in efforts to enhance the photoconversion effi ciency. There have been many efforts to engineer the microstructure of TiO 2 electrodes. Engineered mesopores on the order of 10 nm, such as ordered pores and nanotubes, show faster transport of charge carriers than conventional nanocrystalline TiO 2 (nc-TiO 2 ) electrodes, in which random diffusion is the dominant mechanism. [ 3 , 4 ] In nanotube TiO 2 electrodes, the suppression of the recombination of photogenerated electrons was observed to be ten times higher than in conventional nanocrystalline TiO 2 (nc-TiO 2 ) electrodes, which resulted in an enhancement of the charge collection effi ciency. [ 4 ] The incorporation of macropores or particles that are several hundred nanometers in diameter induces Mie scattering of incident photons, thereby enhancing the light absorption, particularly at infrared wavelengths where the extinction coeffi cients of dyes are low. The introduction of these pores also improves the electrolyte diffusion effi ciency in TiO 2 electrodes. [ 5 , 6 ] Moreover, control of the macropore morphology (i.e., the size of macroscale particles or pores, as well as their organization) can be advantageous for the application of solidstate electrolytes with high viscosities and larger molecular volumes. [ 7 ]

Facile fabrication of sub-100 nm mesoscale inverse opal films and their application in dye-sensitized solar cell electrodes.

Inverse opal (IO) films with mesoporous structures hold promise as high-performance electrodes for various photoelectrochemical devices because of their high specific area as well as their fully connected pore structure. A great challenge to their use is obtaining an intact film of mesoscale colloidal crystals as a template. Here, using the plate-sliding coating method coupled with hot air flow, we successfully deposited mesoscale colloidal crystals onto the substrate. A TiO2 mesoscale IO (meso-IO) with 70 nm pores was then successfully fabricated via atomic layer deposition of TiO2 and subsequent removal of the template. As a photoelectrochemical electrode, the meso-IO structure exhibits enhanced charge transport properties as well as a high specific area. Moreover, dye-sensitized solar cells fabricated using the meso-IO electrode exhibit a higher photocurrent and cell efficiency than a cell constructed using a conventional TiO2 nanoparticle electrode. This meso-IO film provides a new platform for developing electrodes for use in various energy storage and conversion devices.

Holographically defined TiO2 electrodes for dye-sensitized solar cells.

We describe a multibeam interference lithography for creating 3D polymeric porous structures. The coating of a TiO(2) shell and subsequent removal of the template produce holographically defined TiO(2) (h-TiO(2)) electrodes. We analyze the morphological features of the h-TiO(2) electrodes and consider their applicability to dye-sensitized solar cells (DSSCs). Specifically, the performance of the h-TiO(2) electrode was evaluated by comparison with a macroporous TiO(2) electrode produced from colloidal crystals. The h-TiO(2) structure possesses a larger specific area than the inverted colloidal crystals because of a bicontinuous air network with the TiO(2) shell. Consequently, the h-TiO(2) electrode can produce a 30% higher photogenerated electron current.

Characterization of charge transport properties of a 3D electrode for dye-sensitized solar cells.

Electron transport and recombination in three-dimensionally-ordered (3D-ordered) structure electrodes were investigated using intensity-modulated photocurrent and photovoltage spectroscopy. The surface-modified TiO2 inverse opal structure was applied as a 3D electrode. The morphology, crystalline structure and surface states of the 3D-ordered structure were characterized by SEM, TEM and XPS and compared to those of the conventional nanoparticulate TiO2 structure. The performance of the 3D electrode was also evaluated by comparing the transport time and recombination lifetime to those of the conventional electrodes. Remarkably, the recombination lifetime in inverse opal was found to be greater than in nanocrystalline TiO2 by 4.3-6.2 times, thus improving the electron collection efficiency by 10%. Comparing the photovoltaic performance, although the dye adsorption of the 3D-ordered porous electrode is lower, the electrode achieves a photocurrent density comparable to that of a nanoparticulate TiO2 electrode due to the higher light scattering as well as the higher collection efficiency.

Bottom-up Growth of Hierarchical Electrodes for Highly Efficient Dye-Sensitized Solar Cells

The nonconventional bottom-up growth of TiO2 was first demonstrated in the preparation of hierarchical TiO2 electrodes for use in highly efficient dye-sensitized solar cells. The simple immersion of a substrate in a precursor solution enabled the growth of TiO2 particulate films. Here, we have implemented a hierarchical growth strategy in which two stages of controlled growth yielded first macroscale TiO2 particles, followed by mesoscale TiO2 particles. We successfully fabricated electrode films up to 20 μm thick via a growth rate of 0.3 μm/min. The specific area of the electrodes was controlled via the deposition of mesoscale TiO2 particles. The deposited particles displayed a rutile phase with an average size of several tens of nanometers in diameter, as confirmed by XRD and high-resolution TEM imaging. After depositing the second layer of mesoscale TiO2 particles, the photocurrent density increased by a factor of 3. A maximum efficiency of 6.84% was obtained for the hierarchically structured TiO2 electrodes under 1 sun illumination. The hierarchical TiO2 electrodes were compared with macroporous TiO2 electrodes, revealing that the higher photocurrent density could be attributed to a longer electron recombination lifetime and a high specific area. The longer recombination lifetime was supported by the presence of fewer defective TiO2 surfaces, as confirmed by the XPS spectrum.

