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Photoelectric and Electrochemical Performance of Al-Doped ZnO Thin Films Hydrothermally Grown on PET–GR Bilayer Flexible Substrates Wei Wang, Taotao Ai, Wenhu Li, Ran Jing, Yanhan Fei, and Xiaoming Feng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08181 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Photoelectric and Electrochemical Performance of Al-Doped ZnO Thin Films Hydrothermally Grown on PET–GR Bilayer Flexible Substrates Wei Wang, Taotao Ai,* Wenhu Li, Ran Jing, Yanhan Fei, Xiaoming Feng* (School of Materials Science and Engineering, Shaanxi University of Technology, Shaanxi Hanzhong 723000, P.R. China)

ABSTRACT: Al-doped zinc oxide thin films with Al contents in the range of 0-15% were synthesized on the PET–GR flexible substrates by hydrothermal method at 90 ℃ for 5 h, and their performance as p-n heterojunction electrode was evaluated. The results revealed that the Al atoms doped into the ZnO lattice were present in the form of oxidized state, which together with PET–GR substrate affected the internal stress and lattice constant of the ZnO. The effect of Al contents on the morphological evolution of the product was investigated, and the growth mechanism of AZO NRs/PET–GR was also analyzed. The PL spectra of AZO/PET–GR showed wide near-UV emission peak at about 390 nm and the mechanism of free exciton recombination luminescence was discussed. AZO/PET–GR exhibited excellent mobility, the highest mobility was 154.109 cm2V-1s-1 when the Al doping content was 12%, and its optimal resistivity and carrier concentration were 0.656 Ω·cm and 4.668 × 1017 cm-3, respectively. The photo-generated current and EIS measurement of the electrode indicated that the photocurrent densities of 9% Al-doped and un-doped ZnO/PET–GR were 0.7 µA/cm2 and 0.4 µA/cm2, the internal resistance of these two

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electrodes were 7 Ω and 13 Ω, and the slope of the impedance spectra curves in the low frequency region were 4.66 and 2.00, which illustrated that the incorporation of Al could promote the further separation of photo-induced electron-hole pairs, and AZO/PET–GR electrode had better charge conductivity and interface bonding than ZnO/PET–GR.

1 INTRODUCTION Optoelectronic devices based on one-dimensional nanostructured zinc oxide (ZnO) have received considerable scientific attentions because of their enormous potential in a variety of applications, such as nano-generators,1 dye-sensitized solar cells,2 ultraviolet (UV) photo-detectors,3 and super-capacitors.4 Among them, the most critical component of these devices is the ZnO semiconductor with a direct wide band gap (3.37 eV), higher exciton binding energy (60 meV) and short wavelength absorption limit (370 nm), which is an important member of the transparent conductive oxides, and is also deemed to be the most promising candidate to replace indium-tin oxide (ITO). It can achieve stimulated emission of excitons at room temperature, and also obtain more efficient radiation recombination in the manner of inter-band direct transition. Nevertheless, carriers that can move freely in the crystal of intrinsic ZnO are few, resulting in poor electrical conductivity, which can’t satisfy the requirements of ZnO-based optoelectronic devices. In order to improve the electro-optical performances of the ZnO thin films including electrical conductivity and optical transmittance, n-type doping elements such as B, Al, Ge, Si and F are usually introduced into ZnO to obtain impurity semiconductors. In contrast, the 2


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presence of natural defects or hydrogen impurities caused the donor level have a great self-compensating effect on the acceptor level, resulting in p-type doping of ZnO more difficult to implement.5 Surprisingly, Al-doped ZnO (AZO) nanomaterials had lower resistivity and better stability, and the optical transmittance and crystalline of ZnO after doping Al was not be deteriorated.6,7 Rezaie et al. reported the resistivity of the ZnO/ITO bilayer was reduced from 1.2 × 10-3 Ω·cm to 6 × 10-4 Ω·cm after doping of the Al element.8 Peng et al. found that the electrical properties of Al-doped ZnO thin films were better than that of B-doped ZnO films, and their sheet resistance (166 Ω/sq) and resistivity (1.99 × 10-3 Ω·cm) were both lower than that of B-doped ZnO thin films (360 Ω/sq, 4.01 Ω·cm).9 Thus, Al-doped ZnO is regarded as an optimal choice for the current transparent conductive materials. In recent years, with the increasing demand for high-quality flexible and wearable electronic devices, the studies of transparent conductive films with flexible materials as substrates have become hotspots. Tungsten oxide nanowires with excellent field emission properties were prepared on carbon cloth using physical vapor deposition by Wang et al.10. Wu et al. successfully fabricated the Ga-doped ZnO with carrier concentration of 3.39 × 1021 cm-3 and Hall mobility of 7.97 cm2/V·s on the flexible polyethersulfone substrate by RF sputtering technology.11 The most essential

difference

between

flexible

electronic

equipment

and

traditional

optoelectronic device is the replacement of rigid substrate with flexible material substrate. There are many kinds of flexible substrates, including conductive polymer film, ultra-thin glass after thinning processing, graphene, metal foil and carbon fiber

