Flexible ZnO photoelectrode for photo electrochemical energy conversion

Photoelectrochemical properties have been investigated for exible ZnO/ITO/PET photoelectrodes. ZnO was spin coated on ITO/PET substrate with thickness of about 310 nm. The high crystalline structure of ZnO was studied using x-ray diffraction pattern. A value of 3.4 eV has been estimated for optical band gap from its absorption spectra. The exible ZnO photoelectrode was demonstrated to generate photoelectrochemical current. Values of 1.022 and 0.714 were found to be for photo switching and photoresponsivity, respectively. ZnO/ITO/PET can be used as a substrate for making exible hybrid PEC devices to generate solar power and solar fuels.


Introduction
PECs are one type of solar cells that include photoelectrodes and electrolyte [1]. Photoelectrodes can be fabricated from various materials including organic and inorganic materials. The materials of photoelectrodes decide the performance of PECs [2]. Semiconducting photoelectrodes are mostly used in PECs for conversion of photons into electron-hole pairs [3]. First PEC was demonstrated using TiO 2 . Then after, many nanostructured metal oxides have been used in PECs because the advantages of high surface to volume ratio in nanomaterials [4].
Zinc oxide (ZnO) is a n-type semiconducting material with a wide direct energy band gap with a value of 3.37eV at room temperature [5]. Values of 3.96×10 20 cm −3 and 17.7cm 2 V − 1 s − 1 have been reported for electron concentration and mobility of co-doped RF sputtered ZnO lms Optical transmission value is similar to 85%-90% in the range 400-700 nm for these lms [6]. Because of its low toxicity and high electronic conductivity, it has been extensively studied for exploring applications in optoelectronics as solar cells, gas sensors, varistors and a phosphor for colour displays [7]. The On-Off ratio as high as 2.2×10 4 has been reported for the ZnO memristor in a sandwich con guration with sputtered gold top electrode [8].
Flexible photo-electrochemical cells of ZnO on indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates, incorporating muga silk nanoparticles of sizes ranging between 28nm -142nm have been recently studied under white light illumination of power 56 mWcm − 2 . The value of photoresponsivity is found to be 0.119 µAW − 1 . The rise time is 0.9s while the corresponding decay time is 0.6s [9]. Similarly con gured photo-electrochemical cells using basil sensitized ZnO show 0.45% energy conversion e ciency. The photocurrent and photovoltage were enhanced by 1.33nA and 3 mV, respectively. Values of 1.3 and 0.024 µA/W have been reported for On-Off ratio and photoresponsivity, respectively [10]. ZnO lms drop-cast on uorine doped ITO substrates was coated with a layer of natural dye for hybrid photoelectrodes with improved performance [11]. The dye-sensitised solar cells have been fabricated using natural dye extracted from basil or ocimum leaves. These may also be used as photoelectrochemical cell for the applications of water splitting [12]. This article presents the photochemical properties of 310 nm thick spin coated ZnO lms using commercially available nanoparticles.

Materials and Device fabrication
ZnO thin lms were deposited on ITO coated PET substrates (purchased from Sigma Aldrich) with prior cleaning through spin coating technique. Small volume of 40 weight% dispersion of ZnO nanoparticles (130 nm) dispersed in ethanol was spin coated on ITO/PET substrate at 250 rpm and 2000 rpm in a sequence for 15 s and 60 s respectively. The samples were kept for oven drying at 60 0 C for 15 mins [13][14].
Characteriation techniques X-Ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance scanning in the 2θ range of 5-1001, with a step size of 0.021 per second using CuKα radiation of wavelength 0.15406 nm. ITO coated PET substrate was used for the UV-Visible. The absorption and transmittance spectra were recorded using UV-Visible spectrophotometer (Shimadzu 6120, Japan). The photoelectrochemical (PEC) measurements were performed in a standard three electrode con guration using Scanning 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 con rms ZnO lm with high crystallinity.

Optical properties
The absorption spectra of ZnO thin lm 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 lm. The optical band gap of the lms 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 lm. The various optical parameters of ZnO/ITO/PET device are summarized in Table 1. The transmittance of the lm was very high due to transparent behavior of lm 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 lm 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 lm are found to be 2.0506 and 2.0501 at 492 nm and 802 nm wavelengths, respectively from equation (1).
where N = 2n s [(T M -T m )/T M T m ] + (n 2 s +1)/2 and n s is refractive index of substrate. The values of refractive index are consistent with those reported recently [18].

Urbach energy calculation
The exponential dependence on the photon energy (ℎ ) by the absorption coe cient (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 coe cient as a function of the photon energy for ZnO lm. The value of Urbach energy ( ) 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 lm is about 0.2890 eV.
where 0 is a constant, is an energy which is interpreted as the width of the tail of localized states in the forbidden band gap, 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]

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 Na 2 SO 4 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 con rms that the lm was photoactive lm. 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 eld present at the semiconductorelectrolyte interface. The photocurrent is directly dependent on the properties of semiconductor layer.
The I-V curves show the at-band potential V FB. It is the potential where there is no eld in the semiconductor) shifts towards the negative values, which shows n-type behavior of semiconducting thin lm in PEC device. The at 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 at-band potential increased by 0.8 V due to the illumination. The exible 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.  Schematic representation of three electrode electrochemical system.   Linear Sweep voltametry of ZnO/ITO/PET in dark (black) and light (red).

Figure 6
Photoamperometric at 0.25 V of ZnO/PET