## 4 Back Contact for CdTe NC Solar Cells  
It is well known that the CdTe thin film has high resistance due to the difficulty in element doping. Furthermore, the work function of CdTe thin film can be as high as 5.7 eV [50] and no metal exists that has a higher work function to form stable and low resistance contact than CdTe. In order to decrease the electric energy loss and maximize the voltage output, energy level matching between CdTe and the contact electrode is of fundamental importance. In general, the surface of CdTe is etched with nitric and phosphoric acids or Br\({}_{2}\)/methanol solution to generate a Te-rich interface which helps to form low resistance ohmic contacts [52; 53]. However, compared to CdTe thin film fabricated by the vacuum technique, the grain size is small, and a high crystal boundary exists in solution-processed CdTe NC thin film. The acid molecules will move quickly along the crystal boundary and damage the whole NC thin film, leading to device shunt or thin film peeling off the substrate, which has been confirmed in previous work [33]. For a device with normal structure, by using chemical wet etching, the energy obstacle between ITO and CdTe NC film is eliminated and leads to the highest efficiency CdTe solution processed solar cell with a PCE of 12.7% [8; 47; 48; 49]. For devices with inverted structure, the use of dielectric materials as a hole transport layer (HTL) before the deposition of back contact is also preferred to obtain low ohmic contact to CdTe NC. For example, by using high work function MoO\({}_{\rm{x}}\) as the HTL, electrons are blocked while holes can transfer easily from CdTe to the gold electrode, which is essential to reduce carrier recombination on the interface of CdTe NC and to improve the device's performance [25]. Organic hole transport materials can also be used as the HTL for CdTe NC solar cells for low-temperature and low-cost solution processing. [PERSON] et al. first developed p-type Spiro[9H-fluorene-9,9\({}^{\prime}\)-[9H]xanthene]-2,2\({}^{\prime}\),7,7\({}^{\prime}\)-tetramine (spiro-OMeTAD)as an HTL for solution-processed CdTe NC solar cells, and an improvement in open circuit voltage was obtained compared to devices with other HTLs [54]. It was found that the soft base spiro-OMeTAD bonds tightly to the CdTe atoms and forms a dipole layer, which is essential to decrease interface defects in the NC solar cells. The electrical potential between the HTL and the CdTe layer was investigated by Kelvin probe microscopy, and it was found that the dipole moment pointed toward the anode, which reinforces the Vbi (build in field) and facilitates carrier collection. Following this, the commercially available polyelectrolyte poly-[3-(potassium-6-hexanoate) thiophene-2, 5-diyl] (P3 KT) was used as the HTL to improve the performance of aqueous-processed CdTe NC solar cells [55]. P3 KT polyelectrolytes possess suitable energy levels that match well with CdTe, and the S atoms on the polymer backbone can coordinate with the CdTe NCs, which can reduce interfacial recombination and facilitate hole transfer and collection. By further introducing P3 KT/MoOOx double HTLs, a PCE of 5.9% was obtained. It is noted that the Highest Occupied Molecular Orbital (HOMO) level of the polymer mentioned above is ~5.2 eV, which is still lower than that of CdTe NC film (~5.3 eV). To solve this problem, HTL materials with higher HOMO levels are preferred. Triphenylamine-based conjugated polymers are excellent hole transfer materials with a high HOMO level, and they have been widely used in organic solar cells as an HTL [3; 56]. Recently, a new organic hole transport material, poly(diphenylsilane-co-4-vinyl-triphenylamine) (Si-TPA), was used as the hole transport layer (HTL) for CdTe NC solar cells [57]. Its HOMO value is ~5.4 eV, which matches well with the VB of CdTe NC thin film. Studies have shown that Si-TPA can bond to the CdTe NC film by forming N-Cd covalent bonds, and the hole mobility in bulk is enhanced for NC devices.