3



cloth.12,13 Among, graphene (GR) is two-dimensional carbon material that is periodically closely packed by carbon atoms, it exhibits a series of peculiar electronic and physical properties such as electron mobility of 2 × 105 cm2V-1s-1, which is 140 times the mobility of silicon,14 conductivity of up to 108 Ω/m and optical transmittance of about 97.7%. Therefore, graphene is the best choice for flexible growing substrate. In this paper, in order to obtain a new generation of semiconductor optoelectronic devices with high performance and extensibility based on ZnO/PET–GR structure, the double-layer film of graphene-coated polyethylene terephthalate (PET–GR) is acted as growth substrate, the inorganic semiconductor material ZnO is integrated into the PET–GR substrate to promote the physical and chemical properties of the entire composite structure. Al-doped ZnO nanorods (AZO NRs) thin films were grown on PET–GR substrate by low-temperature hydrothermal method. The influences of Al-doping and the presence of flexible substrates on the crystal structure, internal stress and microstructure of ZnO nanomaterials were investigated, and the growth mechanism of ZnO nanorods were also analyzed. Combining the photoluminescence (PL) spectra of ZnO thin films, the optical performances and theoretical model of exciton recombination luminescence were discussed. The electrical conductivity of AZO/PET–GR was studied by means of analyzing the change trend of resistivity, carrier concentration and mobility of the films with the change of Al doping concentration. The n-type AZO/p-type PET–GR heterojunction was constructed and used as the working electrode for electrochemical

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test, and the electrochemical properties were studied based on the energy band theory.

2 EXPERIMENTAL 2.1 Synthesis of AZO NRs/PET–GR In order to fabricate the AZO/PET–GR composite structure easily, ZnO seed layer was first sputtered on the surface of the as-cleaned PET–GR substrate with a size of 1 cm × 2 cm utilizing an ion-sputtering apparatus, which was conducive to reduce the lattice mismatch between the ZnO nanostructures and the substrate material, and induce the growth of ZnO. Sputtering process parameters were as follows: time of 15 s, sputtering current of 10 mA and vacuum pressure of 0.1 mbar. It is worth noting that if the sputtering time is too long, it will cause the thickness of the seed layer to be large, so that the ZnO film is easy to crack off. If the sputtering current is large, the flexible substrate will be bent or even burned. Then, a given mass of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT, C6H12N4) were accurately weighed and dissolved in deionized water to prepare 30 mL of the precursor aqueous solution at a concentration of 0.05 M. Aluminum oxide (Al2O3) at the concentration of 0%, 3%, 6%, 9%, 12% and 15%, which acted as dopant, was added into the as-prepared mixed solution and stirred for 30 min to obtain experimental growth solution. Here, the doping concentration of 3% is defined as 0.03 mol/L. The solution was transferred into a polytetrafluoroethylene liner inside the autoclave. Finally, PET–GR substrate coated with ZnO seed layer was clamped by a specimen holder in order to maintain a vertical state in the solution. After sealing, the reactor was placed in an electric oven and heated at 90 ℃ for 5 h. The sample was 5



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taken out after cooling and the surface-attached impurities were washed with deionized water, and then dried at room temperature to obtain Al-doped ZnO nanomaterials. 2.2 Characterization The crystalline structure of AZO thin film electrodes was analyzed by X-ray diffractometer

(XRD)

with

Cu-Kα

radiation

(Bruker,

D8-FOCUS).

X-ray

photoelectron spectroscopy (XPS) was used to study the surface elemental compositions and chemical valence of the samples on an ESCALAB 250Xi spectrometer (excitation source of Al-Kα). The surface morphology and structure of AZO thin films were observed via field-emission scanning electron microscopy (FE-SEM, Hitachi, SU-8010). Photoluminescence (PL) spectra was recorded at room temperature employing a fluorescence spectrophotometer at an excitation wavelength of 325 nm. The electrical performance parameters of the AZO/PET–GR composite structure were measured on a Hall effect tester (Hall-8800) at a temperature of 300 K using a Van der Pauw method. The electrochemical performances of the p-n heterojunction electrode were determined on a CHI660E electrochemical workstation utilizing three-electrode model, which included the measurements of photocurrent and electrochemical impedance spectroscopy (EIS). The experiments were implemented at room temperature with 0.5 M Na2SO4 solution as electrolyte, among them, the as-prepared samples, platinum wire and saturated calomel electrodes served as working, counter and reference electrodes, respectively. The frequency of the A. C. impedance and the amplitude of the sine wave were set to 0.01~1.0 × 105 Hz and 5 6


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mV, respectively.

