3.3 Optical absorbance and bandgap

Figure 6 displays typical optical absorption spectra for the as-synthesized samples UZ and DZ NPs. It is clear that the samples have a very high absorbance in the UV region. They decrease exponentially with increasing of the wavelength in the visible region. An enhance photon absorption can be noted in the visible region in

Figure 6. UV–Vis spectra of UZ and DZ NPs.

Nanoplasmonic for Solar Energy Conversion Devices DOI: http://dx.doi.org/10.5772/intechopen.84953

the presence of Ag NPs. Furthermore, the highest absorption band can be observed over the range of 350–359 nm, which is ascribed to the band-to-band transition of the ZnO NPs [24]. This exciton absorption peak is being less than of the bulk ZnO (388 nm) which indicates the monodispersing of ZnO NPs as suggested by Brintha et al. [25]. The edge of allowed direct optical bandgap energy (Eg) of UZ and DZ NPs was determined using the Tauc Davis and Mott Equation [26]:

$$\left(h\nu\right)^{2} = \mathbb{S}\left(h\nu - E\_{\text{g}}\right) \tag{3}$$

where S is constant, α is the absorption coefficient of the material, and hv (eV) = 1239.7∕λ (nm) is the photon energy. Eg was estimated by plotting (αhv)2 versus hv as shown in Figure 7. Eg of UZ and DZ NPs were estimated by extrapolating the linear region to the abscissa, i.e., (αhv)2 = 0, yielding Eg = 3.21 to 3.31 eV. The estimated energy bandgaps agreed well with the obtained results in Türkylmaz et al. [27]. It can be seen that the addition of Ag NPs decreased the Eg of ZnO (the inset of Figure 7). This implies that the presence of Ag NPs has downshifted the Fermi level of ZnO toward the valence band [1]. As a result, the driving force for electron injection from dye-excited state to ZnO conduction band is increased [1].

Figure 7. (αhν)2 versus photon energy (hν), the inset is the estimated values of Eg.

#### 3.4 Photoluminescence study

It is well-known that the PL technique is one of the best optical methods for probing electron transitions between high- and low-energy levels as well as for finding electron densities of states [28]. On one hand, it can harness in the material science to identify defects and traps in materials, as well as to estimate the bandgaps of materials. Typically, the PL emission (PLE) spectrum of ZnO NPs exhibits a linear emission in the near ultraviolet (NUV) range (originating from exciton mechanism) and in the visible region (nonradioactive recombination) that is frequently originated from several, intrinsic, and extrinsic defects of ZnO NPs [29]. Different defects or interior energy traps (IETs) can be found within the bandgap of the ZnO NPs, such as vacancies, impurities, imperfections, interstitials, Zn residues, and antisites, mainly from various oxygen vacancies [30, 31]. It is worthy to mention that these IETs play a crucial role in various applications such as the performance of the DSSCs. Therefore, it is hard to recognize the IETs that may be produced, simultaneously, during the synthesization method [29]. Figure 8

#### Figure 8.

PLE spectrum and deconvoluted peaks of UZ NPs was excited at 320 nm.

demonstrates the room temperature PLE spectrum of the as-synthesized UZ, which was registered at excitation wavelength of 320 nm at room temperature. The deconvolution of the PLE spectrum recognizes eight peaks (I to VII) as shown in Figure 8. The main features of the PLE spectrum of UZ NPs can be resolved into two divisions: NUV and visible. For the PLE spectrum (peak I) of NUV region or the near band edge (NBE) transition, it is designated to the radiative recombination of free excitons through an exciton-exciton collision process [32]. The intensity of such NBE excitonic emission depends on the size of the exciton binding energy of ZnO [33]. Different kinds of IETs are supposed to elucidate the observed transition in the visible range. The transition at peak II is allocated to the movement of the excited electrons from the donor (bottom) of conduction band to the acceptor energy level of Zn vacancy (VZn), which concludes that the ZnO NPs are n-type semiconductors [34], whereas peak III is produced by the transition of electrons from conduction band edge to the IETs complex (VZnO) [28], while peak IV is created by the transition of electrons from interstitial of (Zni) to the IETs complex (VZnO). However, the peak V is originated from transition of electrons from conduction band edge to the IETs complex OZn. The transition at peak VI is ascribed to deep IETs (a shallow surface defect level) of electrons from conduction band edge to the IETs complex vacancies of oxygen (VO). Ultimately, peak VII is attributed to the transition of electrons from interstitial of (Zni) to the IET complex (VO). The center, width, and area of the fitted peaks for the PLE spectrum and the corresponding energy for each peak are tabulated in Table 2. As shown in this table, the emission of the visible region is overshadowing NBE transition. The percentage areas of the NBE and the IETs transitions are an indicator of the quality of ZnO (i.e., the ratio of the defects in the ZnO). This is very imperative to facilitate the usage of the ZnO such as DSSCs; a high ratio of defects (IETs transition) is required. This indicates that the defects in ZnO NPs are large, which is deduced from the XRD analysis. Generally, FWHM of intrinsic emission in PL spectrum is related to the crystal quality. A small FWHM indicates that the crystal is of high quality, and its large value suggests that the crystal is imperfect, showing it may have point defects. Meanwhile, at FWHM, the values for both samples are nearly close to each other [35]. The schematic energy band diagram of the PLE spectrum constructed from the data is illustrated in Figure 9. The comparative room temperature PLE spectra (excitation at 320 nm) of DZ:1.0 mol% Ag NPs are shown in Figure 10. PLE spectrum of DZ:1.0 mol% Ag NPs is used to verify the quality crystal and possible

