Multifunctional CZTS thin films: Structural, optoelectrical, electrical and photovoltaic properties

Multifunctional CZTS thin films: Structural, optoelectrical, electrical and photovoltaic properties

Accepted Manuscript Multifunctional CZTS thin films: Structural, optoelectrical, electrical and photovoltaic properties S.S. Fouad, I.M. El Radaf, Pan...

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Accepted Manuscript Multifunctional CZTS thin films: Structural, optoelectrical, electrical and photovoltaic properties S.S. Fouad, I.M. El Radaf, Pankaj Sharma, M.S. El-Bana PII:

S0925-8388(18)31711-0

DOI:

10.1016/j.jallcom.2018.05.033

Reference:

JALCOM 46006

To appear in:

Journal of Alloys and Compounds

Received Date: 5 March 2018 Revised Date:

21 April 2018

Accepted Date: 3 May 2018

Please cite this article as: S.S. Fouad, I.M. El Radaf, P. Sharma, M.S. El-Bana, Multifunctional CZTS thin films: Structural, optoelectrical, electrical and photovoltaic properties, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.05.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Multifunctional CZTS Thin Films: Structural, Optoelectrical, Electrical and Photovoltaic properties S.S.Fouad a, I.M.El Radaf b, Pankaj Sharma c*, M.S.El-Bana a,d a

Nano-Science & Semiconductor Laboratories, Department of Physics, Faculty of Education, Ain Shams

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University, Cairo, Egypt b

Electron Microscope and Thin Films Department, Physics Division, National Research Centre, Dokki, Giza

12622, Egypt c

Department of Physics & Materials Science, Jaypee University of Information Technology, Waknaghat -

173234, India

Materials Physics and Energy Laboratory, College of Sciences and Art at Ar Rass - Qassim University, Ar

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d

Rass 51921, Kingdom of Saudi Arabia

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Abstract

High-quality Cu2ZnSnS4 (CZTS) thin films of different thicknesses (240 nm to 418 nm) have been prepared onto pre-cleaned glass substrates using a low-cost chemical bath deposition technique. Influence of deposition conditions on the variation of the structural, optical, optoelectrical and electrical properties of the CZTS films has been inspected. The XRD

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patterns revealed that CZTS thin films exhibited a polycrystalline nature and follow the kieserite crystal structure. The studied films have exhibited direct energy gap transition that decreased from 1.54 to 1.48 eV. Profound analysis has been made to investigate the variation of the optical and optoelectrical properties of CZTS as a function of deposition time. A

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prominent increase in the nonlinear parameters has been noticed with the increase in the deposition time. This finding could shed lights on the possibility of using CZTS in nonlinear

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devices. The variation of the DC conductivity with temperature for the CZTS thin films has been discussed. The values of the activation energies have been decreased with increasing the film thickness which confirms the increase in film uniformity. Good correlations have been established between the structural aspects of the films to their optical and optoelectrical parameters. Cu2ZnSnS4 film (418 nm) has been selected to be utilized in fabricating the Al/nSi/P-CZTS/Au heterojunction. Photovoltaic behavior was observed for the fabricated device which had a solar conversion efficiency of 3.37 %. Keywords: CZTS thin films; Chemical bath deposition; Linear and nonlinear optics; Optoelectrical properties; Photovoltaic Device. *Corresponding author: [email protected] 1

ACCEPTED MANUSCRIPT 1. Introduction Solar energy has been conceived as the most prominent future energy assets owing to its amply inexhaustible existence, inexhaustible power, and small effects on the environment. Irrespective of the high efficiency of Si-based solar cells, the use of these solar cells is finite due to less flexibility, heavy weight, high cost of installation etc. Considering this a large number of

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alternatives have been enquired to develop flexible, light weight, low cost, high efficiency solar cells.

Further, there has been an increased recall on utilizing abundant, inexpensive and environmentally friendly materials in fabricating thin film solar cells. For almost last decade, the thin film solar cell has been utilized the CuInxGa(1−x)Se2 (CIGS) as an absorber material since it achieved the highest efficiency beyond 20% [1, 2]. The seek for cost reduction for

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mass production and the need for free of toxic elements contribute to rising up the Cu2ZnSnS4 (CZTS) based solar cell as one of the next-stage thin film solar cells [3]. CZTS

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reveals a direct band gap between 1.45 eV and 1.6 eV and large absorption coefficient over 104 cm−1 in the visible light range [4]. Moreover, CZTS is considered as a good candidate in the forthcoming thin film solar cells since its theoretical conversion efficiency is 32% [5]. Cu2ZnSnS4 thin films have been prepared by different techniques viz. sputtering [6], thermal evaporation [7], spray pyrolysis [8], electrodeposition [9], dip coating [10], SILAR method [11], spin coating [8], sol-gel [12], solvothermal method [13] and chemical bath deposition

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(CBD) [14]. The highest conversion efficiency of CZTS achieved using vacuum techniques such as sputtering, thermal evaporation, etc. was about 9.6 % [15]. Whereas the highest achieved efficiency of CZTS thin film solar cell by non-vacuum techniques such as spray

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pyrolysis, sol-gel, etc. was about 12.6 % as reported by Todorov et al [16]. Among the nonvacuum techniques, CBD process is simple, low cost, produces large deposition area, and an effective method for producing of nano-sized thin films [17]. In the present study, we prepare

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Cu2ZnSnS4 thin films by chemical bath deposition technique. The influence of deposition time on the structure, morphology, optical, optoelectrical and electrical characteristics of Cu2ZnSnS4 thin films has been investigated in order to determine the best film suitable for a solar cell as an absorber layer. In addition, to the best of our knowledge, we report here one of the few trials made to investigate the correlations between the optical and optoelectrical properties of CZTS thin films.