3 RESULTS AND DISCUSSION 3.1 The crystalline structure of samples Fig. 1a demonstrates the XRD patterns of ZnO NRs thin film grown on PET–GR substrates with different Al doping concentrations. The XRD curves are indexed against the standard diffraction spectrum (JCPDS 89-1397), the marked diffraction peaks are consistent with the standard peaks, which are assigned to the characteristic peaks of ZnO, except for the short-wide diffraction peak at 54° attributed to the PET–GR substrate, thereby verifying that all film samples have a typical hexagonal wurtzite structure. No characteristic peaks of metal Zn or Al2O3 are observed from all XRD patterns in Fig. 1a, indicating that the doping of Al atoms is effective. The angles of the diffraction peak of the AZO/PET–GR are slightly changed comparing to the location of the un-doped ZnO peak, which may be due to the fact that the Al atoms exist in the ZnO lattice in the form of substitution or interstitial,15 causing lattice distortion and changing the internal stress of the crystal. In order to compare, the XRD patterns of pure and 9% Al-doped ZnO powders are shown in Fig. 1b. According to the interplanar spacing formula (1), the hexagonal system can calculate the lattice constant of the sample:

d=

a (1) 2 a 4 2 (h + hk + k 2 ) + l 2 2 3 c

Here, d represents the interlayer spacing between adjacent lattice planes of ZnO and (hkl) is the Miller indices of the crystal plane. The lattice constants of pure and 9% 7



Al-doped ZnO powders are calculated as a = 3.244 Å, c = 5.205 Å and a = 3.240 Å, c = 5.201 Å, separately, illustrating that the lattice constant decreases after Al doping. It is interesting to note that the numerical values are a = 3.239 Å, c = 5.201 Å and a = 3.249 Å, c = 5.207 Å, respectively, when the corresponding doping concentration of ZnO NRs are grown on a PET–GR substrate. At this time, the lattice constant of 9% AZO/PET–GR is increased. This change is the result of the stress effect arising from the mismatch of thermal expansion coefficient between AZO thin film and flexible substrate,16 revealing the existence of internal stress in the sample. The stress formula (2) of the hexagonal lattice based on the biaxial strain model can be applied to estimate the stress along c-axis of the ZnO films. Stress magnitude of un-doped and 9% Al-doped ZnO NRs film grown on PET–GR substrate are -0.044 GPa and -0.299 GPa, and are all expressed as pressure stress. In equation (2), σ denotes the stress in the film, Cfilm and C0 are the c-axis lattice parameters obtained by XRD measurement and in a state of unstrained, respectively. The elastic constants cij of single-crystal ZnO are fixed values,17 which are c11 = 208.8, c12 = 119.7, c13 = 104.2 and c33 = 213.8, and their units are all GPa.

σ=

2c132 − c33 (c11 + c12 ) C film − C0 × (2) 2c13 C0

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Fig. 1 XRD patterns of AZO NRs/PET–GR at different Al doping concentrations (a) and the comparison of AZO/PET–GR with powdered ZnO in the case of doping 0% and 9% Al (b).

3.2 Analysis of elemental compositions and chemical valence Fig. 2 depicts the XPS spectra of the 9% AZO NRs films hydrothermally fabricated at 90 ℃for 5 h. The strong photoelectron peaks corresponding to the Zn, O and C elements can be clearly seen on the XPS survey spectrum (Fig. 2a), where the C element peak located at about 290 eV may be caused by the reason that the surface of sample adsorbed a handful of CO2 in the air. The obvious Al element peak is not observed at the relevant positions in the survey spectrum, which mainly because the incorporation of Al atoms and the scattering cross section of it are small. As seen in the inset of Fig. 2a, the photoelectron peak at 73.77 eV belongs to the Al 2p peak because it is higher than the 2p3/2 binding energy of the simple-substance Al (72.3 ~ 72.9 eV) and is almost in conformity with the as-reported value for Al2O3: Al 2p at 74.7 eV18 and 74.9 eV19. This means that the Al element is successfully incorporated into the ZnO lattice and present in the form of oxidized state, that is, Al is existing as trivalent Al3+. The atomic concentration of Al element in the lattice can be 9



quantitatively analyzed by element peak area and the sensitivity factor, and the concentration is 0.63 at.%, exposing that the concentration of Al3+ doped into ZnO lattice is restricted under the hydrothermal growth conditions. The two peaks at 1021.27 eV and 1044.37 eV in the high-resolution spectrum (Fig. 2b) of the photoelectron peak corresponding to Zn 2p are attributed to Zn 2p3/2 and Zn 2p1/2, respectively, and the spectral peak of metal Zn with binding energy of 1021.50 eV is not found on the graph, which proves that most of the Zn elements in the AZO/PET–GR exhibit positive divalent oxidation states. The high-resolution XPS spectra of O 1s is depicted in Fig. 2c, the higher peak at 529.97 eV is close to the electron binding energy of O 1s in the Zn–O bond (530.70 eV20), thus ascribing the existence form of O elements to O2-. While, the lower peak at 531.07 eV can be assigned to oxygen defects, indicating that there is a small amount of oxygen vacancy in the AZO NRs film.21

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Fig. 2 XPS spectra of the AZO NRs with the concentration of 0.09 M: (a) XPS survey scan spectrum; (b) and (c) high-resolution spectra at Zn 2p and O 1s state energies, respectively. The illustration in (a) corresponds to the Al2p peak.

