## 3 Hybrid Bulk Heterojunction Solar Cells Using Low Bandgap Nanocrystals  
HSCs based on polymer: CdSe (or other wide bandgap semiconductors such as ZnO, CdS and TiO\({}_{2}\)[38,39]) NC BHJ have been well explored, and PCEs up to 4% have been obtained by optimizing hybrid layer morphology and ligand engineering. However, HSCs can only absorb visible light and the onset of the spectrum response is mainly determined by donor materials, which restrict HSCs for largely usage of long-wavelength infrared photons. Recently, PbS [40], PbSe [41] and PbS\({}_{\text{x}}\)Se\({}_{\text{1-x}}\)[42,43] alloy NCs have demonstrated potential application in low cost, highly efficient photovoltaic products as they contain special properties, for example, a size-tunable bandgap, high charge carrier mobility, multiple charge generation and solution processable ability. A PCE of 12% for PbS NC bulk homojunction photovoltaics has been certificated recently by surface ligand engineering and an optimized design of device structure [44]. By incorporating PbS or PbSe into HSCs, the efficient use of both short and long (>700 nm) wavelength photons was achieved [45]. In 2010, [PERSON] et al. [46] described new HSCs containing a mixture of colloidal PbS NCs and donor polymers including poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-N-octyldithien[3,2-b:2\({}^{\prime}\),3\({}^{\prime}\)-d] pyrrole) (PDTPQx), PDTPBT and P3 HT. It was found that when the polymer PDTPQx was employed, the PCE was 10-100 times higher than that based on traditional polymers of P3 HT or PDTPBT. It was confirmed that the PDTPQx:PbS NC HSC behaved as the real bulk heterojunction rather than as other junctions such as Schottky diodes. Although the early ligand exchange method (for example, the alkyl acid ligands of PbS NCs were substituted by amine ligands before mixing) can improve HSC performance, the butyl amine ligand still affects the polymer matrix during the film deposition. [PERSON] et al. [47] demonstrated a simple method to improve the performance of HSCs using PbS NCs and PDTPBT as the blends. In this case, the blend film was treated by direct post-deposition ligand exchange using 1,2-ethanedithiol (EDT). The OA ligand was selectively replaced by EDT without affecting the polymer matrix. A PCE as high as 3.78% was achieved after further optimizing the device structure by inserting an ETL TiO\({}_{2}\) (device structure: ITO/PEDOT:PSS/PDTPBT:PbS EDT/TiO\({}_{2}\)/LiF/Al). In truth, the observed interparticle spacing is reduced after the alkyl acid ligand is substituted by EDT or MPA ligands (~4.37 nm for OA ligands and ~3.36 nm  
Figure 4: Performance enhancement in polymer: NC hybrid solar cells (HSCs) upon EDT treatment. (**a**) Schematic structure of polymer: NC HSCs. (**b**) TEM image of CdSe nanorods (scale bar: 20 nm) and chemical structures of conjugated polymers (P3 HT and PCPDTBT) and EDT. The nanorods have an aspect ratio of 7. \(J\)-\(V\) characteristics of (**c**) P3 HT:CdSe and (**d**) PCPDBT:CdSe HSCs with/without EDT treatment. (**e**,**f**) Dependence of illumination power on PCE and \(J_{\text{sc}}\)/\(P_{0}\) for P3 HT:CdSe and PCPDTBT:CdSe HSCs with/without EDT treatment, respectively. Reproduced with permission from [37], Copyright © 2013, American Chemical Society. for EDT or MPA ligands), which has been confirmed in the previous work [48]. The -SH can be tightly bonded to the surface of the PbS NC and replace the OA ligand, which can reduce the distance between neighboring NCs and decrease interface recombination. Moreover, [PERSON] et al. [49] demonstrated that the charge separation efficiency was nearly the same as the organic-organic blends for the PbS-polymer hybrid blends. The low performance of HSCs is greatly affected by worse morphology and more traps in the hybrid blends. Most recently, [PERSON] et al. [50] used iodidatum such as lead iodide (PbI\({}_{2}\)) and ammonium iodide (NH\({}_{4}\)I) coupled with n-butylamine to make ligands exchange for PbS NCs.