Figure 3 TEM images of CdTe NT/CdSe QD hybrids. They are prepared by spin coating the hybrid solution on copper net, (a, b, c) without and (d, e, f) with ligand

exchange. Based on the formation of HBH structure, the solar cells were fabricated with the following structure: ITO/CdTe/CdTe: CdSe/ZnO/Al. Firstly, dark I-V characterization was conducted, and the results were shown in semi-log mode in Figure  4a. A smaller dark current at inverse bias and low forward bias is generated in the MPA-treated solar cells. Besides, an increased diode characteristic is also observable from the dark I-V curve in the insert of this website Figure  4a. The corresponding rectifying property is improved due to the enhanced charge collection ability as a consequence of ligand exchange. Figure  4b shows the I-V characteristics of solar cells under 100-mW/cm2 illumination. Improved photovoltaic

performance of NT/QD HBH solar cells is obtained after ligand exchange. A drastic increase in J sc from 1.8 to 3.3 mA/cm2 enables efficiency enhancement from 0.26% to 0.53%. Besides, a slight increase in FF and V oc is also found after MPA treatment of the NT/QD solar Tariquidar concentration cells. Figure 4 Current–voltage characteristics of NT/QD HBH structured solar cells under (a) dark and (b) 100-mW/cm 2 illumination. Data are taken for eight different devices. In order to access the influence of ligand exchange on the performance of NT/QD HBH solar cells, electrochemical impedance spectroscopy (EIS) was used to analyze the dynamic behavior of charge transportation (Figure  5). One semicircle with a frequency variation mainly from 100 to 10 KHz is observed in the Arachidonate 15-lipoxygenase Nyquist plot of each solar cell. This frequency response is correlated with a charge transfer process that occurred at the CdTe/CdSe hybrid interface [15, 16]. Thus, an equivalent circuit with just one parallel component is given in the insert of Figure  5a, in which R s represents the series resistance, R re is the charge transfer recombination resistance,

and C is the capacitance. The Nyquist plot has an enlarged semicircle diameter after ligand exchange, which indicates an increased electron recombination resistance (R ct) [17, 18]. Besides, the effective recombination rate constant (k eff), which is estimated to be equal to the peak frequency (ω max) of this arc [15, 19], is a little smaller in the MPA-treated NT/QD HBH solar cell than that in the OA-capped hybrids. Thus, the electron lifetime (τ) evaluated as τ = 1/2πω max is accordingly increased after MPA treatment. A larger R ct as well as τ value means a smaller leakage current and reduced charge trapping, elucidating the smaller dark current at inverse bias and low forward bias in Figure  4a.