# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 7 Other NC Heterojunction Solar Cells  
CdS is an n-type semiconductor with a bandgap of 2.45 eV. As CdS has a similar structure as that of CdTe, the lattice mismatch is less than 5% between CdS and CdTe. CdS is the most commonly used acceptor for CdTe/CdS heterojunction solar cells. Although up to 20% PCE has been obtained in CdTe/CdS thin film solar cells prepared by chemical source deposition techniques, still few report on solution-processed CdTe NCs/CdS solar cells with satisfactory PCE. [PERSON] et al. [60], for the first time, reported solution-processed CdTe NCs solar cells with a structure of ITO/CdS/CdTe/Au and negligible PCE was obtained in this case due to the poor interface between CdTe NCs and CdS. In 2013, [PERSON] et al. [61] introduced indium-doped ZnO thin films in CdTe/CdS solar cells with a structure of ITO/ZnO-In/CdS/CdTe/MoO\({}_{x}\)/Ag, by optimizing annealing conditions, a PCE of 3.73% coupled with high-stability was obtained. More recently, [PERSON] et al. [62] mixed CdTe and CdS NCs to form a nanoscale bulk CdTe-CdS heterojunction. They fabricated CdTe NC solar cells with a structure of ITO/TiO\({}_{2}\)/CdTe-CdS/MoO\({}_{3}\)/Au, the device showed a PCE of 3.98%, \(V_{\rm{oc}}\) of 0.50 V, \(J_{\rm{sc}}\) of 14.81 mA/cm\({}^{2}\), and FF of 0.538. The IQE values were about 100% in the range of 350-700 nm, implying the efficiency of charge separation. The CdTe NCs/CdS solar cells discussed above suffered from low \(V_{\rm{oc}}\) due to large defects existing on the surface of CdTe NCs and the CdS layer. In 2015, [PERSON] et al. [63] fabricated CdTe/CdS solar cells based on CdTe and CdS NC aqueous solution. By  
Figure 8: **(a)** Schematic of the CdTe-CdSe NC solar cell devices and (b) energy diagram of the above device used in the study. Reproduced with permission from [60], Copyright 2015, Royal Society of Chemistry.  
adjusting the annealing temperature and changing the CdTe to CdS ratio, which might affect the charge dynamics and doping density, the CdTe/CdS junction quality was improved. They found that the content of S might influence the doping density of CdTe-CdS BHJ and, therefore, the depletion width of TiO\({}_{2}\)/CdTe. After optimizing the film thickness to 320 nm, they obtained the champion device with a PCE of 5.73%.  
In the case of CdTe/ZnO solar cells with an inverted structure, ZnO nanorod (NR) is often used as an acceptor due to its high electron mobility, which provides direct pathways for efficient charge carrier transport [64]. However, the CdTe/ZnO devices shows low efficiency due to the poor carrier separation and collection. In order to overcome this drawback, [PERSON] et al. [65] introduced a thin CdS interface layer into ZnO/CdTe core-shell nanorod array solar cells, a PCE of 0.72% was obtained, which was three times of the devices without a CdS interlayer. More recently, [PERSON] et al. [11] adopted a similar method to fabricate ZnO/CdS (interlayer)/CdTe solar cells with a three-dimensional structure. They found that the CdS layer could redistribute the depletion regions and eliminate interface defects. A PCE of 1.16% was obtained in optimized devices. The device's performance may be further improved by tuning the length of the ZnO NRs.