**3. ZnO nanostructures for dye-sensitized solar cells**

DSCs were prepared by immersing the area-defined ZnO NW specimens into a solution of 0.5 mM cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II)bis-tetrabutylammonium (N719, Solaronix) in acetonitrile/*tert*butanol (1:1) for 20 minutes. After rinsing with acetonitrile and natural drying, the sensitized electrodes were sandwiched together with thermally platinized FTO counter electrodes separated by 25-μm-thick hot-melt spacers (Surlyn, Dupont). The electrolyte solution (0.1 M LiI, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide, 0.03 M I2, and 0.5 M *tert*-butylpyridine in acetonitrile) was filled into the internal space.

The comparison of photocurrent-voltage (J-V) characteristics for solar cells which was constructed using the bare ZnO NWs and the branched ZnO NWs with AM 1.5 illumination at 100 mW/cm<sup>2</sup> from a xenon lamp was shown in **Figure 11**. The overall light conversion efficiency and short-circuit current density (*Jsc*) of the branched ZnO nanowire DSCs were 1.51% and 4.27 mA/ cm<sup>2</sup> , respectively, which are almost twice higher than that of the bare ZnO NWs. Increased photon absorption is associated with an increase in internal surface area, resulting in enough dye loading as a major factor in the increase in short-circuit current density. Although the density of ZnO structures exhibits the insignificancy compared with some previous studies, the shortage can be complemented via the extra branches. The values of fill factor (*FF*) for ZnO DSCs are generally low (~0.5) which is attributed to recombination between photoexcited carriers and triiodide ions in the photoanodes and electrolyte, respectively. No obvious difference of the shunt resistance *Rsh = (dV/dI)V = 0* from the J-V curves under illumination revealed almost the same interfacial recombination; however, the series resistance *Rs = (dV/ dI)I = 0* for branched ZnO nanowire DSCs (25.64 Ωcm<sup>2</sup> ) was significantly lower than the bare ZnO nanowire ones (46.13 Ωcm<sup>2</sup> ).

#### **Figure 11.**

*Current density against voltage (J-V) characteristics of the bare ZnO nanowires and the branched ZnO nanowire DSCs [4].*

**49**

**Figure 12.**

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

Some interior parameters of DSCs can be analyzed by the impedance data of the Nyquist plots. The modified equivalent circuit of nanowire DSCs was suggested by Wu et al. [39, 40]. **Figure 12** shows the impedance data for bare and branched ZnO nanowire DSCs performed by applying a 10 mV ac signal over the frequency

transport resistance of the electrons in the ZnO electrode and the charge-transfer resistance of the charge recombination between electrons in the ZnO electrode and

<sup>−</sup> in the electrolyte, respectively. The thickness *LF* of all anodes are about 8 μm; *Cμ = (cμLF)* is the chemical capacitance of the ZnO electrode; *Rs* is a summary resistance for the transport resistance of FTO and other resistances out of the cell; *ZN* is

capacitance and the charge-transfer resistance at the counter electrode (platinized FTO glass)/electrolyte interface, respectively; *CFTO* and *RFTO* are the interfacial

*Nyquist plots of the bare ZnO nanowires and the branched ZnO nanowire DSCs. The solid lines are the fitting* 

*results based on the equivalent circuit model (modified from ref. [41] as shown in the inset) [4].*

Hz under illumination at the applied bias of *Voc*. *rw* and *rk* are the

<sup>−</sup> in the electrolyte; *CPt* and *RPt* are the interfacial

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

range of 10<sup>−</sup><sup>2</sup>

I3

–105

the impedance of diffusion of I3

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

Some interior parameters of DSCs can be analyzed by the impedance data of the Nyquist plots. The modified equivalent circuit of nanowire DSCs was suggested by Wu et al. [39, 40]. **Figure 12** shows the impedance data for bare and branched ZnO nanowire DSCs performed by applying a 10 mV ac signal over the frequency range of 10<sup>−</sup><sup>2</sup> –105 Hz under illumination at the applied bias of *Voc*. *rw* and *rk* are the transport resistance of the electrons in the ZnO electrode and the charge-transfer resistance of the charge recombination between electrons in the ZnO electrode and I3 <sup>−</sup> in the electrolyte, respectively. The thickness *LF* of all anodes are about 8 μm; *Cμ = (cμLF)* is the chemical capacitance of the ZnO electrode; *Rs* is a summary resistance for the transport resistance of FTO and other resistances out of the cell; *ZN* is the impedance of diffusion of I3 <sup>−</sup> in the electrolyte; *CPt* and *RPt* are the interfacial capacitance and the charge-transfer resistance at the counter electrode (platinized FTO glass)/electrolyte interface, respectively; *CFTO* and *RFTO* are the interfacial