3.3 Morphologies and growth habits The FE-SEM images in Fig. 3 display the morphology of the ZnO nanorods with different Al doping concentration, the illustrations demonstrate the corresponding high magnification images (1.0 × 105 times). As seen in Fig. 3, all samples present significant nanorod-like structure with inerratic hexagon-shaped cross section, which are distributed at higher densities on the flexible substrate and have a smooth surface and a flat top. However, most of the ZnO NRs are growing with irregular arrangement rather than completely perpendicular to the substrate, which is generated by the presence of stress in the flexible substrate. As the amount of Al doping increases from 11



0% to 15%, the diameter of AZO NRs is gradually increasing, and the diameter of nanorods ranges from 0.257 µm to 1.130 µm. For studying the effect of multi-layer substrate on the morphology of the products, the nanostructured ZnO at different Al doping content are hydrothermally fabricated on a single layer PET substrate. The FE-SEM images of the AZO/PET are shown in Fig. 4, the insets are the high magnification images (5.0 × 104 times). It can be found that, the morphology of the un-doped ZnO grown on the PET substrate presents nanocone and their top is tapering (Fig. 4a), which is different from the morphology of nanorods for pure ZnO/PET–GR (Fig. 3). The diameter range of AZO/PET is 0590 ~ 2.179 µm, which is distinctly larger than that of AZO NRs/PET–GR. The surface of the Al-doped ZnO crystals only grown on PET substrate is likely to appear more defects, their side face (Fig. 4b and Fig. 4e) and the upper surface (Fig. 4c) are uneven and coarse. The distribution uniformity of AZO/PET is poor, and some of the crystals grow abnormally. This is triggered by the fact that the surface of PET substrate without graphene layer is relatively rough, and its thermal expansion coefficient is larger than that of ZnO. In addition, with the increase of the Al doping content, the regularity of the ZnO morphology for the AZO/PET does not change significantly. Therefore, the graphene layer as a interlayer plays an important role in improving the morphology and structure of Al-doped ZnO nanorods.

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Fig. 3 FE-SEM images of AZO NRs grown on PET–GR substrate at various Al doping concentrations: (a) 0%; (b) 3%; (c) 6%; (d) 9%; (e) 12%; (f) 15%. The inset of upper right corner is corresponding images at high magnification.

Fig. 4 FE-SEM images of Al-doped ZnO films only grown on PET substrate at different Al doping concentrations: (a) 0%; (b) 3%; (c) 6%; (d) 9%; (e) 12%; (f) 15%. The inset of upper right corner is corresponding images at high magnification.

Fig. 5 describes the growth mechanism of the AZO nanorods fabricated on the PET–GR substrate by hydrothermal method at 90 ℃ for 5 h. In the precursor solution, the Zn2+ is provided by Zn(NO3)2·6H2O, and HMT is susceptible to decomposition at reaction temperature of 90 ℃ and slowly releases hydroxyl ions (OH-). After Zn2+ react with OH-, the coordinative tetrahedral growth units of the [Zn(OH)4]2- are 13



formed and continuously transported to the substrate surface. At the same time, Al3+ react with OH- to form the Al(OH)4-. When the concentration of the complexes of [Zn(OH)4]2- and Zn2+ in the solution reaches a critical value, ZnO will be generated by hydrolysis reaction. The involved chemical reactions in the growth process are shown below (equation 3 to 7). Because heterogeneous nucleation only needs to overcome the lower activation energy barrier, it is easier to nucleate on the substrate than the homogeneous nucleation at lower supersaturation. Once the nucleation is accomplished, the crystal nucleus begin to grow up. The ionic polar structure of ZnO with hexagonal wurtzite structure can be described as O atoms are arranged in the hexagonal closest packed manner, Zn atoms fill in the tetrahedral voids, and half of the eight tetrahedral lattice voids are occupied. When Al atoms are doped into ZnO, the majority of the doped Al atoms replace the Zn atoms located in the lattice point and become the substitutional impurities. A small number of Al atoms turn into interstitial impurities. This special structure results in the wurtzite-structured ZnO has Zn polar surface (0001) with the positive charge and O polar face (000 1 ) with negative charge, and allowing the crystal to grow anisotropically along c-axis. The sequence of the growth rates of each crystal planes is summarized as follows: V(0001) > V(10 1 1) > V(10 1 0) > V(000 1 ).22 Hence, the ZnO crystal grows rapidly along the (0001) direction, and forms ZnO nanorods with hexagonal cross sections gradually.