## Nanoplasmonic for Solar Energy Conversion Devices DOI: http://dx.doi.org/10.5772/intechopen.84953


#### Table 2.

Center, energy, width, height, and area of the measured PLE spectrum of UZ NPs.

Figure 9. Schematic energy band diagram from PLES spectra.

Figure 10.

PLE spectrum for UZ and DZ:1.0 Mol% Ag NPs was excited at 320 nm.

effects of DZ:1.0 mol% Ag NPs [36]. It is worthy to mention that the intensity and the position of the PLE spectrum DZ:1.0 mol% Ag NPs are dependent on different factors such as the concentration of the noble metal dopants, temperature,

compressive strain, tensile strain, and shape of the particle. It is reported that the decrease in UV intensity was due to the interactions between the excited ZnO NPs and Ag NPs in the grain boundaries, which created a large amount of defects, as confirmed [37]. These kinds of interactions via the Schottky contact, metal–semiconductor diode effect decrease the recombination of electrons and holes generated from UV light irradiation and improved the photocatalytic activities [37]. It could be seen from the PLE spectra that the UV emission in DZ:1.0 mol% Ag NPs decreases which indicates the decrease in electron–hole recombination. This decrease in emission intensity is in accordance with the Stern-Volmer quenching, and similar results were previously reported [35]. The intensity of visible emission initially decreases for 0.5% nominal silver doping and then increases for higher Ag concentrations. Such anomalous intensity variation in Ag-doped ZnO NPs can be understood considering the Ag incorporation process in the NPs. The Ag+ ions can be incorporated into the ZnO NPs in two different ways: substituting Zn+2 ions creating doubly ionized oxygen vacancies VO or incorporating as interstitials (Agi) [36]. In addition, PL intensity decreases with increasing amount of Ag doping, indicating that the core of Ag NPs captures the excited electrons before they recombine, causing the so-called electron-sink effect. This causes the accumulation of electrons on the Ag NPs, which improves electron life time and can thus reduce charge recombination, ultimately, improving electron transport [6]. While for low doping concentrations, e.g., 0.5%, most of the incorporated Ag<sup>+</sup> ions might occupy ZnO lattice through Zn2+ ion substitution; for higher concentrations, the excess amount of Ag + ions is incorporated into the NPs interstitially, creating larger amount of lattice defects [36]. By increasing Ag concentration, the intensity of defect decreased. The reduction of defects with Ag doping represented a decline in the defects and improved crystallization [37], which agreed well with XRD results. Compared to the PLE spectra of UZ NPs and DZ:1.0 mol% Ag NPs, it reveals a redshift (longer wavelength) which is consistent with previous observations [38, 39]. This shift was due to the variation of the energy bandgap of UZ NPs [40], which agree with the UV–Vis results. The dopant Ag has a great effect on the separation and recombination process of photo-induced charge carriers of ZnO, which can further effect on PL performance [41]. It indicates that the PLE mechanism of Agdoped/ZnO is very complex, and further research is needed.

