## 3 Improving p-n Junction Quality by Using an n-Type NC Partner
In previous work [25], it was found that after optimizing the annealing temperature and the thickness of CdS NC film, a device with a configuration of ITO/ZnO/CdS NC/CdTe NC/Au showed a PCE of 5.14%, which is significantly higher than that of devices with CBD-CdS as an n-type partner (3.73%) [38]. The improvement in device performance was mainly attributed to the increases in fill factor (FF) and short circuit current density (\(J_{\mathrm{sc}}\)). Besides CdS NC, CdSe NC has also been selected as an n-type partner for CdTe NC solar cells. The bandgap of bulk CdSe is about 1.7 eV, which does not lead to efficient CdTe thin film solar cells due to parasitic absorption of the CdSe layer. However, as the diffusion of Se into Te occurs more easily than S into Te, a CdSeTe\({}_{\mathrm{1-x}}\) alloy layer can be easily formed when growing and post-treating the CdTe/CdSe thin film [39; 40]. Due to the \"light bowing\" effects, CdSeTe\({}_{\mathrm{1-x}}\) shows a narrower bandgap than that of CdTe, which will extend the spectrum response to a longer wavelength [41; 42], and a higher \(J_{\mathrm{sc}}\) is expected in this case. The CdTe/CdSe solar cells prepared through magnetron sputtering techniques show a high efficiency of 15.2% coupled with a high \(J_{\mathrm{sc}}\) of 26.3 mA/cm\({}^{2}\)[43]. Researchers have shown that the photo-currents are enhanced in both short and long wavelength regions when CdS is replaced by CdSe in the CdTe thin film solar cells [44]. In the early work of solution-processed CdTe/CdSe NC solar cells [22; 45], however, similar improvement in photo-current were not found, which was mainly attributed to the device architecture and device fabrication techniques used. In the CdTe NC device with a normal ITO/CdTe/CdSe/Al structure, the junction is far from the illumination and the CdSe NC layer acts as an electronic transfer layer. The light absorbed by the CdSe NC layer will not generate any photonic current due to the interface recombination. Recently, a new inverted device structure (ITO/ZnO/CdSe/CdTe/Au) was developed using a layer-by-layer sintering process [46]. In this device structure, the presence of the ZnO layer provides better electrical stability, eliminating the catastrophic shorting of CdTe NC solar cells. As the CdTe NC has a similar size, morphology, and lattice structure as that of CdSe NC, a high junction quality is preferred when the NC thin film is treated under suitable conditions. Furthermore, the formation of the CdSe\({}_{\mathrm{x}}\)Te\({}_{\mathrm{1-x}}\) layer is photo active, which will increase the external quantum efficiency (EQE) value in both short and long wavelengths. By optimizing the CdSe NC layer thickness and annealing temperature, a PCE of 5.81% was obtained in the champion devices. From the EQE spectrum, one can see that the device showed a good spectrum response in the short and long wavelength regions, which is in accordance with the previous report [41]. Due to the toxicity of the Cd element, using safer salts (such as NaCl, KCl or NH\({}_{4}\)Cl) instead of CdCl\({}_{2}\) during the processing steps is believed to be important for decreasing the Cd contamination during the NC solar cells fabrication [8; 47; 48; 49]. On the other hand, finding a new Cd-free n-type partner is also preferred for CdTe NC solar cells. Recently, solution-processed ZnSe NC was introduced as an n-type partner for CdTe NC solar cells [50]. However, compared to CdTe/CdSe, CdTe/ZnSe shows a lower junction quality and device performance. For the CdTe NC/TiO\({}_{2}\) solar cells, the mismatch in crystalline lattice parameters between CdTe and TiO\({}_{2}\) leads to large interface defects. To solve this problem, a thin layer of CdS NC (\(\sim\)5 nm) is deposited on the TiO\({}_{2}\) film before deposition of the CdTe NC layer, which decreases the interface defects as CdS NC has a similar size and morphology to CdTe NC. The efficiency of CdTe NC solar cells increases to 5.14%, which is significantly higher than that of solar cells without the CdS NC interlayer [51].