# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 4 Schottky Junction Solar Cells  
In this device structure, a solar cell is created from the Schottky junction between CdTe and a metal. Metal oxide with high work function is used as back contact to collect hole while aluminum (Al) or other metal with low work function is used as front contact to collect electrons. In 2010, [PERSON] et al., for the first time, reported a Schottky junction solar cell fabricated by using CdTe NCs thin film as an active layer [7]. The NC thin films were fabricated by spin-casting CdTe nanorods onto clean indium-tin-oxide (ITO) coated substrates, after sintering and solvent treatment, Al contacts were evaporated onto the films. The back electrical (Al or Ca/Al) [32] contact was a low work function metal, which could form a rectifying junction with the CdTe NCs active layer. The thickness of CdTe NC layer is around 360 nm, about 10% of the material used in more established CdTe deposition techniques [33], which is less than the minority carrier diffusion length [34]. The optimal device had power conversion efficiency (PCE) of 5.0%. Comparing to alky-phosphine acid-capped CdTe NCs, fatty acid (such as oleic acid)-capped CdTe NCs [35] have many advantages, such as low cost, low fabrication temperature, and fatty acid is easily removed during the ligands' exchange process, which improves purity of the final NCs products. [PERSON] et al. [12] synthesized CdTe NCs by this method and fabricated solar cells with the Schottky structure of ITO/CdTe/Al. The best device showed a PCE of 5.15% under AM 1.5 G illumination. They found that a very smooth surface was formed when using NCs with an appropriate aspect ratio.  
It should be noted that the CdTe NC Schottky solar cells reported above have low \(V_{\text{oc}}\) (less than 0.6 V) due to the Fermi-level pinning, low fill factor (below 60%), and short device lifetime, which is mainly caused by the energy barrier between ITO and CdTe NCs. The work function of ITO and CdTe are 4.7 and 5.3 eV, respectively, which means that contact is not ohmic. In order to overcome this energy barrier, high work function metal or metal oxide should be introduced between ITO and the CdTe NCs thin film. [PERSON] et al. [36] introduced a novel strategy to produce a low-resistance contact to p-CdTe by using Au, C\({}_{60}\), or MoO\({}_{x}\) as the buffer layer between the ITO and CdTe active layer. PCE of 5.3% was obtained in an optimized device with 1 nm Au as interlayer between ITO and CdTe. Comparing to device without interlayer materials, the 1 nm Au modified devices show higher \(V_{\text{oc}}\) (0.65 V) and high FF (64%). Other interlayer materials, such as C\({}_{60}\) or MoO\({}_{x}\) were investigated and higher \(V_{\text{oc}}\) and FF were also obtained in this case comparing to those devices without interlayer materials. Figure 2a,b show the device structure and energy diagram of the CdTe NCs solar cells. By introducing interlayer materials (Figure 2d), the contact between ITO and CdTe is ohmic, while energy band bending occurs in the case without interlayer materials (Figure 2c). Figure 3a shows the J-V characteristic for the best CdTe NCs Schottky solar cell with 1 nm Au as interlayer materials. Figure 3b shows the Mott-Schottky plots C\({}^{-2}\)-V. The flat-band potentials are 0.65 V and 0.50 V for devices with 1 nm Au interlayer, or without, which agrees well with the J-V measurement.