Page - 168 - in Photovoltaic Materials and Electronic Devices

aspect ratios of the NRs were in the range of 76–110 nm, 1.5–5µm, and 20.7–47.9, respectively. Greene et al. indicated that the growing temperature influences the uprightgrowthofZnONRs[16]. Figure 4a depicts the Nyquist plots of the impedance spectra. To characterize the AZO/dye/electrolyte interface, the open-circuit voltage (Voc) levels of the DSSCs were evaluated under AM 1.5 illumination by conducting EIS measurements. The Nyquist plots indicate a small semicircle at high frequencies and a large semicircle at low frequencies. The inset in Figure 4a shows the equivalent circuit. Usually, all the spectra of the DSSCs exhibit three semicircles, whichareascribedtotheelectrochemicalreactionatthePtcounterelectrode,charge transfer at the TiO2/dye/electrolyte, and Warburg diffusion process of I´/I3´, respectively [17,18]. In the present study, the charge transfer resistance at the ZnO/dye/electrolyte interface (Rct2) decreased when the aspect ratio of the ZnO NRs was varied from 20.7 to 47.6. This may be attributable to the increase in the diameter size, length, and quality of ZnO NRs, which led to an increase in the dye adsorption as well as penetration of electron mobility into the pores of the AZO electrode (Figure 4a). The better collected and transported electrons had a lower possibility of recombination, and the electron lifetime was increased [19]. Figure 4b shows Bode phase plots indicating the characteristic frequency peaks (1–104 Hz). Thecharacteristicfrequencypeakshiftedtoalowerfrequencywhentheaspectratio increased, and the characteristic frequency can be considered as the inverse of the electron lifetime (τe) or recombination lifetime (τr) in an AZO film [20,21]. This implies that the NRs with an aspect ratio of 47.6 (grown for 27 h) had the longest electron lifetime in the AZO film. The results indicate that the ZnO NRs, which were grown for 27 h (aspect ratio: 47.6), on the AZO film had a lower transport resistanceandalongerelectronlifetimeintheAZOelectrode. Theelectronlifetimes intheAZOfilmsincreasedfrom3.25to6.12mswhentheaspectratioincreasedfrom 20.7 to 47.6. This result is consistent with the following results obtained from cell performanceandEISanalysis. 168