Zn( NO3 )2 ⋅ 6 H 2 O + H 2 O → Zn2+ + 2 NO3− + 7 H 2 O (3) (CH 2 )6 N4 + 10 H 2 O → 6HCHO + 4 NH 4 + + 4OH − (4)

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Zn2+ + 2OH − + 2 H 2 O → Zn(OH ) 4 2− + 2 H + (5) Al 3+ + 4OH − → Al (OH )4 − (6) Zn(OH )4 2− → ZnO + H 2 O + 2OH − (7)

Fig. 5 Growth mechanism of the AZO NRs/PET–GR prepared by hydrothermal method.

3.4 Room-temperature PL properties The room temperature PL spectra in the wavelength ranges from 360 nm to 630 nm of AZO films doped with different Al content on PET–GR and PET substrates are shown in Fig. 6. A broad near UV emission peak at about 390 nm is discovered in the PL spectra in Fig. 6, which can be explained by the near-band-edge (NBE) transition of ZnO nanorods with a wide band gap.23 In other words, The NBE emission is produced by the free exciton recombination in the course of exciton-exciton collision. The visible emission peak induced by various defects in the ZnO lattice is not very obvious. From Fig. 6a we can know that with the increase of Al doping content, the NBE emission peak moves distinctly toward the direction of the high wavelength, and the intensity of the peaks is enhancing. This may be caused by two reasons: (℃) The band gap is transformed toward the orientation of low energy under the influence of 15



sp-d electron interaction;24,25 (℃) The face or the interface of ZnO nanorods will appear a lot of dangling bonds, adsorbents and other defective state, the photo-generated carriers are captured by these surface defects at an extremely fast rate after being excited by light, thus resulting in redshift phenomenon. However, the PL spectra in the near UV region of the AZO/PET (Fig. 6b) are discontinuous and split. The position of the highest emission peak is slightly changed with the increase of Al concentration, due to the lack of the graphene layer as an electron collector.

Fig. 6 Room-temperature PL spectra of AZO NRs thin film with different Al doping concentrations prepared on PET–GR (a) and only PET (b) substrate.

The schematic of the radiative recombination luminescence for free exciton in ZnO NRs films is shown in Fig. 7. When UV light (3.815 eV) is irradiated onto the ZnO nanorods, a portion of the light is reflected and scattered, and another portion enters into the ZnO NRs. The rest of the light is absorbed by ZnO except the transmitted portion. The electrons leap from the valence band to the conduction band after the ZnO absorbs the excited photons, producing abundant photo-excited electron-hole pairs. These carriers are first relaxed to the edge of the band by the emission of phonons to form free excitons, which are essentially electron-hole pairs 16


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bound by Coulomb’s interaction. Affecting by the small size effect, the mean free path of the electrons in nanostructured ZnO with the wide band gap is confined to the nano-space, which is equivalent to the excitation wavelength, and then causing the overlap of the wave function between the electrons and holes. In this case, it is easier to form Wannier exciton with weakly bound rather than Frenkel exciton. Under the combined action of the quantum confinement effect and Coulomb between electrons and holes, the high concentration exciton produces discrete exciton energy level near the conduction band in the energy gap (Eex in Fig. 7). The exciton luminescence band at near UV region is generated after that the excitation energy is absorbed by the center of separation in the forbidden band.

Fig. 7 Schematic diagram of radiative recombination luminescence for free exciton in ZnO NRs films.

3.5 Electrical performance The Al-doped ZnO nanorod films synthesized on the PET–GR substrate in this experiment are n-type conductance by the Hall effect measurement, which may be the

17



result of the interaction of the two conductive mechanisms. (℃) Intrinsic carriers, when the stoichiometric ratio of ZnO crystals deviates from the ideal ratio, there may be the following six kinds of point defects in intrinsic ZnO,26 including oxygen vacancies (VO), interstitial zinc (Zni), zinc trans (ZnO), zinc vacancies (VZn), interstitial oxygen (Oi) and oxygen trans (OZn). As the formation energy of Vo and Zni is lower relative to that of other defects, they are more likely to produce in the ZnO lattice. The energy level of the intrinsic point defects was calculated by Xu et al. using full-potential linear Muffin-tin orbital (FP-LMTO) method,27 where VO and Zni produced donor levels at 1.62 eV and 0.47 eV below the bottom of the conduction band, respectively, which were the main source of n-type conductivity for ZnO. The reaction equations for intrinsic defects are formulas (8) and (9), and each VO and Zni provides with two conductive electrons. (℃) The carriers formed by the substitutional incorporation of Al atoms, doped Al3+ instead of Zn2+ in ZnO crystal to produce free carriers. The effect of the Al atoms substitution can be expressed by equation (10), where each substitutional Al3+ provides a conductive electron. In formulas (8) to (10), VO indicates that the position of the O atom is empty, Zni denotes that the Zn atom locates in the lattice gap position, AlZn represents Al atoms occupy the site of the Zn atoms, “﹒” means a unit positive charge and “ˊ” signifies a unit negative charge.