#### **Figure 12.**

*Nyquist plots of the bare ZnO nanowires and the branched ZnO nanowire DSCs. The solid lines are the fitting results based on the equivalent circuit model (modified from ref. [41] as shown in the inset) [4].*

*Nanostructures*

internal space.

cm<sup>2</sup>

**3. ZnO nanostructures for dye-sensitized solar cells**

NWs with AM 1.5 illumination at 100 mW/cm<sup>2</sup>

ZnO nanowire DSCs (25.64 Ωcm<sup>2</sup>

).

nanowire ones (46.13 Ωcm<sup>2</sup>

DSCs were prepared by immersing the area-defined ZnO NW specimens into a solution of 0.5 mM cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II)bis-tetrabutylammonium (N719, Solaronix) in acetonitrile/*tert*butanol (1:1) for 20 minutes. After rinsing with acetonitrile and natural drying, the sensitized electrodes were sandwiched together with thermally platinized FTO counter electrodes separated by 25-μm-thick hot-melt spacers (Surlyn, Dupont). The electrolyte solution (0.1 M LiI, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide, 0.03 M I2, and 0.5 M *tert*-butylpyridine in acetonitrile) was filled into the

The comparison of photocurrent-voltage (J-V) characteristics for solar cells which was constructed using the bare ZnO NWs and the branched ZnO

in **Figure 11**. The overall light conversion efficiency and short-circuit current density (*Jsc*) of the branched ZnO nanowire DSCs were 1.51% and 4.27 mA/

, respectively, which are almost twice higher than that of the bare ZnO NWs. Increased photon absorption is associated with an increase in internal surface area, resulting in enough dye loading as a major factor in the increase in short-circuit current density. Although the density of ZnO structures exhibits the insignificancy compared with some previous studies, the shortage can be complemented via the extra branches. The values of fill factor (*FF*) for ZnO DSCs are generally low (~0.5) which is attributed to recombination between photoexcited carriers and triiodide ions in the photoanodes and electrolyte, respectively. No obvious difference of the shunt resistance *Rsh = (dV/dI)V = 0* from the J-V curves under illumination revealed almost the same interfacial recombination; however, the series resistance *Rs = (dV/ dI)I = 0* for branched

from a xenon lamp was shown

) was significantly lower than the bare ZnO

**48**

**Figure 11.**

*nanowire DSCs [4].*

*Current density against voltage (J-V) characteristics of the bare ZnO nanowires and the branched ZnO* 

capacitance and the charge-transfer resistance at the exposed FTO/electrolyte interface, respectively; *CFZ* and *RFZ* are the capacitance and resistance at the FTO/ ZnO contact, respectively.

The specific equivalent circuit might be more complicated while operating the DSCs. Mora-Seró et al. have reported the conductivity modulation of the electrolyte-induced negative capacitance which comes from by injected electrons from the photoelectrode while providing high forward bias with low frequency [42]. In order to avoid the unnecessary interference from the inductor, the lowfrequency part of impedance data was currently ignored. The fitted results of the first-order reaction rate constant for the loss of electrons(*keff*), the electron lifetime (*τ = 1/keff*), the electron transport resistance (*Rw = rwLF*), and the charge-transfer resistance related to recombination of an electron at the ZnO/electrolyte interface (*Rk = rk/LF*) were listed in **Table 2**. *Rk* and *Rw* are quite similar for both DSCs which meant the same crystallinity and interfacial recombination for either bare ZnO NWs or branched ZnO nanostructures. *keff* in the branched ZnO nanowire DSCs was smaller than the bare nanowire ones to cause the smaller effective diffusion length [43] (*Deff = (Rk/Rw)LF 2 keff*) in branched ZnO nanowire DSCs. But the electron lifetime (*τeff = 1/keff*) was prolonged by the additional transport distance of branched ZnO nanowire DSCs.

