Controlling electron transport rate and recombination process of TiO2 dye-sensitized solar cells by design of double-layer films with different arrangement modes

TiO2 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 TiO2 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-TiO2 nanoparticles as the under-layer and anatase-TiO2 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 TiO2 nanoparticles had uniform and nanometer grains with particle size around 20 nm, whereas TiO2 nanowires with length of several μm had diameter in the range 20–50 nm. The optical properties and band gap energies of TiO2 nanoparticles and nanowires were studied through UV–vis absorption. The indirect optical band gap energy of TiO2 nanowires, anatase-TiO2 and rutile-TiO2 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 TiO2 nanostructures will open a new concept for improvement of power conversion efficiency of dye sensitized solar cells.