Influence of Substrates on the Photoelectrochemical Performances of

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Influence of Substrates on the Photoelectrochemical Performances of AgZnSnSe Thin Films 2

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Johnson Henry, Kannusamy Mohanraj, and Ganesan Sivakumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11239 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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The Journal of Physical Chemistry

Influence of Substrates on The Photoelectrochemical Performances Of Ag2ZnSnSe4 Thin Films J. Henry1, K. Mohanraj1,* and G. Sivakumar2 1Department

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of Physics, Manonmaniam Sundaranar University, Tirunelveli 627 012, Tamil Nadu, India

Centralised Instrumentation and Service Laboratory (CISL), Department of Physics, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India * Corresponding author: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

This work describes the photoelectrochemical (PEC) performance of Ag2ZnSnSe4 (AZTSe) thin films prepared on various conductive substrates such as Al, Cu, Ag and conventional glass by thermal evaporation method. The X-ray diffraction (XRD) analysis confirms that the deposited films are pure tetragonal structure of AZTSe. The Scanning Electron Microscope (SEM) images show spherical rich particles with polygonal shaped which are uniformly distributed over the substrates and are densely packed. By comparing the SEM images, the average size of Ag/AZTSe film seems to be higher than the other two films. The Energy dispersive X-ray (EDX) analysis is witnessed for nearly stoichiometric values for the films. The optical absorption coefficient of AZTSe thin films is found about 104 cm-1 and its band gap is found in the range of 1.54 eV-2.15 eV. The Mott-Schottky Plot, shows n-type conductivity for all the films. The J-V plot confirms the photoactivity for all the films. Among the films, the Ag/AZTSe film shows higher Power Conversion Efficiency (PCE) about 0.32%. However, further investigation is needed to optimize the thickness of the conductive layer for improving the efficiency of Photoelectrochemical Cell (PEC) in comparison to the industrially established materials. 1. INTRODUCTION Cu-based kesterite materials (Cu2ZnSnSe4) belong to I2-II-IV-VI4 group1 and it is found to be a potential candidate in photovoltaic devices such as photoelectrochemical cells (PEC) and solar cells. However its efficiency is registered only 12.7 %2 as limited by the deficit in the open circuit voltage. Actually, the absorber Cu2ZnSnSe4 (CZTSe) exhibits severe band tailing, which causes a high density of defects (antisite defect) and disorder on the I-II (CuZn) sublattice.3 The former defect is due to the closeness of ionic radius among Cu+ (0.91 Å), Zn2+ (0.88 Å) and Sn4+ (0.83 Å) and hence more non radiative recombination centers are


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produced.4 Besides, the CuZn antisite defect has low formation energy and high concentration due to the small size and chemical difference between Cu and Zn, making the samples always exhibit p-type conductivity. The formation of CuZn is a bottleneck problem to achieve higher efficiency. In order to solve this issue, a larger size, atoms (eg. Ag, Cd) can be used to replace either Cu to Zn. Importantly, this approach does not alter the co-ordination among the cation elements and crystal structures, while it alters both electronic structures4 and the optical bandgap (1 eV – 2.5 eV) of the parent materials.5-9 It was reported the substation of Ag in the CZTSe decreases the band tailing and improve minority carrier lifetime and it has registered solar cell efficiency of about 10.7%.10 The substation of Ag in the CZTSe decreases the band tailing and improve minority carrier lifetime. Qi et al., 2017 reported that the power conversion efficiency of the CZTSSe thin films increases from 7.39 to 10.36% when increasing the Ag concentration.11 Gershon et al., 2016 reported about 10.2% efficiency for (AgxCu1-x)2ZnSnSe4 thin films by co-evaporation technique.12 In spite of Ag2ZnSnS4 & Ag2ZnSnSe4 (AZTS & AZTSe) is a n-type material which shows noticeable photoconversion efficiency.12-18 Surprisingly, there are some authors have reported that AZTS thin films show p-type conductivity5,19 as similar to the CZTSe thin films. The conductivity of AZTSe is not clearly understood, hence its necessary to study the conductivity of the AZTSe thin films for application in photovoltaic devices. Further, film substrates play an important role, because the electrical and optical properties depend on the conductive nature of the substrates.20 The selection of appropriate substrates can be challenging because not only the manufacture and performances but also economic criteria must be taken into account. Several authors have deposited AZT(S,Se) substrates on commercial conductive (FTO, ITO and Soda lime) substrates. However these substrates are more costly. Metallic substrates offer a number of advantages over commercial conducting substrates, they have superior electrical properties.21