VO → VO•• + 2e ' (8) Zni → Zni •• + 2e ' (9) ZnO Al2 O3  → 2 AlZn• + VZn ''+ 2OO + 1/ 2O2 ( g ) (10)

The relationship curve of resistivity, majority carrier concentration and mobility

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in regard to the Al doping amount is obtained as shown in Fig. 8, in order to further study the electrical behavior of the Al-doped ZnO NRs film grown on PET–GR substrate by hydrothermal method. The maximal carrier concentration of AZO NRs/PET–GR is 4.668 × 1017 cm-3 at the Al doping concentration of 9%, but is lower than that of the boron-doped ZnO nanorods prepared by Wang et al. (5.74 × 1019 cm-3).28 The number of oxygen vacancy continues to increase and Al3+ will persistently displace Zn2+ as the increase of Al3+ doping concentration, so that more free carriers generate in the Al-doped ZnO thin film, which is reflected in the rising stage of the blue curve in Fig. 8. The solubility of Al atoms doped in ZnO is restricted, and if the dopant concentration is excessive, Al3+ will form the defects in the lattice, inducing the crystal structure of the thin film to be distorted and the increase of the interface state. Thus, partial charge carriers are captured by them, resulting in the drop in the carrier concentration curve.

Fig. 8 Resistivity, carrier concentration and Hall mobility of the AZO NRs/PET–GR at different Al doping content.

The mobility first increases and then decreases with the increasing Al doping 19



concentration, the maximal mobility is 154.109 cm2V-1s-1, and the minimum value of 61.347 cm2V-1s-1 can also be achieved. Al-doped ZnO nanostructures were prepared on the double-layer flexible substrate PET–ITO by the same method, its mobility data is gained by Hall-effect measurement and depicted in Fig. 9. We can clearly observe that the mobility of AZO/PET–GR is much larger than that of AZO/PET–ITO, and its minimum is 1.54 times of the maximum of the latter (39.818 cm2V-1s-1) and also 3.47 times of the largest mobility (17.7 cm2V-1s-1) reported by Banerjee et al.29. This performance results from that the graphene coated PET substrate itself has higher mobility. The variation in mobility as regards Al doping amount can be explained as follows: When the doping concentration is small, the potential barrier of the grain boundary reduces with the increase of doping content, which gives rise to the enhancement of mobility. As the doping amount is further increased, the high-angle boundary will arise, numerous Al3+ will gather at the grain boundaries, leading to the enlargement of grain boundary scattering, and free electron migration is hampered. Moreover, it can be seen from the formula (10), oxygen molecules are generated while Zn2+ is replaced by Al3+. The oxygen is difficult to escape from the reactor because the autoclave is a confined environment, additionally O atom has small radius, so it is easy to produce Oi served as impurity scattering center in the ZnO lattice. At this point, the impurity scattering impact will be enhanced to cause the decline in mobility.

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Fig. 9 Comparison between the AZO/PET–GR and AZO/PET–ITO in the mobility.

The resistivity decreases persistently with the Al doping content from 0% to 9%, indicating that the proper incorporation of Al atoms as donor impurity is beneficial to increase the conductivity of ZnO NRs thin films. However, the resistivity shows an increasing trend when the doping concentration is overmuch. The lowest resistivity of 0.656 Ω·cm for the AZO NRs/PET–GR is acquired, which is superior to that of the B-doped ZnO thin films (4.01 Ω·cm)9, but is much lower than the resistivity of the 1.2 at.% Al-doped ZnO thin film fabricated by N. Baydogan et al. (10 × 10-3 Ω·cm)30. According to the calculation formula of resistivity for the n-type semiconductor:

ρ = 1/ neµn , we can know that the magnitude of the resistivity depends on the size of the carrier concentration and the mobility, and is inversely proportional to them. Therefore, the resistivity exhibits a change state opposite to that of the carrier concentration and the mobility. 3.6 Electrochemical properties The Fermi level of the intrinsic graphene is near the Dirac point, and it is a

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zero-bandgap semiconductor. However, due to the adsorption of other molecules, such as water molecules in the air, the presence of a small amount of impurities and graphene-substrate interaction, the graphene tends to have a very small band gap at room temperature and atmospheric pressure.31,32 The substrate of PET–GR used in this experiment exhibits p-type electric conduction through the Hall-effect measurement. The heterojunction structure is produced when the n-type Al-doped ZnO is in close contact with the p-type PET–GR. Since the dielectric constants of the two materials are different, the electrons generate additional potentials at each point in the space charge region (SCR), so as to cause the energy band in the SCR to be bend. The energy band diagram of Shockley model for the AZO NRs/PET–GR is demonstrated in Fig. 10. Under UV light irradiation at the wavelength of 365 nm, electrons after absorbing photon energy directly jump from the valence band top to the conduction band, because the band gap of 3.37 eV for ZnO is less than the excitation energy. As a result that holes (electrons) appear on the valence band (conduction band) of ZnO (PET–GR substrate) to form an electron-hole (e- - h+) pair. Due to the existence of the built-in electric filed (Eb) triggered by the diffusion motion of electrons or holes at the interface, its direction is from the n-type ZnO side to the p-type substrate side, forcing the photo-generated electrons (holes)migrate from the conduction band (valence band) of the PET–GR (ZnO) to the conduction band (valence band) of ZnO (PET–GR), so that resulting in the p-region (n-region) potential elevates (reduces). This process is equivalent to adding a positive voltage to the p-n heterojunction, which eventually leads to the generation of photo-current.