Tetrapod CdSe-sensitized macroporous inverse opal electrodes for photo-electrochemical applications

Quantum dots (QDs) possess promising characteristics that are important to light harvesting, but their mesoscale size limits their application in the direct sensitization of TiO2 porous films for photo-electrochemical cells (PECs). Here, inverse opal (IO) TiO2 structures were sensitized by tetrapod-CdSe (tp-CdSe) QDs, which were used in visible-light PECs. Because of the interconnected macropores in the IO structure, tp-CdSe penetrated the entire film and deposited on its surface. In contrast, infiltration was limited to the surface of the conventional mesoporous TiO2 film. The amount of tp-CdSe deposited was dependent on the immersion time of the film in the tp-CdSe dispersion. Optimum deposition of tp-CdSe was observed at the highest photocurrent density. Light harvesting, thus the photocurrent, increased with increasing amounts of deposit, but there was a corresponding decrease in electron lifetime. The maximum photocurrent density per Cd mass was 0.474 mA cm−2, which is greater than previous results from experiments using QD-sensitized PECs. We thus believe that a combination of tetrapod QDs and the TiO2 IO film may provide a new platform for PEC electrodes.

Highly improved photocurrents of dye-sensitized solar cells containing ultrathin 3D inverse opal electrodes sensitized with a dithienothiophene- based organic dye†

Ultrathin but highly efficient dye-sensitized solar cells (DSCs) may be beneficial for making flexible devices and lowering production costs. Here, a uniform, ultrathin inverse opal (IO) electrode sensitized with a dithienothiophene (DTT)-based sensitizer (Carbz-PAHTDTT) was successfully prepared and demonstrated as a high-efficiency DSC electrode. The ultrathin IO was fabricated using a polystyrene opal template subsequently coated with TiO2 via a chemical vapor deposition approach. The Carbz-PAHTDTT-sensitized IO film was compared to an IO film sensitized by a conventional N719 dye. The charge collection efficiency of the Carbz-PAHTDTT-sensitized TiO2 IO electrode was comparable to the N719-sensitized electrode. However, the Carbz-PAHTDTT TiO2 IO electrode DSCs exhibited remarkably high light-harvesting efficiency due to high adsorption density and visible light absorption features of the Carbz-PAHTDTT-sensitized electrode. The Carbz-PAHTDTT DSCs containing a 3.5 μm thick IO electrode displayed a photocurrent density of 11.23 mA cm−2, which was 1.5 times higher compared to the N719-sensitized DSCs.

Bilayer quantum dot-decorated mesoscopic inverse opals for high volumetric photoelectrochemical water splitting efficiency

Nanostructured heterojunction semiconductor/metal oxide electrodes are desired for use in efficient photoelectrochemical (PEC) water splitting. We introduced a mesoscopic IO (meso-IO) structure and sensitized the TiO2 IO structure using a CdSe/CdS nanoparticle bilayer for use as PEC water splitting electrodes. The CdS nanoparticle shell was deposited on the IO surface using successive ionic layer absorption and reaction and subsequently, the CdSe layer was grown by chemical bath deposition. The CdSe/CdS double layer sensitization revealed the extension of visible light absorption up to 680 nm. The meso-IO structures provide an ideal geometry for highly compact composite materials; the CdSe/CdS meso-IO TiO2 structure produced a highly compact film with a volume fraction of approximately 82%. Thus, upon application as a PEC water splitting electrode, due to this highly compact structure, the CdSe/CdS meso-IO TiO2 electrodes exhibit a high volumetric photocurrent density of 1.95 mA cm−2 μm−1, which was higher than any previous reports in the literature. Moreover, we obtained improved long-term stability during water splitting: the CdSe/CdS meso-IO TiO2 electrode that was decorated using 3 at% iridium oxide colloidal particles displayed a photocurrent of 98% of the initial photocurrent for 2 h.

3D bicontinuous SnO2/TiO2 core/shell structures for highly efficient organic dye-sensitized solar cell electrodes

A photoelectrode for high performance photon-to-electron conversion devices has been developed with various material and structural aspects. For this purpose, one facile approach is introducing hybrid nanostructures. Here, we demonstrated SnO2/TiO2 core/shell hybrid structures with a 3D bicontinuous morphology. We evaluated the effect of the electrode film thickness and the TiO2 shell thickness on the photovoltaic properties using photocurrent–voltage measurements and intensity-modulated photovoltage/photocurrent spectroscopy. As the film and the shell thicknesses increase, we observe a decrease in the charge collection efficiency, whereas the amount of dye adsorbed increases. At the optimum conditions, we obtain the highest photocurrent density of 19.06 mA cm−2 and a conversion efficiency of 8.21% with a 12 μm-thick film and 180 nm-thick shell electrode for dye-sensitized solar cells. The 3D bicontinuous SnO2/TiO2 core/shell electrode is compared with the TiO2/TiO2 electrode to evaluate the effect of the SnO2 core on the photovoltaic properties, and the presence of the SnO2 core enhances the trap-free charge transport mode and significantly enlarges the charge diffusion length by up to 5 times. We believe these results show that the 3D bicontinuous core/shell electrode may be coupled with other sensitizing dyes or quantum dots, as well as redox ions or hole transport materials, to obtain highly efficient photovoltaic or photoelectrochemical devices.