## 2 Hybrid BHJ Solar Cells with a Large Bandgap Semiconductor NC as an Acceptor
This film was spin-cast from a pyridine chloroform solution of 90 wt% CdSe nanorods in P3 HT. Reproduced with permission from [18], Copyright © 2002, The American Association for the Advancement of Science. In most cases, CdSe NCs are prepared by the solvothermal method. Long-chain organic ligands such as alkyl acid, alkyl phosphoric acid or trioctylphosphine oxide (TOPO) are used to stabilize the NCs and prevent their aggregation [24; 25; 26; 27; 28]. However, the side chains of polymers (usually alkyl chains) cannot cap on the surface of NCs, which makes the separation of the organic/inorganic phase uncontrollable in the hybrid active layer and hinders the transfer of charges between the NC and polymer and thus reduces device performance. To address this problem, [PERSON] et al. [29] fabricated P3 HT with different functional groups that can easily control the morphology of polymer/CdSe NCs. Figure 3a shows the synthesis process of P3 HT with a -NH2 functional group. When blended with an NC acceptor, the amino end group of the P3 HT donor caps on the surface of the NC and forms intimate nanocomposites with a favorable morphology. The HSC shows a significantly higher PCE than the control device (Figure 3b). More importantly, the highest efficiency occurred at lower concentrations of CdSe NCs, which is mainly determined by the high degree of homogeneity of hybrid blend layers. Figure 2: Photoluminescence intensity of MEH-PPV: CdSe nanocrystal (NC) blends as a function of the weight fraction of CdSe NCs under excitation at 500 nm: (**a**) CdSe NCs prepared by Se powder, (**b**) CdSe NCs prepared by trioctylphosphine/Se injection. Current density–voltage (_J–V_) characteristics of a MEH-PPV: CdSe NC device with the weight ratio of 1:8 (**c**) without and (**d**) with annealing at 180 \({}^{\circ}\)C for 20 min. The dotted line was measured in the dark and the solid line under 1.5 AM solar illumination. The insets are the _J–V_ curves with current density (_J_) expressed in logarithmic coordinates. Reproduced with permission from [22], Copyright © 2006, IOP Publishing. An alternative way to improve the quality of the HSC active layer is to eliminate the insulated ligands (such as oleic acid) on the CdSe NC, which will reduce the interface recombination between the NC and polymer, and improve the carriers' collecting ratio. In recent years, solution-processed PbS colloidal quantum dot (CQD) solar cells have attracted much attention and excellent achievement has been obtained (with PCEs exceeding 12%), which is mainly attributed to the ligand chemistry engineering. The insulated ligands oleic acid (OA) can be replaced by 1,2-ethanedithiol (EDT), mercaptopropionic acid (MPA), NH\({}_{4}\)I, etc. [30, 31, 32, 33]. Inspired by these fruitful works, [PERSON] et al. [34] demonstrated a facile method to fabricate nanoscale morphology of P3 HT nanowires (NWs) and CdS NC blends adopting the ligand exchange method and solvent-assisted chemical grafting. It was found that an interpenetrating BHJ network was formed in the blends, which increased the carriers' separating and collecting ratios, leading to efficient PCE. The HSC with a configuration of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/P3 HT NW:CdS/bathocuproine (BCP)/Mg:Ag delivered an energy conversion efficiency exceeding 4%. It should be noted that P3 HT or PPV was widely used as the donor material for HSCs in early works [19, 22, 29]. Unfortunately, the onset of the spectrum response for these materials (with bandgaps exceeding 1.9 eV) was below 650 nm, which significantly reduced photon absorption in the longer wavelengths and thus rendered the short-circuit current density (\(J_{\mathrm{sc}}\)) of HSCs. Poly [2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiazole)](PCPDTBT), a conjugated polymer with a low bandgap, has been widely used in OSCs [35]. PCPDTBT used as a donor in HSCs should expand the spectrum responsibility into the near-infrared field and thus high \(J_{\mathrm{sc}}\) is expected.