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Many studies have focused on FTO/ITO substrates coated with AZTSe. The corresponding literature on metal substrates coated with AZTSe is limited. Also, the effect of various conducting substrates on the PEC response of AZTSe films are scarcely addressed in the literature.22 It is motivated the authors to prepare tunable conductive AZTSe thin films. To the best of our knowledge there is no report available for PEC performances of AZTSe thin films deposition of on a conductive substrate. In this work we reported vacuum evaporated AZTSe thin films obtained on lab made Cu, Al and Ag substrates and studied its photoelectrochemical properties. Cu, Al and Ag substrates have high electrical conducting nature and less cost, hence we selected this substrates. Here we have prepared Al, Cu, Ag substrates by the thermal evaporation method. AZTSe thin films were deposited on the above conductive substrates by the same thermal evaporation method and annealed at 300 ˚C for 2 h. The key factor of this approach is to study the electrical conductivity of the AZTSe thin films over various conductive substrates. The film deposited on Cu substrates shows n-type conductivity due to the Cu inclusion from the substrates whereas films obtained from other conducting surface shows p-type conductivity. As expected that the p-type AZTSe film shows higher efficiency of about 0.32 % than n-type AZTSe. This work indicates that the conductivity of the AZTSe can be tuned by varying the conductive substrates which alter the photoconversion performances of the AZTSe film. 2. EXPERIMENTAL SECTION Al/Cu/Ag substrates were prepared by thermal evaporation using Al/Cu/Ag metal precursor. A known amount of Al or Cu or Ag powder was placed in a Mo boat for evaporation. HIND HIVAC (Model 12A4D) coating unit was used for the deposition with an operating current of 160 Amp and pressure of about 10-4 mbar.


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The stoichiometric ratio (2:1:1:4) of metal powders Ag, Zn, Sn and Se were ground wellusing agate mortar and pestle and placed in a Mo (5cm x 1cm x 1cm) boat for evaporation. The glass substrates were kept at 15 cm from the Mo boat. HIND HIVAC (Model 12A4D) coating unit was used for the deposition of the AZTSe thin films with the operating current of 160 Amp. The substrates were kept at ambient temperature and the pressure during evaporation was maintained at 10-4 mbar. The films were deposited on glass, Al, Cu and Ag substrates. The deposited films were annealed at 300 ˚C for 2h in the air atmosphere. The prepared AZTSe thin films with various substrates were characterized by using X-ray diffraction (XRD, PANalytical X‘ PERT- PRO diffractometer), RAMAN spectra was recorded using FT-RAMAN spectrometer (BRUKER RFS 27: Stand-alone FT-Raman Spectrometer), UV-Visible (UV-Vis, UV-2400 PC series UV-Visible spectrometer), and Scanning electron microscope (SEM, Carl Zeiss EVO 18 SEM). Photoelectrochemical (PEC) measurements were carried out in a standard 3-electrode configuration. To prepare electrolyte 0.5 M NaOH and 0.5 MNa2S was mixed well then 0.5 M of S powder was introduced into the solid solution. The above mixture is heated at 50 ˚C in 100 ml of distilled water. Then the solution was filtered and the filtrate solution was used as electrolyte to study the PEC performance of AZTSe thin films. The photoelectrochemical cell was fabricated with the configuration of (Ag, Cu, Al)/AZTSe thin films as working electrode along with Ag/AgCl reference electrode, while Pt wire is used as a counter electrode. The light source emits the wavelength ranging from visible to near infrared region and intensity of the light used in this study was 35.4 mW/cm2.

