indium tin oxide hybrid electrodes

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Enhancement in photovoltaic properties of bismuth ferrite/zinc oxide heterostructure solar cell device with graphene/indium tin oxide hybrid electrodes A.M. Afzala,∗, Yasir Javedb, Sajad Hussainc, Adnan Alid, M.Z. Yaqoobd, Sohail Mumtaza a

Department of Electrical and Biological Physics, Kwangwoon University, Seoul, 01897, Republic of Korea Department of Physics, University of Agriculture, Faisalabad, 38000, Pakistan c Department of Physics, Division of Science and Technology, University of Education, Lahore, 54000, Pakistan d Department of Physics, Government College University Faisalabad, 38000, Faisalabad, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Atomic layer deposition Annealing atmosphere Leakage current Graphene Transparent electrode Power conversion efficiency

Integrating of ferroelectric thin films with two-dimensional materials may provide a novel and unique characteristics in the field of optoelectronics due to the coupling of their distinctive intrinsic features. A heterostructure (bismuth ferrite/zinc oxide) device is fabricated with different types of the electrode to enhance the power conversion efficiency (PCE). A single-phase multiferroic BFO thin film is grown by atomic layer deposition (ALD) method and annealed in different environments such as helium, nitrogen, and oxygen. We investigated the effect of annealing parameters and different types of electrodes on solar cell applications. We observed that the leakage current 10 orders of magnitude was reduced by decreasing in the dielectric loss. Further, the power conversion efficiency (PCE) is improved from 4.1% to 7.4% with a hybrid transparent electrode (graphene/ indium tin oxide). The value of PCE is further increased at a low temperature. So, the improvement in the key parameter of bismuth ferrite thin-film evidently highlights the importance of annealing atmosphere and graphene as an electrode in BFO thin film applications in optoelectronics.

1. Introduction Bismuth ferrite (BFO) nanostructure has giant magnetization and electric polarization which makes it an intriguing candidate for photocatalytic activity, perovskite solar cell, and spintronic devices. Singlephase BFO multiferroic materials have attained much attention among the researcher because the ferroelectric and ferromagnetic characteristics are modulated to each other and due to lead-free materials [1–5]. Bismuth ferrite has a rhombohedral perovskite structure with bandgap 2.2–2.7 eV and also exhibiting high Curie and Néel temperatures such as 1103 and 643 K, respectively. In literature, it is controversial that BFO is considered as n-type or p-type material. On the other hand, the DFT calculation has described the exact characteristics and provided evidence that BFO is more likely p-type than n-type [6,7]. Although, the various low dimensional system of BFO, such as thin films and nanoparticles may show unambiguous changes in structural, ferroelectric and magnetic properties [8]. In addition, BiFeO3 thin films can be utilized in spintronics, ferroelectric, magnetoelectric, magnetic memories and magneto-electric random access memory applications

∗

because it shows fascinating multiferroic behavior at room temperature [4,9,10]. So, large leakage current and small value of magnetization have made it limited as a point of application. These problems have arisen due to the oxygen (O2) vacancies and the formation of parasitic phases (Bi2O3) [11–14]. However, an elusive challenge in this field is the realization to control leakage current and value of magnetization. In previous reports, both quantities (leakage current and magnetization) have been improved in many ways like the substitution of Bi+3 and Fe+3 ions on Asite and B-site respectively in the perovskite ABO3 structure. For example, the multiferroic BiFeO3 was doped with different elements such as Lu, Gd, Sm, Ho, Nd, Dy, and La or co-doped with Gd and Ti [5,15–18]. In some doped cases, the leakage current and magnetization improve and along with the reverse effect on the other parameters. So, mostly the doping process can introduce impurity phases and structural phase transition [19]. On the other hand, useful efforts are being made toward the improvement of materials functionality by modified the synthesis conditions and develop a different route to grow. In this respect, the different group have studied the effect of different growth

Corresponding author. E-mail address: [email protected] (A.M. Afzal).

https://doi.org/10.1016/j.ceramint.2019.12.166 Received 14 September 2019; Received in revised form 21 November 2019; Accepted 21 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: A.M. Afzal, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.166

