| The efficient use of solar energy for the sustainable production of clean energy is an attractive approach to address the currently global energy crisis and environmental issues.While high-efficiency,low-cost quantum dot sensitized solar cells(QDSCs)represent a major opportunity to address this challenge.Colloidal quantum dots(QDs)are considered a promising and ideal materials to be applied as light harvester in excitonic solar cells due to multiple appealing properties such as tunable band gap and high absorption coefficient over broad spectral range.More importantly,the possibility of multiple exciton generation(MEG)by single photon absorption and the theoretical photoconversion efficiency(PCE)of the excitonic solar cells can reach over the Queisser-Shockley limit of 32%within a fluorescence quantum yield higher than 100%.In the past few years,QDSCs have achieved a significant enhancement in photovoltaic(PV)performances.The PCE of the QDSCs is still quite lower than the commercial silicon-solar cells,which is mainly attributed to undesirable carrier recombination occurring at semiconductor metal oxide/QDs/electrolyte interfaces and within the QDs.In addition,the poor long-time stability of QDSCs due to leakage of volatile electrolytes and QDs corrosion is a major issue for large scale and practical application of QDSCs.Based on these problems,we conduct this research of adjusting the band gap of CdSe/CdS core/shell structure to boost the PCE performance of the QDSCs.The thesis includes 4 parts:(1)To conduct the study on how the shell thickness affect the optical and electrochemical properties by adding CdS shell on the CdSe core QDs.The results show that the 6 cycles of CdS shell(correspond to 1.96 nm thickness)exhibits the best PCE among 2 cycles,6 cycles and 13 cycles CdS shell which short circuit current density,open circuit voltage and fill factor reached up to 9.3 mA/cm2,0.539 V,and 60%respectively.And the final PCE reached to 3.01%.(2)Based on the CdSe/CdS core/shell structure,we design the CdSe/(CdSexS1-x)4/(CdS)2 core/shell structure to explore the PCE performance.Typically,CdSe/(CdSexSi-x)4/(CdS)2 can be prepared by maintaining the same CdSe core size,then replacing the first 4 cycles of CdS shell with CdSexS1-x alloyed layer(x=0.5)and then coating the same 2 cycles of CdS again.The advantage of this structure is to use the gradient band structure to accommodate the transfer efficiency of electrons and holes pairs to increase the PCE.Finally,15.48 mA/cm2 for short circuit current density and 0.575 V for open circuit voltage and the final PCE were obtained for the CdSe/(CdSexS1-x)4/(CdS)2 solar cell.(3)Based on the above two studies,we further designed a consistent gradient band structure to further accelerate the transition of electrons and holes so that get higher PCE.Finally,we obtained 21.08 mA/cm2,0.562V for short circuit current density and open circuit voltage,respectively.And the PCE raise up to 6.86%.(4)We applied two alloy QDs to Photoelectrochemical(PEC)hydrogen generation and obtained 15 mA/cm2 for 4 alloy layers QDs and 17 mA/cm2 for gradient alloy QDs compared to 6 CdS shell QDs which achieved only 6 mA/cm2. |
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