Eui-Hyun Kong

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.

The nature of hydrogen-bonding interaction in the prototypic hybrid halide perovskite, tetragonal CH3NH3PbI3

In spite of the key role of hydrogen bonding in the structural stabilization of the prototypic hybrid halide perovskite, CH3NH3PbI3 (MAPbI3), little progress has been made in our in-depth understanding of the hydrogen-bonding interaction between the MA+-ion and the iodide ions in the PbI6-octahedron network. Herein, we show that there exist two distinct types of the hydrogen-bonding interaction, naming α- and β-modes, in the tetragonal MAPbI3 on the basis of symmetry argument and density-functional theory calculations. The computed Kohn-Sham (K-S) energy difference between these two interaction modes is 45.14 meV per MA-site with the α-interaction mode being responsible for the stable hydrogen-bonding network. The computed bandgap (Eg) is also affected by the hydrogen-bonding mode, with Eg of the α-interaction mode (1.73 eV) being significantly narrower than that of the β-interaction mode (2.03 eV). We have further estimated the individual bonding strength for the ten relevant hydrogen bonds having a bond critical point.

Cauliflower-like SnO2 hollow microspheres as anode and carbon fiber as cathode for high performance quantum dot and dye-sensitized solar cells.

Cauliflower-like tin oxide (SnO2) hollow microspheres (HMS) sensitized with multilayer quantum dots (QDs) as photoanode and alternative stable, low-cost counter electrode are employed for the first time in QD-sensitized solar cells (QDSCs). Cauliflower-like SnO2 hollow spheres mainly consist of 50 nm-sized agglomerated nanoparticles; they possess a high internal surface area and light scattering in between the microspheres and shell layers. This makes them promising photoanode material for both QDSCs and dye-sensitized solar cells (DSCs). Successive ionic layer adsorption and reaction (SILAR) method and chemical bath deposition (CBD) are used for QD-sensitizing the SnO2 microspheres. Additionally, carbon-nanofiber (CNF) with a unique structure is used as an alternative counter electrode (CE) and compared with the standard platinum (Pt) CE. Their electrocatalytic properties are measured using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and Tafel-polarization. Under 1 sun illumination, solar cells made with hollow SnO2 photoanode sandwiched with the stable CNF CE showed a power conversion efficiency of 2.5% in QDSCs and 3.0% for DSCs, which is quite promising with the standard Pt CE (QDSCs: 2.1%, and DSCs: 3.6%).

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.

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.