The PEC analysis was performed by the

Electrochemical Analyzer (CHI604E electrochemical workstation) trough Linear Sweep Voltammetry technique (LSV).


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3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the AZTSe thin films prepared on various substrates. In the XRD patterns, polycrystalline peaks appear at 2θ=26.8˚, 45.0˚ and 53.4˚ belong to the reflection planes (112), (204) and (312) respectively of tetragonal structured AZTSe phase. In the case of film deposited on Ag substrate shows some additional peaks belong to the hexagonal structure of SnSe2 and ZnSe (JCPDS Card No.: 230602 & 150105). Importantly, the conducting substrates do not alter the position of the prominent plane (112) whereas the intensity of the peaks varies with substrates. Similar type of results were reported on Cd0.5Fe0.5Se thin films deposited on various substrates.23 The average crystallite size was estimated by using Scherer’s formula,24 0.9 𝜆

(1)

𝐷 = 𝛽cos 𝜃

where β is full width at half maximum (FWHM), λ is the wavelength of the X-ray source, θ is the Bragg angle. The microstrain can be calculated from the following relation. 𝜀=

𝛽 tan 𝜃

(2)

The dislocation density of the prepared films can be calculated from the following relation 𝛿=

1

(3)

𝐷2

Lattice parameter for the tetragonal CZTSe system can be calculated by the following formula 1

= 𝑑2

ℎ2 + 𝑘2 𝑎2

𝑙2

(4)

+ 𝑐2

and cell volume can be calculated by 𝑣 = 𝑎2𝑐

(5)


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The calculated crystallite size, dislocation density, microstrain and the lattice parameters values are tabulated in Table 1. The obtained lattice parameter value is well agreed with the earlier report of AZTSe thin films.25 The crystallite size is found to be decreased for the films deposited at conductive substrates. Tetragonal distortion of the crystal structure is revealed by the deviation of the structural factor (c/2a) from unity.26 It is observed from table 1 that the structural factors of the films increases from 0.992 to 1.007 with various conductive substrates. The results suggest that the AZTSe thin films on conductive substrates process more strains on the films which agrees with strain values given in Table 1 XRD is not alone considered as a sufficient tool to analyse AZTSe films, hence Raman spectra is also recorded for the prepared films. Figure 2 shows the Raman spectra of the AZTSe thin film deposited on various conductive substrates. In the spectrum of AZTSe, some bands appear at 170-174 cm-1, 188-199 cm-1, and 206 cm-1 that confirm the formation of AZTSe thin films16 and the band observed at 237 cm-1 is due to Ag2Se as secondary phase.27 While, in the case of films deposited on Cu, Al and Ag films shows band shift with additional phases ZnSe (at 248-257 cm-1) and SnSe (at 130-156cm-1).28,

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A band is observed at 291 cm-1 in the

Cu/AZTSe film due to CuSe.30 The observed results are coinciding well with XRD pattern of the AZTSe films. It is noticed from our XRD and Raman analysis; that the Al does not react with chalcogenide whereas Ag and Cu atoms partially react with chalcogenide. Similar type of results was reported for CZTSSe thin films deposited on Au, W and Pt substrates.31 Figure 3 shows the SEM images of AZTSe thin films deposited on various conductive substrates. The SEM images of the film deposited on Ag substrates show more denser particles when compared to other films. The individual spherical grains with boundaries are clearly seen which are uniformly distributed over the surface. Some irregular shape grains were also found over the substrate surface. Similar results were recorded for CZTS thin films deposited on


