Nanocrystal (NC) solar cells have received extensive attention in the last several years due to their many advantages—such as low cost, environmental friendliness, solution process, simple device technics, and compatible roll-to-roll manufacturing [
1,
2,
3,
4,
5,
6,
7,
8]. The most efficient NC based solar cells nowadays are fabricated by using CdTe or PbS NC as donors with wide bandgap semiconductor materials as acceptors [
9,
10,
11,
12,
13]. Compared to PbS NC, CdTe NC is less complex and more stable in air, which permits devices to be fabricated under ambient conditions. Thus, CdTe NC thin film has been studied intensively [
14,
15,
16,
17,
18,
19,
20]. The working principle of efficient CdTe NC solar cells is based on the p-n heterojunction that contains two media: an electron donor and electron acceptor. To prepare a high quality CdTe NC donor absorber, a layer by layer sintering process should be carried out to eliminate stress and defects in CdTe NC thin film [
21,
22]. The electron acceptor materials—typically ZnO, CdSe or other n-type semiconductor thin films—are thus the key to improve the device performance. The most efficient CdTe NC–ZnO solar cells are fabricated by using a normal structure of ITO/CdTe/ZnO/Al [
10]. In this device architecture, Zn
2+ precursor is deposited on the CdCl
2 treated CdTe NC thin film and annealed at a moderate temperature of 300 °C, which enables high quality junction formation and avoids large current leaks. However, efforts try to duplicate this device with an inverted structure of ITO/ZnO/CdTe/Au failed due to the poor junction quality in the ZnO–CdTe interface, which had been confirmed in our previous work [
23]. It is noted that there are many merits for solar cells with inverted structure, such as a charge separating interface close to the illumination and usage of metal or metal oxide with high work function as a hole-collecting electrode, which endows device long lifetime. Solution processed CdTe NC based solar cells with inverted structure of ITO/CdSe/CdTe/Au were reported for the first time by Towsend et al. [
19]. Although as high as 3.8% PCE was obtained in the above devices, they suffered from low
Voc (< 0.5 V) value, much lower than that (up to 0.8 V) of CdTe/CdS devices prepared by close space sublimation (CSS) method [
24]. With Cr/Au as hole collecting electrode, the
Voc of devices with the structure of ITO/CdSe/CdTe/Cr/Au can be further increased to 0.62 V [
20]. By using ZnO as the interlayer, devices with an architecture of ITO/ZnO/CdSe/CdTe/Au obtain a high
Voc of up to 0.6 V in conjunction with a high PCE of ~6% reported in our recent work [
25,
26]. Recently, Yang et al. [
27,
28] developed an in situ route to construct CdTe–CdS NC bulk heterostructure solar cells by direct thermal treatment of mercaptoethylamine stabilized CdTe NC. In this device structure, the formation of n-CdS shells on CdTe NC eliminated the recombination of carrier and improved the device performance. Recently, [
29] introduced p type spiro-OMeTAD as the hole transport layer between CdTe NC thin film and Au electrode, as high as 0.71 V of
Voc was obtained, which was the highest
Voc value ever reported for solution-processed CdTe NC based solar cells. Another way to improve the
Voc of CdTe NC based solar cells is to engineer the bandgap structure of the photo-generated electron-accepting materials. As previously reported [
12], the trap state density of CdTe was 10
14 cm
−3, high doped density of n-type materials was necessary to fully deplete the CdTe NC film in order to increase the carriers’ separating and collecting efficiencies. It had been found that, by using n type doped TiO
2 as the electron acceptor, the performance of PbS colloidal quantum dot/TiO
2 heterojunction solar cells can be tailored and improved substantially [
30]. Bulk hetero-junction structure was built as porous structure formed during the decomposition of Ti-sol, which improved carrier collecting efficiency. Herein, we introduce solution-processed CdTe NC/TiO
2 hetero-junction solar cells consisting of ZnO film, thermal decomposition Sb-doped TiO
2 film, and solution processed CdTe NC thin film. The carrier separating/collecting efficiency of solar cells can be improved by introducing Sb-doped in TiO
2 as the electron acceptor. It is found that
Voc up to 0.7 V can be obtained with a Sb-doped TiO
2 electrode and as high as 0.74 V is obtained in the case of 8% Sb-doped TiO
2 devices, which is the highest
Voc ever reported for solution-processed CdTe NC based solar cells.