The current density of a DSC is determined by the amount of photogenerated carriers, the electron injection efficiency from the dye molecules to the semiconductor, and the recombination rate between the injected electrons and the oxidative dye or redox species in the electrolyte. The initial amount of photogenerated carriers may have a significant effect on the light-harvesting ability of photoanodes of different structures. **Figure 13** displays the comparison of incident monochromatic photon to current conversion efficiency (IPCE). The peaks at approximately 400 nm were due to direct light harvesting by ZnO semiconductor. The photogenerated electrons diffused through ZnO and the holes in the valence band were replenished directly by charge transfer from the I3 <sup>−</sup>/I<sup>−</sup>electrolyte [44]. The maximum peak at approximately 525 nm is contributed by the dye absorption, corresponding to the visible *t2* → *π\** metal-to-ligand charge transfer (MLCT). The IPCE obtained for the branched ZnO nanowire DSCs was almost 1.5 times that of the bare ones. This improvement is primarily due to sufficient dye loading of the branched ZnO NWs, which increases the internal surface area within the photoelectrode. From the dye loading measurement, as shown in **Figure 14**, the concentration of dye in the branched ZnO nanowire electrode was found to be 2.9 × 10<sup>−</sup><sup>9</sup> mol/cm<sup>−</sup><sup>2</sup> as measured from dye-desorption experiments, which is almost 40% higher than the obtained value of 2.1 × 10<sup>−</sup><sup>9</sup> mol/cm<sup>−</sup><sup>2</sup> for the bare ZnO nanowire electrode.

It is worth noting that even though the current density and energy conversion efficiency of branched ZnO DSCs is twice that of bare ZnO ones, the dye loading


#### **Table 2.**

*Performances and electron transport properties of the bare ZnO nanowire and the branched ZnO nanowire DSCs determined by photocurrent density-voltage (J-V) characteristics and electrochemical impedance spectroscopy (EIS) analysis [4].*

**51**

**Figure 13.**

**Figure 14.**

*the branched ZnO nanowire DSCs [4].*

*and dissolved in 0.1 M NaOH solution [4].*

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

*The incident monochromatic photon to current conversion efficiency (IPCE) of the bare ZnO nanowire and* 

of branched ZnO DSCs is not twice that of bare ZnO ones. The differences may be that the internal surface of the bare ZnO NWs is insufficient and the excess dye results in the formation of Zn2+/dye complexes rather than an effective chemical bond between the ZnO and dye molecules. To avoid the excessive reaction, Chou et al. [45] reported the shorter immersion time for deficient chemical stability ZnO electrode as compared with TiO2 electrode. Therefore, the excess immersion time of the insufficient internal surface of bare ZnO NWs cannot achieve more dye loading but seriously deteriorates the performance of the DSCs. Although the secondary branches have a nonuniform distribution due to the simple dip-coating process, the branches emitted from a portion of the ZnO NWs could still provide a greater effective surface area for dye adsorption than the bare NWs. The dc or radio frequency (RF) magnetron sputtering or atomic layer deposition (ALD) is suggested to apply great benefit to the pre-coating processes for the optimization of the filling factor of the ZnO NWs. Further improvement of light harvesting, current density, and

*Optical absorption of dye detached from the bare ZnO nanowire and the branched ZnO nanowire substrates* 

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

#### **Figure 13.**

*Nanostructures*

ZnO contact, respectively.

length [43] (*Deff = (Rk/Rw)LF*

branched ZnO nanowire DSCs.

the obtained value of 2.1 × 10<sup>−</sup><sup>9</sup>

*Jsc* **(mA/ cm2 )**

*Voc* **(V)** *2*

band were replenished directly by charge transfer from the I3

capacitance and the charge-transfer resistance at the exposed FTO/electrolyte interface, respectively; *CFZ* and *RFZ* are the capacitance and resistance at the FTO/

