## 2 NC Solar Cell Fabrication Technology Determines the Electronic Properties  
The size and morphology of CdTe NC thin film is quite different from that prepared by vacuum techniques. It is comprised of thousands of atoms. Therefore, the optical properties and bandgap of NC show a wide distribution due to the quantum size effects [9]. The assembly of CdTe NC during the fabrication of NC thin film is not perfect, and impurities within the surface of NC affect the optical and electrical properties of NC thin film [14; 15; 16]. In addition, the bandgap of bulk CdTe is \(\sim\)1.45 eV, which is in the ideal bandgap region for single junction solar cells to achieve high efficiency. The decrease in size will shift the bandgap of NC to a wide region (\(>\)1.45 eV), which may affect the performance of NC solar cells. A chemical/annealing process is usually adopted to eliminate bulk defects in the CdTe NC thin film to increase the diffusion length of carriers. This is different from the fabrication of PbS quantum dots solar cells [17] under low temperatures (\(<\)100 \({}^{\circ}\)C). It is well known that CdCl\({}_{2}\) treatment at \(\sim\)400 \({}^{\circ}\)C can significantly improve the junction quality of CdTe thin film solar cells prepared by the close space submission (CSS) method [18; 19]. However, as the grain size, melting point, or surface chemical properties of CdTe NC are quite different from those of CdTe thin film prepared by vacuum technology, the optimal chemical/annealing strategy should be different. To prepare CdTe NC thin film by solution processing, the first key issue is to obtain a well-dispersed NC solution. Thanks to the development of NC separation techniques, homogeneous CdTe NC solution can be prepared by refluxing the as-prepared NC to remove the impurities and insulated ligands and then disperse it into a solvent such as pyridine or deionized water [20; 21]. In the early research, CdTe NC thin film was prepared by spin-casting a single layer of CdTe NC solution on the substrate and undergoing a chemical/annealing process [22]. The drawback of this method is that the increased stresses that develop during the annealing step may cause the formation of cracks, voids, and/or holes in the NC film, which will lead to device shunt or weaken in performance. Furthermore, the bulk defects in the CdTe NC thin film limit carrier transfer, and the solar cell PCEs are usually below 3% [23; 24]. Recently, a layer-by-layer approach was adopted to overcome this problem [25; 26]. As shown in Figure 1, this involves several drops of CdTe NCs solution being put on top of an ITO/ZnO/n type semiconductor (such as CdS, CdSe, or TiO\({}_{2}\)) substrate and spin-casted. Then, the substrate is placed on a hot plate and annealed at a temperature of \(\sim\)150 \({}^{\circ}\)C for several minutes to eliminate any organic solvent. After that, the substrate is dipped into a saturated CdCl\({}_{2}\)/CH\({}_{3}\)OH solution for \(\sim\)10 s and taken out, rinsed in n-propanol, and blow-dried under N2 flow. The substrate is then placed immediately on a hotplate at \(\sim\)350 \({}^{\circ}\)C for \(\sim\)10-60 s. This process permits the growth of CdTe NC into larger sizes (typically \(\sim\)100 nm), which is important to decrease the interface defects between NC grains. This NC thin film fabrication process is repeated for several cycles in order to obtain CdTe NC thin film with the desired thickness. In general, 5-7 layers are needed to obtain an optimal thickness for light harvesting. This layer-by-layer approach can alleviate stresses in each layer, and the residual defects are filled by the subsequent layers. The high quality CdTe NC thin film obtained in this case is beneficial for carrier transfer, and high device performance is expected. On the other hand, this technique permits the electronic properties of each layer to be adjusted, which is important for efficient solar cell design.