The as-synthesized nanomaterials with their composites and fabricated devices are characterized by various characterization tools. In order to determine the optical properties of the nanomaterials, ultra-violet visible diffuse reflectance spectroscopy (UV-Vis DRS) is used with 200 to 800 nm wavelength range. X-ray diffractometer (XRD) is used to analyse crystallite size and phase purity. Morphology of the ZnO NRs is determined by using SEM images. After fabrication of sandwich solar cell devices, the photovoltaic measurement is carried out under solar simulator.
3.1 Optical Absorption Study
The optical properties of the bare ZnO and CdS sensitized ZnO NRs are analysed by UV–Vis DRS as shown in Fig. 1. From DRS spectra, it’s clear that ZnO NRs shows absorption spectra at 390 nm and obtained band-gap value is 3.17 eV. For bare ZnO case, the spectral curve does not display any significant deviation. However, ZnO:CdS thin film shows the absorption edge is higher than that of bare ZnO indicating the influence of CdS QDsas displayed in Fig. 1aof the binary composite [24]. The DRS spectra of ternary nanocomposites shows broad visible region, due to N719 dye (Fig. 1b) [25]. Therefore, the existence of N719 affects optical properties of the ternary nanocomposites that is responsible for the red shift in absorption spectrum. This improved absorption in the visible wavelength region results in the generation of abundant electron–hole pairs in the ZnO:CdS and ZnO:N719 based device under UV-visible illumination, which can lead to greater photo-current activity.
3.2 FTIR analysis
Figure 2represents the FTIR patternsfor bare MPA, MPA capped CdS QDs and bare ZnO NRs as well as CdS QDs sensitized ZnO NRs. For MPA (Fig. 2a) the bands observed at ~ 3400 − 3000 cm− 1,~2948 cm− 1,~2570 cm− 1, ~ 1704 cm− 1 and ~ 1402 cm− 1 are due to stretching vibrations of the functional moieties such as -OH, -CH2, S-H, C = O and C-O, respectively. In addition, a small band value at ~ 2670 cm− 1 is due to bending vibration of -OH. However, in FTIR pattern of MPA capped CdS QDs (Fig. 2b), the stretching band of carboxylic O-H group is disappeared due to its deprotonation; while the peak position of C = O group is shifted from 1704 to 1545 cm− 1 [26]. In addition, the stretching band of S-H is also not observed because of it’s high pKa value; which reveals that the interaction of sulfhydryl moieties of MPA with Cd2+ ions and hence, the co-ordination between the different moieties avoids the deprotonation of the sulfhydryl group of MPA [27].
Similarly, in FTIR spectra of bare ZnO NRs, the broad absorption band at ~ 3400 − 3000 cm− 1and ~ 1400–1500 cm− 1 are due to O–H stretching and deformation of C–OH groups of water molecules. Zn–O bond stretching vibrations appears at 690 cm− 1. However, ZnO:CdS composites also shows O–H stretching vibrations in the range of 3000–3500 cm− 1 range, while Zn–O bond ranges between 500 and 600 cm− 1. The peak at 1392 cm− 1 in the binary composite is assigned to C–O bending vibration. From the FTIR spectra of ZnO:CdS composite, the composites have the characteristic peak of oxygen containing functional groups particularly at 1704 cm− 1 are weakened and the O–H stretching peak decreases. This is mainly attributed to the loss of oxygen containing functional groups. Overall, present study reveals that, surface of ZnO NRs are covered through CdS QDs.
3.3 XRD analysis of ZnO: CdS thin film
X-ray diffraction (XRD) technique is used to determine crystallinity, phase of the ZnO NRs, CdS QDs and their composites. Typical XRD pattern of the synthesized ZnO NRs, ZnO:CdS composites are shown in Fig. 3. The various reflections are appeared at 2θ values of 31.86, 34.58, 36.32, 47.63, 56.71, 62.86, 68.08, 69.26 and 76.88, with corresponding reflection of (100), (002), (101), (102), (110), (103), (112), (201) and (202), respectively. All these peaks are indexed and these are well matches with Wurtzite (hexagonal) structure of ZnO (JCPDS card no. 36-1451) [28]. Furthermore, absence of any characteristic impurity peaks indicates formation of high quality ZnO NRs.
Interestingly, ZnO:CdS composites shows an additional peak at 27.42°, 44.86° which is due to presence of (111) and (220) crystal plane of CdS QDs on ZnO NRs; which clearly indicate that the proper depositions of CdS QDs take place on surface of ZnO NRs. The crystallite size of bare ZnO NRs and ZnO:CdS composites determined from the Debye Scherrer’s equation and is shown in Table 3.1. Moreover, after sensitization; the crystallite size of ZnO:CdS is decreases.
Table 3.1
Structural parameter of ZnO:CdS thin film.
| Sample | (hkl) | d value | Parameters | Crystal size (nm) | Dominance crystal structure |
| a (Å) | c (Å) | V(Å3) |
| Bare ZnO | 101 | 2.471 | 3.23 | 5.18 | 54.04 | 9.34 | Wurtzite hexagonal |
| 002 | 2.590 |
| 110 | 1.621 |
| Bare CdS | 111 | 2.38 | 5.85 | - | 200.20 | 1.89 | Cubic |
| 220 | 2.07 |
| 311 | 1.76 |
| ZnO:CdS | 101 | 2.482 | 3.21 | 5.19 | 54.57 | 9.39 | Wurtzite hexagonal |
| 002 | 2.585 |
| 110 | 1.602 |
3.4 Scanning electron microscopy
The surface morphology of all pristine materials as well as the binary (ZnO-CdS) nanocomposites was carried out using SEM technique, as depicted in Fig. 4. ZnO shows NRs shaped morphology without sensitization of CdS QDs as shown in Fig. 4a. It is observed that the surfaces of the bare ZnO NRs appear clear and soft. Furthermore, SEM images of CdS QDs deposited on ZnO surfaces are shown in Fig. 4b, that appears rough and tarnished surfaces, which indicates that deposition of the CdS QDs on the surface of ZnO NRs. Randomly scattered spherical particles with uniform size-distribution of CdS QDs are good resolved on the ZnO surface helps for the maximum anchoring of N719 dye molecule.