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ACCEPTED MANUSCRIPT 2. Experimental techniques 2.1. Films preparation and characterizations The Cu2ZnSnS4 thin films have been deposited onto pre-cleaned microscopic glass substrates by the chemical bath deposition (CBD) technique. The CuNO3, ZnNO3, SnCl2, and C2H5NS are used as sources for Cu+, Zn+2, Sn+4 and S-2 ions respectively. Copper nitrate (0.1

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M) was dissolved in 20 ml de-ionized water and stirred for 5 min by a magnetic stirrer. 40 ml of 0.2 M C2H5NS was added slowly in Copper nitrate. Then 0.05 M of zinc nitrate was added to the solution and stirred well. Finally, 0.05 M of SnCl2 was combined with the mixture and stirred well for 30 min. The deposition was carried out at a constant bath temperature of 80 o

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C. The pH of the solution adjusted at 10. After 30 minutes a brown thin film formed onto

both sides of the substrate. At the end of the deposition, the substrates were washed well with

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double distilled water and dried. Alpha step D 500 stylus profilometer was utilized to evaluate the thickness of the film. Films kept in solution for 3, 5 and 8 hours have recorded thicknesses of 240, 351 and 418 nm, respectively. The structural of Cu2ZnSnS4 thin films have been examined by X-ray diffraction (XRD), using Philips X-ray diffractometer (X’Pert) with monochromatic CuKα radiation, operating at 40 kV and 25 mA. The field-emission scanning electron microscopy (FESEM, Quanta FEG 250, FEI, USA interfaced with Energy

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dispersive X-ray (EDS) unit) has been utilized to inspect the chemical compositions of the prepared films. Also, the surface morphology of the films has been investigated using the recorded images by the high-resolution field emission scanning electron microscopy (FESEM) (SEM; Inspect S, FEI, Holland). Furthermore, the morphology and the grains size

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of the as-deposited thin films have been studied using the high-resolution transmission electron microscopy (HRTEM) (JEM 2100, Jeol, Japan). The transmission (T(λ and

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reflection (R(λ spectra of the films have been recorded in the wavelength range 400 nm –

2500 nm at normal light incidence using a double beam spectrophotometer (JASCO, v-570). The electrical conductivity has been measured by a four-probe configuration in the temperature range 300 to 500 K. The resistance (R) has been recorded using a digital electrometer (Keithley 614). 2.2. Preparation and characterization of the Al/n-Si/P-CZTS/Au hetero-junction In this work, Al/n-Si/P-CZTS/Au hetero-junction has been fabricated. Si wafer (doped

with phosphorus) substrate with (100) orientation, 0.5 mm thickness and 1–10 Ω.cm

resistivities was utilized in the device fabrication. HF has been used to etch the n-type single 3

ACCEPTED MANUSCRIPT crystal silicon wafer to eliminate any oxide layers [18, 19]. Then, the Si wafer was cleaned out by methyl alcohol and distilled water respectively. Then, the Ohmic contact was made by depositing Al electrode on the back side of the Si substrate. Cu2ZnSnS4 thin film (418 nm) has been chemically deposited onto the top of the Si wafer. Finally, an Au electrode has been evaporated onto the Cu2ZnSnS4 thin film in a shape of the grid using shadow mask made

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from aluminum foil. The current-voltage (I-V) characteristic of Al/ n-Si/ P-Cu2ZnSnS4/ Au heterojunction was measured by using high impedance electrometer (Type Keithley 614). 3. Results and Discussion

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3.1. Structural Investigations 3.1.1 X-ray diffraction studies

The crystallographic structure of the studied films has been analyzed by X-ray

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diffraction technique (XRD) in the range 2θ = 10 − 80°. Figure 1(a) shows the XRD patterns of Cu2ZnSnS4 thin films. The patterns showed that the Cu2ZnSnS4 thin films are polycrystalline in nature and the crystallinity increases as deposition time increases. The XRD peaks are attributed to the diffraction pattern of kieserite (tetragonal) structure of CZTS (JCPDF card No. 26-0575) which reveals the formation of CZTS phase.

equation [20]:

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The grain size (D) has been calculated for Cu2ZnSnS4 thin films via Scherer’s

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=

0.9  (1   

where D is the grain size, β is the full-width at half maximum (FWHM) of the peak (in

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radians), θ is the Bragg’s diffraction angle at the peak position and λ = 1.5406 Å is the wavelength of the X-ray radiation. The obtained values of the grain size (D) are tabulated in Table 1. As noticed the increase of the film thickness (240 nm to 418 nm) is accompanied by the increase in the grain size (19.84 nm to 38.32 nm). Furthermore, the lattice strain,  and

dislocation density, δ of CZTS thin films have been evaluated using the following relations [21, 22]: = =

   (2 4 1 (3  4

ACCEPTED MANUSCRIPT The values of lattice strain, ε and dislocation density, δ have been found to decrease as the deposition time increase (see Table 1). Figure 1(b) reveals that the increase in the deposition time is accompanied by an increase in the grain size D and a decrease in the lattice strain . This means that the crystallization approaches more perfect with increasing the film

thickness. This returns to the decrease in the interspacing between film grains which reveals

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the formation of higher quality films. In other words, with the increase of deposition time, the dislocation density and the micro-strain are reduced because stress is released in the film during deposition. 3.1.2 Morphology analysis

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FESEM and HRTEM have been utilized to analyse the morphology of the Cu2ZnSnS4 films. The HRSEM micrographs of the CZTS thin films (see Fig. 2(a-c)) display that the

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films are homogeneous and uniform. The films contain circle-shaped grains that increase in size due to the increase of deposition time. The average grain size indicated that the Cu2ZnSnS4 thin films have nanostructure feature. The elemental composition of Cu2ZnSnS4 thin films has been looked into by energy dispersive X-ray spectrometry (EDS). Figure 2(d) presents the EDS of CZTS thin film deposited at 8 hours as a representative example. The pattern affirms the existence of copper, zinc, tin, and sulfur. The atomic ratio of Cu, Zn, Sn

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and S in the as-deposited Cu2ZnSnS4 thin films is near 2:1:1:4 which is close to the stoichiometric composition. Figure 3(a-b) shows the HRTEM and the corresponding electron diffraction pattern of the as-deposited Cu2ZnSnS4 thin film deposited at 30 minutes. As evident from TEM micrograph, the grains have spherical shapes and the grain size of the as-

