Controlling electron transport rate and recombination process of TiO ₂ dye-sensitized solar cells by design of double-layer films with different arrangement modes

TiO ₂ dye-sensitized solar cells (DSSCs) in the form of double-layer films, containing an under-layer and an over-layer, with various crystal structures (i.e., anatase and rutile phases) and morphologies (i.e., nanoparticle and nanowire) were reported. It was found that the photovoltaic performance of TiO ₂ DSSCs depends on the morphology, crystal structure, light scattering effect, optical band gap energy and arrangement of the under- and over-layer films. The double-layer solar cell made of anatase-TiO ₂ nanoparticles as the under-layer and anatase-TiO ₂ nanowires as the over-layer (i.e., AW solar cell) showed the highest power conversion efficiency and fill factor of 6.34% and 62.6%, respectively. High electron lifetime, rapid transportation and less recombination of photogenerated electrons are the factors affect the efficiency improvement of AW film and was demonstrated by electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) analyses revealed that TiO ₂ nanoparticles had uniform and nanometer grains with particle size around 20 nm, whereas TiO ₂ nanowires with length of several μm had diameter in the range 20-50 nm. The optical properties and band gap energies of TiO ₂ nanoparticles and nanowires were studied through UV-vis absorption. The indirect optical band gap energy of TiO ₂ nanowires, anatase-TiO ₂ and rutile-TiO ₂ nanoparticles was calculated 3.61, 3.47 and 3.41 eV, respectively. The design of double-layer solar cells by manipulation of morphology and crystal structure of TiO ₂ nanostructures will open a new concept for improvement of power conversion efficiency of dye sensitized solar cells.