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various conductive substrates.32 Average particle size is measured to be less than 150 nm for the AZTSe thin films deposited on commercial glass and Ag substrates. While the average particle size is measured to be less than 100 nm for Al/AZTSe and Cu/AZTSe. The result is in accordance with the crystallite size calculated from XRD analysis. The presence of larger particle size in Ag/AZTSe is then expected to be beneficial to conductivity.33 As the particle size of the thin films increases, this implies that the amount of grain boundary decreases. The extent of grain boundary scattering decreases because of the decrease in the amount of grain boundary. It is well known that the polycrystalline materials are composed of a large number of crystallites linked by grain boundaries. Potential barriers formed at the grain-grain interfaces can strongly suppress the flow of majority carriers and provide efficient recombination centers for the minority carriers. The thickness of the films was measured using the cross sectional view of the SEM images as shown in Supporting Information (Fig. S1). The cross section image clearly shows the bilayer for Ag/AZTSe, Cu/AZTSe and Al/AZTSe thin films. The thickness of the AZTSe layers was found to be about 1 μm. EDX pattern of AZTSe, Al, Cu and Ag are also given in supporting information (Fig. S2). The EDX spectra of the Al, Cu and Ag films show 100% Al, Cu and Ag respectively. The EDX elements of AZTSe thin films deposited on glass substrate show Ag-8.03, Zn-37.98, Sn-17.58 and Se- 36.43 values of Ag2ZnSnSe4 compound. From the value, it is found that the sample has excess Zn and Se vacancies. Figure 4 shows the UV-Visible spectra of AZTSe thin films deposited on various substrates that indicates a good absorption in the visible region. The films deposited on glass substrates shows higher absorption in the visible range. It can be seen from the figure that the optical absorption declines around 700 nm owing to the onset of fundamental absorption and the onset absorption edge shifts towards higher wavelengths for the films deposited on Cu and Al substrates. Thus, widening of the absorption signal. The absorption coefficient ‘α’ was calculated by using the following formula,


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𝛼=

(1𝑡 )𝑙𝑛(𝑇1)

(6)

Where α is the absorption coefficient, t is the thickness of the films. The α value of the the AZTSe films is found to be greater than 104 cm-1 (Fig.S3 given in supporting information). The increase in‘α’ causing a decrease in the transition energy of the charge carriers. In the visible region, the absorption coefficient enhances significantly with decreasing wavelength because of the band-to-band absorption of incident light.32 The result confirms that the deposited films can be used for the optical application. Band gap energy (Eg) is defined as the optical energy band gap between the valence band (V.B) and the conduction band (C.B). Optical band gap energy was calculated by using the following equation, (𝛼ℎ𝜈) = 𝐴(ℎ𝜈 ― 𝐸𝑔)𝑛

(7)

The obtained bandgap value was given in Table 1. The bandgap of the AZTSe films deposited Cu and Ag substrates show higher value attributed to smaller crystallite size (Table. 1) and may contain secondary phases.34 The formation of secondary phase (ZnSe, CuSe, SnSe & SnSe) presented in the material was affected both optical and electrical conductivity. The bandgap of ZnSe & CuSe was found to be 2.7 eV & 2.2 eV respectively.35,

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Due to the

presence of these impurities, the bandgap energy of the AZTSe deposited on conductive substrates increases about 23% than AZTSe thin films deposited on glass substrates. At the same, time the presence of impurities SnSe (1.3-1.7 eV) & SnSe2 (1.6 eV) does not affect the band gap of the AZTSe.37-39 The formation of secondary phase will affect the conductivity of the materials as supported with there are several reports available in the literature. Ahn et al. reported the Eg of quaternary material depends on the existence of secondary phase in the film.40 Tao et al. 2014 reported that the increase in Eg of CZTS is due to the presence of


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secondary phases SnS & ZnS.41 The presence of secondary phase ZnSe (ZnS) in CZTSe (CZTS) increases the Eg value of the materials.42 While the band gap of the Al/AZTSe film shows less band gap (1.5 eV). This may be due to the film contains only major crystalline as confirmed by XRD and Raman analysis. C-V profiling is a key electrical characterization method for studying CZTSSe thin film solar cells. It is also a very useful technique for investigating the space charge density and depletion width. Figure 6 shows the C-V measurements of the AZTSe thin films deposited on different substrates. It is seen from the figure that the capacitances decrease gradually with the increase in reverse bias, demonstrating that the bias voltage can expand the depletion with and shrink the capacitance in AZTSe devices. The space charge density and depletion width can be calculated from C-V graph using the formula43 𝑊𝑑 =