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deposited film has been found in the range 50 nm. The electron diffraction reveals the nanocrystalline nature of the films as can be noticed from the rings pattern in the selected

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area of electron diffraction. Also, the discrete spots in the ring pattern confirm that the film consists of nano-sized grains. 3.2 Optical characterizations 3.2.1 Transmittance and reflectance analysis The application of CZTS thin films in solar cells requires the detailed study of the optical properties. Here we have investigated the interaction of light with the material using the UV-Visible-NIR spectroscopy by obtaining the transmission and reflection spectra. The spectrophotometric measurements of T(λ and R(λ have been utilized in determining the

optical parameters such as; the absorption coefficient (!), extinction coefficient (k) and the 5

ACCEPTED MANUSCRIPT refractive index (n) of CZTS thin films. The variation of optical reflectance and transmittance as a function of wavelength for Cu2ZnSnS4 thin films deposited at different deposition time have been represented in Fig. 4(a-b). From these plots we can observe that the transmittance decreases with increasing of deposition time from 3 to 8 hours while the reflectance has been found to increase with the increase of deposition time from 3 hours to 8 hours.

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The absorption coefficients, α, of the CZTS thin films deposited at different deposition time (3, 5 and 8 hours) has been determined from the transmittance T (λ) and reflectance, R (λ) and film thickness d by the following formula [23]: (#()* +

+-

(#(). /+*

+ R 0

#⁄

2 (4)

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#

α = $ ln '

Figure 5(a) explains the spectral dependence of the absorption coefficient on the wavelength

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for the Cu2ZnSnS4 thin films. It has been observed that the absorption coefficient of Cu2ZnSnS4 thin films increases as wavelength increases beside it also increases with the increase in the deposition time. Also, films reveal large absorption coefficient over 104 cm−1 in the visible light range which is in agreement with the literature [2, 4]. This confirms the high quality of the prepared films to be employed as an absorber film in the solar cells.

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The extinction coefficient (k) has a significant role in determining different optical parameters, as those are connected with the absorption of light waves and the dielectric constants. The extinction coefficient (k) of the Cu2ZnSnS4 thin films deposited at different deposition time (3, 5 and 8 hours) has been evaluated from the relation [24, 25]: 4 (5 45

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3 =

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Figure 5(b) represents the dependence of the extinction coefficient on the wavelength of CZTS thin films. The extinction coefficient has been found to decrease with the increase of wavelength λ, whereas, it increases with increasing the deposition time from 3 to 8 hours. For the increased time of deposition, there may have been agglomeration of particles which leads to an increase in extinction coefficient. The calculation of refractive index is important for the materials which used for the fabrication of optical devices, optical filters, and switches. The refractive index (n) of the Cu2ZnSnS4 thin films deposited at different deposition time (3, 5 and 8 hours) has been

determined from the extinction coefficient (k and reflectance (R) by the following relation [26]: 6

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* (1 + 9 49  8= −: − 3 ; (6 (1 − 9 (1 − 9

Figure 5-c illustrates the spectral variation of the reflective index of CZTS thin films as a function of wavelength. It has been detected that the refractive index of Cu2ZnSnS4 thin films has an anomalous dispersion for λ<675 nm and normal dispersion for λ>675 nm. The

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refractive index has been detected to increase with the increase of deposition time. This has been explicated by the simple concept that with increasing the deposition time, the density of film increases and thereby leads to an increase in refractive index.

The interaction of light with the materials leads to a transition of electrons from the

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lower energy band (valence band) to higher energy band (conduction band). For understanding the form of transition and the energy required (optical band gap) for the

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transition, Tauc’s relation has been employed. The optical band gap (>? ) of CZTS thin films deposited at different deposition time (3, 5 and 8 hours) can be obtained by employing Tauc's relation [27, 28]:

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4ℎA = BCℎA − >? D (7

where B is a constant, and G presents the type of optical transition process. After fitting

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different values of G, all studied films have been found to obey equation (7) with G = 1/2 which presents the case of direct transition (cf., Fig. 5(d)). It has been observed that the values of >? (cf., Table 2) is very close to the optimum band gap value “1.5 eV” of semiconductor used for photovoltaic conversion, and have been found to decrease from 1.54

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eV to 1.48 eV as the deposition time increases from 3 hours to 8 hour. The decrease in >?

values could be due to the formation of defects and accordingly affects the optical properties

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of the materials, where the optical band gap is a compositional dependence. There may be

other causes for band gap fluctuation like variation in film morphology. The decrease of >? in our study may be interpreted to the presence of unstructured defects, which subsequently increases the grain size of the film as given in [29] and this has been confirmed by the increase in the value of grain size given in Table 1 for the samples under study.

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ACCEPTED MANUSCRIPT 3.3 Optoelectrical characterizations 3.3.1 Dielectric study of Cu2ZnSnS4 thin films: The obtained values of refractive index and the extinction coefficient have been applied to measure the values of the real (# ) and imaginary ( ) parts of electronic dielectric constant of the Cu2ZnSnS4 thin films through the following expressions [30-32]  = 283 (9

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# = 8 − 3  (8

Figure 6(a, b) displays the photon energy dependence of real and imaginary parts of dielectric

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constant. It has been noticed that the values of both # and  become larger with increasing the deposition time. Also, obvious peaks have been observed in both graphs. The peaks position in the real dielectric constant (Fig. 6a) is seen at λ<675, which is the value of

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conversion from the anomalous to normal dispersion areas. This is expectable as the value of

ε# is mainly dependable on the refractive index value. Whereas, the peaks positions have shifted to a bit higher wavelength (see Fig. 6b) as the imaginary dielectric constant ε is

mainly dominated by the values of the extinction coefficient.