The specific equivalent circuit might be more complicated while operating the DSCs. Mora-Seró et al. have reported the conductivity modulation of the electrolyte-induced negative capacitance which comes from by injected electrons from the photoelectrode while providing high forward bias with low frequency [42]. In order to avoid the unnecessary interference from the inductor, the lowfrequency part of impedance data was currently ignored. The fitted results of the first-order reaction rate constant for the loss of electrons(*keff*), the electron lifetime (*τ = 1/keff*), the electron transport resistance (*Rw = rwLF*), and the charge-transfer resistance related to recombination of an electron at the ZnO/electrolyte interface (*Rk = rk/LF*) were listed in **Table 2**. *Rk* and *Rw* are quite similar for both DSCs which meant the same crystallinity and interfacial recombination for either bare ZnO NWs or branched ZnO nanostructures. *keff* in the branched ZnO nanowire DSCs was smaller than the bare nanowire ones to cause the smaller effective diffusion

electron lifetime (*τeff = 1/keff*) was prolonged by the additional transport distance of

The maximum peak at approximately 525 nm is contributed by the dye absorption, corresponding to the visible *t2* → *π\** metal-to-ligand charge transfer (MLCT). The IPCE obtained for the branched ZnO nanowire DSCs was almost 1.5 times that of the bare ones. This improvement is primarily due to sufficient dye loading of the branched ZnO NWs, which increases the internal surface area within the photoelectrode. From the dye loading measurement, as shown in **Figure 14**, the concentration

of dye in the branched ZnO nanowire electrode was found to be 2.9 × 10<sup>−</sup><sup>9</sup>

mol/cm<sup>−</sup><sup>2</sup>

*FF* **η (%)**

as measured from dye-desorption experiments, which is almost 40% higher than

*Performances and electron transport properties of the bare ZnO nanowire and the branched ZnO nanowire DSCs determined by photocurrent density-voltage (J-V) characteristics and electrochemical impedance* 

It is worth noting that even though the current density and energy conversion efficiency of branched ZnO DSCs is twice that of bare ZnO ones, the dye loading

> *keff* **(s<sup>−</sup><sup>1</sup> )**

2.37 0.636 0.498 0.75 38.31 0.026 92.12 3.63 6.23 × 10<sup>−</sup><sup>4</sup>

4.27 0.675 0.522 1.51 26.31 0.038 86.85 3.36 4.35 × 10<sup>−</sup><sup>4</sup>

The current density of a DSC is determined by the amount of photogenerated carriers, the electron injection efficiency from the dye molecules to the semiconductor, and the recombination rate between the injected electrons and the oxidative dye or redox species in the electrolyte. The initial amount of photogenerated carriers may have a significant effect on the light-harvesting ability of photoanodes of different structures. **Figure 13** displays the comparison of incident monochromatic photon to current conversion efficiency (IPCE). The peaks at approximately 400 nm were due to direct light harvesting by ZnO semiconductor. The photogenerated electrons diffused through ZnO and the holes in the valence

*keff*) in branched ZnO nanowire DSCs. But the

<sup>−</sup>/I<sup>−</sup>electrolyte [44].

for the bare ZnO nanowire electrode.

*Rk* **(Ω)**

*R<sup>w</sup>* **(Ω)**

**τ***eff* **(s)** mol/cm<sup>−</sup><sup>2</sup>

*Deff* **(cm2 /s)**

**50**

**ZnO DSCs**

Bare nanowires

**Table 2.**

Branched nanowires

*spectroscopy (EIS) analysis [4].*

*The incident monochromatic photon to current conversion efficiency (IPCE) of the bare ZnO nanowire and the branched ZnO nanowire DSCs [4].*