𝐴𝜀𝑜𝜀𝑠 𝐶

Where C, A, ɛo and ɛs stands for the measured capacitance, the active area, the vacuum dielectric constant (~8.854 x 10-12 F/m) and relative dielectric constant respectively. The depletion width Vs space-charge density graph is shown in Fig 7 for AZTSe thin films deposited on various conductive substrates and the estimated depletion width value is presented in Table 2. The obtained depletion width is much lower than the thickness of the electrodes. Cu/AZTSe thin film shows higher depletion region. The photocurrent of the film depends on the depletion layer. A thicker depletion layer is favorable to separate electron-hole pairs but results in higher resistivity.44 Figure 8 shows the Mott-Schottky analysis of AZTSe thin films deposited on various substrates. From the plot, the depletion and accumulation region can be seen for all the films. The depletion regions are related to the depletion of the charge carriers whereas accumulation is related to diffusion of the charge carriers.45 The nature of the depletion region of films shows


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positive slope. Thus the films are n-type conductivity. It is reported that AZTSe and AZTS are n-type material.12-18 While, some reports show p-type conductivity for AZTS (Ma et al., 2017 and Ma et al., 2018, 5, 19). Variation of capacitance (C) with applied potential may be represented by the Mott−Schottky equation,46 1/(Csc)2 = (2/eεε0NAS2)(V– Vfb− kT/e) for n type

(8)

1/(Csc)2 = -(2/eεε0NAS2)(V – Vfb-kT/e) for p type

(9)

where Csc is the space-charge capacitance (in F/cm2), e is the electronic charge (C); ε is the dielectric constant of the semiconductors; ε0 is the permittivity of free space; NA is the carrier density in cm−3; S is the surface area of the electrode, V is the applied potential (V); Vfb is the Flat Band potential (V); k is the Boltzmann constant, and; T is the temperature (K). According to the Mott-Schottky equation, the carrier concentration is inversely proportional to the slope of C-2 vs. E, where C is the space-charge layer capacitance of the film and E is the potential of the electrode.47 From the slope of the plot, we can determine the conductivity, carrier concentration and Flat Band potential of the materials and presented in Table 2. The Flat band potential shows negative values for all AZTSe thin films. The negative values of Vfb indicate reduced recombination

rate

and

better

separation

and

transportation

of

charge–carrier

at

semiconductor/electrolyte junction. A thin depletion layer enables the diffusion of photogenerated charge–carriers, which is a favorable characteristic for PEC activity.48 PEC is one such method to utilize sunlight into electrical/chemical energy. Usually, a two different electrolytic solutions namely polysulphide and iodide based electrolytes are widely used in PEC analysis. In this work, the iodide based electrolyte was preferred owing to thin film working electrodes having higher stability in this electrolyte than that of polysulphide electrolyte solution. The photoactivity of the AZTSe thin films was evaluated using three


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electrode PEC cells. The solid-liquid junction was formed when the AZTSe electrodes as the photoelectrode contact with polysulfide solution as the electrolyte. Figure 9 shows the J-V characteristics of AZTSe thin films deposited on various substrates under visible light. During illuminating the cell, AZTSe semiconductor absorbs the photons with energy equal to or greater than the band gap energy of AZTSe. This causes the creation of electron-hole pairs in the depletion region and in the diffusion layer (e− + h+). They are driven separately by the electric field at the interface.49 The generated minority carriers are driven by the electric field within the space charge region. These carriers move toward the electrode/electrolyte interface and are transferred across the interface to reduce one of the redox species. Here polysulfide solution is acted as mediator in PEC. The oxidized mediator forms iodide/triiodide (Sn2−/S2−) which in-turn obtained an electron at the counter electrode after the electron moved through the electrical load. This reaction occurred repeatedly while the PEC was exposed to light. J-V parameters such as short circuit current, open circuit voltage fill factor and efficiency were calculated and the results are given in table 2. The fill factor and efficiency were calculated by using the following formula23 and their calculated values are given in Table 2. 𝐹𝐹 =