### 3.5 Photovoltaic performance

The radiant light can be enhanced after being effectively coupled with plasmon absorption due to the increase of the optical density around Ag NPs. According to that, there were more photons, which were usually absorbed by the dye molecules placed in the vicinity of Ag NPs, resulting in the improvement of the device performance [23]. The system so constructed therefore provides a more efficient charge-transfer process with enhanced light absorption, thus leading to a better overall performance of Ag NPs photoanode [11]. The performance of the UZ and DZ NPs photoanodes that were sensitized by EY dye was scanned from 0 to 0.6 V. The characteristic photocurrent density-voltage (J-V) curves of different fabricated DSSCs devices are plotted in Figure 11. The calculated values of the photovoltaic performance parameters (efficiency (η), fill factor (FF), maximum power (Pm), maximum voltage (Vm), maximum current density (Jm), open-circuit voltage (VOC), and short circuit photocurrent density (JSC)) for all fabricated DSSC devices are plotted in Figure 12. The cell with the doped photoanode DZ:1.0 mol% Ag exhibited the highest η and best performance, which has the following performance parameters: Voc = 0.46 V, Jsc = 7.81 mA.cm<sup>2</sup> , Pm = 1.91, FF = 51%, and η = 1.91% (Figure 12). Through small additions of metal nanostructures (<2 wt %), the active Nanoplasmonic for Solar Energy Conversion Devices DOI: http://dx.doi.org/10.5772/intechopen.84953

Figure 11. J-V behavior for DSSCs based on UZ and DZ NPs photoanode sensitized with EY.

material required to achieve high efficiency solar conversion can be drastically reduced [5]. However, when the concentration was increased further, the photocurrent decreased, presumably because the Ag NPs act as recombination sites of excited electrons [14]. Consequently, to maximize the plasmonic effect in DSSCs, the optimal concentration of the plasmonic structure was decided to be 0.5 wt% [14]. Previously, it is reported that plasmon-enhanced dye excitation by an electromagnetic field decreased as the distance between dye molecules and plasmonic nanoparticles increased [14]. The enhanced Jsc is related to the light-harvesting capability of dye molecules by plasmon-enhanced excitation [14]. Decrease in Jsc and photon-to-current efficiency beyond 1.0 mol% Ag NPs can also result in a loss of absorbing dye volume due to the large volume of Ag NPs.

In addition, aggregation of Ag NPs may also occur resulting in electron trapping [1]. This process would diminish the electron density, thus decreasing Jsc [1]. This is probably due to plasmonic-induced photocurrents, in that the hot electrons driven from the plasmon-induced charge separation of Ag NPs can be injected into the conduction band of ZnO, resulting in plasmon-assisted photocurrent generation [14]. The improved Jsc is resulted from higher plasmon resonance effect caused by coverage of Ag NPs on the ZnO NPs, which increases the number of photoelectrons generated and improves the effective separation of electron–hole pairs. Therefore, the loaded Ag nanoparticles can improve Jsc rather than the Voc in these DSSCs. Interestingly, the efficiency of DSSC based on Ag-modified ZnO NPs array decreases again. Such agglomeration of Ag NPs on ZnO NPs results in less surface area for dye loading as well as ineffective light absorption and charge separation [10]. It can be concluded that the improved Jsc may be attributed to not only the increase in the dye excitation by the effects of NPL associated with near-field enhancement and scattering but also to the generation of additional photocurrent owing to the NPL-induced direct hot-electron transfer from plasmonic structures to MO conduction bands [1]. For the higher concentrations of Ag NPs, it may cluster to form larger Ag NPs groups with lower electron storage capability, reducing Voc [15]. Consequently, the probability of electrons and holes to recombine may increase, so Jsc and Voc would decrease. Therefore, some of the Ag NPs in the AgZnO network structure may be corroded by electrolyte and oxidized to Ag + ions [1, 15]. This can also increase the recombination rate that could lead to a reduction in electron density in the conduction band of ZnO [1]. This diminishes the number of charge carriers, thereby acting as recombination centers that could lead to a

Figure 12. Photovoltaic parameters of UZ and DZ NPs for the DSSCs sensitized with EY dye (a) η%, (b) FF%, (c) Pm, (d) Vm, (e) Jm, (f) VOC, and (g) JSC.

reduction in electron concentration in the conduction band of ZnO, thus resulting in a decrease in Jsc and Voc [1, 15]. There are contradictory reports on the increase or decrease in the Voc value induced by the presence of metal NPs in the photoanode [14]. Furthermore, in the case of a further decrease in Voc as well as FF was observed, it may imply that exposed Ag NPs can act as recombination centers [14]. Voc is typically in proportion to the difference between the quasi-Fermi level and the Nernst potential of the redox couple. Thus, the slight decrease in Voc can be explained by the downshifted Fermi level due to the incorporation of plasmonic particles [6, 13, 14]. The FF of the fabricated cells changes from 27 to 51% for 5% Ag and 1% Ag, respectively. Generally, the low values of the current density may be attributed to the anchoring group missing in the chemical structure of EY dye. These groups make a strong link between ZnO semiconductor surface and the dye molecule, in order to enhance electron injection from the LUMO energy level of the dye molecule to the conduction band of the semiconductor layer [42]. Nevertheless, Nanoplasmonic for Solar Energy Conversion Devices DOI: http://dx.doi.org/10.5772/intechopen.84953

Figure 13. Power density for DSSCs based on UZ and DZ NPs photoanode sensitized with EY.

metallic NPs have been reported to exhibit electron-sink (or photo charging) effect [6]. It was predicted that due to the electron-sink effect of the Ag NPs, a reduction in charge recombination and improved charge transport are evidenced in improving FF value with increasing plasmonic NP doping. Thus, this would account for the slight increase in Jsc. Figure 13 presents the power density for DSSCs based on UZ and DZ NPs photoanode sensitized with EY.

