However, the quantum size effect cannot be used to explain the increased light absorption of the ITO/nc-TiO2/CdS(5) and ITO/nc-TiO2/CdS(10)/P3HT:PCBM films in near-infrared (NIR) region (wavelength >700 nm) see more because bulk CdS with an absorption onset of 2.42 eV mainly absorbs in the visible region (wavelength from roughly 400 to 700 nm). The increased light absorption of these

films with CdS in the NIR region may be probably due to the electron coupling between the TiO2 and CdS heterostructure [29, 30]. As shown in Figure 1b, the photogenerated electrons can effectively transfer from the conduction band (CB) of CdS to that of TiO2 because of the lower CB level (−4.2 eV) of TiO2 than that (−3.7 eV) of CdS, which may most probably be due to a superposition of the electronic states of TiO2 and CdS. Therefore, an electronic interaction between the TiO2 and CdS exists and makes the bandgap of the TiO2/CdS composite system different from that of TiO2 or CdS. For example, as reported previously by Luo et al. [30], the bandgap of the TiO2/CdS composite system is 2.39 eV, which is even smaller than that of bulk CdS and leads to a weak absorption of the TiO2/CdS film in the NIR region. These results show that the deposited CdS nanoparticles effectively improve

the light absorption of the ITO/nc-TiO2 and ITO/nc-TiO2/P3HT:PCBM films, which is beneficial to the improvement of the performance of the cells. Figure 4 UV–vis absorption

spectrum of the ITO/nc-TiO 2 , ITO/nc-TiO 2 /CdS(5), and ITO/nc-TiO 2 /CdS( n )/P3HT:PCBM films ( AZD3965 n  = 0 and 10). In order to more clearly investigate the influence of CdS QDs on the optoelectronic performance of the prepared solar cells, the I-V characteristics of the ITO/nc-TiO2/CdS(n)/P3HT:PCBM solar cells without the PEDOT:PSS layer under 100-mW/cm2 white light illumination were first measured as shown in Figure 5 (n = 0, 5, 10, and 15). Four factors concerning cell performance: V oc, I sc, fill factor (FF), NADPH-cytochrome-c2 reductase and power conversion efficiency (PCE), extracted from the I-V characteristics are shown in Table 1. It can be found that the PCE of the ITO/nc-TiO2/P3HT:PCBM/Ag cell under white light illumination with an intensity of 100 mW/cm2 is only about 0.15%, which is comparable to the reported PCE value of 0.13% [11]. Moreover, the V oc (0.15 V), I sc (3.81 mA/cm2), and FF (0.27) are also very close to the reported values, i.e., V oc = 0.15 V, I sc = 4 mA/cm2, and FF = 0.27 [11]. Figure 5 I – V characteristics of the ITO/nc-TiO 2 /CdS( n )/P3HT:PCBM devices ( n  = 0, 5, 10, and 15). Table 1 Summary of PV cell performance under white light illumination with an intensity of 100 mW/cm 2 Cells V oc(V) I sc(mA/cm2) PCE (%) FF ITO/nc-TiO2/P3HT:PCBM/Ag 0.15 3.81 0.15 0.27 ITO/nc-TiO2/CdS(5)/P3HT:PCBM/Ag 0.60 5.81 1.57 0.5 ITO/nc-TiO2/CdS(10)/P3HT:PCBM/Ag 0.40 4.93 0.68 0.35 ITO/nc-TiO2/CdS(15)/P3HT:PCBM/Ag 0.33 4.90 0.61 0.