# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 9 Perspectives and Conclusions  
Recent developments of CdTe NCs for photovoltaic applications have promoted synthesis methods, material and film processing techniques, and characterization and optimization of optoelectronic properties. These have enabled the solution-processed CdTe NC solar cells with AM 1.5 PCE up to 12%, which offers a new and powerful combination of power conversion efficiency and cost. CdTe NCs champion solar cell devices with all kinds of structure are summarized in Table 2.  
In order to further improve the PCE and stability of CdTe NC-based solar cells, progress remains to be made on a number of key fronts. It is suggested that CdTe NCs solar cells with bulk heterojunction structure can eliminate the band edge and improve the carrier collection efficiency, which has been confirmed by our research group in the case of CdTe:CdSe NC bulk nano hetero-junction solar cells [81]. On the other hand, as the bandgap is limited, light above 900 nm cannot be used in CdTe NCs' active \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Structure** & \(\mathbf{V_{\infty}}\) **(V)** & \(\mathbf{J_{\infty}}\) **(mA/cm\({}^{2}\))** & **FF (\%)** & **PCE (\%)** & **Reference** \\ \hline ITO/PEDOT:PSS/PPV:CdTe/ZnO/Al & 0.50 & 10.7 & 40 & 2.14 & [69] \\ ITO/TiO\({}_{2}\)/CdTe/PPV:CdTe/MoO\({}_{3}\)/Au & 0.591 & 16.08 & 50.1 & 4.76 & [71] \\ ITO/TiO\({}_{2}\)/PNV:CdTe/MoO\({}_{3}\)/Au & 0.531 & 12.5 & 53.1 & 3.61 & [72] \\ ITO/TiO\({}_{2}\)/CdTe/PPV/MoO\({}_{3}\)-sol/Au-sol & 0.45 & 10.11 & 37 & 1.71 & [74] \\ ITO/TiO\({}_{2}\)/CdTe/PPV:CdTe/MoO\({}_{3}\)/Au & 0.50 & 17.1 & 47.2 & 4.0 & [75] \\ ITO/TiO\({}_{2}\)/CdTe/P3 HT:CdTe/MoO\({}_{3}\)/Au & 0.54 & 16.95 & 47.2 & 4.32 & [76] \\ ITO/TiO\({}_{2}\)/PPV:CdSe\({}_{x}\)Te\({}_{1-x}\)/MoO\({}_{3}\)/Au & 0.552 & 13.94 & 43.5 & 3.35 & [78] \\ ITO/TiO\({}_{2}\)/MPPV:CdTe/MoO\({}_{3}\)/Au & 0.624 & 15.15 & 54.8 & 5.18 & [80] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Selected performance parameters of HSCs (hybrid solar cells) based on aqueous-processed polymer NCs (nanocrystals). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Device Structure** & **PCE** & **Published Years** & **Reference** \\ \hline ITO/Au/CdTe/Al & 5.30\% & 2014 & [37] \\ ITO/CdTe/ZnO/Al & 12.3\% & 2014 & [40] \\ ITO/CdTe/CdSe:Te\({}_{1-x}\)/ZnO/Al & 7.1\% & 2012 & [9] \\ ITO/ZnO/CdSe/CdTe/Au & 5.81\% & 2015 & [60] \\ ITO/TiO\({}_{2}\)/CdTe-CdS BHJ/Au & 5.73\% & 2015 & [64] \\ ITO/TiO\({}_{2}\)/PPV:CdSe\({}_{x}\)Te\({}_{1-x}\)/MoO\({}_{3}\)/Au & 3.35\% & 2015 & [79] \\ ITO/TiO\({}_{2}\)/MPPV:CdSe\({}_{x}\)Te\({}_{1-x}\)/MoO\({}_{3}\)/Au & 5.18\% & 2015 & [81] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Selected champion CdTe NC solar cell devices. layer, and multi-junction or graded bandgap devices should be designed to absorb all photons in ultraviolet (UV), visible, and infra-red regions of the solar spectrum. Recently, [PERSON] et al. [82] developed a new graded bandgap device structure of FTO/n-ZnS/n-CdS/n-CdTe/Au. In this device structure, charge carrier annihilation is drastically reduced and photo-generated charge carriers have highest possible mobility values in nano crystalline rod-type CdS and CdTe. Their preliminary work on this graded bandgap devices showed over 10% efficiency with extremely high \(J_{\rm sc}\sim\)48 mA/cm\({}^{2}\). In the case of organic-inorganic NCs hybrid solar cells, metal oxides have the ability to attain high efficiency as they have several scales of pores which may shorten the carrier diffusing length [83]. Using different passivation layers (ZnSe or CdS) and changing different metal oxide thicknesses are also possible ways of improving the efficiency of CdTe NC-based solar cells [84; 85]. With the rapid rate of progress on CdTe NC-based solar cells, combined with further device optimization, solution-process CdTe NCs solar cells have a promising future. This work is supported by the National Natural Science Foundation of China (Nos.