## 5 HSCs Using Organic Materials as the HTL  
For NC solar cells, the carrier recombination between the active layer and the contact electrode is serious, which renders the weak collection of carriers. For example, the CdTe thin film has a high work function (up to 5.3 eV) due to its self-compensation effects [64], which makes it difficult to attain ohmic contact between CdTe NC thin film and a metal electrode. There are several ways to obtain ohmic contact for CdTe-based solar cells with inverted structure. Among which, introducing a low cost and stable HTL for reducing carriers recombining is preferred. It is well known that MoO\({}_{\rm x}\)[65], V\({}_{2}\)O\({}_{5}\)[66], NiO [67], WO\({}_{3}\)[68] and other metal-organic dielectric materials [69; 70] are widely used as HTLs in CdTe thin film solar cells and improved performance is usually obtained. However, researchers have found that these metal oxide materials are not stable when exposed in ambient conditions and the energy levels are difficult to be changed, which is not preferred for commercial application. On the contrary, there are many advantages for organic hole transport materials with a high HOMO level, such as being stable, low energy-consuming and they can be fabricated by a solution process. Recently, [PERSON] [71] et al. developed PEDOT: PSS as an HTL between CdTe thin film and a contact electrode, and the device showed improved efficiency compared to the control devices. Spiro-OMeTAD(2,2,7,7-tetrakis(N,N-di-4-methoxyphenylamino)-9,9-spirobituorene), as an important hole transfer material, has been applied successfully in perovskite solar cells, which is also preferred for NC solar cells as the high work function (-5.2 eV) [72]. Based on potential change results presented by Kelvin probe microscopy between Spiro and the CdTe NC film, it was found that a dipole field is formed between CdTe NC and HTL, which strengthens the build-in electric field and increases the NC device's performance. However, like many metallic oxides, PEDOT: PSS or Spiro-OMeTAD is not stable under wet environments. [PERSON] [73] and coworkers, for the first time, incorporated a crosslinkable conjugated polymer as an HTL for decreasing carrier recombination in the interface of the NC thin film and contact metal. Compared to other organic HTLs, crosslinkable conjugated polymers are very stable after the crosslinking reaction, and can be adhered to the NC surface by forming N-Cd bonds. Moreover, this material has high carrier mobility and easy tunability of energy levels by molecular design. In this case, a poly(diphenylsilane-co-4-vinyltriphenylamine) HTL is deposited on the CdTe film and forms a Cd-N covalent bond, which reduces the interface trap state and carrier recombination. More importantly, a dipole layer is formed between CdTe NC film and the HTL, which facilitates holes' transport with a small barrier and improves carriers' collecting efficiency. As high as 8.34% PCE has been obtained in NC solar cells with a Si-TPA interlayer, which is significantly higher than the controlled device or device with PEDOT:PSS or PVK as an HTL. In addition, it was demonstrated that CdTe solar cells with CdSe as the n type partner may be subjected to low \(V_{\text{oc}}\) owing to a large electron injection barrier and interface recombination in metal oxide/CdSe\({}_{\text{x}}\)Te\({}_{\text{1-x}}\) alloys [74]. By incorporating a CdS/CdSe NC double ETL and TPA HTL, the interface recombination was decreased and high \(V_{\text{oc}}\) expected [75]. The device structure and preparing process are presented in Figure 6. These two aspects' optimization led to reduced interface trap-assisted recombination both in the ZnO/CdS and CdTe/contact electrode. As shown in Figure 7, a very high PCE of 9.2% was attained by simultaneously optimizing the ETL and HTL of the HSC. Besides, it is noted that the EQE spectrum of the optimized device showed a high EQE value from 400 to 900 nm, which is significantly higher than other control devices.