Yong-June Chang

Size-tunable mesoporous spherical TiO2 as a scattering overlayer in high-performance dye-sensitized solar cells

Size-tunable mesoporous spherical TiO2 (MS TiO2) with a surface area of ∼110 m2 g−1 have been prepared through combination of “dilute mixing”-driven hydrolysis of titanium(iv) tetraethoxide and solvothermal treatment. The hierarchically structured MS TiO2 are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MS TiO2 (587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO2 film, bi-layered dye-sensitized solar cells (DSCs) have been fabricated. Since the MS TiO2 particles are comprised of ∼10 nm nanocrystallites that cluster together to form large secondary spheres, they can function as light scatterers without sacrificing the surface area for dye-uptake. As a result, the present MS TiO2-based cells perform a noticeable improvement in the overall efficiency: maximum 9.37% versus 6.80% for the reference cell made of a TiO2 nanocrystalline film. This extraordinary result is attributed to the dual effects of enhanced dye loading and light scattering.

Broadband light confinement using a hierarchically structured TiO2 multi-layer for dye-sensitized solar cells

Herein, we present a simple strategy for broadband light confinement without sacrificing dye-loading capacity by suitably combining multi-layer architecture with hierarchically structured TiO2. For this purpose, three distinct TiO2 hierarchical nanomaterials were exploited to simultaneously realize high internal surface area and a graded series of optical properties (in terms of reflectance and transmittance). The present hierarchically structured multi-layer showed a remarkable improvement in the overall efficiency for dye-sensitized solar cells (DSCs): a maximum of 11.43% at 1 Sun (12.16% at 1/8 Sun) versus 8.15% at 1 Sun (8.26% at 1/8 Sun) for the reference cell made of a nanocrystalline TiO2 single-layer. This notable result is attributed to the synergetic effects of the enhanced broadband light confinement, dye-loading, and charge-collection efficiency.

A light scattering polymer gel electrolyte for high performance dye-sensitized solar cells

In this work, a light scattering polymer gel electrolyte (LS-PGE) is fabricated by suitably mixing hierarchically structured TiO2 microspheres and poly(lactic acid-co-glycolic acid) (PLGA). The newly synthesized TiO2 microspheres exhibit high reflectance in the visible light region and contain nanopores inside to facilitate the ion diffusion. Also, PLGA helps the microspheres disperse in the electrolyte without precipitation and prevents the solvent from evaporating. The LS-PGE renders more efficient light harvesting than conventional scattering layers so that it can show high efficiencies of 8.2% in comparison with the liquid electrolyte of 7.7%. In view of long-term stability, the efficiencies lie in ±10% of the initial value for 1500 h under ambient conditions. Thus, the LS-PGE can replace conventional scattering layers to give compact and flexible photoanodes, and further assure long-term stability of dye-sensitized solar cells during practical operation.

Bandgap Tuning with Thermal Residual Stresses Induced in a Quantum Dot

Lattice distortion induced by residual stresses can alter electronic and mechanical properties of materials significantly. Herein, a novel way of the bandgap tuning in a quantum dot (QD) by lattice distortion is presented using 4-nm-sized CdS QDs grown on a TiO2 particle as an application example. The bandgap tuning (from 2.74 eV to 2.49 eV) of a CdS QD is achieved by suitably adjusting the degree of lattice distortion in a QD via the tensile residual stresses which arise from the difference in thermal expansion coefficients between CdS and TiO2. The idea of bandgap tuning is then applied to QD-sensitized solar cells, achieving ≈60% increase in the power conversion efficiency by controlling the degree of thermal residual stress. Since the present methodology is not limited to a specific QD system, it will potentially pave a way to unexplored quantum effects in various QD-based applications.

Bandgap Tuning by Using a Lattice Distortion Induced by Two Symmetries That Coexist in a Quantum Dot

Among the interests in the application of quantum dots (QDs), the bandgap tuning is of key importance in controlling their material properties. The bandgap of a QD can be adjusted by adopting a variety of different physicochemical methods. Herein, a novel way of the bandgap tuning is developed in an Ag2S-based QD system by suitably quenching the transformation from monoclinic Ag2S to cubic Ag and by subsequently inducing a lattice-distorted region of ≈1-nm-scale in a QD. The two distinct crystalline phases of Ag2S and Ag coexisting with the lattice-distorted region are experimentally demonstrated by visually showing this remarkable coexistence in a QD. A new approach is presented to the bandgap tuning (2.51 to 1.64 eV) and enhancing optical properties by suitably tailoring the degree of the lattice-distorted region in a QD. This conceptual method could pave a new way to utilizing quantum effects in various QD applications.

Hierarchically Nanostructured Photoelectrodes for Quantum-Dot-Sensitized Solar Cells

Quantum-dot-sensitized solar cells (QDSSCs) recently have attracted a great deal of attention owing to their advantages that include high molar extinction coefficient of quantum dots, tunable energy gaps, and multiple exciton generation. However, various architectures have not yet been proposed as alternative working electrodes for QDSSCs. In this article, two hierarchical nanostructures will be presented as next-generation photoelectrodes for highly-efficient QDSSCs: (a) the tertiary-hierarchically-structured mesoporous spherical (MS) TiO2 and (b) the sea urchin TiO2 (SU TiO2) particles composed of radially aligned rutile TiO2 nanowires. The MS TiO2 offers a high surface area, a high internal reflectance in the visible region, and good pore accessibility. A conversion efficiency of 1.9 % was achieved by CdS QDSSCs made with the MS TiO2 photoelectrode, which corresponds to ~58 % improvement as compared with the values obtained from the conventional devices made with 20-nm-sized nanocrystalline TiO2 film. Secondly, SU TiO2 was incorporated into the TiO2 nanoparticle (NP) network to construct the SU–NP composite film and applied to the CdS/CdSe/ZnS QDSSCs. A conversion efficiency of 4.2 % was achieved, which corresponds to ~20 % improvement as compared with the values obtained from the reference cell made of the NP film. We attribute this extraordinary result to the light scattering effect and efficient charge collection. Thus, the SU TiO2 and MS TiO2 can be promising materials for the photoanodes of QDSSCs.

A tri-functional TiO2 photoelectrode: single crystalline nanowires directly grown on nanoparticles for dye-sensitized solar cells

We prepared a hybrid photoelectrode with single crystalline TiO2 nanowires (NWs) directly grown on TiO2 nanoparticles (NPs) for efficient dye-sensitized solar cells (DSCs). For this purpose, the NWs were grown on the commercial NPs, P25 Degussa through use of a solvothermal treatment. The NWs offer the dual functions of optical confinement and rapid electron transport, while the incorporated NPs provide a high surface area for dye loading. As a result, the NP–NW hybrid electrode showed 6.6% of the conversion efficiency under AM 1.5 illumination of 100 mW cm−2, which corresponds to a noticeable enhancement over the efficiency of the reference device with the NPs. Thus, we expect that the hybridization of two distinct nanomaterials will provide an effective strategy in designing the photoelectrode of DSCs.