# Solution-Processed CdTe Thin-Film Solar Cells Using ZnSe Nanocrystal as a Buffer Layer
## 1 Introduction  
Solution-processed thin-film solar cells based on nanocrystals (NC), perovskites, or organic light absorbers have many merits such as low cost, easy large-area manufacturing, flexibility and high efficiency, which make them competitive when compared with traditional crystal Si or thin-film solar cells based on vacuum technics [1; 2; 3; 4; 5; 6; 7]. Among different kinds of NCs, CdTe NC solar cells have attracted greater attention in recent years [8; 9; 10; 11]. CdTe possesses an idea bandgap of \(\sim\)1.45 eV and a large absorption coefficient (>10\({}^{4}\)/cm in the visible region), which are promising with regard to achieving impressive power conversion efficiency (PCE). Nowadays, CdTe thin-film solar cells with efficiency >22% has been realized by optimizing the fabricating techniques, while other thin-film solar cells such as CuIn\({}_{x}\)Ga\({}_{(1-x)}\)Se\({}_{2}\) (CIGS) or perovskite with efficiency up to 23% are also realized, which is prospective for low-cost and efficient solar cell products [12]. By contrast, with the development of NC thin film treatment technics and device structure design, the PCE of solution-processed CdTeNC solar cells has been elevated to \(\sim\)12% with a normal structure of ITO/CdTe/In:ZnO/Al [13; 14; 15], which shows potential compared with amorphous Si solar cells. However, in most cases, in order to obtain a PCE up to 10%, a light/voltage treatment must be taken out before the device measurement in order to eliminate the energy barrier between the ITO and the CdTe active layer. Therefore, device stability is a vital issue in this case. It has been recently reported that stable and efficient CdTe NC/ZnO solar cells with a configuration of ITO/ZnTe/CdTe/In:ZnO/Al can be attained by inserting a thin layer of ZnTeCu, etched CdTe:Cu or Te buffer layer between CdTe and ITO [16]. For CdTe NC solar cells with an inverted structure, satisfactory ohmic contact between the CdTe and the metal can be realized by adopting metals and metal oxides as well as an organic hole transport layer for collecting holes [17]. The most successful CdTe NC solar cells with inverted structures using CdSe, CdS, or TiO\({}_{2}\) buffer layers have been well developed. [PERSON] et al. developed CdTe NC solar cells by using chemical bath deposition (CBD) CdS [18] as the electron acceptors. A PCE as high as 6.66% is obtained in this case. It has also been found that the substitution of CdS NC for CBD-CdS can improve the quality of the CdTe/CdS hetero-junction and suppress the dark current [19]. Following this, CdTe/CdSe NC solar cells with inverted structures such as ITO/CdSe/CdTe/Au were developed and improved performance was demonstrated [20]. Later, the CdTe:CdSe NC hybrid layer and the graded-bandgap structure designs were introduced into the CdTe/CdSe NC solar cells. Additionally, PCEs of 6.25% and 6.34% were achieved in device structures of ITO/ZnO/CdSe/CdSe:CdTe/CdTe/Au and ITO/ZnO/CdSe/CdSe\({}_{\mathrm{x}}\)Te\({}_{\mathrm{1-x}}\)/CdTe/MoOx/Au, respectively [21; 22]. More recently, a novel cross-linkable conjugated polymer has been applied as the hole transport layer (HTL) for CdTe NC solar cells and a very high PCE of 8.34% obtained with the structure of ITO/ZnO/CdSe/CdTe/HTL/Au due to the reduced recombination in the back contact and the interface dipole effect [23]. It is noted that the incorporation of CdSe (with thickness below \(\sim\)100 nm) is helpful in enhancing the short-circuit current due to the easy formation of CdSeTe alloy with low bandgap and the reduced interface recombination. The formation of CdSeTe alloy will increase the external quantum efficiency spectrum (EQE) response both in the short wavelength and long wavelength regions, which has been confirmed in the literature and our previous reports [22; 24; 25]. However, CdSe thickness up to 100 nm will result in low device performance due to incomplete formation of CdSeTe alloy. TiO\({}_{2}\) is also found to be a good n type partner for CdTe NC solar cells. [PERSON] et al.