## 3 Improving p-n Junction Quality by Using an n-Type NC Partner  
CdTe can exist easily in both n- and p-type materials by controlling the doping concentration of Cd or Te during thin film formation. However, due to the self-compensation effect, n-type CdTe is not stable; therefore, efficient polycrystalline CdTe thin-film solar cells are generally fabricated with p-n heterojunction configurations to effectively collect photo-carriers from devices in which the diffusion coefficients are small [27; 28]. As confirmed in the literature, CdTe NC thin film remains a weak p-type with a hole concentration of around 10\({}^{15}\)/cm\({}^{3}\)[29]. The first completely solution-processed NC solar cells with a PCE of 2.9 % in a configuration of ITO/CdTe/CdSe/Au were fabricated by employing CdTe and CdSe NCs as donor and acceptor materials [23]. The solar cells were fabricated by depositing one layer of CdTe NC and one layer of CdSe NC on the ITO substrate and annealing at 400 \({}^{\circ}\)C with CdCl\({}_{2}\)/methanol solution. Later on, the effect of the selected electrode or thickness of NC on the device's performance was further investigated and similar device performances were obtained [30; 31]. There are several obvious drawbacks for these NC solar cells. The charge-separating interface is far from the illumination, which results in high recombination, as the carriers must travel almost the whole active layer before they are collected. On the other hand, the CdTe/CdSe NC active layers are over-treated, leading to unexpected carrier recombination due to large defects in the NC thin film. Following this, a layer-by-layer approach was adopted to fabricate CdTe NC/ZnO heterojunction solar cells with a configuration of ITO/CdTe/ZnO/Al [32; 33; 34]. In this process, the n-type partner ZnO thin film is fabricated by depositing Zn\({}^{2+}\) precursor on the CdTe thin film and annealing at a moderate temperature of \(\sim\)300 \({}^{\circ}\)C. The optimized annealing temperature for CdTe NC thin film is \(\sim\)350 \({}^{\circ}\)C. A further increase or decrease in the annealing temperature will deteriorate the performance of the CdTe NC thin film solar cells, resulting in device shunt or inadequate annealing, leading to poor NC grain size. When the ZnO was replaced by In-doped ZnO, a PCE of 8.54% was recorded [29], which was a drastic improvement comparing with previous work. Researchers also found that the PCE can be further elevated to 12% via light/current treatment. Unfortunately, the PCE will degrade in one day due to the recovery of energy obstacles for carrier transfer from CdTe to the ITO electrode. It is found that interdiffusion from the ITO into the CdTe layer at temperature higher than 250 \({}^{\circ}\)C may result in unfavorable electronic doping in CdTe NC film [14; 15; 16]. To overcome this drawback, introducing metal oxide, Au, ZnTeCu or Te interlayers between CdTe NC and ITO will significantly improve stability of CdTe NC solar cells with normal structure [35; 36; 37]. It is noted that the grain size of CdTe NC is around \(\sim\)100 nm after chemical/heat-treatment. On the contrary, the typical n-type partner for CdTe thin film solar cells prepared by the chemical bath deposition (CBD) method, CdS, showed significantly larger grain sizes (several \(\upmu\)m) and poor smoothness when compared with CdTe NC thin film. As shown in Figure 2a, when CdTe NCs are deposited on CBD-CdS thin film, lots of pin holes may be generated due to the mismatch in grain size between CdTe NC and CBD CdS, which reduces the junction quality of CdTe-CdS. On the contrary, a smooth and compact thin film is obtained when  
Figure 1: NC solar cells with inverted structure fabricated by a layer by layer solution process. CdS NC ink is adopted (as shown in Figure 2b), which is important to eliminate interface defects and decrease carrier recombination in the interface.