## 6 Outlook  
Unlike the traditional CdTe thin film solar cells fabricated from the evaporation of semiconductor materials to construct an active layer, CdTe NC solar cells are prepared by a simple layer-by-layer solution and sintering process. Therefore, the electronic properties of NC thin film can be rationally controlled by varying the thin film fabricating method. The highest and most stable PCE of solution-processed CdTe NC based solar cells obtained is around 9%; there still exists great potential for commercial application. To further improve the efficiency up to 15%, more research should be done. Firstly, the \(V_{\text{oc}}\) of most CdTe NC solar cells is around 0.6-0.7 V, which is significantly lower than most CdTe thin film solar cells prepared by CSS or other vacuum techniques (\(\sim\)0.8-0.9 V). As the carrier doping concentration for CdTe NC thin film is \(\sim\)10\({}^{15}\)/cm\({}^{3}\), it is difficult to make good ohmic contact without further interface treatment or increased doping density. Creating a Te-rich layer is therefore important to create good ohmic contact to further increase the performance of the NC solar cells. Unfortunately, there are still no reports on the successful application of these methods to solution-processed CdTe NC solar cells. In a previous report, [PERSON] et al [29] attempted to create a Te-rich layer by wet-etching the CdTe NC thin film; the device showed exclusively shunt action and no efficiency. Therefore, a new chemical etching method must be developed to further improve the ohmic back contact to CdTe NC thin film. In the previous reports, introducing a interlayer (such as ZnTe:Cu or Te) between ITO and CdTe NC thin film has led to significantly improvement in performance of CdTe NC solar cells [36]. As shown in Figure 4a-d, by modeling heavily doped CdTe at the interface between ITO and the CdTe NC thin film, a good Ohmic contact to CdTe is formed, which can significantly improve the CdTe NC based solar cells [12]. How to apply this technology to inverted NC solar cells is a still challenge. On the other hand, increasing the doping concentration of CdTe NC thin film is also crucial to further improve the performance of NC solar cells. In previous work, ZnTe:Cu or Cu\({}_{\text{x}}\)Te was selected as the back contact for NC solar cells and a PCE of \(\sim\)6% was obtained [24]. The low PCE is mainly attributed to the low \(J_{\text{sc}}\), which is due to the absorption of the n-type partner. By using a solution-processed n-type semiconductor NC and optimizing the Cu doping in the CdTe NC thin film, the PCE of NC solar cells could improve towards 10%. Recently, [PERSON] et al. [68] and our group developed a new device structure with a graded bandgap structure. In this device structure, the separation of photo-generated charge carriers is easy, and the spectrum response is extended. More work should be done to further optimize each active layer to further improve the spectrum response to the long wavelength region and to improve the performance of NC solar cells towards 15%. In order to realize the commercial application of CdTe NC solar cells, improved stability, material processing, and scale-up fabricating techniques (such as inkjet printing or spray coating) are required. However, due to their simple, low-cost fabrication techniques and low material consumption, CdTe NC-based solar cells have a promising future. **Author Contributions:** [PERSON], [PERSON] and [PERSON] examined the literature; [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] compiled the paper. **Funding:** This research received no external funding. **Acknowledgments:** We thank the financial support of the National Natural Science Foundation of China (No. 21875075, 61774077, 61274062, 61775061), Guangdong Province Natural Science Fund (No. 2018A0303130041), Guangzhou Science and Technology Plan Project (No. 201804010295), National Undergraduate Innovative and Entrepreneurial Training Program (No. 201810561013).