Fabrication of Bilayer ZnO Based Hybrid Photoanode for Enhanced Photovoltaic Performance in CdS
5.3 RESULTS AND DISCUSSIONS
5.3.5 PHOTOLUMINESCENCE ANALYSES OF THE MATERIALS
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Figure. 5.3.8 Diffused reflectance spectra of the ZnO NW (red line) ZnO HMSP (violet line), ZnO HMSP and ZnO NW (olive line) films on glass substrates.
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Figure 5.3.9 (A) Steady-state PL spectra for the photoanodic materials ZnO HMSP–CdS (black line), ZnO NWCdS (blue line) and ZnO NWZnO HMSP–CdS (olive line) composite on glass slides. (B)Steady-state PL spectra of pure CdS NPs, ZnO NWs and ZnO HMSPs. All the spectra are recorded at an excitation wavelength of 410 nm.
From the figure 5.3.9 (A), we have observed a significant quenching in the PL intensity for ZnO NWZnO HMSPCdS [trace (c)] as compared to ZnO HMSPCdS [trace (a)] and ZnO NWCdS
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Wavelength (nm)
(c) (b) (a)
Normal iz ed Inte ns ity
(a) ZnO HMSPCdS (b) ZnO NWCdS
(c) ZnO NWZnO HMSPCdS
(A)
(B)
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Nornal iz ed Inte ns ity
Wavelength (nm)
CdS ZnO NW ZnO HMSP
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[trace (b)], indicating the inhibited recombination of photoinduced electron and hole and a possible excited state electronic interaction between ZnO HMSP–CdS layer and ZnO NWs in the photoanode. A possible reason for the inhibited recombination of photoinduced electron and hole in the photoanode could be attributed to an efficient charge migration to 1D ZnO NWs from ZnO HMSPCdS layer. Efficient charge migration or separation effect is also explicit in the photoanode ZnO NWCdS [trace (b)] because of a faster photogenerated electron migration along the 1D single crystalline ZnO NWs which is not in case of ZnO HMSPCdS photoanode [trace (a)]. Please note that,although we have performed the PL experiments in order to excite only the CdS QDs (rather than ZnO) at an excitation wavelength of 410 nm, the effect of defect mediated visible light emission by ZnO cannot be ruled out completely. In the fluorescence spectrum of ZnO NWCdS [trace (b); figure 5.3.9 (A)] the origin of the peak below 450 nm (i.e., blue emission) could be from transitions involving Zn interstitial defect states (Zni, Shallow donor) to the valance band levels while the shoulder in the range 450500 nm (green emission) observed is may be due to transition from the conduction band to deep trap levels of ZnO (created by oxygen vacancies).40 For better understanding we have carried out PL analysis of pure CdS, ZnO NW and ZnO HMSP excited at a wavelength of 410 nm as shown in figure 5.3.9 (B). From figure (B) it is clear that the emission peaks for all the photoanodes in the range spanning from 500-650 nm are originated from CdS QDs deposited on the photoanodes and not from ZnO. ZnO NWs (red line) and ZnO HMSPs (blue line) exhibit a defect mediated emission peaks at a wavelength ~448 nm with satellite peaks in the range of 455 nm to 480 nm.
Further, we have deconvoluted the PL emission spectra of the hybrid photoanode, i.e., trace (c) in figure 5.3.9 (A), as it is very broad with a clear indication of the overlap of two or more peaks. Figure 5.3.10 represents the deconvoluted steady-state PL spectra of the photoanode ZnO NW-ZnO HMSP-CdS. The strong PL emission at ~550 nm exhibits the band gap emission of CdS QDs arising from radiative excitonic recombination. Another broad emission band occurs at 605 nm, which is most likely due to the recombination of trapped carriers generated by defect states.41
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Figure 5.3.10 Deconvoluted Steady-state photoluminescence spectra of the photoanode ZnO NW-ZnO HMSP-CdS.
Dynamic PL spectra for all the samples excited at a wavelength 405 nm are recorded to confirm the enhanced charge separation in the photoanode ZnO NW–ZnO HMSP–CdS as shown in the figure 5.3.11. The PL decay traces of all the samples are fitted with a bi-exponential function to calculate the exciton lifetime. The exciton lifetimes (1 and 2) as well as average lifetime (<>) for the samples are tabulated in the table 5.3.2. The average lifetime values are calculated by using the expression in equation (1).39
< 𝜏 >= 𝛼1 𝜏12+ 𝛼1 𝜏22
𝛼1 𝜏1+ 𝛼1 𝜏2 (1)
It is well known that photoexcited CdS QDs exhibit electron (e)hole (h) pair separation followed by charge carrier recombination via radiative or non-radiative processes.42 When CdS QDs are anchored to ZnO surface the photoexcited CdS QDs, which are capable of injecting electrons into the conduction band level of ZnO via the delocalization of electron wave function because of the lower conduction band offset of ZnO, yields a substantial decrease in exciton lifetime of the system.
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550 nm
605 nm
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Wavelength (nm)
ZnO NW-ZnO HMSP-CdS Deconvoluted Peak 1 Deconvoluted Peak 2
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Figure 5.3.11 Time resolved PL spectra of (a) ZnO HMSP–CdS (black line), (b) ZnO NW-CdS (blue line) and (c) ZnO NW–ZnO HMSP–CdS (olive line) recorded at an excitation wavelength of 405 nm.
Table 5.3.2 values of fitting parameter (2), exciton lifetimes (1, 2), pre-exponential factors (1, 2) and average exciton lifetimes (<>).
Sample 2 1 (ns) 2 (ns) 1 2 <> (ns)
(a)ZnO HMSP–CdS 1.03 1.03 9.1 19.5 80.5 8.88
(b)ZnO NW-CdS 1.05 0.96 8.02 24.8 75.2 7.75
(c)ZnO NW–ZnO HMSP–CdS 1.04 0.99 5.54 44.0 55.9 4.97
From the table 5.3.2, faster exciton average lifetime for ZnO NWCdS (7.75 nm) than ZnO HMSPCdS (8.88 ns) indicates a stronger electronic interaction between ZnO NWs and CdS QDs and a favorable photogenerated charge migration along the 1D ZnO NWs.43 A significant decrease in average exciton lifetime for ZnO NW–ZnO HMSP–CdS (4.97 ns) has been observed as compared to ZnO HMSP–CdS (8.88 ns). The static quenching in PL lifetime confirms the favorable excited state electronic interaction between ZnO NWs and ZnO HMSPCdS layer and participation of ZnO NWs in efficient and spontaneous collection of photogenerated electrons from the ZnO HMSPCdS layer.
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Count s
Time (ns)
(a) ZnO HMSPCdS (b) ZnO NWCdS
(c) ZnO NWZnO HMSPCdS
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