To further elucidate the films dielectric constants, the dissipation factor (tan δ) for

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Cu2ZnSnS4 thin films can be calculated by the relation [31]: tan δ =

ε (10 ε#

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The dissipation factor evaluates the loss rate of the power of a mechanical mode, similar to oscillation in a dissipation system. Figure 6(c) shows the dielectric loss dependence on photon energy of Cu2ZnSnS4 thin films. The dissipation factor tan δ is found to increase with

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the increase of the deposition time while it is almost constant with increasing the value of hν till 1.75 eV, then the value of the dissipation factor increases. The observed upturn at 2.6 eV

is obtained at the same value of peaks position in ε graph, which is predictable as tan δ is

dominated by the variation of ε . Furthermore, it is worth noted that the imaginary dielectric

constant ε is corresponding to the density of states within the band gap of the non-crystalline materials which is correlated to the defects level in the material. In addition, the aforementioned structure and its optical parameters confirmed the increase in defects with the increase in film thickness. Therefore, the increase of the dielectric loss with the increase of the film thickness means maximizing the energy losses due to the increase in the films intrinsic defects. This trend was observed for other reported research. 8

ACCEPTED MANUSCRIPT When the energetic electron beam passes through the thin film they loss amount of energy related to the free electron density of the material. The characteristics of energy loss of quick electrons moving via the volume and surface of the material are denoted by the volume (VELF) and surface (SELF) energy loss functions respectively. They are correlated

to the real (ε#  and imaginary (ε ) parts of the electronic dielectric constant through the

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following expressions [33]: 1 ε VELF = −Im : ; =  (11 ε ε# − ε

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1 ε SELF = −Im : ;= (12 ε+1 (ε# + 1 + ε

Figure 7(a, b) explains the spectral dependence of VELF and SELF on the energy for

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Cu2ZnSnS4 thin films. It has been observed that the increase in photon energy, as well as the film thickness, is accompanied by an increase in both the volume and surface energy loss functions. The nature of curves is similar but VELF is higher than SELF for the studied films. This is predictable in semiconductor films where the charge carriers suffer from more collisions inside the material due to the long traveling distance they have through the bulk material. This results in more loss of energy [31]. Also, the increase in both VELF and SELF

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with the increase in film thickness could be interpreted to the increase in intrinsic defects in the films which results in increasing the collisions of carriers during its journey in the films. Hence, an increase in both VELF and SELF is observed.

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3.3.2 Optical and electrical conductivities

The optical response is mainly studied for any material in terms of the optical

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conductivity which is obtained for Cu2ZnSnS4 thin films using the following formula [34,

σTUV =

αnc (13 4π

It is noticed that the optical conductivity increases with increasing the deposition time as well as with increasing photon energy for all films (see Fig. 8a). The high magnitude of the optical conductivity assures the high photo-response of the films under study. Also, this behavior could be due to the increase of electrons excitation by increasing the incident photon energy. These findings are inconsistent with the results of other semiconductors thin films. The CZTS (418 nm) film reveals the highest optical conductivity behavior (see Fig. 8a) as well as the highest absorption coefficient as compared to other films (cf., Fig. 5a) 9

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from optical conductivity σopt and absorption coefficient 4, by using the following expression [36]: YZ =

2Y[\ (14 4

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Figure 8 (b) shows the change of the electrical conductivity as a function of ℎA for CZTS thin films. It can be noticed that the electrical conductivity decreases as the photon energy increases for all films, while it increases with the increase of the deposition time. The decrease in electrical conductivity with the increase in photon energy confirms the

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semiconducting nature of the prepared films. 3.3.3 Nonlinear optical properties of Cu2ZnSnS4 thin films

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The nonlinear optical parameters have acquired much emphasis in the field of optoelectronic applications. The linear optical susceptibility χ(# for Cu2ZnSnS4 thin films has been evaluated via the following relation [37]: χ(# =

n − 1 (15 4π

Figure 9 (a) explains the dependence of linear optical susceptibility χ(# on the photon energy

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for CZTS thin films. It has been observed that the χ(# increases with the increase of the

deposition time. The third order nonlinear optical susceptibility χ(^ for Cu2ZnSnS4 thin films

has been obtained by the following relation [38]: /

n − 1 = C' 2 (16 4π

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(^

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where (C) is constant equal to 1.7×10-10 esu, and 8 is the linear refractive index of the material.

The dependence of third-order nonlinear optical susceptibility χ(^ on the photon

energy for Cu2ZnSnS4 thin films is shown in Fig. 9(b). It can be seen that the values of χ(^ increases with the increase of both the photon energy and deposition time. The nonlinear

refractive index n for CZTS thin films has been determined using the relation [39]: n =

#`a(b cd

(17)

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ACCEPTED MANUSCRIPT where n0 is the static refractive index. The variation of nonlinear refractive index with photon energy for Cu2ZnSnS4 thin films is presented in Fig. 9 (c). It is noticed that the values of nonlinear refractive index increases with the increase of both the photon energy and deposition time as well. A prominent peak is observed at 1.83 eV in each of χ(# , χ(^ & n graphs. This peak is correlated to the conversion from the anomalous to the normal dispersion

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of the studied films (see Fig. 5c). This is totally accepted as the behavior of the three parameters is followed by the values of the films refractive index. CZTS film (418) reveals the highest nonlinear optical parameters, two order higher than the 240 nm film, which fit in the same range of different semiconducting materials [31, 32, 40, 41]. This refers to the rise

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up of the nonlinear behavior of bound electrons in the investigated films to the applied electric field that correlated to the increase in the three-dimensional network. This finding

3.4 Electrical characterizations

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could shed lights on the possibility of using Cu2ZnSnS4 in nonlinear devices.

The electrical properties of Cu2ZnSnS4 thin films have been studied using the dc conductivity measurement in the temperature range (300 K - 450 K). The electrical resistance of the studied films has been measured as a function of temperature and then used to calculate the conductivity (σ$e ) [42]:

t (18 R A

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σ$e =

where R is the resistance of CZTS thin film, t is the film thickness and A is the area of the

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film.

The values of activation energy (Eg ) and the pre-exponential conductivity factor (σT )

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for Cu2ZnSnS4 thin films have been determined using the formula [43, 44]: σ$e = σT exp(Eg ⁄k k T (19

where k k is the Boltzmann constant.