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Fig. 10 Energy band diagram of the AZO NRs/PET–GR heterojunction in a thermal equilibrium state. e-, electron; h+, hole; Ec1 and Ev1, conduction band and valence band of a PET–GR substrate; Ec2 and Ev2, conduction band and valence band of the ZnO; EVAC, vacuum level; Eb, built-in electric field.

The photo-current transient response spectra of pure and 9% Al-doped ZnO/PET–GR heterojunctions under UV illumination are measured, as shown in Fig. 11. The photo-generated current density of 9% Al-doped ZnO/PET–GR is 0.7 µA/cm2, which is significantly higher than that of pure ZnO/PET–GR (0.4 µA/cm2), indicating that the photoelectric conversion efficiency of the p-n junction is improved after Al doping. This phenomenon can be attributed to that after Al atoms are incorporated into n-type ZnO, the concentration of the donor impurity in the n-region of the p-n heterojunction increases, and more electrons can be ionized comparing to the un-doped, therefore, the diffusion movement of electrons from the n-region to the p-region is strengthened, resulting in a larger built-in electric field than that of before doping. After several on-off intermittent irradiation, the current densities of pure and Al-doped ZnO/PET–GR both have disturbances with different amplitude and the fluctuation of the AZO/PET–GR heterojunction electrode is larger, declaring that its 23



stability is worse than that of the pure ZnO/PET–GR electrode. The enhancement of photo-current density indicates that the separation efficiency of photo-generated electron-hole pairs in Al-doped ZnO is increased and the recombination probability is relatively low, which is suitable for the application of photodiode.

Fig. 11 Photocurrent transient response spectra of ZnO/PET–GR and AZO/PET–GR with switching the 365 nm UV light illumination on and off.

The analysis of EIS is considered to be one of the main methods for detecting the basic properties of electrode materials for supercapacitor. The typical EIS of ZnO/PET–GR and ZAO/PET–GR electrodes and the corresponding function fitting in the partial regions are shown in Fig. 12. The results of the fitting are recorded in Table 1. From the intersection point of the impedance curve and the horizontal axis in the high frequency region in Fig. 12, it can be seen that the internal resistance of ZnO/PET–GR and AZO/PET–GR electrodes are 13 Ω and 7 Ω, respectively. Lower internal resistance plays an important role in reducing the energy loss of supercapacitors. The circular arc radius of the EIS in the high frequency region refects the size of the interface layer resistance produced on the surface of the working 24


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electrode. The arc radius of the impedance curve for the sample doped with 9% Al is found to be 3806.134 after the function fitting y = B + sqrt[R2 - (x - A)2], where R represents the radius of the circle, which is much smaller than that of the un-doped sample (6426.55). Because the fitting similarity of different specimens is 0.99997 and 0.99588, respectively, and they are close to 1, signifying that the above fitting results are reliable. Consequently, the doping of Al atoms can effectively reduce the interfacial layer resistance of the sample and promote the charge transfer ability of the surface. The slope of the EIS curve in the low frequency region of ZnO/PET–GR and AZO/PET–GR are 2.00 and 4.66, which indicates that the diffusion rate of Al-doped ZnO electrode is larger than that of former, AZO/PET–GR has better electrochemical performance. The curvein the low frequency region of the two electrodes are neither a vertical line nor a line with an inclination angle of 45°, revealing that the formed capacitance is not an ideal electric double-layer capacitor, and may be composed of an electric double-layer capacitor and a Faradaic pseudo-capacitance together.33-35

Fig. 12 Nyquist plots of the electrochemical impedance data of ZnO/PET–GR and AZO/PET–GR.

