## 2 Hybrid BHJ Solar Cells with a Large Bandgap Semiconductor NC as an Acceptor  
The active layer of HSCs is an interpenetrating network structure in the early time, which is similar to OSCs [16]. It is well known that the OSCs consist of a BHJ active layer which includes a polymer as a donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor [17]. The drawback in this structure is the relative low mobility of electron acceptor PCBM that may affect electron transfer and collection in the OSCs. By replacing PCBM with semiconductor NCs as the acceptor, the polymer NC hybrid thin film may combine the merits of high light absorbance of organic polymers and high electron mobility of the stable semiconductor NC, which is promising for achieving inexpensive, stable and high-performance solar cells. In the early work of HSCs, [PERSON] et al., for the first time, reported a novel HSC with a device configuration of ITO/poly (3-hexylthiophene) (P3 HT): CdSe NC/Al by solution processing [18]. Right here, electrons transferred along CdSe nanorods while holes were transported in the polymer. Figure 1a-c shows the structure of regioregular P3 HT, energy diagram of the polymer and NCs and device structure. Photogenerated electrons are transferred from the conduction band of the polymer into the conduction band of the NC while holes from NC to polymer. It was found that the CdSe NC and P3 HT in the blend film have a complementary absorption property in the visible spectrum. In addition, by altering the diameter of CdSe NCs, the onset of the absorption spectrum can be tuned in a large range. By carefully controlling both the diameter and length of CdSe NCs, a PCE of 1.7% was obtained in a champion device. The experimental results indicate that the length of nanorods has a bigger effect on the PCE of HSCs than the diameter of NCs. The onset of the spectrum response was extended up to ~720 nm by improving the diameter of NCs. [PERSON] et al. [19] adopted the ultrafast transient spectroscopy to investigate the charge transfer (CT) dynamics between the polymer and NCs of the hybrid film. Through systematically analyzing the bleaching signal of CdSe NCs, the electron transfer was found to occur in a very fast way (less than 65 fs). The morphology of CdSe NCs has a significant effect on HSC performance. [PERSON] et al. [20] found that the HSC processed from the mixture of multi-armed CdSe NCs and a traditional polymer P3 HT showed enhanced properties than the device from nanorod/polymer blends, which was attributed to the improved electron extraction since CdSe tetrapods with four arms can facilitate the electron transfer vertical to the plane of the active layer. When the polymer P3 HT was replaced by poly(p-phenylenvinylene) derivative (OC1C10-PPV), [21] and a solvent of 1,2,4-trichlorobenzene with a high boiling point (up to 200 \({}^{\circ}\)C), instead of chloroform used to prepare the HSC, a high PCE of 2.8% was obtained. Subsequently, the crystalline structure and the solubility of CdSe NCs were found to greatly affect the HSC performance [22; 23]. [PERSON] et al. [22] developed a new receipt for the fabrication of spherical CdS NCs using cadmium carboxyl as the Cd precursor. From Figure 2a,b, the photoluminescence (PL) intensity of hybrid blends decays linearly with the increase of CdSe NC content, which is irrespective to the CdSe NC synthesis method. The HSC based on MEH-PPV: CdSe NC blend film showed improved efficiency when post-annealing at 180 \({}^{\circ}\)C was carried out (Figure 2c,d). Figure 1: (**a**) The molecular structure of regioregular P3 HT. (**b**) The energy level diagram of CdSe nanorods and P3 HT with a schematic drawing of electron transfer to CdSe and hole transfer to P3 HT. (**c**) The device structure consists of a 200 nm film sandwiched between an aluminum electrode and a transparent hole transfer layer (HTL) of PEDOT:PSS (Bayer AG, Pittsburgh, PA), which was deposited on the indium tin oxide electrode. The active area of the device is 3 mm\({}^{2}\).