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 ß-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 ß-interaction mode (2.03 eV). We have further estimated the individual bonding strength for the ten relevant hydrogen bonds having a bond critical point.

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.

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%).

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.

Bi-functional ion exchangers for enhanced performance of dye-sensitized solar cells.

Ion exchange using aerosol OT (AOT) offers dye adsorption twice as fast as known methods. Moreover, it suppresses the dye-agglomeration that may cause insufficient dye-coverage on the photoelectrode surface. Consequently, its dual function of fast dye-loading and higher dye-coverage significantly improves the power conversion efficiency of dye-sensitized solar cells.

Sea urchin TiO2-nanoparticle hybrid composite photoelectrodes for CdS/CdSe/ZnS quantum-dot-sensitized solar cells.

The sea urchin TiO(2) (SU TiO(2)) particles composed of radially aligned rutile TiO(2) nanowires are successfully synthesized through the simple solvothermal process. SU TiO(2) was incorporated into the TiO(2) nanoparticle (NP) network to construct the SU-NP composite film, and applied to the CdS/CdSe/ZnS quantum-dot-sensitized solar cells (QDSSCs). A conversion efficiency of 4.2% was achieved with a short-circuit photocurrent density of 18.2 mA cm(-2) and an open-circuit voltage of 531 mV, 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.