# Solution-Processed CdTe Thin-Film Solar Cells Using ZnSe Nanocrystal as a Buffer Layer
## 3 Results and Discussion  
The ZnSe NC samples are prepared by injecting TOP-Se into the Zn\({}^{2+}\) carboxylic precursor and adopting HDA as the solvent at a high temperature of 300 \({}^{\circ}\)C. Using TEM images as shown in Figure 1a, we observe that the ZnSe NCs have a spherical morphology with an average size of \(\sim\)30 nm. We also found large aggregation, which may be due to the large polarity of ZnSe NC in toluene. The ZnSe NC samples are refluxed in pyridine for 12 h and dried under a vacuum oven at 120 \({}^{\circ}\)C for one day before XRD measurement. The diffraction patterns have peaks at about 25.98\({}^{\circ}\), 27.24\({}^{\circ}\), 45.26\({}^{\circ}\), 48.74\({}^{\circ}\) and 53.55\({}^{\circ}\) as identified from the XRD pattern (Figure 2b), corresponding to the (100), (002), (110), (103) and (112) facets, which agree well with the standard wurtzite structure of ZnSe (JCPDS 97-1463). The ultraviolet (UV) absorption peak is located at 412 nm for ZnSe NC in the toluene solution, which corresponds to 3.0 eV for the ZnSe NC bandgap (calculated from the absorption peak of Figure 1c). It is noted that the bandgap calculated from the absorption peak is not accurate, as NC with different diameters existed in this case. To investigate the transmission and bandgap of sintering NC thin films, ZnSe NC with different thicknesses are deposited onto the ITO/ZnO substrate and annealed at 380 \({}^{\circ}\)C for 10 min to remove any organic solvent and impurity. The average thickness of ZnSe NC thin film is measured by using a profilometer. The typical transmission spectra of the ZnSe NC thin films (ITO/ZnO/ZnSe), which have 60 nm, 100 nm, and 120 nm thicknesses, are shown in Figure 1d (inset shows the plot of \(\langle\alpha\mathrm{h}\rangle\)\(\rangle\)\({}^{2}\) versus the photon energy). It is clear that the NC thin films are nearly transparent in a wavelength range higher than 500 nm and the films show high transparency for wavelengths below 450 nm, which is beneficial for light harvesting of the CdTe NC active layer. The transmission decreases in wavelengths below 500 nm with an increased thickness of ZnSe NC. From the inset of Figure 1d, the bandgap of the ZnSe thin film is 2.81 eV, which is derived by taking a tangent of a straight line at A = 0 (A = (\(\alpha\mathrm{h}\)\(\rangle\)\({}^{2}\)). Figure 1: (**a**) Transmission electron microscope (TEM) image of ZnSe nanocrystal (NC); (**b**) X-ray diffraction (XRD) pattern of the ZnSe NC (nanocrystal) thin film; (**c**) absorbance of ZnSe NCs in toluene; and (**d**) transmission spectrum of the ZnSe NC thin film with different thicknesses (ITO/ZnO/ZnSe, inset shows the plot of bandgap). The architecture and band alignment of the NC device are presented in Figure 2a,b. In this device structure, carriers are mainly generated in the CdTe NC active layer. Unlike CdS or CdSe, the conduction band of ZnSe is higher than that of CdTe, a type I heterojunction is formed for CdTe/ZnSe heterojunction, and therefore band bending at the ZnSe interface is spike like. It is noted that the hole barrier levels for CdTe/ZnSe heterojunction depends on both the valence band levels and the Fermi levels of the CdTe and ZnSe. A small positive conduction-band offset may help maintain good cell efficiency as it created a large hole barrier adjacent to the interface and reduced interface recombination [32; 33; 34]. Efficiency up to 11% has been attained in the case of Cu(In,Ga)Se\({}_{2}\) thin-film solar cells with ZnSe buffer layer [35] while >21% efficiency has been achieved for the ZnS buffer layer [36]. Therefore, CdTe NC solar cells with a ZnSe NC buffer layer is expected to obtain efficiency once the CdTe/ZnSe junction quality is improved. The MoOx buffer layer blocks electrons and facilitates hole collecting, which was confirmed in the previous report [27]. The cross-sectional SEM image of ITO/ZnO/ZnSe/CdTe/MoOx/Au is presented in Figure 2c. It is clear that the CdTe NC and ZnSe NC layers are compact and pin-hole free.