**Figure 14.** *Optical absorption of dye detached from the bare ZnO nanowire and the branched ZnO nanowire substrates and dissolved in 0.1 M NaOH solution [4].*

of branched ZnO DSCs is not twice that of bare ZnO ones. The differences may be that the internal surface of the bare ZnO NWs is insufficient and the excess dye results in the formation of Zn2+/dye complexes rather than an effective chemical bond between the ZnO and dye molecules. To avoid the excessive reaction, Chou et al. [45] reported the shorter immersion time for deficient chemical stability ZnO electrode as compared with TiO2 electrode. Therefore, the excess immersion time of the insufficient internal surface of bare ZnO NWs cannot achieve more dye loading but seriously deteriorates the performance of the DSCs. Although the secondary branches have a nonuniform distribution due to the simple dip-coating process, the branches emitted from a portion of the ZnO NWs could still provide a greater effective surface area for dye adsorption than the bare NWs. The dc or radio frequency (RF) magnetron sputtering or atomic layer deposition (ALD) is suggested to apply great benefit to the pre-coating processes for the optimization of the filling factor of the ZnO NWs. Further improvement of light harvesting, current density, and

overwhelm energy conversion efficiency could be implemented through adjusting denser and longer branches to fill the interstitial voids between backbone NWs.

Hierarchically packed ZnO NPs were formed in the condensation reactions of the sol-gel process that was mentioned previously. The diameter of the ZnO NPs in the range of 160–680 nm was used as shown in **Figure 15a**. **Figure 15b** shows the spherical-shaped secondary ZnO NPs with the diameter of 680 nm. The similar ZnO architectures have been elucidated as the random lasing applications in which the cavities were formed by multiple scattering (UV range) between ZnO primary particles [47]. The laser action comes from an efficient amplification along the closed-loop light-scattering path within a secondary ZnO nanoparticle. Recently, Cao et al. have demonstrated that the aggregation of ZnO nanocrystallites performs an effective scheme to generate multiple light scattering (sunlight range) within the photoelectrode film of DSCs without using any other scattering layers [48, 49], as shown in **Figure 15c**. With utilization of ruthenium complex *cis*-[RuL2(NCS)2] (L = 4,4′-dicarboxy-2,2′-bipyridine), N3 dye, Cao achieved the maximum energy conversion efficiency of 5.4%.

In this present research, a broad size distribution of secondary ZnO NPs with mean radius of 360 nm, as shown in **Figure 16a**, is controlled to provide the widerange absorption of visible sunlight within the preferable packing of the ZnO photoelectrode. The hierarchical ZnO photoelectrode provides the multiple scattering of light, and therefore the light-traveling distance can be significantly prolonged. Furthermore, the primary ZnO nanocrystallites could supply internal surface area to allow enough adsorption of dye molecules. From the optical absorption spectra, the intrinsic exciton absorption (direct transition of energy bandgap of ZnO) could

**53**

**Figure 17.**

*Molecular structures of indoline D149 and D205 dyes.*

**Figure 16.**

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

be particularly identified for 2 μm film. However, the absorption at wavelengths around 400–650 nm caused by the light scattering is enhanced dramatically with increasing the thickness of the ZnO photoelectrodes from 2 to 12μmm as shown in **Figure 16b**. Through a significant light scattering from the hierarchical structure, the thicker ZnO films provide the optical gallery that could provide more photon

The fabrication procedure of DSCs for ZnO NPs were similar to the ZnO NWs, but the photoelectrodes were prepared by screen-printing method. The molecular structures of the indoline-based organic dyes employed in this hierarchically packed ZnO photoelectrode are depicted in **Figure 17**. Both D149 and D205 sensitizers have double rhodanic acid as an anchor moiety. However, D205 is designed by introducing an octyl substitute into the terminal rhodanine ring to replace the ethyl group of D149 [51, 52]. In order to improve the DSCs performance, optimization of the thickness of the ZnO photoelectrode is necessary, because the photovoltaic characteristics exhibit significant variation depending on the thickness. **Figure 18a**, **b**

*(a) Diameter distribution for the ZnO secondary nanoparticles. (b) The corresponding optical absorption* 

*spectra of ZnO photoelectrodes with various film thicknesses, from 2 to 12 μm [50].*

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

absorption in the visible region by the dye molecules.

#### **Figure 15.**

*(a, b) The FESEM and TEM images for the self-assembled ZnO secondary nanoparticles, respectively. (c) The schematic multiple scattering of light within the hierarchical ZnO photoelectrode composed by self-assembled ZnO secondary nanoparticles [46].*

### *Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

be particularly identified for 2 μm film. However, the absorption at wavelengths around 400–650 nm caused by the light scattering is enhanced dramatically with increasing the thickness of the ZnO photoelectrodes from 2 to 12μmm as shown in **Figure 16b**. Through a significant light scattering from the hierarchical structure, the thicker ZnO films provide the optical gallery that could provide more photon absorption in the visible region by the dye molecules.