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minute. To make the thin film heterostructure Gr/BFO/ZnO/Gr/ITO/ substrate, the BFO of the thin film was deposited on ZnO thin film by the ALD method. Moreover, the CVD doped graphene was used as the top electrode and a hybrid electrode with ITO as the bottom electrode. The CVD graphene was used as a transparent electrode to enhance the photovoltaic characteristics. The copper (Cu) foil from the backside of graphene was removed by ammonium persulfate. The graphene with Poly (methyl methacrylate) (PMMA) layer was transferred into deionized water to remove the residue. The graphene with PMMA on the top surface was transferred onto the target place by wet transfer method. To investigate the BFO thin film deeply, we were designed heterostructure devices with different combinations of electrodes with Au and graphene. The field emission scanning electron microscope (FE-SEM), Xray diffraction (XRD), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the thin films. Standard remanent polarization (Pr) and PUND testing procedures were used to measure the ferroelectric properties and IV characteristics. To check the magnetic properties of BFO thin films, magnetization was studied as a function of annealing atmosphere, temperature and applied magnetic field. For magnetic measurement, superconducting quantum interference devices (SQUID) were employed and the temperature ranging from 1.5 to 400 K. For Characterization and device fabrication, we used the XRD to characterize the crystal structure and size of BFO nanostructures by using the model JDX-11 of Joel Company Ltd., Japan which performed at 35 kV and 30 mA. This is a powder type X-ray diffractometer with CuKα (1.5418 A˙ ) source. The specimen (S) is utilized in the form of a glass slide (2.5 × 5 cm2). To compute the DC magnetization measurement of BFO nanostructures at room temperature, the Quantum Design physical properties measurement system (PPMS) was used. For the photoconversion efficiency of the BFO device, the current-voltage (I–V) measurement for the solar cell was measured with a voltmeter (Keithley 2400) under AM 1.5 illuminations (100 mWcm−2). The active area of the solar cell devices is 3 mm × 5 mm .

parameters such as temperature, distance, oxygen pressure, deposition rate, thickness and nonstoichiometry on ferroelectric and ferromagnetic properties of BiFeO3 thin films grown with different techniques [20–29]. However, in these growth techniques, the high temperature, electrode film interface, low control over thickness, domain boundaries and vacancy formation (oxygen vacancies) have a strong diverse effect on functional properties of BFO thin film. Basically, the explanation of such kind of properties is coupled with the destruction of the modulation cycloid due to the spatial confinement in BFO nanocrystals. Highly specialized substrates and epitaxial growth methods have required for most of the antiferromagnetic nature and weak electro-electric coupling of bulk BFO. So, the suitable growth method and approaches with precise control over the stoichiometry and critical dimensions have been unrevealed [30,31]. Further, the performance of optoelectronics devices is based on the controlling of leakage current, contact resistance, the electron-hole (eh) pair generation in the active region, depletion region and drifting of charges (e-h pair) to the outside circuit. Recently, the hybrid transparent bi-film composed of conductive metal oxide and carbon complexes has been reported [32]. So, high performance with small leakage current and low contact resistance optoelectronic device is still an elusive challenge. In the present research, we fabricate a heterostructure (bismuth ferrite (BFO)/zinc oxide (ZnO)/indium tin oxide (ITO)/substrate) device. The single phased BFO thin film is deposited on ZnO by the ALD technique. We design multiple devices to check the effect of annealing atmosphere and electrodes on BFO thin film as well as on heterostructure. After the fabrication, the samples are annealed in a different atmosphere such as helium (He), nitrogen (N) and oxygen at 600 °C. In addition, the samples are annealed for different duration 1 h to a prolonged 24 h in a different atmosphere to increase the grain size and filling the oxygen vacancies effectively. Furthermore, we also checked the effect of cooling rate on BFO thin films. In this while, the sample was cooled using a rate of 3 °C per minute under the constant flow of gases. In order to analyze the surface morphology, thickness, and composition, the field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD), atomic force microscope (AFM) and Xray photoelectron spectroscopy (XPS) was used. We also measured the leakage current, magnetic and dielectric properties of BFO thin film to estimate the effect of growth by ALD and annealing conditions. Further, the heterostructure is measured to estimate the power conversion efficiency (PCE). Thus, the ALD method with suitable annealing conditions and graphene electrode provides the more effective method for the highly controlled deposition of BFO over the other methods and various geometries that can be applied for the unique optoelectronics applications. So, these findings provide a unique platform to use BFO thin films in optoelectronics and spintronics applications.

