# Recent Progress on Solution-Processed CdTe Nanocrystals Solar Cells
## 8 Hybrid Solar Cells  
Polymer/NC hybrid solar cells (HSCs) have received much attention due to the low-cost fabrication techniques, low material consumption, and potential application on flexible solar cells [66; 67; 68]. The presence of semiconductor NCs may provide high charge transfer rates as high-mobility inorganic semiconductor materials. Conjugated polymers, such as poly(3-hexylthiophene) (P3 HT) [66], poly (1,4-phenylene vinylene) (PPV) derivatives, or poly [2-methoxy-5-(3,7-dimethyloctyl oxy)]-1,4-phenylenevinylene (MDMO-PPV), etc. [67], are often used as electron donors, while NCs, such as CdSe, CdTe, TiO\({}_{2}\), ZnO, PbS, PbSe, etc., are used as electron collectors. The first notable HSC with a structure of ITO/P3 HT:CdSe/Al was reported by [PERSON] in 2002 and a PCE of 1.7% was obtained in the this case [66]. Following this, many research groups developed new donor materials and acceptor materials, used different device structures, and optimized the phase separation of the hybrid active layer to improve the performance of HSCs. With the development of water-soluble polymer and NC fabrication techniques, a lot of devices based on water-soluble polymer and NCs were fabricated. These solar cells devices are attractive for the environmentally-friendly processing method as using water as solvent. In 2011, [PERSON] et al. [69] fabricated water-soluble HSCs with a structure of ITO/PEDOT:PSS/PPV:CdTe/ZnO/Al and obtained a PCE of 2.14%. They found that annealing temperature, annealing time, and active layer thickness were critical factors for the hybrid solar cells. Subsequently, this research group used an aqueous-solution-processed poly(p-phenylenevinylene) (PPV) precursor as the donor, while 2-mercaptoethanol/hydamine (MA)-stabilized CdTe NCs were used as the acceptor to fabricate an inverted planar heterojunction device and obtained a high PCE of 3.75% [70]. At the same time, they also took advantage of the materials to construct an n-i structure device. They found that before depositing the PPV:CdTe layer, the hydrophilicity of the annealed CdTe layer was improved by spin-coating an aqueous solution of CdCl\({}_{2}\), which greatly increased the FF and \(J_{\rm sc}\) of devices. A record PCE of 4.76% for HSC was achieved in this case. They found that interface modification using TiO\({}_{2}\) as electrons transport layer and MoO\({}_{3}\) as holes transport layer could improve the extraction of free carriers and, thus, lead to an enhancement in the \(V_{\rm oc}\) and FF [71]. To investigate the function of TiO\({}_{2}\) and MoO\({}_{3}\), they fabricated three different devices. Without the TiO\({}_{2}\) layer, the device exhibited a low \(J_{\rm sc}\), \(V_{\rm oc}\), and FF, resulting in a low PCE. The TiO\({}_{2}\) layer provided an electron-selective contact and blocked the holes. PEDOT:PSS was also selected as hole transport layer (HTL ) to replace MoO\({}_{3}\). However, a very pronounced S-shape in the J-V curve was obtained, which reduced the fill factor significantly. The poor performance was a result of the surface tension between PEDOT:PSS and PNV. Following this, [PERSON] [72] and other groups [73] made a number of attempts to improve the device performance. It is worth mentioning that dip-coating gold nanoparticle (NP) films as the back electrode can be used for large-area aqueous-solution-processed HSCs [74]. Subsequently, [PERSON] et al. [75] developed a new water-soluble poly(2,5-thienylene vinylene) (PWTV). In this case, the positive charges on the side chains of the polymer precursor could be easily eliminated after annealing under a nitrogen atmosphere. The CdTe NCs layer was employed to block the holes and increase light absorption. After annealing, effective secondary bonding was established between PWTV and CdTe, resulting in a high PCE of 4.3%. HSCs based on the aqueous-solution-processed P3 HT dots and CdTe (NCs) [76] was also developed. Alloy NCs were also used as acceptors for HSCs.