3.1 Structural properties of ZnO/ITO thin film
XRD pattern of ZnO thin film grown on ITO-PET substrate is shown in Fig. 2. XRD experiments were performed with X-ray diffractometer. XRD pattern was used to study the orientation and phase of the sample. The marked diffraction peaks are corresponding to ZnO. The (101) peak at 2θ = 33.58° was observed, which corresponds ZnO (JCPDS card No. 89–1397) and veriries the hexagonal wurtzite structure of ZnO. The two diffraction peaks at 47° and 54.6° that are corresponding to the ITO-PET substrate. The XRD pattern confirms ZnO film with high crystallinity.
3.2 Optical properties
The absorption spectra of ZnO thin film deposited on ITO coated PET substrate were measured by the UV-Visible spectrometer in the wavelength range 200-1100 nm as shown in Fig. 3a. The absorption curve shows the absorption peak about 387 nm for ZnO thin film. The optical band gap of the films was determined using the Tauc plot as shown in Fig. 3b. The obtained band gap was found about 3.4 eV for the ZnO thin film. The various optical parameters of ZnO/ITO/PET device are summarized in Table 1. The transmittance of the film was very high due to transparent behavior of film as shown in Fig. 3c. The Urbach curve is shown in Fig. 3d and Urbach energy is found at 0.2890 eV.
Refractive index calculation
ZnO film exhibits good transparency in the visible and infrared region (~78%). The refractive index (n) at different wavelengths was calculated using the envelope curve method in the transmission spectra [17]. The equation for refractive index is expressed as equation (1). Values of refractive index n for ZnO film are found to be 2.0506 and 2.0501 at 492 nm and 802 nm wavelengths, respectively from equation (1).
n = [N + (N2-n2s)1/2]1/2 (1)
where N = 2ns [(TM-Tm)/TMTm] + (n2s+1)/2 and ns is refractive index of substrate. The values of refractive index are consistent with those reported recently [18].
Thickness calculation
The thickness of the ZnO film was calculated using the equation (2) [19]. The thickness of ZnO film was found as 310 nm.
t = (λ1λ2)/[2 (λ1n2-λ2n1)] (2)
where n1 and n2 are the refractive indices corresponding to wavelengths λ1 and λ2, respectively.
Urbach energy calculation
The exponential dependence on the photon energy (ℎʋ) by the absorption coefficient (α) near the band edge for noncrystalline materials follows the Urbach relation as expressed in equation (3). Urbach curve shows the variation in the logarithm of the absorption coefficient as a function of the photon energy for ZnO film. The value of Urbach energy (𝐸𝑢) is normally calculated by considering the reciprocal of the slope of the linear portion in the lower photon energy region of these curves [20]. The calculated values of Urbach energy for ZnO film is about 0.2890 eV.
α(ʋ) = α0 exp(hʋ/Eu) (3)
where 𝛼0 is a constant, 𝐸𝑢 is an energy which is interpreted as the width of the tail of localized states in the forbidden band gap, ʋ is the frequency of radiation, and ℎ is Planck’s constant. The value of 0.2890eV for Urbach energy is believed to be relative to the degree of crystallinity in the spin-coated ZnO [21]
Table 1 Optical parameters of ZnO/ITO/PET.
Device
|
Band gap (eV)
|
Refractive index (n)
|
Urbach energy (Eu) eV
|
n at 492 nm
|
n at 802 nm
|
ZnO/ITO/PET
|
3.4
|
2.0506
|
2.0501
|
0.2890
|
3.3 Photoelectrochemical properties
The energy band diagram of ZnO/ITO/PET is shown in Fig. 4, which explains the charge transfer mechanism in PEC device. The photo electrochemical (PEC) response of the ZnO/ITO/PET with Na2SO4 electrolyte was recorded using linear sweep photovoltammogram and photoamperometric technique as shown in Fig. 5 and 6, respectively. The dark and red color is shown for without illumination and under illumination, respectively. The PEC response confirms that the film was photoactive film. A negative slope of I-V shows the n-type semiconducting behavior of ZnO. At high photon energy, the maximum photovoltage dimishes due to the limited penetration depth of ZnO. Then excited electron enters into the ZnO region and diffuses at back contact. As a result, there is a generation of photocurrent and photo voltage. The photo generated carriers are collected due to the electric field present at the semiconductor-electrolyte interface. The photocurrent is directly dependent on the properties of semiconductor layer.
The I-V curves show the flat-band potential VFB. It is the potential where there is no field in the semiconductor) shifts towards the negative values, which shows n-type behavior of semiconducting thin film in PEC device. The flat band potential was shifted towards higher value under illumination from 2.6 to 3.4 V. In the photoamperometric measurement, the working electrode is held at a constant 0.25V potential and illuminated with white LED light for a short period of time. The photocurrent was observed at 1.85 μA and 1.89 μA for dark and illuminated condition, respectively. The photocurrent is enhanced by 4% and flat-band potential increased by 0.8 V due to the illumination. The flexible ZnO photoelectrode was demonstrated for photoswitching properties and photoresponsive behavior. The ON/OFF ratio and photoresponsivity were calculated about 1.0216 and 0.7142 μA/W, respectively. The measured photoelectrochemical parameters are summarized in Table 2.
Table 2 Photoelectrochemical parameters for flexible ZnO photoelectrode.
Device
|
Photocurrent (μA)
|
VF (V)
|
Increment due to illumination
|
ON/OFF ratio (ILight/IDark)
|
Photoresponsivity R
[= ILight-IDark) / PLight ] μA/W
|
Dark
|
Light
|
Dark
|
Light
|
Photocurrent
|
VF (V)
|
ZnO/ITO
|
1.85
|
1.89
|
2.6
|
3.4
|
4%
|
0.8
|
1.0216
|
0.7142
|
ZnO can only absorb ultraviolet (UV) light (4% in the sun light spectrum) due to its wide band gap [20]. Therefore, ZnO photoelectrodes can be modified with organic or inorganic materials to absorb visible and other spectrum of sun light. Biomaterials have high absorption in visible spectrum. Natural dyes can absorb the light in a wide range of wavelengths. Flexible ZnO/ITO/PET photoelectrodes can be used as a substrate for developing next generation hybrid solar energy conversion devices as well as wearable optoelectronic devices. [21-23].