# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 6 CdTe/CdSe NCs Heterojunction Solar Cells  
In all kinds of CdTe NC heterojunction solar cells, CdTe/CdSe NCs solar cells are the most researched system. Unlike the CdTe/ZnO system, CdTe and CdSe NCs have similar sizes and structures (zinc-blende or wurtzite), so a high-quality p-n junction is expected due to the low lattice mismatch between CdTe/CdSe NCs. In 2005, [PERSON] et al. [6] first reported all solution-processed CdTe/CdSe NC heterojunction solar cells with structure of ITO/CdTe/CdSe/Ca/Al. By varying system parameters, the best device showed \(J_{sc}\) of 13.2 mA/cm\({}^{2}\), \(V_{oc}\) of 0.45 V, FF of 0.49, and a PCE of 2.9%. Low \(V_{oc}\) and FF implied that the junction of CdTe/CdSe and quality of NC thin film should be further improved. Following this, many studies on the mechanism of the junction were carried out [45; 46; 47; 48; 49; 50; 51; 52; 53].  
Surface photo-voltage spectroscopy was used to investigate the charge separation in Type II tunneling multilayered structures of CdTe and CdSe NCs [54]. It was found that electrons in CdSe NC layers diffuse quicker than holes in CdTe NCs layers. [PERSON] et al. further investigated the back contact metal for electrons collected [55]. They found that Ag, Ca/Al, Au, and Al electrodes all yield similar \(V_{oc}\) and device performance. Recently, [PERSON] et al. [56] studied the thickness dependence on the performance of CdTe/CdSe solar cells. A low short circuit current was obtained in the case of CdTe thickness over 500 nm while very low \(V_{oc}\) was obtained in 200 nm CdTe devices.  
It is noted that all CdTe/CdSe NC solar cells discussed above have a normal structure of ITO/CdTe/CdSe/metal. Light absorption begins at the ITO, rather than the junction, many minority carriers must travel the thickness of the entire film before they are collected. Furthermore, the mismatch work function between CdTe and ITO make it difficult to obtain high \(V_{oc}\) in this case. In2014, [PERSON] et al. [57] developed a new inverted structure of ITO/CdSe/CdTe/Cr/Au, with a high \(V_{\rm{oc}}\) of 0.62 V and an efficiency of 2.1% obtained for the optimized devices. The low external quantum efficiency (EQE) obtained in the region of 400 \(\sim\) 900 nm resulted in low \(J_{\rm{sc}}\) (below 10 mA/cm\({}^{2}\)), which was mainly caused by the large thickness (up to 100 nm) CdSe NC film used. In this case, the visible-wavelength response of the CdTe layers will be decreased. [PERSON] et al. [56] fabricated CdTe NCs solar cells with structure of ITO/CdSe/CdTe/Au fully by solution processing and obtained a highest PCE of 4.20%, coupled with a high \(V_{\rm{oc}}\) of 0.60 V, by carefully designing the annealing strategy. [PERSON] et al. [58] fabricated CdSe/CdTe NC solar cell devices treated with non-toxic salt of NH\({}_{4}\)Cl. They demonstrated minimal differences in film morphology and device performance in devices fabricated by using toxic salt of cadmium chloride or non-toxic salt of NH\({}_{4}\)Cl. More recently, [PERSON] et al. [59] developed a novel inverted device structure of ITO/ZnO/CdSe/CdTe/Au. This devices structure has many benefits. The charge-separating interface is close to the illumination, permitting efficient collection of carriers. As shown in Figure 8, 40 nm ZnO is first deposited on ITO by decomposition of Zn precursors, following this a thin layer of CdSe NCs is deposited on ZnO. The ZnO layer provides a smooth and pinhole-free pre-layer on which a smooth CdSe film can be formed, which eliminates catastrophic shorts. In an optimized devices, \(J_{\rm{sc}}\) of 15.28 mA/cm\({}^{2}\), \(V_{\rm{oc}}\) of 0.65 V, and FF of 0.585 was obtained, resulting in a high PCE value of 5.81%, which was the best result ever reported for solution-processed CdTe/CdSe NC solar cells.