Doping CdS / CdSe Quantum Dot Sensitizing Agent Thin Film Solar Cells

Quantum dot sensitized solar cells (QDSCs) is a new kind of dye-sensitized solarcells (DSCs). Quantum dots (QDs) exhibit significant advantages, such as size-dependentbandgaps, easy fabrication, and extremely high molar extinction coefficient, etc.Especially the theoretical efficiency of QDSCs is estimated up to44%due to multipleexcition effect of QDs. So the QDSCs have received considerable attention recently.However, the efficiency of QDSCs is still lower than that of DSCs. From the workingmechanism of QDSCs, it is found that the efficient charge separation is an importantfactor for the device performance.In the thesis, focusing on how to optimize the charge transfer path in QDSCs andbasing on CdS/CdSe co-sensitized system, doping experiments on quantum dotsensitizers of thin film solar cells were designed. It mainly includes the following threeaspects:(1) The successive ionic layer adsorption and reaction (SILAR) method wasemployed to fabricate CdS/CdSe co-sensitized solar cells. When CdS deposition times is8, and CdSe deposition times is4, short-circuit current (JSC), open-circuit voltage (VOC)and fill factor (FF) reach to8.86mA/cm2,0.48V, and0.34, respectively. The efficiency ofsolar cell reaches to1.43%. After adding a ZnS passivation layer on the surface of QDs,JSCis significantly increased, reaching to10.59mA/cm2, VOCremains unchanged. But theFF decreases sharply, leading to the overall solar cell efficiency is only1.30%.(2) Based on CdS/CdSe sensitization experiments, experiments on solely dopingmetal cation in CdS or CdSe were conducted. Mn-doped-CdS/CdSe co-sensitized solarcells, La-doped-CdS/CdSe co-sensitized solar cells, CdS/Mg-doped-CdSe co-sensitizedsolar cells and CdS/Co-doped-CdSe co-sensitized solar cells were fabricated. Amongthese works, used by SILAR method, doping CdS with La, doping CdSe with Mg and Co,and assembling them into La-doped-CdS/CdSe co-sensitized solar cells,CdS/Mg-doped-CdSe co-sensitized solar cells and CdS/Co-doped-CdSe co-sensitizedsolar cells, respectively, these three works have not been found in the literature. Afterdoping in quantum dot sensitizers, JSCand VOCare all improved, especially the JSCincreases significantly. Through a series of characterization and tests, it is found that afterdoping metal cation in CdS or CdSe, the conduction band of quantum dot improves, thisis conducive to charge transport in solar cell, reducing the recombination between electrons and holes, improving the ability of photoanode to capture light. Thus, thephotocurrent is improved, and ultimately, the photoelectric conversion efficiency isenhanced.(3) Based on the design idea of solely doping metal cation in CdS or CdSe, theresearch in simultaneously doping CdS with Mn, and doping CdSe with Co was also didfor the first time. It is found that after simultaneously dope CdS with Mn, and dope CdSewith Co, the light absorption range and light intensity of the photoanode are better thanthat of solely doping Mn in CdS or doping Co in CdSe and the undoped photoanode.When assembled into QDSCs, the corresponding monochromatic incidentphoton-to-electron conversion efficiency (IPCE) and solar cell efficiency is higher, too. Itis shown that after simultaneously doping CdS with Mn and doping CdSe with Co, thecharge transport in solar cell is much more efficient, the recombination between electronsand holes further decreases. Thus, the performance of solar cell gets improved.