Figure 10 shows a plot of ln (σ) vs. 103/T for the CZTS thin films. The values of

activation energy (Eg ) and the pre-exponential conductivity factors have been calculated from the slopes and the intercepts, respectively. It has been noticed that the conductivity rises with the increase of temperature which confirms the normal semiconducting behavior. It can be seen that the conductivity curves for Cu2ZnSnS4 thin films show two distinct linear regions which indicate the presence of two conduction mechanisms. The first region extended in the temperature range 300-375 K; while the second region in the temperature ranges 375-450 K. 11

ACCEPTED MANUSCRIPT In low-temperature region (Region I), it has been found that the calculated activation energy ∆E decreases from 0.32 to 0.26 eV as the deposition time increases from 3 to 8 hour respectively. In high-temperature region (Region II), it can be seen that the calculated activation energy ∆E decreases from 0.71 to 0.65 eV as the deposition time increases from 3 to 8 hour respectively. The thermal activation energies and the pre-exponential factor for the

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two regions have been calculated and summarized in Table 3. The calculated values of the activation energy in both regions reveal opposite trend to the pre-exponential factors. A close inspection of data presented in Table 3 manifest that the conduction mechanism in region I could be dominated by the hopping of carriers at the extended states nears the mobility edge.

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Whereas in region II, the conduction mechanism could be controlled by the hopping of carriers in the localized states near the Fermi level. From another point of view, the reduction in the activation energy values with the increase in deposition time means the increase of the

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film quality. This could be due to the decrease in the interspacing between film grains and decreasing the stress in the prepared film.

3.5 Photovoltaic properties of CZTS/n-Si hetero-junction

The photovoltaic properties have been studied by analyzing the current-voltage (I-V)

measurements in dark and under light illuminations for the fabricated hetero-junction. Figure

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10(b) illustrates the I-V characteristics of n-Si/P-Cu2ZnSnS4 heterojunction when exposed to light and dark conditions. It is evident that the value of current for the n-Si/P-CZTS heterojunction when exposed to light is more prominent as compared to that of dark conditions. This reveals that the absorbed light generates carrier-contributing photocurrent because of the

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creation of electron-hole pairs.

The performance of the fabricated hetero-junction can be evaluated by determining its

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power conversion efficiency. The key parameters that describe the power conversion energy are lm (the open circuit voltage), nom (the short circuit current density), and pp (the fill

factor). The lm of the solar cell can be prescribed as the voltage that balances the current

flow through the external circuit. The nom can be described as the current during the external circuit in the absence of applied voltage. To estimate the efficiency of our solar cell, an illuminated cell has been brought together to a load resistance from zero to infinity. Figure 10(c) shows the J–V plot of Al/n-Si/P-CZTS/Au hetero-junction with area 0.5×0.5 cm2 under light illumination of 1000 W/cm2. The efficiency (η) of a solar cell is a parameter used to examine the functioning of one solar cell with another solar cell. Also, it equals the ratio of

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ACCEPTED MANUSCRIPT output energy from the solar cell to the input energy from the sun. The solar cell efficiency (η) can be determined via the following equation [45]: q=

rstu pp × lm × nom = × 100% (20 rvw rvw

where rstu is the output energy from the solar cell, and rvw is the input energy from the sun.

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The Fill factor (pp) of the fabricated cell is known as the ratio between the maximum power point (rstu ) and the product of the open circuit voltage (lm ) & the short circuit current

density (nom ). It has been calculated using the expression [46]:

rstu ls × ns = (21 lm × nom lm × nom

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pp =

The device parameters estimated for our solar cell are lm = 0.594 V, nom =5.46 mAcm-2,

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ls = 0.43 V, ns = 3.93 mAcm-2, pp = 0.52 and efficiency q = 3.37 %. The good value of the

fill factor could be attributed to the good interface between Cu2ZnSnS4 film and electrodes surface. 4. Conclusion

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In this work, Cu2ZnSnS4 thin films have been prepared by CBD technique with different deposition time 3, 5, and 8 hours. The influence of deposition time on the structural, morphology, optical, optoelectrical and electrical properties of Cu2ZnSnS4 thin films has been investigated. The EDS of these films confirmed a good stoichiometry in the prepared

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compositions. The X-ray structural analysis of the CZTS thin films revealed the polycrystalline nature of studied films with the tetragonal crystal structure (kieserite

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structure). The values of the grain size, strain function and the dislocation density of the Cu2ZnSnS4 films have been calculated. The HRTEM of the CZTS thin film showed that the films are homogenous and the grains are definable. The refractive index (8, extinction coefficient (k, absorption coefficient (α have been found to increase with the increase in

the deposition time. The Cu2ZnSnS4 thin films have revealed a direct energy gap in the range of the optimum gap (1.5 eV) for the thin films solar cell. The value of the direct energy gap has been decreased with increasing the deposition time. The real and imaginary parts of the dielectric constant have increased with the increase in deposition time. They have been

utilized to extract other interesting optoelectrical parameters such as tan δ, VELF, SELF,

σTUV , and σTUV . All of the determined non-linear parameters have been found to rise with the 13

ACCEPTED MANUSCRIPT increase in film thickness. The high values of the non-linear parameters referred to the opportunity of using CZTS in nonlinear devices. In addition, the temperature dependence of the dc conductivity of Cu2ZnSnS4 thin films has been studied in the temperature range 300 to 450 K. It has been found that the variation of conductivity versus temperature showed two linear parts at two different temperature ranges, resulting in two different ranges of the

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activation energies. The values of the activation energies have been decreased with increasing the deposition time and film thickness. This assures the increase in film uniformity with the increase in deposition time. Since CZTS is an attractive material for thin film hetero-junction based solar cells, and according to the obtained data of the studied films. The Al/n-Si/P-

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Cu2ZnSnS4/Au hetero-junction has been fabricated. The device reveals an efficiency about 3.37 %.