3. Results and discussion 3.1. Structural and composition characterization To get information about samples thickness, we used AFM and FESEM to view the top surface and cross-sectional image respectively. Fig. 1 (a) illustrates the schematic diagram of the final heterostructure device. In this heterostructure device, few-layer graphene was used as a top and bottom electrodes. Fig. 1 (b) shows the cross-sectional FE-SEM image of a thin-film heterostructure device. The thickness of BFO and ZnO thin films grown by ALD are in the range of 80 nm and 50 nm respectively. In Fig. 1 (c) shows the AFM image which demonstrates the surface morphology and roughness of BFO thin film grown by ALD. We observed that the thin film was uniformly grown and surface roughness was small. Further, we also analyzed the effect of annealing on grain growth and alteration in surface structure. We observed that the annealing conditions strongly affect the grain size and surface roughness of the thin film. So, we analyzed that the film surface was the flattest in case of nitrogen and helium annealing, while the surface was the roughest result from ramping. In the case of nitrogen, helium and ramping, the grain sizes of the thin film are identical and slightly increased in case of oxygen annealing. We observed the maximum grain size in case of oxygen annealing for 24 h. Fig. 1 (d) shows the XRD pattern of BFO thin film by ALD. We observed the effect of annealing on microstructure from XRD traces. The peaks become prominent after the annealing in a different atmosphere. But in the case of oxygen annealing, a large change has been observed. In many studies, parasitic phases have been reported produced during the growth [11,12,37,38]. So, we did not observe any parasitic peaks in XRD results. To identify the structural

2. Experimental details BFO film was grown by ALD. Bismuth (III) 2,3-dimethyl-2-butoxide, iron (III) tert-butoxide was used as a metal precursor and water precursor as the oxygen source. The BFO thin film was deposited on Gr/ SiO2/Si substrate at low temperature (150 °C) as compared to other growth techniques. The thin film was deposited until the desired film thickness (60 nm) was reached by interchanging ultrathin Fe–O and Bi–O layers. The BFO film was grown as a binary oxide layer and the growth rates of BiOx layer and FeOx layer were 0.4 A/cycle and 0.2 A/ cycle respectively. To limit the Bismuth (Bi) out-diffusion during annealing at high temperature, a thin layer of FeOx (4 nm) was deposited at the start and as the final top layer [33]. The pulse durations for Bi, Fe, and water were 0.4s, 0.5s, and 0.5s respectively [34–36]. After completing the deposition, the samples were annealed in the different atmospheres such as He, N2 and O2 ambient at 600 °C at atmospheric pressure 1 h to 24 h and measured with gold and graphene electrode. Furthermore, one set of devices was cooled at a slow rate of 3 °C per 2

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Fig. 1. (a) Schematic diagram of Gr/BFO/ZnO/Gr/ITO/glass heterostructure device (b) FE-SEM image of the heterostructure device. The thickness of BFO and ZnO is 80 nm and 70 nm respectively (c) Atomic force microscopy image of the top surface of BFO thin film by grown by ALD (d) X-ray diffraction of BFO thin film (e) Raman spectra of the graphene layer. The inset figure shows the graphene layer on the ITO substrate.

the magnetization as shown in Fig. S2 (b). We observed that the saturation magnetization enhanced in oxygen atmosphere annealed samples which were in the range of 52 emu/g. It was observed that the saturation magnetization was increased as the annealing time increased. The enlargement of magnetic moments in the BFO system is due to the presence of Fe+3 due to surplus O vacancies. So, the magnetization was improved dramatically due to the reduction of O2 vacancies in the nanostructures.

characteristic of few-layer (1.2 nm) graphene, the Raman spectrum of graphene was performed. The G and 2D Raman peaks of graphene appear around 1586 cm−1 and 2684 cm−1, respectively as shown in Fig. 1 (e). The inset figure shows the FE-SEM image of graphene on the ITO film. The graphene was transferred by wet transfer method. To check the chemical state of BFO thin film, the X-ray photoelectron spectroscopy (XPS) was used. Fig. S1 (a) shows the XPS data of BFO thin films grown by ALD. Fig. S1 (b) represents the position of peaks Fe 2p3/2 , Fe 2p1/2 and satellite of Fe 2p3/2 at 710.4, 724.4 and 718.4 eV respectively. Fig. S1 (c) and (d) shows the Bi 4f5/2 and 4f7/2 peaks for Bi+3 centered at 158.9 and 164.2 eV respectively and have a good agreement with previously reported values [39–42]. Fig. S2 (a) shows the magnetic hysteresis (M-H) loops at the different annealing atmosphere. The samples were annealed in air, nitrogen, helium, and oxygen. Further, the samples were heated and cooled down with a specific rate called the ramping. The ramping process further improved

