# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 5 CdTe/ZnO Heterojunction Solar Cells
It was found that In-doped ZnO devices had better device performance than those of ZnO NC devices due to the high mobility of In-doped ZnO, which improved the carrier collection efficiency (as shown in Figure 6c). To eliminate the energy barrier between ITO and CdTe, the as-prepared devices would undergo a current/light soaking process. This process resulted in a decrease in series resistance and eliminating roll-over in J-V curve. The \(J_{\rm SC}\), \(V_{\rm oc}\), and FF of the best devices were 25.8 mA/cm\({}^{2}\), 0.684 V and 0.71, respectively, resulting in a very high PCE of 12.3%, which was the highest PCE ever reported for CdTe NC solar cells. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) were used to analyze the composition of CdTe devices and films. They found that 0.2% Cu accumulated at the sharp interfaces (Figure 7). In comparison, Cu concentrations of \(5\times 10^{17}\) cm\({}^{-3}\) were observed in the SIMS profiles of highly-efficient CdTe devices. It was really interesting that Cu tended to migrate toward the CdTe/ITO interface, which confirmed the cause of the current/light soaking effect. Figure 6: Schematic (**a**) and cross-sectional SEM (**b**) of a superstrate style device with the following structure: glass/ITO/CdTe/ZnO/Al (false color added for clarity). J-V characteristics (**c**) and external quantum efficiency (**d**) of devices prepared with colloidal nanocrystal-based ZnO (green) and sol-gel In-doped ZnO (In:ZnO; purple). Reproduced with permission from [40]. Copyright 2014, American Chemical Society. Figure 7: Depth profiling using (**a**) X-ray photoelectron spectroscopy (XPS) and (**b**) secondary ion mass spectrometry SIMS. The XPS data and CdCl\({}_{2}\)-treated SIMS samples (panel b, blue and black) were obtained from In:ZnO/CdTe/ITO stacks, while the untreated sample (panel b, open squares/circles) did not have a ZnO layer deposited to rule out In:ZnO film as the Cu source. The SIMS Te counts (circles) are shown as a guide to show the location of the interfaces. Reproduced with permission from [40]. Copyright 2014, American Chemical Society. [PERSON] and co-workers further studied physical, chemical, and optical properties of sintering CdTe/ZnO NCs solar cells [41]. Studies showed that the CdTe grain growth was strongly dependent on NC surface chemistry and chemical environment. Thermal annealing and CdCl\({}_{2}\) treatment were found to be key factors to remove the organic capping ligands and grain growth of CdTe. By carefully designing the treatment of CdTe NCs thin film, solar cells with a PCE up to 7.7% and internal quantum efficiency approaching 95% were obtained. Following this, they used indium-doped ZnO as the acceptor, and a PCE above 8% was obtained. They also found that the device efficiency was up to 9.8% under forward-bias treatment/light soaking. More recently, [PERSON] et al. [42] investigated the effects of different size, shape, and crystal structure of the initial NCs on the CdTe NC solar cells. They found that inks of predominantly wurtzite tetrapod-shaped NCs with arms about 5.6 nm in diameter formed better films than inks composed of smaller tetrapods, irregular faceted NCs, or spherical zinc blende NCs, which was consistent with the previous reports [12]. A smooth and low-defect CdTe NC film can be formed in the case of using spherical CdTe NCs because spherical NCs could assemble with higher packing density and, therefore, more easily convert into continuous films with less void space and larger grains. After optimizing the ink formulation and deposition conditions, [PERSON] et al. found that devices annealed at 350 \({}^{\circ}\)C for 30 s leads to the largest grain growth and highest efficiency of 11.3% [42]. The Shockley-Queisser limit [43; 44] for single and multi-junction solar cells dictates that controlling the bandgap of individual absorbing layers is vital for achieving high efficiency.