## 6 HSCs with Tandem Structure  
With the rapid development of molecular and device design, up to 18% efficiency has been realized in single junction OSCs [76]. However, the further improvement of single junction solar cell performance is limited, and thus double, triple or more junction solar cells should be carried out to improve the HSC performance [77; 78; 79]. For solar cells with tandem structures, broader spectrum photons can be harvested after using different bandgapsvactive materials. Organic and some NCs such as PbS have similar fabricating technology, and low-cost, light-weight solar modules can be realized using the same coating method. Compared to organic thin film, PbS NC layers benefit from a tunable bandgap by adjusting the size of PbS quantum dots, which is appropriate for tandem cell design. In 2014, [PERSON] and coworkers [80], for the first time, reported a new tandem HSC employing PbS NCs as the front sub-cell while a P3 HT: PCBM blend was the bottom cell (device structure: ITO/PbS/WO\({}_{3}\)/Al/P3 HT:PCBM/Al), which allowed the usage of sunlight from ultraviolet to near-infrared. However, only a 1.8% PCE was attained for this device, which shows also lower performance than the PbS NC or P3 HT:PCBM solar cells. The main reason for low efficiency originates from the interface recombination and inadequate fabricating parameter optimization for each single junction. Later on, [PERSON] et al. [81] developed a different device design consisting of PbS NCs as the bottom cell and a polymer (with large bandgap): fullerene blend as the top cell (device structure: ITO/TiO\({}_{2}\)/PbS NC/MoO\({}_{3}\)/ZnO/PFN/polymer:fullerene/MoO\({}_{\text{x}}\)/Ag). The HSC-based tandem structure delivered a high \(V_{\text{oc}}\) (1.3 V) and PCE greater than 5% after optimizing the tunnel junction layer and individual organic/inorganic cells. The effective connection of two single sub-cells in hybrid tandem solar cells is very important for decreasing interface recombination and internal losses. It was found that when MoO\({}_{\text{x}}\)/Au/ZnO was selected as the interface recombination layer in the hybrid tandem cell with a device architecture of ITO/AZO/PbS/MoO\({}_{3}\)/Au/PFN/PCBM:PDPP3T/MoO\({}_{3}\)/Ag, an 8.3% PCE was obtained after optimizing the optical and electrical properties of each sub-cell [82]. However, in the previous reports, one noteworthy limitation was the inaccurate arrangement of the PbS NC (with low bandgap) layer as the front cell, which limited the light harvested and prevented the tandem cells to create more photocurrent. The main reason for this structure design is the incompatibility of the PbS NC solution to organic bulk heterojunction layer [83; 84]. Secondly, in order to improve the PbS NC film quality, ligand (such as MPA or EDT) exchange must be taken out to eliminate trap states, which cause additional damage of underlayers [85]. To overcome these challenges, PbS NC ink utilizes hexane and the deposition of PbS NCs is simplified by eliminating the many steps required for PbS NC layer deposition. Figure 8a presents the device structure of an HSC with a tandem configuration. The organic PTB7:PCBM blend device acts as the front cell while PbS NCs act as the bottom cell. Figure 8b shows the band alignment of the hybrid tandem solar cell. Here, an ultrathin (0.5 nm) Au film was inserted between the AZO and MoO\({}_{\text{x}}\) layer, as the effective recombination centers to decrease the collection of photo-generated carriers in the active layers. After optimizing device fabricating parameters, the tandem solar cell reached a high PCE of 9.4%. Most recently, [PERSON] et al. [86] presented a hybrid tandem cell in which organic BHJ worked as the back-cell collected the NIR photons while the CQD layer absorbed the visible light photons. This device with a configuration of ITO/ZnO/PbS/EDT-PbS/Au/ZnO/PTBT-Th:IEICO-4F/MoO\({}_{3}\)/Ag reinforced the photon-to-current conversion from visible to NIR wavelengths (from 350 to 1000 nm).