3.2. Dielectric properties Further, we checked the influence of annealing conditions and electrodes on the current density (J) on the thin films grown by ALD. Fig. 2 (a) shows the current density with Au and graphene electrode. In the case of graphene, we observed that the J of the device was improved due to low contact resistance and clean interface. Fig. 2 (b) shows the 3

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Fig. 2. (a) Electric-field dependences of the current density of BFO thin film at room-temperature with Au and graphene electrode (b) Electric-field dependences of the current density of BFO thin film at room-temperature under different annealing conditions with graphene electrode (c) Electric-field dependences of the current density of BFO thin film at a different temperature of a sample annealed in oxygen at 24 h. (d) Change in leakage current with annealing time in different environments. In all systems, the leakage current decrease with annealing time.

dipoles due to inertia are not agreed to align themselves in the applied field direction [43,44]. The improvement in dielectric constant at low f is also related to a large dissipation factor (tan δ). Fig. 3 (a) & (b) shows the variation in dissipation factor with frequency for all samples which have a tendency to increase without appearing any peak. The dissipation factor decreases at low frequency can be explained by Koop's phenomenological model [44]. At lower frequency the resistivity (ρ) is large and the grain boundary effect is dominant. For the exchange of electrons located at grain boundaries between Fe2+and Fe3+ ions are acquired more energy. So, this acquirement increases the energy loss. The resistivity is small at high f and a small amount of energy is required for the electrons hopping between the Fe2+ and Fe3+ ions situated in a grain. Therefore, the dissipation factor is also small.

effect of annealing conditions on current density. We observed that the oxygen annealing has a robust effect on the leakage current and after ramping, the effect becomes more prominent (Fig. 2 (c)). But in case of nitrogen and helium annealing, the effect is small as compared to oxygen annealing as shown in Fig. 2 (d). This reduction in leakage current is due to a decrease in oxygen vacancies and Fe+2 ions. For dielectric analysis, the dielectric constant (ε ) and dissipation factor (tan δ) of BFO nanoparticles annealed in a different atmosphere with different cooling rates were measured with respect to frequency (f) variation (1 KHz-1MHz) at 300 K. We observed that the dielectric constant decreased continuously with enhancing the frequency. This type of mechanism is explained by the dispersion due to MaxwellWagner type interfacial polarization [43]. The local displacement of charge carriers in the direction of the electric field happens due to the electron transfer mechanism between Fe2+ and Fe3+ ions. So, this local displacement of charges determines the polarization. The effect of the hopping process is also defined as the variation in dielectric constant with frequency (f) in distinctive ferrites. The electrons which contribute to the hopping process between Fe2+↔Fe3+ ions are locally aligned in the electric field direction which defines the polarization mechanism at lower frequencies. We observed a high dielectric constant due to this process. On the other hand, at a higher frequency, the polarization reduces and in the long run reaches a constant value. Basically, the f of hopping electrons between Fe2+↔Fe3+ ions don't allow the fast-changing of the alternating field. The access time (time taken by the field to switch its polarity) is decreased with increasing frequency. Hence, it becomes small than the electrons response time. The electrons are not responded to the applied electric field. If we increased the frequency further (> 105 Hz), the dielectric constant remains constant because the

3.3. Electrical and photovoltaic properties Fig. 3 (c) illustrates the current-voltage characteristics of ITO, Gr and hybrid electrode of Gr/ITO. First of all, we compared the performance of electrodes with a hybrid transparent electrode of Gr with ITO on the bases of electrical measurement by Hall measurement system. The hybrid electrode of Gr and ITO showed better results due to a decrease in sheet resistance and an increase in charge density as summarized in Table 1. Further, the hybrid electrode of n-doped graphene (Fig. S5) and ITO shows Ohmic contact with n-type ZnO because the hybrid electrode has the same work function. Fig. 3 (d) shows the I–V curves of Au and graphene with BFO films. We analyzed that the BFO (ΦBFO ≈ 4.7 eV ) film showed non-Ohmic contact with Au (ΦAu ≈ 5.1 eV ) due to the difference in work function. In the case of p-doped graphene (Fig. S4), the BFO thin films show Ohmic contact due to same work 4

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Fig. 3. (a) Frequency dependence of real part of dielectric constant for annealed samples under a different environment at room temperature (b) Frequency dependence of dissipation factor (tan δ) for annealed samples under a different environment at room temperature (c) Current-voltage (I–V) characteristics of different electrode (d) Current-voltage (I–V) characteristics of different electrode with BFO thin films.

