在薄膜太陽能技術中，Cu(In,Ga)Se2(CIGS)太陽能電池是取代矽晶太陽能電池最有潛力的候選者之一。一般CIGSe結構中，因為其氧化鋅窗口層和緩衝層的寄生吸收，在短波長的光譜響應普遍較弱。使用下轉換螢光(LDS)材料可以減少短波長的光學損耗，該材料可吸收短波長光子並且發出對吸收層較有利的光子。 LDS一般鍍製在元件頂部，為增強短λ響應的被動方式，且不論太陽能電池的製程為何，皆可以增強元件效率。
現今對於不同螢光材料的研究主要使用無機量子點(QDs)有機染料和稀土離子/複合物。無機量子點可藉由調整尺寸改變其吸收和發射能譜，且具有高發射強度。然而另一方面，由於吸收和發射能譜的大幅重疊，導致大量的吸收損失，且材料本身為高度毒性元素，如鎘和鉛。而有機染料具有相對高的吸收係數並接近於一的PL QY，但其較窄吸收能譜和相對小的Stoke shift，阻礙了有機染料的應用。此外，稀土離子表現出高PL QY但極低的吸收係數且其原料太過昂貴。
因此，碳核奈米點更受歡迎，其中石墨烯量子點(GQDs)表現出獨特的半導體特性。因此，GQDs 可用於光電，生物傳感，有機光伏和聚合物裝置。CQDs 具有功能穩定性，並且具低毒性，高螢光，高化學惰性，以及在極性和非極性溶劑中的優異溶解性。然而，此材料很難在固體基板上獲得均勻薄層分佈。例如，使用CQDs水溶液度沈積均勻薄層特別困難，因為CQDs在乾燥後易於團聚。為了克服這個缺點，我們在聚合物中參雜，例如羧甲基纖維素(CMC)和聚乙烯醇(PVA)，有助於保持由表面官能基以及核心產生的CQDs的光學性質.

Thin-film solar cell based on Cu(In,Ga)Se2 (CIGS) absorber is one of the most attractive candidate among the thin film solar technologies having the potential to replace crystalline silicon solar cell. The typical structure of a CIGS cell shows poor spectral response at short wavelengths due to the parasitic absorption losses in the commonly used zinc oxide window and buffer layers. The optical losses in short wavelength can be reduced using a luminescent down-shifting (LDS) material, which absorbs short-wavelength photons and re-emits them at a more favorable wavelength. LDS layer is applied on top of the cell structure, therefore it is a passive approach to enhance the short-λ response, which is supposed to enhance the cell performance irrespective of any modulation in the growth and fabrication of active material of solar cell.
Investigation for various luminescent materials have been done for LDS mainly utilizing inorganic quantum dots (QDs) organic dyes and rare-earth ions/complexes. Inorganic QDs possess tunable absorption and emission bands according to their size, with high emission intensity. On the other hand, they result in high reabsorption losses due to the large overlap of absorption and emission bands, and involve toxic elements. Organic dyes show relatively high absorption coefficients and close to unity PL QY (photoluminescence quantum yield) but their narrow absorption bands and relatively small Stokes shifts hinders their application. Further, rare-earth ions exhibit high PL QY but have extremely low absorption coefficients and are generally too expensive.
Nano dots (CQDs) based on carbon core more popularly, the Graphene quantum dots (GQDs) exhibiting unique semiconducting properties. Therefore, GQDs are potential candidates for use in optoelectronic devices, biological sensing, organic photovoltaics, and polymeric devices. CQDs exhibits functional stabilities and also have very mild toxicity, Strong fluorescence, good chemical inertness, and excellent solubility in non-polar and polar solvents. On the other hand, it is quite difficult to obtain a uniformly thin layer or distribution of such material on solid substrates. For example, producing thin layers of aqueous CQDs is especially difficult because CQDs are prone to agglomeration after drying. To overcome this drawback, we used the method of incorporating into polymer matrix such as Carboxymethyl cellulose (CMC) and Polyvinyl alcohol (PVA) which can help to retain the optical properties of CQDs arising from the surface functional groups as well as the core.

Abstract 2
List of figures 7
List of tables 9
List of abbreviations and notations 10
1. Introduction 11
1.1. Forward 11
2. Fluorescent Nano dots 14
2.1. Mechanism and origins of photoluminescence 15
2.1.1. Photoluminescence in pure quantum dots 16
2.1.2. Photoluminescence in CQDs/GQDs 16
2.1.3. Photoluminescence in CNDs 17
2.2. Similarities and differences among types 17
2.3. Luminescence Quenching 19
2.3.1. Förster resonance energy transfer 19
2.3.2. Aggregation‐caused PL quenching 20
2.4. Carbon Dots 20
2.4.1. Overview 20
2.5. Synthesis of C-dots 21
2.6. Purification of Carbon Dots 23
2.7. Surface passivation and/or functionalization (SPF) strategy 23
3. Characterization 24
3.1. UV/vis Absorption Spectroscopy 24
3.2. Fluorescence Emission Spectroscopy 25
3.3. Atomic force microscopy (AFM) 26
3.4. Transmission electron microscopy(TEM) 27
3.5. Raman Spectroscopy 27
3.6. X-ray photoelectron spectroscopy (XPS) 27
3.7. Photoluminescence Quantum Yield (PLQY) measurement 27
3.8. External quantum efficiency (EQE) 28
4. Results and discussion 29
4.1. Experimental Procedure 29
4.1.1 Hydro-thermal process(N-CQDs) 29
4.1.2. N-CQDs/PVA films 32
4.1.3. N-CQDs/CMC films 32
4.1.4. BN-CQDS 33
4.2. AFM analysis 36
4.3. X-ray photoelectron spectroscopy (XPS) 38
4.4. Transmission electron microscopy(TEM) 39
4.5. EQE measurement of CIGS solar cell with LDS 41
5. Conclusion and Future Perspective 46
6. References 47
7. Appendix 50

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