The fabrication procedure of DSCs for ZnO NPs were similar to the ZnO NWs, but the photoelectrodes were prepared by screen-printing method. The molecular structures of the indoline-based organic dyes employed in this hierarchically packed ZnO photoelectrode are depicted in **Figure 17**. Both D149 and D205 sensitizers have double rhodanic acid as an anchor moiety. However, D205 is designed by introducing an octyl substitute into the terminal rhodanine ring to replace the ethyl group of D149 [51, 52]. In order to improve the DSCs performance, optimization of the thickness of the ZnO photoelectrode is necessary, because the photovoltaic characteristics exhibit significant variation depending on the thickness. **Figure 18a**, **b**

#### **Figure 16.**

*Nanostructures*

conversion efficiency of 5.4%.

overwhelm energy conversion efficiency could be implemented through adjusting denser and longer branches to fill the interstitial voids between backbone NWs. Hierarchically packed ZnO NPs were formed in the condensation reactions of the sol-gel process that was mentioned previously. The diameter of the ZnO NPs in the range of 160–680 nm was used as shown in **Figure 15a**. **Figure 15b** shows the spherical-shaped secondary ZnO NPs with the diameter of 680 nm. The similar ZnO architectures have been elucidated as the random lasing applications in which the cavities were formed by multiple scattering (UV range) between ZnO primary particles [47]. The laser action comes from an efficient amplification along the closed-loop light-scattering path within a secondary ZnO nanoparticle. Recently, Cao et al. have demonstrated that the aggregation of ZnO nanocrystallites performs an effective scheme to generate multiple light scattering (sunlight range) within the photoelectrode film of DSCs without using any other scattering layers [48, 49], as shown in **Figure 15c**. With utilization of ruthenium complex *cis*-[RuL2(NCS)2] (L = 4,4′-dicarboxy-2,2′-bipyridine), N3 dye, Cao achieved the maximum energy

In this present research, a broad size distribution of secondary ZnO NPs with mean radius of 360 nm, as shown in **Figure 16a**, is controlled to provide the widerange absorption of visible sunlight within the preferable packing of the ZnO photoelectrode. The hierarchical ZnO photoelectrode provides the multiple scattering of light, and therefore the light-traveling distance can be significantly prolonged. Furthermore, the primary ZnO nanocrystallites could supply internal surface area to allow enough adsorption of dye molecules. From the optical absorption spectra, the intrinsic exciton absorption (direct transition of energy bandgap of ZnO) could

*(a, b) The FESEM and TEM images for the self-assembled ZnO secondary nanoparticles, respectively. (c) The schematic multiple scattering of light within the hierarchical ZnO photoelectrode composed by self-assembled* 

**52**

**Figure 15.**

*ZnO secondary nanoparticles [46].*

*(a) Diameter distribution for the ZnO secondary nanoparticles. (b) The corresponding optical absorption spectra of ZnO photoelectrodes with various film thicknesses, from 2 to 12 μm [50].*

**Figure 17.** *Molecular structures of indoline D149 and D205 dyes.*

compares the IPCE spectra of ZnO DSCs constructed using two indoline dyes with different film thicknesses. For both two indoline DSCs, IPCE increases significantly with thicker photoelectrodes but saturated at the thickness above 30 μm due to the limitation of electron diffusion length. The spectra at wavelengths shorter than 400 nm are deteriorated due to the UV cutoff effect caused by the thick glass substrate. The photocurrent peak at approximately 367 nm belongs to direct light harvesting of ZnO semiconductor, which remains almost unchanged due to the short penetration depth of UV light. **Figure 18a**, **b** also shows the maximal IPCE value increase gradually with the thickness of ZnO photoelectrode from 71 to 74% and 77 to 79% at wavelength of 550 nm for D149- and D205-sensitized DSCs, respectively. The optimal IPCE obtained for the D205-sensitized ZnO DSCs are

