# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 5 CdTe/ZnO Heterojunction Solar Cells  
Due to the existence of defects on the interface, Fermi-level pinning generally occurs in the case of CdTe Schottky solar cells, which results in devices with low efficiency and device lifetime. CdTe solar cells with a p-n junction configuration are generally adopted in order to effectively collect photo-induced carriers. CdTe/ZnO heterojunction solar cells with CdTe as the donor and ZnO as the acceptor, is one simple p-n junction solar cell. In order to eliminate the inner stress formed by CdTe NC film with large thickness, [PERSON] et al. [8] developed a new layer-by-layer sintering approach to prepare CdTe NC thin film. As shown in Figure 4, one thin layer CdTe film was prepared by spin-casting CdTe NC solution onto the ITO substrate, then it was annealed at moderate temperature (150 \(\sim\) 350 \({}^{\circ}\)C) and chemical treatment, which alleviated stresses and defects formed within subsequent layers. This process was repeated several times to obtain a desired thickness (\(\sim\)400 nm). CdTe NC thin film prepared by this mean showed to be more compact and had few defects. ZnO film was  
Figure 3: **(a)** J-V curve of the CdTe solar cells with a 1 nm Au interlayer (solid lines represent the simulation results, while open circles represent the experiment results); and **(b)** Mott-Schottky curves in dark conditions measured at a constant frequency of 1000 Hz for the CdTe NC solar cell device without an Au interlayer. Reproduced with permission from [37]. Copyright 2010, IOP Publishing. Figure 2: A schematic image of the CdTe nanocrystals (NCs) solar cells: **(a)** the device’s structure; **(b)** the energy diagram; **(c)** the band diagram without an interlayer; and **(d)** the band diagram with an interlayer. Reproduced with permission from [36]. Copyright 2014, IOP Publishing. then deposited onto the CdTe NCs film by solution processing. Ultraviolet irradiation of ZnO films was carried out for the desorption of surface-bound oxygen or carbon dioxide which, in turn, created highly-doped n-type ZnO [37; 38]. They finally obtained a champion device with a PCE of 6.9% and up to 90% internal quantum efficiency. From the capacitance-voltage (C-V) curves, they concluded that the CdTe/ZnO system operated not like a p-n junction, but more like an i-n junction due to full depletion of the CdTe layer (Figure 5). As discussed above, the contact between ITO and CdTe is not ohmic, which will decrease the device performance, especially the \(V_{\mathrm{oc}}\) of the device. More recently, [PERSON] and co-workers [39; 40] adopted this layer-by-layer technique to fabricate CdTe NC solar cells with a structure of ITO/CdTe/ZnO/Al. Figure 4: A general schematic of the layer-by-layer process for developing sintered inorganic thin-film solar cells from colloidal nanocrystals. Reproduced with permission from [8]. Copyright 2011, American Chemical Society. (**A**) Synthesis of colloidal Nanocrystals. (**B**) Deposition of thin-films. (**C**) Chemical and thermal treatment. (**D**) Deposition of top metal electrode. (**E**) Electronic structure. Figure 5: (**a**) Schematic representations of partially- and fully-depleted abrupt heterojunctions based on the device architecture explored in this work. (**b**) The spatial and wavelength-dependent optical field density that has been normalized by the incoming field and the simulated exciton generation rate within the CdTe layer of an optimized device with 400 nm CdTe and 60 nm ZnO layers. The exciton generation rate is simulated with the standard AM 1.5 100 mW/cm\({}^{2}\) irradiance as the incoming light source. Reproduced with permission from [8]. Copyright 2011, American Chemical Society. To find out the effect of ZnO on the performance of solar cell devices, they prepared ZnO thin film by using a ZnO NC solution or a sol-gel In-doped ZnO. They obtained a certified PCE of 8.54% with In-doped ZnO thin film.