Fabrication Of Ordered Structure TiO₂Films Photoanode For Quantum Dot Sensitized Solar Cells

Evolving from dye-sensitized solar cells （DSSC）,quantum dots sensitized solar cells（QDSSC） replace the organic dye with inorganic quantum dots such as CdS, CdSe, PbSand Sb2S3have showed several new advantages over DSSC such as tunable band gap,high extinction coefficients, and large intrinsic dipole moment. The quantum effect canachieve multiple excitons generation for a single photon and push thermodynamicefficiency limit of these devices up to66%instead of the current31%of theSchockley-Queisser detailed balance limit. However, the maximum reported efficiency ofQDSSC is not more than7%yet, which is much lower than that of the DSSC, far less theefficiency limit. Poor electrons transport properties in the photoanode have beendemonstrated to be one of the key problems.One way to solve this problem is the use ofone dimensional nanostructure such as nanorods, nanowires and nanotubes. They havebeen proved to have superior electrons transport properties to its traditional nanoparticlecounterpart, as they can transport electrons directly from top to the electrode which isunlike the nanoparticle photoanode that electrons have to cross many grain boundariesbefore being collected.Thus, our research work mainly focused on the following aspects:Rutile TiO₂nanofilms, which were composed of many nanosheet-array domains withdifferent orientations, were synthesized directly on fluorine-doped SnO2conductive glass（FTO） substrates by a chemical deposition method in a short time. The average thicknessof the nanosheets is about10nm; the nanosheets in each domain were parallel to eachother and perpendicular to the substrate.The size and profile of the domains have a goodcorrespondence to those of the FTO grains of the substrate, indicating a coherentnucleating and epitaxial growing nature of the films. The nanosheets split gradually andfinally developed into nanofibers as prolonging the growing time to20h. Open circuitvoltage decay measurement of the nanosheet-array film exhibited an electrons lifetime of60ms, together with an electrons difussion length of7.7μm.Highly ordered titania nanotube films were fabricated by anodic oxidation. It was foundthat the anodic voltage and duration played a vital role in controling the diameter, length and morphology of the nanotube films. Under the optimizedcondition of4h anodicoxidation duration in60V, the nanotubes were115nm in diameter and18μm in length.Subsequently, large-scale free-standing TiO₂nanotube films were detached from anodizedTi foils via HgCl2soak treatment, which can be further transferred and bonded onto FTOby TiO₂sol containing Ti（OBu）4and polyethylene glycol for QDSSC application.Sequential chemical bath deposition was adopted to sensitize the nanotube films withCdS quantum dots. The obtained CdS/TiO₂/FTO electrodes showed an enhancedabsorbance in visible region, which indicated an effective adsorption of CdS. The samplewith10-cycles sequential chemical bath deposition demonstrated both the highest shortcircuit current density and conversation efficiency, which were3.21mA/cm2and0.38%,respectively. Compared with their nanoparticle counterparts, the quantum dots solar cells（QDSSC） based on this free standing titania nanotube films exhibited a longer electron lifetime and better electron transport properties, as well as a promising application prospect.