Studying On Modification Of Pbs/CdS/TiO₂Photoanode And Photoelectric Performance

Developing more clean and renewable energy is an immediate emergency,because of fossil energy shortage and environmental pollution. The radiation comesfrom the sun is the source of human energy needs, and it is a kind of clean greenrenewable energy. At present, the main way to use solar energy is solar cell. Solar cellis a specific device to convert the solar energy into electricity, and an efficient methodto replace the traditional fossil energy. Among the series of solar cells, quantum dotssensitized solar cell (QDSSC) is a promising solar cells due to simple preparationmethod, easy to operate, low cost, non-toxicity, and have higher theoreticalconversion efficiency. It can provide a new lifestyle for mankind, and it will be a newera of green and sustainable development in energy for human society. However,hitherto we have not found an effective way to further improve the energy conversionefficiency for QDSSC, further study to promoting energy conversion efficiency ofQDSSC is significant for solar cell and development of clean and renewable energy.QDSSC is composed of photoanode, counter electrode and electrolyte. Theprimarily function of photoanode is generating photogenerated charge carriers andeffectively transfering, collecting the photogenerated electrons. So, the quality ofphotoanode can directly affects the performance of solar cells.Titanium dioxide (TiO2) is used in solar cells, which owing to cheap, chemicalstability, and non-toxicity. TiO2is a broadband gap semiconductor oxide, the optical band gap is about3.2eV. One way to improve the energy conversion efficiency ofsolar cell is sensitizing semiconductor quantum dots (CdS, PbS) on TiO2bysuccessive ion layer adsorption and reaction (SILAR) method, which can formheterojunction between TiO2and semiconductor quantum dots with narrow band gap.The heterojunction has the special energy level with a ladder-like distribution. It isbeneficial to inject photogenerated electrons from the quantum dots (QDs) conductionband to the TiO2conduction band. The photogenerated electron-hole pairs can rapidseparation and transport in the heterojunction area. Another method is introducingdefection in TiO2, including impurity and oxygen vacancy. These defections can causeimpurity energy level, which can improve the electrical property of TiO2, and mayfinally improve energy conversion efficiency of solar cell.According to the two methods, we study the energy conversion efficiency ofQDSSCs (PbS/CdS/TiO2) and TiO2after hydrogen treatment. Finally the energyconversion efficiency is improved efficiently after we combine these two ideastogether. The main work of this paper is as follows:(1) The perpendicular TiO2nanorod arrays is prepared on FTO substrate via onestep hydrothermal method by mixture of Ti(OC4H9)4and HCl as a precursor solution.The crystal structure of TiO2nanorod arrays, morphology and optical absorptionproperties are studied in this paper. The method of SILAR is used to sensitize TiO2nanorod arrays with CdS and PbS QDs. The impact of the number of sensitized cyclesfor the solar cells photoelectric properties is discussed. The photoelectric performanceof solar cells is achieved under the simulated sunlight AM1.5light at100mW/cm2.Among CdS QDs sensitized on TiO2nanorod arrays, the best performance occurs at7cycles of CdS QDs deposited on TiO2nanorod arrays. The corresponding short-circuitcurrent density, open-circuit voltage, fill factor and energy conversion efficiency are7.41mA/cm2,0.46V,51.95%and1.77%, respectively. On this basis, PbS QDssensitized on CdS/TiO2is studied, the best performance occurs at6cycles of PbSQDs deposited on CdS/TiO2nanorod arrays. The corresponding short-circuit currentdensity, open-circuit voltage, fill factor and energy conversion efficiency are15.52mA/cm2,0.37V,46.07%and2.65%, respectively. The energy conversion efficiency of co-sensitized solar cell is1.5times of CdS/TiO2solar cells. The results indicate thatco-sensitized is an efficient way to improve the performance of solar cells.(2) As synthesized TiO2nanorod arrays are treated with hydrogen. Afterhydrogen treatment, the colors of TiO2are gradually darkened with increasing thehydrogen treatment temperature. The UV-vis spectrums demonstrate the absorptionband edge gradually red shift, and the intensity of optical absorption is increased. Thisis because the internal electronic state is introduced by hydrogen treatment in TiO2,which can reduce the forbidden band width of TiO2. XPS and Mott-Schottky curveindicate the oxygen vacancy (hydroxyl group) is introduced in the TiO2surface in theprocess of hydrogen treatment, which introduce an intermediate level in TiO2bandgap. The intermediate level can reduce the energy barrier of electron transition, andalso increase the carrier density of TiO2. Finally we found hydrogen treatment processcan improve the performance of TiO2solar cells.(3) CdS QDs-sensitized on TiO2and PbS/CdS QDs co-sensitized on TiO2nanorod arrays on FTO are prepared by using the successive ionic layer adsorptionand reaction (SILAR) method, their optical absorption and photoelectron chemicalperformance are investigated. The experimental results show that the QDs sensitizedon TiO2can effectively improve the optical absorption performance. With increasingthe number of SILAR cycles, the photoelectric properties of photoanode are incresedfirst then decreased. The short-circuit current density, open-circuit voltage, fill factorand energy conversion efficiency increased obviously with increasing sensitizationcycles of CdS QDs and approached a maximum value of8.85mA/cm2,0.44V,50.87%and1.98%at5cycles of CdS QDs. The optical absorption of PbS/CdS QDsco-sensitized on TiO2nanorod arrays is extended to the near infrared region. WhenPbS QDs deposited on CdS/TiO2photoanode at6cycles, the short-circuit currentdensity and photoelectric conversion efficiency reached a maximum value are19.34mA/cm2and3.98%, respectively. The photoelectric performance of the TiO2withhydrogen treatment is obviously better than without hydrogen treatment.(4) Ni2+doping in PbS/CdS/TiO2is studied. The method is adding0.075M of Ni(CH3COO)2in Cd source, synthesizing Ni2+doped CdS quantum dots (CdS:Ni2+) by using the successive ionic layer adsorption and reaction (SILAR) method. The opticalabsorption, and the photoelectric performance of Ni2+doping in PbS/CdS/TiO2isbetter than PbS/CdS/TiO2due to introduce a new electronic state in CdS quantum dotby Ni2+dopant. The new electronic state can effectively restrain the photogeneratedelectrons-hole pairs recombination and improves the photocorrosion resistance of thesolar cells. Finaly, the energy conversion efficiency of solar cells can be improved.The photoelectric performance of solar cells is also depended on the number ofsensitized cycles. When sensitized CdS:Ni2+QDs for7cycles, then sensitized PbSQDs for6cycles, the solar cells the optimal performance is obtained, thecorresponding short-circuit current density, open-circuit voltage, fill factor and energyconversion efficiency are18.56mA/cm2,0.41V,47.31%and3.60%, respectively.Finally the energy conversion efficiency was reached4.29%, after hydrogen treatmentcombined with CdS:Ni2+used in solar cells.