**Figure 18.**

*Photocurrent action spectra of ZnO DSCs constructed using (a) D149 and (b) D205, with different photoelectrode thicknesses [50].*

**55**

**Figure 19.**

*solid lines are the fitting results [50].*

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

higher than the D149-sensitized DSCs in the visible-wavelength (400–700 nm) region. The numerous cracks in thick photoelectrode films (>32 μm) were also

photoelectrodes and two indoline dyes under AM 1.5 full sunlight illumination

The comparison of photocurrent-voltage (J-V) of DSCs using 27-μm-thick ZnO

Both *Voc* and *Jsc* for D205-sensitized ZnO DSCs are higher than D149-sensitized ones. As a result of the higher IPCE, the *Jsc* for the D205-sensitized ZnO DSCs is higher than the D149-sensitized ZnO ones. The dark current indicates that D205-sensitized ZnO DSCs have a slightly more negative-onset potential for the reduction of I3

*Photovoltaic characteristics of DSCs with 27-μm-thick ZnO photoelectrodes and two different indoline dyes. (a) J-V curves for D149- and D205-sensitized DSCs with AM 1.5 illumination and in the dark, respectively. (b) Nyquist plots of D149- and D205-sensitized DSCs performed under illumination at the applied bias of Voc. The* 

) and in the dark was shown in **Figure 19a**. For D205 uptake, the J-V

, *Voc* = 0.65 V, *FF* = 0.67, and *η* = 5.34%. The J-V plot

, *Voc* = 0.64 V, *FF* = 0.71, and *η* = 4.95%.

<sup>−</sup> than

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

of D149 uptake reveals *Jsc* = 10.94 mA cm<sup>−</sup><sup>2</sup>

(100 mW cm<sup>−</sup><sup>2</sup>

plot reveals *Jsc* = 12.17 mA cm<sup>−</sup><sup>2</sup>

observed due to unpracticed-printing technique.

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*

higher than the D149-sensitized DSCs in the visible-wavelength (400–700 nm) region. The numerous cracks in thick photoelectrode films (>32 μm) were also observed due to unpracticed-printing technique.

The comparison of photocurrent-voltage (J-V) of DSCs using 27-μm-thick ZnO photoelectrodes and two indoline dyes under AM 1.5 full sunlight illumination (100 mW cm<sup>−</sup><sup>2</sup> ) and in the dark was shown in **Figure 19a**. For D205 uptake, the J-V plot reveals *Jsc* = 12.17 mA cm<sup>−</sup><sup>2</sup> , *Voc* = 0.65 V, *FF* = 0.67, and *η* = 5.34%. The J-V plot of D149 uptake reveals *Jsc* = 10.94 mA cm<sup>−</sup><sup>2</sup> , *Voc* = 0.64 V, *FF* = 0.71, and *η* = 4.95%. Both *Voc* and *Jsc* for D205-sensitized ZnO DSCs are higher than D149-sensitized ones. As a result of the higher IPCE, the *Jsc* for the D205-sensitized ZnO DSCs is higher than the D149-sensitized ZnO ones. The dark current indicates that D205-sensitized ZnO DSCs have a slightly more negative-onset potential for the reduction of I3 <sup>−</sup> than

#### **Figure 19.**

*Photovoltaic characteristics of DSCs with 27-μm-thick ZnO photoelectrodes and two different indoline dyes. (a) J-V curves for D149- and D205-sensitized DSCs with AM 1.5 illumination and in the dark, respectively. (b) Nyquist plots of D149- and D205-sensitized DSCs performed under illumination at the applied bias of Voc. The solid lines are the fitting results [50].*