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References

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[1] P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%, physica status solidi (RRL)-Rapid Research Letters, 10 (2016) 583-586. [2] S. Chaudhari, P. Kannan, S.R. Dey, Influence of stabilizing agent on dip coating of Cu2ZnSnS4 thin film, Thin Solid Films, (2017). [3] Y. Wang, J. Ma, P. Liu, Y. Chen, R. Li, J. Gu, J. Lu, S.-e. Yang, X. Gao, Cu2ZnSnS4 films deposited by a co-electrodeposition-annealing route, Materials Letters, 77 (2012) 13-16. [4] M.I. Khalil, R. Bernasconi, L. Magagnin, CZTS layers for solar cells by an electrodepositionannealing route, Electrochimica Acta, 145 (2014) 154-158. [5] K. Ito, T. Nakazawa, Electrical and optical properties of stannite-type quaternary semiconductor thin films, Japanese journal of applied physics, 27 (1988) 2094. [6] J.-S. Seol, S.-Y. Lee, J.-C. Lee, H.-D. Nam, K.-H. Kim, Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process, Solar energy materials and solar cells, 75 (2003) 155-162. [7] C. Shi, G. Shi, Z. Chen, P. Yang, M. Yao, Deposition of Cu2ZnSnS4 thin films by vacuum thermal evaporation from single quaternary compound source, Materials Letters, 73 (2012) 89-91. [8] T. Chtouki, L. Soumahoro, B. Kulyk, H. Bougharraf, H. Erguig, K. Ammous, B. Sahraoui, Comparative Study on the Structural, Morphological, Linear and Nonlinear Optical Properties of CZTS Thin Films Prepared by Spin-Coating and Spray Pyrolysis, Materials Today: Proceedings, 4 (2017) 5146-5153. [9] M. Farinella, R. Inguanta, T. Spanò, P. Livreri, S. Piazza, C. Sunseri, Electrochemical deposition of CZTS thin films on flexible substrate, Energy Procedia, 44 (2014) 105-110. [10] F. Aslan, A. Göktaş, A. Tumbul, Influence of pH on structural, optical and electrical properties of solution processed Cu2ZnSnS4 thin film absorbers, Materials Science in Semiconductor Processing, 43 (2016) 139-143. [11] S.S. Mali, P.S. Shinde, C.A. Betty, P.N. Bhosale, Y.W. Oh, P.S. Patil, Synthesis and characterization of Cu2ZnSnS4 thin films by SILAR method, Journal of Physics and Chemistry of Solids, 73 (2012) 735-740. [12] F. Yakuphanoglu, Nanostructure Cu2ZnSnS4 thin film prepared by sol–gel for optoelectronic applications, Solar Energy, 85 (2011) 2518-2523. [13] H. Guan, H. Hou, F. Yu, L. Li, Synthesis of wurtzite Cu2ZnSnS4 thin films directly on glass substrates by the solvothermal method, Materials Letters, 159 (2015) 200-203. [14] T.R. Rana, N. Shinde, J. Kim, Novel chemical route for chemical bath deposition of Cu2ZnSnS4 (CZTS) thin films with stacked precursor thin films, Materials Letters, 162 (2016) 40-43. 14

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[15] A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I. Kötschau, H.-W. Schock, R. Schurr, A. Hölzing, S. Jost, R. Hock, Cu2ZnSnS4 thin film solar cells from electroplated precursors: novel lowcost perspective, Thin Solid Films, 517 (2009) 2511-2514. [16] T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D.B. Mitzi, Beyond 11% efficiency: characteristics of state-of-the-art Cu2ZnSn(S,Se)4 solar cells, Advanced Energy Materials, 3 (2013) 34-38. [17] S. Kumar, S. Kumar, P. Sharma, V. Sharma, and S. C. Katyal, CdS nanofilms: Effect of film thickness on morphology and optical band gap, Journal of Applied Physics 112, (2012: 123512. [18] R. Gupta, F. Yakuphanoglu, Photoconductive Schottky diode based on Al/p-Si/SnS2/Ag for optical sensor applications, Solar Energy, 86 (2012) 1539-1545. [19] G. Sakr, Characterization of Al/p-Si/n-AgGaSe2/Au thin films heterojunction device, Materials Chemistry and Physics, 138 (2013) 951-955. [20] P. Thakur, R. Sharma, V. Sharma, and P. Sharma, Structural and optical properties of Mn0. 5Zn0. 5Fe2O4 nano ferrites: Effect of sintering temperature, Materials Chemistry and Physics 193 (2017) 285-289.. [21] M. Fadel, I. Yahia, G. Sakr, F. Yakuphanoglu, S. Shenouda, Structure, optical spectroscopy and dispersion parameters of ZnGa2Se4 thin films at different annealing temperatures, Optics Communications, 285 (2012) 3154-3161. [22] A. Sawaby, M. Selim, S. Marzouk, M. Mostafa, A. Hosny, Structure, optical and electrochromic properties of NiO thin films, Physica B: Condensed Matter, 405 (2010) 3412-3420. [23] L. Boudaoud, N. Benramdane, R. Desfeux, B. Khelifa, C. Mathieu, Structural and optical properties of MoO3 and V2O5 thin films prepared by Spray Pyrolysis, Catalysis today, 113 (2006) 230-234. [24] S.S. Fouad, E.A.A. El-Shazly, M.R. Balboul, S.A. Fayek, M.S. El-Bana, Optical parameter studies of thermally evaporated As-Se-Sn glassy system, Journal of Materials Science: Materials in Electronics, 17 (2006) 193-198. [25] M.S. El-Bana, I.M. El Radaf, S.S. Fouad, G.B. Sakr, Structural and optoelectrical properties of nanostructured LiNiO2 thin films grown by spray pyrolysis technique, Journal of Alloys and Compounds, 705 (2017) 333-339. [26] E. Shaaban, N. Afify, A. El-Taher, Effect of film thickness on microstructure parameters and optical constants of CdTe thin films, Journal of Alloys and Compounds, 482 (2009) 400-404. [27] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, physica status solidi (b), 15 (1966) 627-637. [28] A. Ammar, A. Farid, S. Fouad, Optical and other physical characteristics of amorphous Ge–Se– Ag alloys, Physica B: Condensed Matter, 307 (2001) 64-71. [29] S. Mushtaq, B. Ismail, M. Raheel, A. Zeb, Nickel Antimony Sulphide Thin Films for Solar Cell Application: Study of Optical Constants, Natural Science, 8 (2016) 33. [30] A.S. Hassanien, Studies on dielectric properties, opto-electrical parameters and electronic polarizability of thermally evaporated amorphous Cd50S50−xSex thin films, Journal of Alloys and Compounds, 671 (2016) 566-578. [31] P. Sharma, M.S. El-Bana, S.S. Fouad, V. Sharma, Effect of compositional dependence on physical and optical parameters of Te17Se83−xBix glassy system, Journal of Alloys and Compounds, 667 (2016) 204-210. [32] M.S. El-Bana, S.S. Fouad, Opto-electrical characterisation of As33Se67−xSnx thin films, Journal of Alloys and Compounds, 695 (2017) 1532-1538. [33] D.M. Abdel-Basset, S. Mulmi, M.S. El-Bana, S.S. Fouad, V. Thangadurai, Structure, Ionic Conductivity, and Dielectric Properties of Li-Rich Garnet-type Li5+2xLa3Ta2–xSmxO12 (0≤ x≤ 0.55) and Their Chemical Stability, Inorganic chemistry, 56 (2017) 8865-8877. [34] P. Sharma, A. Dahshan, and K. A. Aly, New quaternary Ge–Se–Sb–Ag optical materials: Blue shift in absorption edge and evaluation of optical parameters, Journal of Alloys and Compounds 616 (2014) 323-327. [35] K. A. Aly, and Farid M. Abdel-Rahim, Effect of Sn addition on the optical constants of Ge–Sb–S thin films based only on their measured reflectance spectra, Journal of Alloys and Compounds 561 (2013) 284-290.