annealing). Further, we appraised the optoelectronic features of heterostructure device with a transparent electrode of graphene under a deep ultraviolet (DUV) light illumination source. The wavelength and intensity of the light source were 220 nm and 11 mW/cm−2 respectively. We performed our measurements of our samples in vacuum to stop the external degradation of oxygen or water molecules. We perceived that the photo-induced current density increased gradually by annealing the samples in another environment than air as shown in Fig. 4 (a). In the case of oxygen annealing, we observed the large photocurrent density. Further, we also calculated the rise time and decay time by fitting the raw data with the following equations:

Table 1 Comparison of different materials and hybrid structure. Materials

Sheet resistance (Ω/sq)

Surface carrier concentration (cm−2)

Carrier mobility (cm2/V.s)

ITO Graphene Graphene/ITO

80.21 420.21 69.35

7.2 × 1014 4.54 × 1012 3.24 × 1015

30 19000 94.25

functions with a large improvement in the performance of the device. The demand for the low cost and flexible methodologies for energy storage is mandatory in photo-electronic and nano-electronic devices. For the realization of these requirements, we fabricated the Gr/BFO/ ZnO/Gr/ITO/substrate heterostructure thin films solar cell devices. Fig. 1 (a) illustrates the schematic diagram of the final device. For the confirmation of the thickness of the ALD grown thin films of BFO and ZnO on ITO/substrate, the FE-SEM was used to take the cross-sectional image of the heterostructure device as shown in Fig. 1 (b). The thickness of the BFO thin film is ~80 nm. Further, we fabricated Gr/BFO/ZnO/Gr/ITO/substrate heterostructure solar cell devices to study the solar cell application in different samples annealed in a different atmosphere and with different electrodes. So, the heterostructure device was measured with Au/ITO, Gr/ Gr-ITO electrodes. First, we measured only Au/BFO/ZnO/ITO heterostructure with annealing and without annealing to check the effect of the annealing atmosphere. Secondly, the heterostructure device (Gr/ BFO/ZnO/Gr/ITO) measured for solar cell applications with the hybrid transparent electrode (Gr/Gr-ITO) with annealing and without annealing to check the effect of each component (graphene contact and

IPh (t ) = Idark + Ae

IPh (t ) = Idark + Ae

t

trise

−t

tdecay

(2) (3)

where IPh (t ) is the photocurrent underDUV light, Idark denotes the dark current. Further, t represents the time for switching the light on/off. After fitting, we obtained the values of the rise and decay time. We observed that the rise time changed from 3.5 s to 6 s and decay time varied from 16 s, 18 s and 21 s for air, He, and O2 annealing respectively. Fig. 4 (b) shows the comparison of photo-induced current density with Au and graphene contact. In the case of O2 annealing and graphene electrode, we observed large charge carrier generation/recombination. Basically, the phenomena of photocurrent depend on the charge carrier generation/recombination processes. In the case of graphene electrode and annealing, the contact resistance is decreased due to the small work function of the electrode with BFO and ZnO thin films and clean interfaces. Further, the current density-voltage (J-V) measurement was performed for each solar cell sample in dark and under 5

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Fig. 4. (a) Photo-response of heterostructure device annealed in a different environment. DUV light source is used to measure the photo-induced current (b) Comparison of photo-induced current under DUV light source with Au and graphene electrodes.

Fig. 5. (a,b &c) The current density verse voltage (J-V) characteristics of Air, He, and O2 annealed samples respectively in dark and light solar cell device (d) Opencircuit voltage (Voc) and short circuit current (Isc) of the devices in a different atmosphere with graphene electrode. The inset figure shows the Voc and Isc with Au electrode.

discussed our BFO/ZnO heterostructure solar cell device with Au and ITO electrodes. We observed the PCE value up to 4.1% with Isc, Voc and FF 8 mA/cm2, 0.34 V, 33 respectively without annealing. Further, the PCE value increased up to 5.3% by annealing the samples in an oxygen atmosphere as shown in Fig. 6 (a). The Isc, Voc and FF are also improved up to 14.5 mA/cm2, 0.67 V, 41 respectively. Next, we turned our attention towards the BFO/ZnO heterostructure with the graphene and ITO hybrid electrode. First, we measured the heterostructure device without annealing. The value of PCE more increased up to 6.9%. Further, we calculated the values of Fill factor (FF ) in case Air, He and O2 annealing samples with Au and graphene electrode respectively of the Gr/BFO/ZnO/Gr/ITO/substrate junction device by using the Voc and Isc from Fig. 5 (d). The value of FF is increased up to 46 with the graphene electrode by increasing the Voc ≈ 0.67 V and Isc ≈ 14.5 mA/cm2 . By putting the all values in equation (1) which gives to

the illumination of light as presented in Fig. 5 (a,b & c). We observed that the sample annealed in oxygen with a graphene electrode shows better results as compared to other samples. The PCE is estimated by the following relation [45].