*Nanostructures*

**54**

**Figure 18.**

*photoelectrode thicknesses [50].*

*Photocurrent action spectra of ZnO DSCs constructed using (a) D149 and (b) D205, with different* 

compares the IPCE spectra of ZnO DSCs constructed using two indoline dyes with different film thicknesses. For both two indoline DSCs, IPCE increases significantly with thicker photoelectrodes but saturated at the thickness above 30 μm due to the limitation of electron diffusion length. The spectra at wavelengths shorter than 400 nm are deteriorated due to the UV cutoff effect caused by the thick glass substrate. The photocurrent peak at approximately 367 nm belongs to direct light harvesting of ZnO semiconductor, which remains almost unchanged due to the short penetration depth of UV light. **Figure 18a**, **b** also shows the maximal IPCE value increase gradually with the thickness of ZnO photoelectrode from 71 to 74% and 77 to 79% at wavelength of 550 nm for D149- and D205-sensitized DSCs, respectively. The optimal IPCE obtained for the D205-sensitized ZnO DSCs are

D149-sensitized ZnO DSCs and could also be rationalized as a negative shift in edge of ZnO conduction band caused by D205 dye adsorption. The alkyl chain of the terminal rhodanine moiety of D205 has been extended from ethyl to octyl, which cause the effective suppression of electron recombination between I3 <sup>−</sup> and efficient electrons injection into the photoelectrodes [53]. The dyes with the hydrophobic alkyl chains principally not only could form a barrier layer on the sensitizer dye to protect the dye layer against water intrusion from the electrolyte but also rearrange dyes that are more perpendicular to the ZnO surface.

As shown in **Figure 19b**, the obvious middle semicircle of the Nyquist plots belongs to the electron recombination resistance which means a superior circuit in D205 sensitized ZnO DSCs than that of D149-sensitized ZnO DSC. Like **Figure 12**, some interior parameters of the devices can be further derived by well-fitting the impedance data based on the modified equivalent circuit of DSCs as shown in **Figure 20**. The detail parameters are listed in **Table 3**. The electron loss rate *keff* in the D205-sensitized ZnO DSCs is smaller than the D149-sensitized ones, which causes the prolonged electron lifetime *τeff* in the D205-sensitized ZnO DSCs. The larger charge-transfer resistance *Rk* value for D205-sensitized ZnO DSCs indicates the less interfacial recombination occurring between the injected electrons of ZnO and the I3 − of electrolyte. Moreover, the effective electron diffusion coefficient *Deff* is also enhanced with utilization of D205 sensitizer. It is reasonable that the photocurrent density may be directly affected by changes in the electron recombination rate. The amphiphilic D205 may help the formation of a self-assembled dye monolayer that prevents photoelectrons from being resorbed by triiodide ions in the electrolyte, resulting in a higher *Voc* and *Jsc* [54].

#### **Figure 20.**

*The equivalent circuit model of ZnO DSCs composed with hierarchical nanoparticles [50].*


#### **Table 3.**

*Performances and electron transport properties of the D149- and D205-sensitized DSCs (27-μm-thick ZnO photoelectrode) determined by J-V characteristics and EIS analysis [50].*

**57**

**Author details**

provided the original work is properly cited.

Hsin-Ming Cheng\* and Shun-Wei Liu\*

and swliu@mail.mcut.edu.tw

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Department of Electronic Engineering and Organic Electronics Research Center,

Ming Chi University of Technology, New Taipei City, Taiwan

\*Address all correspondence to: smcheng@mail.mcut.edu.tw

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications…*

Low-dimensional NPs, QDs, and NWs have attracted considerable attention owing to their interesting physical and chemical properties. ZnO NWs can shed the light to conduct electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions because of the excellent electrical transport and photonic interconnection due to their crystallinity. ZnO QDs and NPs are of great interest because of the three-dimensional confinement of carrier, and phonon leads not only continuous tuning of the optoelectronic properties but also improvement in device performance. As a wide-bandgap semiconductor, ZnO has been reported as an alternative for DSCs because ZnO offers a large direct bandgap which is similar to TiO2 and even higher electron mobility. ZnO also can be tailored to various nanostructures that provides a promising means for improving the performance of the photoelectrode in DSCs. It is of great urgency to effectively design and control the process window that enables the seamless integration not only the mass produc-

tion but reproducibility of ZnO nanostructures for the near future.

*DOI: http://dx.doi.org/10.5772/intechopen.85699*

**4. Conclusion**

*Low-Dimensional ZnO Nanostructures: Fabrication, Optical Properties, and Applications… DOI: http://dx.doi.org/10.5772/intechopen.85699*
