Preparation And Photovoltaic Performance Investigation Of PbS Based Quantum-dot Sensitized Solar Cells

In this thesis,the PbS quantum-dots were successfully prepared on TiO₂ nanorod arrays by repeating spin-coating the Pb?NO₃?₂ solution,the mixing solution of Na₂S and 1,2-ethanedithiol?EDT?via successive ionic layer absorption and reaction?SILAR?procedure and the corresponding solid-state quantum-dot sensitized solar cells were fabricated.The chemical composition,morphology,crytal phase,optical absorption of the PbS quantum-dots and the photovoltaic performance of the corresponding solar cells were systematically compared.The gradient-band-gap PbS quantum-dots were first and easily constructed by change the EDT concentration in mixing solution of Na₂S and EDT via two-step spin-coating SILAR procedure and the the corresponding quantum-dot sensitized solar cells were fabricated.The influence of the EDT concentration in the mixing solution of Na₂S and EDT on the crystal sizes,band gap,valence band maximum,conduction band minimum,chemical compositon and morophology of the PbS quantum-dots and the the photovoltaic performance of the corresponding solar cells were systematically investigated.Morever,the Ag₂S and PbS quantum dots were synthesized by the two-phase method at room temperature and their morphology,crystal phase and optical absorption were characterized by TEM,HRTEM,XRD and UV-Vis.Comparing with the PbS quantum dots prepared by the two-step spin-coating SILAR procedure and the three-step spin-coating SILAR procedure,the chemical composition,surface morphology,crystal phase and optical absorption of the PbS quantum dots were not very different,and both of them can deposit the compact and fully-covered PbS quantum dot thin films on the TiO₂ nanorod arrays.The chemical composition of the PbS quantum-dots using the two-step spin-coating SILAR procedure can easily adjust by changing the Na₂S and EDT concentration in the mixing solution.The PbS quantum-dot sensitized solar cells using the three-step spin-coating procedure exhibited a PCE of 3.79%,along with a V_oc of 0.54 V,a J_sc of11.90 mA×cm^-2,and a FF of 0.59.The PbS quantum-dot sensitized solar cells using the two-step spin-coating procedure exhibited a PCE of 3.81%,along with a V_oc of 0.55 V,a J_sc 11.35 mA×cm^-2,and an FF of 0.61.These photovoltaic performance parameters of quantum-dot sensitized solar cells were almost the same whether using the three-step spin-coating or two-step spin-coating procedure.By adjusting the EDT concentration in the mixing solution of Na₂S and EDT,it can be found that the crystal sizes of the PbS quantum dots gradually decreased,the band gap of the PbS quantum dots gradually increased,and the valence band maximum and the conduction band minimum of the PbS quantum dots up-shifted with the increase of the EDT concentration in the mixing solution from 1.5 mmol×dm^-3 to 3 mmol×dm^-3 and 6mmol×dm^-3.For the gradient-band-gap PbS QDs,the conduction band minimum gradually decreased,and the valence band maximum gradually increased.Therefore,transporting of the conduction band electron to FTO and the valence band hole to the spiro-OMeTAD layer was accelerated and the charge separation was improved.Because the conduction band minimum of the PbS QDs directly contacting the spiro-OMeTAD layer was relatively high.The optimized solid-state gradient-band-gap PbS quantum-dot sensitized solar cells achieved a PCE of 6.29%,along with a V_oc of 0.65 V,a J_sc of 15.09 mA·cm^-2,and an FF of 0.64 under 1 sun.The crystal size of the Ag₂S quantum dots prepared by the two-phase method was homogeneous and the size was about 5 nm.However,the prepared Ag₂S quantum dots were difficult to be used in fabricating the quantum-dot sensitized solar cells due to the long ligand on the surface and decrease the conductivity of the Ag₂S quantum dots.The crystal size of the prepared PbS quantum dots was inhomogeneous,and the crystal sizes was in the range of 5-20 nm.It is necessary to further adjust the composition of the precursor solution to synthesize the PbS quantum dots with the homogeneous crystal size.Therefore,the preparation of Ag₂S and PbS quantum dots by two-phase method still need further exploration.