PCE =

Voc × Jsc × FF Pin

(4)

whereas,

FF =

Vmax  ×  Imax Voc   ×  Isc

(5)

where Jsc represents the short circuit current density, FF denotes the fill factor, Voc illustrates the open-circuit voltage, Pin demonstrates the input power of illumination source, Imax and Vmax exhibits maximum current values and the maximum voltage respectively at which the maximum power of the solar cell device can be achieved. First, we 6

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Fig. 6. (a) FF and PCE of the devices in different atmosphere with Au and ITO electrode (b) FF and PCE of the devices in different atmosphere with Gr and ITO/Gr electrode (c) PCE of the devices at different temperature (d) Change in resistance (Rs) with temperature (e) PCE of BFO/ZnO heterostructure thin-film solar cell with Au/ITO and Gr/Gr-ITO electrodes along with annealing effect. Both parameters are enhanced the PCE value. The hybrid transparent electrode prominent effect on PCE.

rise a PCE value. Remarkably in evaluation, the PCE has been enlarged up to 7.4% (Fig. 6 (b)) when the heterostructure with graphene electrode was annealed in an oxygen atmosphere which is much better as compared to the previous study [46–48] (Table 2). The comparison of PCE is shown in Fig. 6 (e).We observed that the PCE value was increased by two factors: annealing in a different atmosphere and hybrid electrode of Gr-ITO. In the first case, the shunt resistance is increased by decreasing the leakage current in the oxygen atmosphere. The annealing phenomenon increased the forward current as well as decrease the reverse current by improving the quality of materials and interfaces. Secondly, the PCE value is increased by Ohmic hybrid electrode which enhanced the performance of the device. Moreover, we also measured the solar cell samples at different temperatures. The performance of the solar cell is more improved. We observed the maximum value of PCE (7.6%) at temperature 250 K as shown in Fig. 6 (c). The performance of

Table 2 Comparison of the performance of the devices. Red fonts show our work. Device structure

Voc

Isc (mA/ cm2)

Efficiency (%)

ITO/Ba dope BFO/Au [49] Graphite/BFO/ZnO/ITO [50] ITO/NiO/Bi2FeCrO6/Nb:SrTiO3 [51] Pt/BLFTO thin films/ZnO:Al [52] ZnO–BFO–N719–CuSCN [53] FTO/TiO2/BFO & FTO/ZnO/BFO [54] Au/BFO/ZnO/ITO (annealed) Gr/BFO/ZnO/Gr/ITO (annealed) (our work)

0.58 0.64 0.53 0.5 0.42 – 0.65 0.67

0.0012 12.47 8 1.3 1.38 3.63

0.006 3.94 2 – 0.38 2.86 5.2 7.4

13.155

7

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the device is decreased at low temperatures due to a rise in series resistance. The change in series resistance is shown in Fig. 6 (d). However, the series resistance changed exponentially with temperature which describes the non-Ohmic behavior due to semiconductors resistance. Further, we have inspected more than one sample based on each atmosphere. We obtained a stable and consistent performance of each solar cell device. We suggested that this development in PCE is endorsed to the annealing in O2 atmosphere due to reduced leakage current and leads to the more rectifying junction.

[15]

[16] [17]

[18] [19]

4. Conclusion [20]

We successfully fabricated the Gr/BFO/ZnO/Gr/ITO/substrate heterostructure device for the improvement of solar cell outcomes. Further, the samples were annealed in a different atmosphere to improve the performance of BFO thin film and the interface of the heterostructure device. We observed that the ramping samples in the O2 atmosphere showed a small leakage current. Furthermore, we measured the junction device under the DUV light source. It is observed that the PCE is improved up to 7.4% in O2 ramping samples with graphene electrode at room temperature. So, the improvement in the crystalline structure, dielectric, magnetic and photovoltaic properties in BFO nanostructure by annealing atmosphere provides a unique platform to explore new findings in the field of optoelectronics and spintronics.

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[22]

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[25]

Declaration of competing interest [26]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.166.

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