# Solution-Processed CdTe Thin-Film Solar Cells Using ZnSe Nanocrystal as a Buffer Layer
## 3 Results and Discussion
In this study, in order to further increase the quality of ZnSe NC film, 1,2-ethanedithiol solution (EDT, 1% \(v/v\) in methanol) is adopted during the layer-by-layer processing for developing the ZnSe NC thin film. As shown in Figure 3a, the -SH is firstly bonded to the Zn\({}^{2+}\) of ZnSe NC, then attached to another layer of ZnSe, which permits smooth and compact ZnSe NC film formation during the deposition of ZnSe NC film. From the infared spectra shown in Figure 3b (transmittance vs. wavenumber), the molecular vibration spectrum corresponding to sulfydryl is found at 559 cm\({}^{-1}\), 1265 cm\({}^{-1}\) and 2746 cm\({}^{-1}\) respectively for ZnSe NC film coverage with EDT, which implies that EDT is bonded to the  
Figure 2: **(a)** Schematic and (**b**) energy band alignment of CdTe NC solar cells with the ZnSe NC buffer layer; (**c**) cross-sectional scanning electron microscope (SEM) image and (**d**) X-ray photoelectron narrow scan (XPS) of NC solar cells. \(J\)-\(V\) curves of CdTe NC solar cells (**e**) with different ZnSe NC thicknesses and (**f**) with different annealing temperatures (inset of Figure 2e,f show the \(J\)-\(V\) curve expressed in logarithmic coordinates.). ZnSe NC thin film. The morphologies of the ZnSe NC thin film w/o ligand treatment are characterized by atomic force microscopy (AFM). As shown in Figure 3c,d, the root-mean-square (RMS) roughness for NC with ligand treatment is 4.8 nm while 6.5 nm for NC without ligand treatment. It is also found that without ligand treatment more aggregations are found (Figure 3c), which may result in a large leakage current and low diode quality. Figure 3: (**a**) A schematic figure of 1,2-ethanedithiol (EDT) bonding to adjacent ZnSe NC film; (**b**) Infrared spectrum of EDT treated ZnSe NC thin film, atomic force microscopy (AFM) images of ZnSe NC thin film (**c**) without EDT treatment (**d**) with EDT treatment. The corresponding _J-V_ curves characteristic of NC solar cells are presented in Figure 4a. The NC solar cells with ligand treatment have a \(J_{\rm sc}\) of 13.55 mA/cm\({}^{2}\), a \(V_{\rm oc}\) of 0.64 V, an FF of 47.33%, and a PCE of 3.58%, which is increased by 16.6% in PCE when compared to devices without ligand treatment. From Table 1, it is found that NC solar cells with ligand treatment show a significant improvement in FF (from 40.79% to 47.33%) and \(J_{sc}\) (from 11.94 mA/cm\({}^{2}\) to 13.55 mA/cm\({}^{2}\)) with the \(R_{\rm s}\) decreasing from 23.50 to 19.48 \(\Omega\)-cm\({}^{2}\) and the \(R_{\rm sh}\) increasing from 246 to 251 \(\Omega\)-cm\({}^{2}\). Therefore, the device with ligand treatment will show less current leaking. It was reported in our previous work that CdTe NC solar cells with efficiency as high as 5.14% are attained with device configuration of ITO/ZnO/CdS/CdTe/MoOx/Au [12]. Comparing these two devices, the low PCE obtained in the ZnSe NC device is mainly attributed to the low \(J_{\rm sc}\) (13.55 mA/cm\({}^{2}\) to 17.26 mA/cm\({}^{2}\) for CdS device) and FF (47.33% to 52.84% for CdS device). It is interesting that the Voc of the ZnSe NC device is slightly higher than that of the CdS NC device (0.64 V to 0.56 V). Figure 4b shows the _J-V_ curve in the dark. It is clear that the current density at a reversed bias of \(-\)1 V for an EDT treatment device is lower than a device without EDT treatment. However, the reverse and forward current is almost the same at V = \(-\)0.5 V and V = 0.5 V. Therefore, EDT treatment can only partly reduce the leakage current and improve the quality of the ZnSe NC film and decrease carrier recombination, which results in improvements for the efficiency of NC solar cells. The EDT treatment techniques here need to be further optimized, such as changing EDT concentration or spin-casting speed, in order to improve the NC solar cells' performance. The external quantum efficiency (EQE) of NC devices without EDT treatment is presented in Figure 4c.