The Synthesis And Application Of Polyoxometalate/Metal Oxide Semiconductor Composite In Dye-sensitized Solar Cells

The overall idea for the energy development "in 13 th Five-Year" plan of China is to optimize the energy distribution.Along with the increasingly prominent energy problems,exploring renewable energy resources becomes an urgent affair.The dye-sensitized solar cells?DSSCs?is based on the dye-sensitized semiconductor materials to convert light energy into electrical energy.As one of the key parts in DSSCs,the sensitizer is important for high power-conversion efficiency?PCE?.However,N719 is expensive and shows a narrow spectral response,which is one of the major stumbling blocks for the further enhancement in the PCE.Thus it is necessary to explore cheap and novel panchromatic sensitizer or co-sensitizer.Besides,many metal oxide semiconductors?MOS?photoelectrodes still suffer from poor conductivity and low PCE,which are greatly limited by the low electron diffusion coefficient and the high interfacial electron–hole recombination.Polyoxometalates?denoted as POMs?,as a large and rapidly growing class of inorganic metal-oxygen nanoclusters,have enormous potential applications in photovoltaic cells.The energy level and response to solar spectrum of POMs could be modulated through controlling their structures or compositions.In this paper,a di-vanadium-substituted Lindqvist-type polyoxometalate was hydrothermally synthesized.They were firstly applied into DSSCs as co-sensitizers to efficiently enhance the photovoltaic.Besides,the high molybdenum nuclear cluster that has strong absorption in the visible region combined with MOSs as a way to construct MOS composite photoelectrodes with light enhanced conductivity for high performance photoelectronic devices.1.A di-vanadium-substituted Lindqvist-type polyoxometalate [Cu(C₁₂H8N2)₂]₂[V2W4O19]·4H2O?1?was hydrothermally synthesized and characterized structurally by single crystal X-ray diffraction analysis.Ultraviolet photoelectron spectroscopy and density functional theoretical studies indicate that the energy level of 1 matches well with the conduction band of the TiO₂.Furthermore,considering the semiconductor-like nature of 1 and the introduction of transition metal element Cu synchronously extends the absorption to the visible region,which should also be beneficial to the photovoltaic device performance.1-doped TiO₂ composites?denoted as 1@TiO₂?have been successfully fabricated by a simple sol-gel method,which were introduced into the DSSCs as co-sensitizers in N719-sensitized photoanodes by mixing 1@TiO₂ with P25 nanoparticles with different weight ratios to enhance the PCE.The investigations show that the DSSCs assembled with 1@TiO₂/19P25/N719 has the best performance and the overall improvement of the efficiency is 21.6% compared with pure P25/N719.Furthermore,the electrochemical impedance spectroscopy and open-circuit voltage decay investigations show that the cosensitization of 1 and N719 can promote electron transfer and restrain charge recombinations in the DSSCs,resulting in the longer electron lifetime.That is,the calculated electron lifetime?about 5.75 ms?of the DSSC based on 1@TiO₂/19P25/N719 is longer than that of P25/N719?about 2.83 ms?.2.Four types of MOS based photoelectrodes were fabricated by a simple wet chemistry method through bounding TiO₂,SnO₂,WO₃,and ZnO with keplerate-type polyoxometalate?NH4?42[MoVI72MoV60O₃72?CH3COO?30?H2O?72]({Mo₁₃₂}),respectively.In these photoelectrodes,{Mo₁₃₂} may act as a photo-induced electron acceptor/donor to accelerate electron transfer and then improve their conductivity efficiently.Further the 5%{Mo₁₃₂}/TiO₂ composite photoelectrodes were introduced into the DSSCs as the photoanode.In this system,{Mo₁₃₂} can enhance light absorption and as electron acceptor to accelerate the electron transfer.The 5% {Mo₁₃₂}/TiO₂ modied photoanode exhibits the largest electron lifetime?5.12 ms?,longer than that of pure TiO₂?2.21 ms?and a high efficiency up to 7.94%,enhanced by 31% compared with that of pure TiO₂?6.06%?.All the results indicate that the electron recombination kinetics have been retarded effectively after adding the {Mo₁₃₂}.