Flexible zinc oxide photoelectrode for photo electrochemical energy conversion

Photoelectrochemical properties have been investigated for flexible photoelectrodes containing 310 nm thick ZnO film on spin-coated ITO/PET. 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 flexible ZnO photoelectrode was demonstrated to generate photoelectrochemical current. The photocurrents are enhanced by 4% whereas flat-band potential is shifted by 8 V due to the illumination. Values of 1.022 and 0.714 AW−1 were found to be for photo switching and photoresponsivity, respectively. ZnO/ITO/PET can be used as a substrate for making flexible hybrid PEC devices to generate solar power and solar fuels.


Introduction
Photoelectrochemical (PEC) cells 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 . Thereafter, 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 nanoparticles of sizes ranging between 28 nm -142 nm have been recently studied under white light illumination of power 56 mWcm -2 . The value of photoresponsivity is found to be 0.119 lAW -1 . The rise time is 0.9 s while the corresponding decay time is 0.6 s [9]. Similarly configured photo-electrochemical cells using basil sensitized ZnO show 0.45% energy conversion efficiency. The photocurrent and photovoltage were enhanced by 1.33 nA and 3 mV, respectively. Values of 1.3 and 0.024 lA/W have been reported for On-Off ratio and photoresponsivity, respectively [10]. Flexible Ultraviolet zinc nanorods photodetectors are reported to show photoresponsivity within the range of 0.13-0.19 A/W under UV power density varying between 0.8 and 12.47 mWcm -2 [11]. ZnO films drop-cast on fluorine doped ITO substrates was coated with a layer of natural dye for hybrid photoelectrodes with improved performance [12]. 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 [13]. This article presents the photochemical properties of 310 nm thick spin coated ZnO films using commercially available nanoparticles.

Characteriation techniques
X-Ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance scanning in the 2h range of 5-1001, with a step size of 0.021 per second using CuKa 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 configuration using Scanning electrochemical microscopy (SECM, CH Instrument, model CHI920D) as shown in Fig. 1. ZnO film was used as working electrode, a saturated calomel electrode (SCE) was used as reference and a Pt wire was used as counter electrode in a standard three electrode system with 0.1 M Na 2 SO 4 aqueous solution as electrolyte. The fabricated ZnO/ITO/PET device was converted into photoelectrode and used as working electrode in PEC cell. The linear sweep voltametry technique was used at scan rate of 0.1 V/s. The white LED light (with intensity of about 56 mW/cm 2 ) was used as light source to measure current-voltage (I-V) behavior of the PEC cell under dark and light [16,17].

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 2h = 33.58°was observed, which corresponds ZnO (JCPDS card No.

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 [18]. The equation for refractive index is expressed as Eq. (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 Eq. (1).

Thickness calculation
The thickness of the ZnO film was calculated using the Eq. (2) [19]. The thickness of ZnO film was found as 310 nm.
where n 1 and n 2 are the refractive indices corresponding to wavelengths k 1 and k 2 , respectively.

Urbach energy calculation
The exponential dependence on the photon energy (ht) by the absorption coefficient (a) near the band edge for noncrystalline materials follows the Urbach relation as expressed in Eq. (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.
where a 0 is a constant, Eu is an energy which is interpreted as the width of the tail of localized states in the forbidden band gap, t is the frequency of radiation, and h is Planck's constant. The value of 0.2890 eV for Urbach energy is believed to be relative to the degree of crystallinity in the spin-coated ZnO [21] Values of optical parameters in Table 1 are consistent with published data [22]

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  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 [23]    The I-V curves show the flat-band potential V FB . 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 V to 3.4 V. In the photoamperometric measurement, the working electrode is held at a constant 0.25 V potential and illuminated with white LED light for a short period of time. The photocurrent was observed at 1.85 lA and 1.89 lA 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 lA/W, respectively. The measured photoelectrochemical parameters are summarized in Table 2.
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. [24].

Conclusions
We have demonstrated flexible ZnO photoelectrode for photoelectrochemical solar energy conversion. The flexible thin film solar cells have greater advantages for commercial productions because of their flexibility and lightweight features. Flexible ZnO PEC device has generated photocurrents of about 1.85 lA and 1.89 lA under dark and illumination, respectively. The flat band potential was shifted from 2.6 to 3.4 V due to the illumination. The photocurrents and flat-band potential are enhanced by 4% and 0.8 V, respectively due to the illumination. The flexible ZnO/ITO/PET has shown photoswitching ON/OFF ratio and photoresponsivity about 1.0216 and 0.7142 lA/W, respectively. These results provide a fundamental understanding on flexible ZnO