𝐽𝑚𝑎𝑥 𝑋 𝑉𝑀𝑎𝑥

(10)

𝐽𝑠𝑐 𝑋 𝑉𝑜𝑐

𝜂 (%) =

𝐽𝑠𝑐 𝑋 𝑉𝑜𝑐 𝑃𝑖𝑛

𝑋 𝐹𝐹 𝑋 100

(11)

Where Jmax is maximum current density, Vmax is maximum voltage, Voc is the open circuit voltage, Jsc is the short circuit current density and Pin the power density of the incident light. The Jmax and Vmax are obtained at the maximum power point on the photovoltaic power output curve.


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It is observed from the table that the film deposited on Ag substrates shows higher short circuit current and open circuit voltage when compared with others. This may be a fact that the rapid interconversion between Sn2− and S2− can accelerate the generation of photoelectrons from AZTSe thin films. Also, Ag/AZTSe film shows higher electrical conductivity which is well agreed with the impedance plot (fig. 10). Ag/AZTSe film shows higher power conversion efficiency (PCE), which may be due to the bigger crystallite size which in turn decreases the grain boundary and allows the electron to move freely on the surface, hence increases the PCE.50 Moreover, the Ag/AZTSe thin film shows higher relaxation time which reduces the recombination of photogenerated electron-hole pairs. The comparison table 3 is given for conductive substrates such as Al, Ti, Cu, Ag etc. By comparing with the reports our films shows higher Jsc and PCE. From our observation we found that the efficiency of the prepared films is lower than the CZTS films attributed to the presence of secondary phases which reduces the optical absorption and lower crystallite size, the bigger size particle is very advantageous for solar cells, which influences higher diffusion length. The presence of p-type impurities (SnSe & CuSe) may form diode/insulating nature in the absorbing layer.58,59 Which causes high photo carrier recombination and hence it affects the PEC performances of the host atoms.60 We further investigated the PEC properties of the photoanodes via EIS. Recent studies have established Electrochemical Impedance Spectroscopy (EIS) as a powerful technique to characterize the dynamics of charge transfer at the electrode/electrolyte interface, as well as charge transport in the bulk of the semiconductor. Data from EIS measurements is often shown in form of Nyquist plots, where the real component and negative imaginary component of the complex impedance Z are plotted on the horizontal and vertical axis, respectively.61 Figure 10 shows the Nyquist plot, equivalent circuit and error calculation of the AZTSe thin films deposited on various conductive substrates. The Nyquist plot shows semicircle nature for all the


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films. The semicircular nature of the AZTSe films substantiates the dominance of the space charge region in the photovoltaic devices.62 It is observed from the films that the diameter of the arc decreases for the films deposited on Al and Ag substrates which results increase in electrical conductivity due to the reduction in resistance of the samples.33 The results are well reflected in our PEC analysis where the film deposited on Ag substrate shows higher open circuit voltage and efficiency. The Nyquist plot is well fitted using ZSimpWin 3.21 software and the resulting equivalent circuits are given in Fig.8 and its parameters are listed in Table.4. The electrochemical equivalence circuit shows resistance (R), constant phase angle element (CPE) (Q), Capacitance (C) and inductance (L). CPE is a practical way to describe the interfacial characteristics. In CPE constant Y0 is a parameter related to the properties of both the surface and the electroactive species. While the exponent n has a non-unity value when the surface is rough when n =1 the CPE becomes a pure capacitance with C= Y0, indicating that true area is the same as the geometric area.63 From table 3 it is found that the resistance value is lower for Ag/AZTSe thin films which imply higher electrical conductivity. The lower resistivity is due to the bigger grain size (from XRD and SEM analysis). Figure 11 shows the Bode plot of AZTe thin films deposited on various conductive substrates. The phase angle value is close to 55º. The ideal capacitance should have phase angle 90º, in our work the obtained phase angle is less than ideal one which supports the replacement of capacitance by constant phase element (CPE) in the electrochemical equivalence circuit. The observed characteristic frequency (f0) of Cu/AZTSe and Al/AZTSe films shifts towards high frequency region which clearly shows the decrease in the electron lifetime.64 The relaxation constant (τ) has been calculated by using the formula and the values are tabulated in Table 2.

