## 4 HSC with More Active Layers  
In the case of the BHJ photovoltaic device, the blend mixture includes a conjugated polymer and a semiconductor NC. The carrier recombination between the hybrid layer and the contact electrode is serious, which renders the performance of the HSC. It was confirmed that the self-assembly property of the active solution in the coating process significantly affects the phase separation of the dried hybrid layer, which is important for the carrier departure and charge migrate [51; 52]. As for the polymer/fullerene blend solar cells, it is well known that the donor/acceptor phase separation in the vertical distribution in the polymer/fullerene blend film is also critical to device performance [53; 54]. Therefore, an optimized device architecture for HSCs may consist of multi-film such as donor: acceptor (BHJ)-donor-acceptor structure (D:A-D-A) for optimizing interface engineering. With this strategy, the charge recombination is reduced, which has been demonstrated in small molecule OSCs [55; 56]. For HSCs, [PERSON] et al. [57] designed a device with a configuration of ITO/TiO\({}_{2}\)/CdTe/MEH-PPV:CdTe/MoO\({}_{3}\)/Au based on aqueous-processed CdTe NCs and MEH-PPV. Figures 5a and 5b show the schematic device structure and band alignment of semiconductor NCs and the polymer. The ratio of MEH-PPV to CdTe in the mixture layer was found to affect the nanoscale morphology of hybrid blend film, which then impacts CT and transport. A PCE as high as 4.20% was attained by controlling the ratio of MEH-PPV: NCs in the blend film (the optimized weight ratio for MEH-PPV: CdTe is 1:12) and annealing temperature (the optimized temperature is 315 \({}^{\circ}\)C). Later on, the PCE was further improved to 4.32% when the MEH-PPV was replaced by another polymer P3 HT [58]. To enhance the internal quantum efficiency and extend the range of the depletion field, an HSC with both sides bulk heterojunction (BHJ) (device architecture ITO/TiO\({}_{2}\)/CdTe:TiO\({}_{2}\)/CdTe/PPV:CdTe/MoO\({}_{3}\)/Au) was recently reported by [PERSON] et al. [59]. They found that employing different acceptors with more active  
Figure 5: (**a**) HSC structure and (**b**) band alignment of different layers. EF represents the Fermi level. Reproduced with permission from [57], Copyright © 2016, Royal Society of Chemistry. layers can simultaneously suppress the interface recombination and promote the carrier extraction in the HSC. Benefiting from the advantages of increased carrier collection and lifetime, a PCE up to 6% was obtained in a champion device. Following this, this research group selected ZnO NCs and sol-gel ZnO as an acceptor/ETL to build an HSC with a device architecture of ITO/ZnO/CdTe:ZnO NC/CdTe/PPV:CdTe/MoO\({}_{3}\)/Au [60]. A high PCE of 6.51% was achieved, which was among the highest PCEs already reported for aqueous-processed CdTe NC HSCs. In the field of PbS NC HSCs, the main reason for high device properties in these cases is mainly attributed to the exchange ligands, which reduce the defect density by passivating the bulk and interface states. It is noted that the PbS NCs become insoluble in most organic solvent after ligand-exchange, which prevents the dissolvement of the polymer: PbS NC layer when the PbS NC layer is deposited on the hybrid blend. Therefore, an optimized structure with the configuration of D-D:A-A can be realized for PbS NC HSCs. Inspired by efficient small molecule OSC design [55], [PERSON] et al. [61] developed a D-D:A-A HSC structure of ITO/PEDOT/DDTPBT: NC/NC/LiF/Al. A PCE as high as 5.50% coupled with improved FF was achieved in this HSC, which is mainly owed to the outstanding properties of the PbS\({}_{\rm x}\)Se\({}_{1-\rm x}\) alloy NCs and the optimized device structure for efficient carrier separation and collecting. The band alignment of the polymer and PbS NCs was also found to be critical for charge separation.