Photosensitization Of TiO₂Nano Arrays With Ⅱ-Ⅵ Semiconductors For Photovoltaic Devices

Increasing demand for energy is forcing us to research into new resources, amongwhich solar energy is ideal to meet the target because of its abundant, clean andinexhaustible characteristics. The solar-to-electric energy conversion is one importantway in solar applications and various solar cells such as photoelectrochemical （PEC）,photovoltaic, and photocatalysis cells, have been widely used. However, theconversion efficiency is always a bottle-neck in these solar cells.TiO₂films have been demonstrated to be promising candidates as photoanodes dueto their appropriate energy band position and both thermal and chemical stability insolution. Among all the TiO₂films, one-dimensional single crystalline array films,including nanotubes, nonorods and nanowires, caught particular attention as they haveenlarged surface area and can provide direct pathway for photogenerated electrontransfer, which accordingly leads to the enhancement of light harvesting and chargeseparation. The morphologies of TiO₂nanotubes with nanowires directly formed ontop （denoted as TiO₂NTWs） have ever been reported and were supposed to beadvantageous in various applications, for example, to enhance photocatalysis and toincrease energy-conversion efficiency for dye or quantum dot （QD） solar cells.However, the large band gap of TiO₂（3.2eV） limits its absorption region to theultraviolet which takes only3⁵%of the whole solar spectrum. To extend theactivity of a photoelectrode into the visible light region, various approaches wereemployed including doping TiO₂with other impurities, using dye-sensitized solar cells （DSSCs）, and very recently using the composite semiconductors such as QDssensitized solar cells （QDSCs）. Many kinds of small band-gap semiconductors thatcan absorb light in the visible region, e.g. CdS, PbSe, CdSe, InP and CdTe, have beenused as sensitizers. Among them, CdS and CdSe are semiconductors with direct bandgap （Egap） of2.25eV and1.70eV respectively, which means CdS and CdSe cantrigger wider light absorption range compared to TiO₂. Based on the aboveunderstanding, we propose that the co-modified TiO₂NTWs with CdS as underlayerand CdSe as outer layer should be a promising photoanode for its wide absorptionspectrum, high electron injection efficiency and fast electrons transference. What’smore, as composite semiconductors, the interfacial structure of the twosemiconductors will affect the efficient charge transfer channel at the interface. As forthe QDs modified TiO₂, the area of the channel is named effective coverage area. Wethink that achieving an effective coverage of the oxide with the QDs is key to theimprovement of this type of QD solar cell.In this paper we prepared TiO₂nanotubes array that with length of2.6².8μm anddiameter¹10nm by anodic oxidation, after annealed at450C for2h the sampleshows mixed structure of anatase and rutile phase. Then by successive ion layerabsorption method （SILAR） we successfully deposited CdSe on the TiO₂NTs, afterannealed at300C for1h CdSe was found for the hexagonal and the cubic mixedphase structure. The introduction of CdSe greatly increased the absorption of thesample in visible region, the absorb edge can be up to720nm. And with the increaseof laps CdSe deposition, the optical and photoelectric conversion efficiency aresignificantly first increases and then decreases trend. When the deposition laps was7,the Jsc achieved maximum value of3.32mA/cm2, photoelectric conversion efficiencyof1.32%. This is because with the increase of the deposition laps, the quantity ofCdSe deposited on the TiO₂NTs increased, then the absorption of light wasenhancement. At the same time heterojunction area between CdSe and TiO₂nanotubes also increased. When CdSe amount to a certain extent, continue increasethe amount CdSe will only make CdSe particles grew up and then lose quantumeffectsr, which leads to the photoelectric convert efficiency decreased. To further increase the absorption in visibe region and photoelectric conversionefficiency, we successfully deposited CdS, CdSe, ZnS nano semiconductor particlesby SILAR method in both inner and outer surface of TiO₂nanotubes, formedZnS/CdSe/CdS/TiO₂cascade structure. The size of aggregated particles was within20nm. The first part of the experiment, we control outer ZnS deposition cycles at3,CdSe at7, increasing CdS cycles, found that when CdS deposition cycles was4,ZnS/CdSe/CdS/TiO₂cascade structure achieve maximum efficiency of2.095%. Thesecond part of the experiment, we control outer ZnS deposition cycles at3, the innerCdS at4, change CdSe deposition cycles, found that as the cycles of CdSe increases,the cover of nanoparticles on TiO₂surface tends to be completely, and the particlesize is also increased and light absorption performance also improved. By control thedeposition cycles we found both the Jsc and photoelectric conversion efficiency ofshowed the trend of first increases and then decreases. Among them, the sampleZnS（3c）/CdSe（5c）/CdS（4c）/TiO₂achived the maximum photocurrent density of4.30mA/cm2, and maximum efficiency of2.408%, which is26times increased comparedto the pure TiO₂nanotube. The reason is that the existence of narrow energy band gapsemiconductors of ZnS CdSe, CdS greatly broadened the spectral response scope. Theincrease of the number of nano semiconductor particles also increased heterojunctionarea between them and TiO₂nanotubes, and ZnS/CdS e/CdS/TiO₂cascade structurealso favors the separation and transmission of photo-induced carriers.