Synthesis And Properties Of Cu-In-S Quantum Dots For Si Solar Cells

Semiconductor colloidal quantum dots (QDs), which are also called semiconductor nanocrystals, are nanoparticles whose properties and size locate between molecule materials and bulk materials. QDs are mainly composed of Ⅱ-Ⅳ, Ⅲ-Ⅴ, Ⅳ-Ⅵ or Ⅰ-Ⅲ-Ⅵ elements. They own many unique optical, mechanical and electrical properties because of confinement effect and quantum size effect and are widely applied to bio-imaging, biosensor, solar cells, LED lighting and display. As a result, it is very significant to synthesize shape and size-controlled QDs with high qualities.The synthesis method for QDs can be divided into two types according to reaction solvent, namely water solution and organic phase. The aqueous approach typically involves low-cost raw materials, less toxicity, and eco-friendly. Also, QDs synthesized in water solution can be directly applied to bio-imaging. However, they own inhomogeneous morphology, easy aggregation, and low quantum yield. On the contrary, QDs synthesized in organic phase have the benefits of good dispersibility, high crystallinity, and high quantum yield, etc. So it is usually used to synthesize QDs with excellent properties. The present of oxygen in air generally affects the properties of QDs. So, most of synthetic methods need involve the protective atmosphere of N2 or Ar,which makes the reaction operation become complex and limits their application in large-scale production. Therefore, a single step method will be optimum and is the pursuit of this study. Recently, due to the high toxicity of cadmium-containing QDs, more attention has been paid to low toxic CuInS2 QDs which does not contain any toxic heavy metals and exhibits the significant quantum confinement effect. This material could therefore offer the opportunity to fulfill the potential of semiconductor QDs without the toxicity limitations encountered by II-VI QDs and provide PL emission ranging from the visible light to the near-infrared light (NIR).In view of the above reasons, this thesis describes the synthesis of high quality CIS QDs with emission ranging from the red to NIR without any inert atmosphere protection via a simple one-pot microwave irradiation and solvothermal method, respectively. After growing ZnS shell on the surface of CIS core, as-prepared CIS/ZnS QDs possess not only a high PLQY but also a large Stokes shift. The optical properties can be easily tailored by controlling the reaction temperature, reaction time, and the Cu/In ratio. Especially, when they were first embedded into polymethyl methacrylate (PMMA) transparent matrix to form a composite film as a luminescent down-shifting (LDS) layer for Si solar cell, the conversion efficiency of the cell was enhanced.The main contents are summarized as follows:(1) Different sizes of CuInS2 QDs were synthesized without any inert atmosphere protection via a simple one-pot microwave irradiation. The influences of reaction temperature and time, the molar ratio of Cu to In, and the amount of sulfur and DDT on the optical properties were discussed in detail. Finally, the optimal preparation condition was achieved:T=220℃, t=5min, S:Cu:In=1:1:2. OA:DDT:ODE=0.5:2:3. The emission wavelength of as-prepared CIS QDs covers from 634 to 712 nm and the highest quantum yield of 6.9% was obtained.(2) CIS QDs were successfully prepared by the modified solvothermal method. The effect of reaction temperature and time on PL properties were investigated in detail. As a result, the optimized preparation conditions for CIS QDs with the maximum quantum yield in the laboratory are T=220 C and t=40min. For industrial-scale production, the best synthesis process is T=200℃ and t=120min. Under this condition, gram-scale QDs can be prepared at one time by using the suitable reaction vessel. The emission wavelength of as-prepared CIS QDs covers from 619 to 769 nm and the highest quantum yield of 8.4% was obtained. The PL properties are much better than that of CIS QDs prepared by microwave irradiation.(3) Based on the above experimental results. CIS/ZnS core/shell QDs were successfully synthesized by shelling ZnS on the surface of CIS QDs. The PL intensity and photostability of CIS/ZnS core/shell QDs were improved greatly and the quantum yield increased from 2.2% of CIS to 72.9%. When they were first embedded into polymethyl methacrylate (PMMA) transparent matrix to form a composite film as a luminescent down-shifting (LDS) layer for Si solar cell, the conversion efficiency of the cell shows an improvement of 3.8%(from 15.60 to 16.21%) in compared to the one with pure glass on the top.