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.

Size-tunable mesoporous spherical TiO 2 (MS TiO 2 ) with a surface area of $110 m 2 g À1 have been prepared through combination of ''dilute mixing''-driven hydrolysis of titanium(iv) tetraethoxide and solvothermal treatment. The hierarchically structured MS TiO 2 are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MS TiO 2 (587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO 2 film, bi-layered dye-sensitized solar cells (DSCs) have been fabricated. Since the MS TiO 2 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 TiO 2 -based cells perform a noticeable improvement in the overall efficiency: maximum 9.37% versus 6.80% for the reference cell made of a TiO 2 nanocrystalline film. This extraordinary result is attributed to the dual effects of enhanced dye loading and light scattering.

Herein, we present a simple strategy for broadband light confinement without sacrificing dye-loading capacity by suitably combining multi-layer architecture with hierarchically structured TiO 2 . For this purpose, three distinct TiO 2 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 TiO 2 single-layer. This notable result is attributed to the synergetic effects of the enhanced broadband light confinement, dye-loading, and charge-collection efficiency.

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%).

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