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Table 1. Local fitting results of the electrochemical impedance spectroscopy of ZnO/PET–GR and AZO/PET–GR Nonlinear function fitting y = B + sqrt[R2 - (x - A)2]

Samples

Fitting similarity (R-Square) ZnO/PET–GR

0.99997

Fitting similarity (R-Square)

0.99679

Radius (R)

Center coordinates X (A)

Center coordinates Y (B)

Slope (b)

Intercept (a)

6426.55

5373.16

-3545.09

2.00

300.33

0.99588

Fitting similarity (R-Square)

0.99988

Fitting similarity (R-Square) Al-dopedZnO /PET–GR

Linear function fitting y = a + b·x

Radius (R)

Center coordinates X (A)

Center coordinates Y (B)

Slope (b)

Intercept (a)

3806.14

3507.79

-1497.30

4.66

-152.33

4 CONCLUSIONS In summary, the Al-doped ZnO thin films had been successfully fabricated on p-type double-layer PET–GR flexible substrate using a simple low-temperature hydrothermal technique. Due to the incorporation of Al atoms and the presence of the graphene layer, the crystal structure and internal stress of ZnO produced slight changes, and the morphologies of AZO/PET–GR were improved compared to that of AZO/PET. Finally, ZnO nanorods with diameters ranging from 0.257 µm to 1.130 µm were obtained. With the increase of Al doping concentration, the red-shift of the near-UV emission peaks occurred and the intensity of the peaks enhanced continuously, and the mechanism of the free-exciton radiative recombination luminescence was also analyzed. The as-prepared AZO/PET–GR had satisfactory Hall mobility, the maximum of 154.109 cm2V-1s-1 and the minimum of 61.347 cm2V-1s-1 were gained, and significantly higher than that of the AZO/PET–ITO prepared by the 26


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same method. This was determined by the fact that the substrate itself had particular electrical properties, making the AZO/PET–GR heterojunction suitable for applications of high-mobility optoelectronic devices. The photo-generated current density and the charge transfer of ZnO thin films were increased after the incorporation of Al atoms, the recombination probability of the photo-produced electron-hole pairs and internal resistance of the p-n heterojunction electrodes were reduced, resulting in AZO/PET–GR had better electrochemical performance than ZnO/PET–GR.

AUTHOR INFORMATION Corresponding Authors *Taotao Ai. E-mail: [email protected]. *Xiaoming Feng. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (Grant No. 51671116), the Project Supported by Key Project of Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2016JZ016), Key Scientific Research Project of Natural Science Foundation of Education Department of Shaanxi Provincial Government (Grant No. 16JS018), and Scientific Research Startup Program for Introduced Talents of Shaanxi University of Technology, China (Grant 27



No. SLGQD16-04).

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TOC graphic

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Fig. 1 XRD patterns of AZO NRs/PET–GR at different Al doping concentrations (a) and the comparison of AZO/PET–GR with powdered ZnO in the case of doping 0% and 9% Al (b). 297x139mm (150 x 150 DPI)



Fig. 2 XPS spectra of the AZO NRs with the concentration of 0.09 M: (a) XPS survey scan spectrum; (b) and (c) high-resolution spectra at Zn 2p and O 1s state energies, respectively. The illustration in (a) corresponds to the Al2p peak. 210x190mm (150 x 150 DPI)


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Fig. 3 FE-SEM images of AZO NRs grown on PET–GR substrate at various Al doping concentrations: (a) 0%; (b) 3%; (c) 6%; (d) 9%; (e) 12%; (f) 15%. The inset of upper right corner is corresponding images at high magnification. 244x122mm (150 x 150 DPI)



Fig. 4 FE-SEM images of Al-doped ZnO films only grown on PET substrate at different Al doping concentrations: (a) 0%; (b) 3%; (c) 6%; (d) 9%; (e) 12%; (f) 15%. The inset of upper right corner is corresponding images at high magnification. 245x123mm (150 x 150 DPI)


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Fig. 5 Growth mechanism of the AZO NRs/PET–GR prepared by hydrothermal method. 250x145mm (150 x 150 DPI)



Fig. 6 Room-temperature PL spectra of AZO NRs thin film with different Al doping concentrations prepared on PET–GR (a) and only PET (b) substrate. 237x90mm (150 x 150 DPI)


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Fig. 7 Schematic diagram of radiative recombination luminescence for free exciton in ZnO NRs films. 208x138mm (150 x 150 DPI)



Fig. 8 Resistivity, carrier concentration and Hall mobility of the AZO NRs/PET–GR at different Al doping content. 297x202mm (150 x 150 DPI)


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Fig. 9 Comparison between the AZO/PET–GR and AZO/PET–ITO in the mobility. 297x229mm (150 x 150 DPI)



Fig. 10 Energy band diagram of the AZO NRs/PET–GR heterojunction in a thermal equilibrium state. e-, electron; h+, hole; Ec1 and Ev1, conduction band and valence band of a PET–GR substrate; Ec2 and Ev2, conduction band and valence band of the ZnO; EVAC, vacuum level; Eb, built-in electric field. 150x124mm (96 x 96 DPI)


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Fig. 11 Photocurrent transient response spectra of ZnO/PET–GR and AZO/PET–GR with switching the 365 nm UV light illumination on and off. 297x229mm (150 x 150 DPI)



Fig. 12 Nyquist plots of the electrochemical impedance data of ZnO/PET–GR and AZO/PET–GR. 297x229mm (150 x 150 DPI)


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TOC Graphic 85x47mm (150 x 150 DPI)


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