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[36] T. Girisun, S. Dhanuskodi, Linear and nonlinear optical properties of tris thiourea zinc sulphate single crystals, Crystal Research and Technology, 44 (2009) 1297-1302. [37] H. Ticha, L. Tichy, Semiempirical relation between non-linear susceptibility (refractive index), linear refractive index and optical gap and its application to amorphous chalcogenides, J. Optoelectron. Adv. Mater., 4 (2002) 381-386. [38] D.C. Sati, S.C. Katyal, P. Sharma, Role of Composition and Substrate Temperature on Nonlinear Optical Properties of GeSeTe Thin Films in 0.4–2.4 µm Wavelength Range, IEEE Transactions on Electron Devices, 63 (2016) 698-703. [39] S. Hegde, A. Kunjomana, K. Chandrasekharan, K. Ramesh, M. Prashantha, Optical and electrical properties of SnS semiconductor crystals grown by physical vapor deposition technique, Physica B: Condensed Matter, 406 (2011) 1143-1148. [40] M.S. El-Bana, R. Bohdan, S.S. Fouad, Optical characteristics and holographic gratings recording on As30Se70 thin films, Journal of Alloys and Compounds, 686 (2016) 115-121. [41] S.S. Fouad, G.A.M. Amin, M.S. El-Bana, Physical and optical characterizations of Ge10Se90−xTex thin films in view of their spectroscopic ellipsometry data, Journal of Non-Crystalline Solids, 481 (2018) 314-320. [42] S.A. Khan, Z.H. Khan, A. El-Sebaii, F. Al-Marzouki, A. Al-Ghamdi, Structural, optical and electrical properties of cadmium-doped lead chalcogenide (PbSe) thin films, Physica B: Condensed Matter, 405 (2010) 3384-3390. [43] D.M. Abdel-Basset, S. Mulmi, M.S. El-Bana, S.S. Fouad, V. Thangadurai, Synthesis and characterization of novel Li-stuffed garnet-like Li5+2xLa3Ta2−xGdxO12 (0≤ x≤ 0.55): structure–property relationships, Dalton Transactions, 46 (2017) 933-946. [44] M.S. El-Bana, G. Mohammed, A.M. El Sayed, S. El-Gamal, Preparation and characterization of PbO/carboxymethyl cellulose/polyvinylpyrrolidone nanocomposite films, Polymer Composites, (2017). [45] F. Zhang, X. Xu, W. Tang, J. Zhang, Z. Zhuo, J. Wang, J. Wang, Z. Xu, Y. Wang, Recent development of the inverted configuration organic solar cells, Solar energy materials and solar cells, 95 (2011) 1785-1799. [46] A. Alkaya, R. Kaplan, H. Canbolat, S. Hegedus, A comparison of fill factor and recombination losses in amorphous silicon solar cells on ZnO and SnO2, Renewable Energy, 34 (2009) 1595-1599.

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ACCEPTED MANUSCRIPT List of Table and Figure Captions Table 1. The average values of the grain size (D), the strain function (z), and the dislocation density ({) of the main plan (112) of the Cu2ZnSnS4 thin films. Table 2.The values of the optical band gap of the CZTS thin films.

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Table 3. The thermal activation energies and the pre-exponential factor of Cu2ZnSnS4 thin films. Figure 1: (a) X-ray diffraction patterns for the CuZnSnS4 thin films deposited at different deposition times (3, 5 and 8 hours), (b) The dependence of the crystallite size and lattice strain on the deposition time of the CZTS thin films.

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Figure 2: (a) HRSEM images of the Cu2ZnSnS4 thin films deposited at 3 hour, (b) 5 hour, (c) 8 hour, (d) EDS spectra of the CZTS thin film deposited at 8 hour.

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Figure 3: (a) HRTEM images of the Cu2ZnSnS4 thin films deposited at 30 minute, (b) The electron diffraction of the CZTS thin films deposited at 30 minute. Figure 4: (a) The transmittance spectra of the CuZnSnS4 thin films deposited at different deposition times (3, 5 and 8 hours), (b) The reflectance spectra of the CZTS thin films deposited at different deposition times (3, 5 and 8 hours).