𝜏=

1 𝑓0


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From the table 2 it is found that the film deposited on Ag substrate shows a higher relaxation time, which is very necessary for photovoltaic applications to avoid recombination loss. 4. CONCLUSIONS In this work we have prepared tunable electrical conductive AZTSe thin films by vacuum evaporation method. AZTSe thin films are deposited on Cu, Al and Ag films and studied its structural, optical and electrical properties. XRD analysis shows tetragonal structure for the AZTSe thin films. Among the films, Ag/AZTSe film shows higher crystallite size. Raman spectra analysis also confirms the formation of AZTSe with some secondary phases. The AZTSe films show higher absorption in the visible region. Mott-Schottky plot shows n-type conductivity for AZTSe thin films deposited on conducting substrates. The impedance spectra show semicircle nature for all the films. The PEC studies show higher efficiency for Ag/AZTSe thin films. The results demonstrate that the Ag film can act as a suitable substrate for AZTSe materials than commercially available substrates.


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FIGURES

Figure 1: XRD pattern of the AZTSe thin films prepared on various substrates


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Figure 2: Raman of the AZTSe thin films prepared on various substrates


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Figure 3: SEM images of the AZTSe thin films prepared on various substrates


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Figure 4: UV-Visible spectra of the AZTSe thin films prepared on various substrates

Figure 5: Bandgap plot of AZTSe thin films prepared on various substrates


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Figure 6: C-V plot of AZTSe thin films deposited on various conducting substrates

Figure 7: Depletion width and space-charge density derived from C-V curves

Figure 8: Mott-Schottky plot of AZTSe thin films deposited on various conducting substrates


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Figure 9: J-V plot of AZTSe thin films deposited on various conducting substrates


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Figure 10: EIS plot of AZTSe thin films deposited on various conducting substrates


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Figure 11. Bode plot of AZTSe thin films deposited on various conductive substrates.


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TABLES Table 1. Crystalline parameters of AZTSe thin films a (Å)

c (Å)

V (Å)3

c/2a

Eg (eV)

6.25 x 1014

5.68

11.31

364.88

0.996

1.771(1)

0.0246

16.0 x 1014

5.68

11.31

364.88

0.995

1.541(5)

Cu/AZTSe 25

0.0246

16.0 x 1014

5.68

11.27

363.59

0.992

2.157(3)

Ag/AZTSe 33

0.0183

9.2 x 1014

5.64

11.36

361.35

1.007

2.180(1)

Code

D (nm)

Microstrain Dislocation Density

AZTSe

40

0.0147

Al/AZTSe

25

Table 2: Flab band potential, Carrier concentration and PEC parameters of the AZTSe thin films Code

Conductivity

VFB

Depletion width (nm)

N

Voc (A)

Jsc (mA)

FF (%)

Efficiency (%)

Al/AZTSe

n-type

-0.169

0.103

2.01x1020

0.11

1.37

25.5 0.128

1.46

Cu/AZTSe

n-type

-0.903

108

1.1x1017

0.06

1.21

26

1.21

Ag/AZTSe

n-type

-0.709

0.476

4.236x1019 0.148 2.58

0.108

25.6 0.32

Relaxation time (μs)

1.33


24

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: Comparison of PEC parameters with different compounds deposited on various substrates

S.No Compound

Substrate

D (nm) 45 15 11.53 10.20 14.55 42 39 20 71.96 63.26 53.74 47.60 45.91 49.95 53.81 64.69 40 50 66 25

