Interface Design And Modification Of TiO₂ Nanowires Photoanode In Quantum Dots Sensitized Solar Cells

Solar cells are one of the effective methods to satisfy the requirement of clean energy in the future as devices to convert sunlight into electricity directly.As the third generation solar battery,quantum dot-sensitized solar cells?QDSCs?show great potential and development prospects in the solar cell field due to the fact that the merits of QDSCs are demonstrated in the cost-effective,simple preparation process and the unique properties of the quantum dot sensitizers.However,the reported highest photoelectric conversion efficiency?PCE?has reached a record of about 12%in the QDSCs,which is still much lower than the theoretical PCE?about 66%?.The further enhancement of the photoelectric properties of solar cells is the key of the development of solar cells.And the charge recombination of QDSCs has a great impact on photovoltaic performance.Therefore,it is significance for the improvement of the performance of QDSCs to reduce the charge recombination and improve the charge transfer performance.The reported method is the surface modification of the barrier layer?such as the Al₂O₃ and MgO?to restrain the recombination process after that the photoexcited electron injected in the oxide film.But the barrier layer against the photoexcited electron in the QDs injection into the oxide semiconductor.So,the effective suppression of the photoexcited charge recombination while maintaining the effective charge transfer is a current key problem for QDSCs.In addition,it often ignores the effect of the Fermi level of TiO₂films on the charge separation and transfer.And there were few reports about the construction of the TiO₂ films/FTO interface electric field for the improvement of the charge separation and transfer.In our work,we applied the surface modification and doping modification of oxide film to construct the interface electric field that favors the charge transfer,therefore achieve the goals of suppressing the charge recombination in the QDSCs and improving the charge separation and transfer.Surface photovoltage?SPV?,transient surface photovoltage?TPV?,and transient photocurrent?TPC?techniques were applied to study the dynamics properties of photoexcited charges in order to analyze the interface barrier layer in the anatase Ti O₂ nanowires?NWs?solar cells and the effect of the photoexcited charge properties at the interface on the photoelectric properties of QDSCs.The specific work contents are as follows:1.In order to deal with the charge recombination in the QDSCs,the surface of the anatase TiO₂ NWs film has been successfully modified by Al-doped Ti O₂ layers to construct the Al-TiO₂/TiO₂ heterojunction,using sol-gel method.The QDSCs devices were assembled with the pre and post modification oxide semiconductor after CdS QDs sensitization.The Fermi level is changed due to the Al-doped and therefore form the interface barrier at the Al-TiO₂/TiO₂ heterojunction.And the interface electric field direction at the Al-TiO₂/FTO interface is from TiO₂ NWs toward Al-TiO₂ film,therefore achieve the goals of suppressing the recombination of the photoexcited electrons in the TiO₂ film with the holes and electrolyte.As a result,the photoelectric properties of QDSCs were enhanced,and the QDSCs based TiO₂ NWs@1.26%Al-Ti photoanode exhibits a maximal energy conversion efficiency of 2.58%,which is 45%higher than the QDSCs based on bare TiO₂ NWs photoanode.This is taken as proof that Al-TiO₂/TiO₂ heterojunction is an effective method to restrain the charge recombination and prolong the electron lifetime in the QDSCs,and it shows the great application prospect in the field of solar cell.2.The preliminary work indicates the work function of the anatase TiO₂ NWs film?4.70 eV?is smaller than the work function of FTO,and the depletion layer probably exists at the anatase TiO₂ NWs film and FTO interface.In order to study the effect of anatase TiO₂ NWs/FTO interface on the properties of solar cells,the Kelvin probe technique was used to test the work function of TiO₂ film and FTO.And SPV and TPV techniques were used to study the dynamics properties of photoexcited charge at the TiO₂/FTO interface for both the TiO₂ film and QD-sensitized TiO₂ film.The results by different illumination methods show that the interface electric field direction at the TiO₂/FTO interface leads to the photoexcited holes transfer to the FTO substrate,which will be at a disadvantage for improving the photoelectric properties of solar cells.3.According to the above work,the interface electric field direction at the TiO₂/FTO interface is against the improvement of the photoelectric properties of solar cells.In order to control the TiO₂/FTO interface,we elaborately designed and constructed the N-doped anatase TiO₂ NWs/FTO interface with the desirable orientations,and the SPV and the Kelvin probe techniques were used to analysis the N-doped anatase TiO₂ NWs/FTO interface.After N-doped anatase TiO₂ NWs film,the interface electric field direction at TiO₂/FTO interface from TiO₂ toward FTO turns from FTO toward TiO₂,which will be in favor of the photoexcited electrons transfer to the FTO substrate.Thus the interface electric field is beneficial to the improvement of the photoelectric properties of solar cells through N-doped anatase TiO₂ NWs film.The QDSCs with the optimal N doping amount exhibit the best photoelectric properties with energy conversion efficiency of 2.75%,which is 1.46times enhancement as compared to the undoped reference solar cells.