#### 3.6 Transient open-circuit voltage decay measurement

Agreeing to the proposed simple model of the DSSCs by Ref. [43] at short circuit, different charge-transition processes can be found in the DSSCs devices, in which the slower recombination process is preferable. In addition, the charge can present more than one kind of motion status at different voltage-dependent regions. Among them, the exponential increase region relates to internal trapping of the photoanode material, which reveals the apparent electron lifetime or response time (τn) or the electron recombination rate (krec) in the photoanode material. The transient open-circuit photovoltage decay (TOCVD) is a suitable method to study τn and krec in DSSC, which can provide some quantitative information on the electron recombination velocity in DSSC [44]. In order to conduct the TOCVD measurement, the DSSCs devices, based on UZ and DZ 1.0 mol% Ag NPs, were illuminated for 2 min to be equilibrium.

Between electron injection and electron recombination at the FTO surface, i.e., a steady-state voltage was obtained and Voc was recorded after that, the subsequent decay of photovoltage during the light source was turned off. The decay of the photovoltage reflects the decrease of the electron concentration at the FTO surface, which is primarily produced by the charge recombination. In other words, the recombination velocity of photoelectron is proportional to the response of the TOCVD [44]. Figure 14 depicts TOCPVD experiment for the DSSCs devices, which follow a pseudoexponential (semilog) form. Two decayed components are distinguished, fast and slow decays, which may be ascribed to their intrinsic material properties [45]. This, also, may be attributed to the different defects of the ZnO NPs that were proved in the PL results earlier. A double exponential function was used as a powerful fitting function for the TOCVD data as depicted in Eq. (4) to extract the decay parameters [24]:

$$\mathbf{V\_{OC}} = \mathbf{V\_0} + \mathbf{V\_1} \mathbf{e^{-\frac{(t-t\_0)}{\tau\_1}}} + \mathbf{V\_2} \mathbf{e^{-\frac{(t-t\_0)}{\tau\_2}}} \tag{4}$$

where V0 is the offset of the VOC. The amplitudes of VOC for fast and slow decay are V1 and V2, respectively; it is the decay time and is the center of the fitting curve. The decay time constant (lifetime) for fast curve and slow curve is τ<sup>1</sup> and τ2, respectively. For the right-hand side of Eq. (4), it describes the fast and slow decays, respectively, the second term for the fast decay and the third term for the slow decay [24]. All extracted decay parameters of the TOCPV are listed in Table 3. As it was reported in Ref. [24], the krec is determined by

$$\mathbf{k}\_{\rm rec} = \left[ \frac{\mathbf{V\_1}}{\tau\_1} \mathbf{e^{-\frac{(t-t\_0)}{\tau\_1}}} + \frac{\mathbf{V\_2}}{\tau\_2} \mathbf{e^{-\frac{(t-t\_0)}{\tau\_2}}} \right] (V\_T)^{-1} \tag{5}$$

where the thermal voltage (VT = KBT∕q), with KB, is Boltzmann constant, T is the absolute temperature, and q is the elementary charge, using the following Equation [36]

$$
\pi\_n = -V\_T \left(\frac{dV\_{as}}{dt}\right)^{-1} \tag{6}
$$

Meanwhile, the recombination rate of the device based on DZ:1.0 mol% Ag NPs photoanode is slower than that of UZ NPs photoanode. The results revealed that the τn in the UZ NPs photoanode-based DSSC was longer than that in the DZ:1.0 mol% Ag NPs photoanode-based DSSCs, demonstrating that the latter possessed a higher

Figure 14.

TOCPVD of the DSSCs cells is the sensitized EY based on UZ and DZ 1.0 Mol% Ag NPs, and the red line is the double exponential decay fitted curve.


Table 3.

The parameters of the TOCPVD.

surface trap density [45] that might result from the larger surface area. The higher recombination rate and shorter electron lifetime within ZnO NPs photoanode were the reason why the FF of the corresponding DSSCs decreased.