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Figure 5: The absorption coefficient (a), the extinction coefficient (b), the refractive index (c) as a function of wavelength for the CuZnSnS4 thin films deposited at different deposition time. (d) Plot of (αhν versus the photon energy hν for the CZTS thin films deposited at

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different deposition time.

Figure 6: The variation of the real dielectric constant (a), the imaginary dielectric constant (b), the dielectric loss as a function of photon energy for the CuZnSnS4 thin films deposited

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at different deposition time.

Figure 7: The dependence of both the VELF (a), and the SELF (b) functions on photon energy of the Cu2ZnSnS4 thin films. Figure 8: The dependence of the optical conductivity (a), and the electrical conductivity (b) on the photon energy for the CuZnSnS4 thin films deposited at different deposition time.

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ACCEPTED MANUSCRIPT Figure 9: Variation of the linear optical susceptibility (a) the third order nonlinear optical susceptibility (b), and the nonlinear refractive index (c) with the photon energy of the investigated CuZnSnS4 thin films deposited at different deposition time Figure 10: (a) Temperature dependence of the dc conductivity of Cu2ZnSnS4 thin films, (b) I–V characteristics of n-Si/ P-CZTS heterojunction device in dark and under illumination; (c)

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J–V characteristic curve for Al/ n-Si/ P-CZTS/ Au heterojunction under illumination of 1000

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W/cm2.

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ACCEPTED MANUSCRIPT Table 1. Sample

D(nm)

ε X 10-3

δ X 10-3 (nm-2)

240 nm (3h)

19.84

5.13

2.54

351 nm (5h) 418 nm (8h)

26.17 38.32

3.48 1.94

1.46 0.68

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Table2.

~€‚  eV

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Sample 240 nm (3h)

1.54

351 nm (5h)

1.51 1.48

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418 nm (8h)

Table 3. The thermal activation energies and the pre-exponential factor of Cu2ZnSnS4 thin films.

∆ E I [eV]

σo Region I

∆ E II [eV]

σo Region II

Region I

[ƒ ƒ-1.cm-1]

Region II

[ƒ ƒ-1.cm-1]

0.32

7.38

0.71

244

0.29

20.15

0.68

403

33.21

0.65

572

240 nm (3h)

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351 nm (5h)

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Sample

0.26

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418 nm (8h)

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ACCEPTED MANUSCRIPT Figure 1

40 240 nm 351 nm 418nm

400

5.0

35

4.5 4.0

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600

Crystallite size D (nm)

(105) (220)

800

5.5 [b]

(312)

1000

(112) (103)

30

3.5 3.0

25

2.5

200

2.0

0

10

20

30

40

50

60

70

80

3

2 θ (Degrees)

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Depositon time (h)

Figure 2

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Intensity (a.u)

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Lattice strain ε

1200

20

[c]

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[a]

Figure 4 100

100

[a]

90

40

R%

50

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60

240 nm 351 nm 418 nm

30 20 10 500

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λ (nm)

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λ (nm)

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6x104 [a]

0.20

4

4x10

K 3x104

0.15 0.10

2x104

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0 500

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0.00 500

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6.0 9

(α hυ ) (eV.m )

-2 2

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3.5 3.0 2.5

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4x10

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[c]

2000

λ (nm)

λ (nm)

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α (cm−1)

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[b]

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22

1.5

2.0

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5

240 nm 351 nm 418 nm

240 nm 351 nm 418 nm

[a]

4

[b]

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3

ε2

ε1

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4 1 2 0 0.5

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2.0

2.5

3.0

0 0.5

3.5

1.0

1.5

2.0

hν ν (eV)

2.5

3.0

3.5

hν ν (eV)

1.0 240 nm 351 nm 418 nm

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3.5

hν (eV)

240 nm 351 nm 418 nm

[a]

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0.08

0.06

SELF

0.30

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Figure 7

240 nm 351 nm 418 nm

[b]

0.04

0.02

0.05

0.00 0.5

1.0

1.5

2.0

2.5

0.00 0.5 3.0

1.0

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2.0

hν ν (eV)

hν (eV)

23

2.5

3.0

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8x10

15

7x10

6

240 nm 351 nm 418 nm

3.0x10

240 nm 351 nm 418 nm

[b]

[a] 6

2.5x10

15

-1

6

σ e (S)

σ opt (S)

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6x10

15

5x10

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1.5x10

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χ

χ

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1.0x10

(3)

(esu)

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(1)

3.0

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Figure 9

1.2

2.5

hν (eV)

hν (eV)

[a]

2.0

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240 nm 351 nm 418 nm

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-9

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hν (eV)

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2.5

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hν (eV)

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3.5

ACCEPTED MANUSCRIPT Figure 10 -2

T (240 nm Film) T (351 nm Film) T (418 nm Film)

[a]

Au/ nSi/ p-Cu2ZnSnS4/ Al

15 [b]

I (mA)

-6 -8 -10

10

5

-12 -1.5

-16

-1.0

-18

0 0.0

0.5

-5 2.2

2.4

2.6

2.8

3 10 /T

3.0

3.2

-1 [K ]

6

AL/ n-Si/ Cu2ZnSnS4/ / Au [c]

4 3 2

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J (mAcm-2)

5

JSC= 5.46 mAcm-2 Jm= 3.93 mAcm-2 Voc=0.594 v Vm=0.43 v

1 0 0.0

FF=0.52 η = 3.37 % 0.1

0.2

0.3

0.4

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25

1.0

1.5

V (volts)

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-0.5

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-1 ln (σ σ ) [Ω Ω .cm]

-4

dark illumination

0.5

0.6

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Highlights:

 Cu2ZnSnS4 thin films are prepared by CBD technique.  XRD revealed the polycrystalline nature with kieserite structure.

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 HRTEM shows that the films are homogenous and the grains are definable.  Optical parameters are determined using transmission & reflection spectra.

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 The fabricated device reveals an efficiency about 3.37 %.