E (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Bi2Se3 Bi2Se3 CdSe CdSe CdSe Sb2S3 Sb2S3 Sb2S3 Cd0.5Fe0.5Se Cd0.5Fe0.5Se Cd0.5Fe0.5Se CZTS CZTS CZTS CZTS CZTS CZTS CZTS CZTS Cu2ZnSnSe4 Cu2ZnSnSe4 Cu2ZnSnSe4 Ag2ZnSnSe4

ITO FTO SS Ti FTO Ti SS FTO Al Cu SS ITO ITO ITO ITO ITO ITO ITO ITO Cu Ag Al Cu

24 25

Jsc (A)

1.9 1.5 1.8 1.95 1.95 1.95 1.31 1.40 1.43 1.51 1.18 1.22 1.31 1.38 1.402 1.196 1.322 2.157

Voc ( V) 462 mV 643 mV 312 mV 303 mV 356 mV 315 66 251 706 mV 391 mV 269 mV 0.42 V 0.46 V 0.51 V 0.39 V 0.28 V 0.30 V 0.31V 0.38 V 100 180 845 0.06

Ag2ZnSnSe4 Ag Ag2ZnSnSe4 Al

Ƞ (%)

References

0.304 mA 0.297 mA 0⋅560 mA 0⋅100 mA 0⋅850 mA 16 x10-6 30 x10-6 1006 x10-6 0.897 μA 0.665 μA 0298 μA 12.88 mA 13.04 mA 15.23 mA 8.81 mA 3.49 mA 4.08 mA 5.53 mA 6.49 mA 4.5x10-3 4x10-3 20x10-3 1.21 x10-3

FF (%) 0.21 0.22 28.81 27.06 39.25 47.14 50.50 59.5 24 22 19 43 48 49 38 0.29 0.31 0.29 0.96 28 24 30 26

0.09 0.14 0.123 0.02 0.297 0.0047 0.0020 0.298 0.33 0.12 0.03 2.33 2.87 3.81 1.31 0.29 0.38 0.50 0.96 0.43 0.58 1.6 0.108

0.148

2.58 x10-3

25.6

0.32

0.11

1.37 x10-3

25.5

0.128

51 51 52 52 52 53 53 53 54 54 54 55 55 55 55 56 56 56 56 57 57 57 Present Study Present Study Present Study

33

2.180

25

1.541


25


Page 26 of 37

Table 4. Equivalent circuit parameters of AZTSe thin films

Parameters

Al/AZTSe

Cu/AZTSe

Ag/AZTSe

R1 (Ω)

26.9

1410

25

C1 (F)

1x10-5

1x10-5

1x10-5

R2 (Ω)

26.9

1410

25

Yo (S.secn)

1x10-5

0.01

1x10-5

n

0.8

10

0.8

R3 (Ω)

26.9

1410

25

L1(H)

10

10

10

R4 (Ω)

26.9

1410

25

C2 (F)

1x10-5

-

1x10-5

R5 (Ω)

26.9

1410

25

L2 (H)

-

10

-

Q1


26

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information (docx.) Supporting information contains the Cross sectional SEM images and EDX patterns of the AZTSe films AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] ACKNOWLEDGEMENT The author J. Henry acknowledged his sincere thanks to University Grants Commission (UGC), New Delhi, India for providing financial support through Basic Scientific Research (BSR) fellowship. The authors are grateful to V.V College of Engineering, Tisaiyanvilai, Tirunelveli, Tamil Nadu, India for recording UV-Visible analysis. The authors are grateful to International Research Center, Kalasalingam University, Anand Nagar, Krishnankoil-626 126, Tamil Nadu, India for recording SEM analysis. The authors are grateful to SAIF-IIT Madras, Chennai, Tamil Nadu, India for Raman analysis. REFERENCES (1) Mali, S.S.; Patil, B.M.; Betty, C.A.; Bhosale, P.N.; Oh, Y.W.; Jadkar, S.R.; Devan, R.S.; Ma, Y.R. ; Patil, P.S. Novel synthesis of kesterite Cu2ZnSnS4 nanoflakes by successive ionic

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Table of Contents (TOC) Graphic


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Influence of Substrates on the Photoelectrochemical Performances of

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