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Cu2O:Na thin film solar cells fabricated by DC magnetron sputtering method
Accepted Manuscript Title: THE CHARACTERISTICS OF IGZO/ZnO/Cu2 O:Na THIN FILM SOLAR CELLS FABRICATED BY DC MAGNETRON SPUTTERING METHOD Authors: Nguyen Huu Ke, Phan Thi Kieu Loan, Dao Anh Tuan, Huynh Thanh Dat, Cao Vinh Tran, Le Vu Tuan Hung PII: DOI: Reference:
S1010-6030(17)30586-5 http://dx.doi.org/10.1016/j.jphotochem.2017.09.016 JPC 10861
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
27-4-2017 29-8-2017 5-9-2017
Please cite this article as: Nguyen Huu Ke, Phan Thi Kieu Loan, Dao Anh Tuan, Huynh Thanh Dat, Cao Vinh Tran, Le Vu Tuan Hung, THE CHARACTERISTICS OF IGZO/ZnO/Cu2O:Na THIN FILM SOLAR CELLS FABRICATED BY DC MAGNETRON SPUTTERING METHOD, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.09.016 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.
THE CHARACTERISTICS OF IGZO/ZnO/Cu2O:Na THIN FILM SOLAR CELLS FABRICATED BY DC MAGNETRON SPUTTERING METHOD Nguyen Huu Ke1*, Phan Thi Kieu Loan1, Dao Anh Tuan1, Huynh Thanh Dat3, Cao Vinh Tran2, Le Vu Tuan Hung1 1
Department of Applied Physics, University of Science, VNU-HCM Laboratory of Advanced Materials, University of Science, VNU-HCM 3 Vietnam National University - Ho Chi Minh City 227 Nguyen Van Cu Street, Award 4, District 5, Ho Chi Minh city, Viet Nam 2
Email: [email protected]
Graphical Abstract
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b) ZnO IGZO
Glass
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Highlights -
The Cu2O:Na thin films are successfully deposited by the reactive DC magnetron sputtering method on glass substrates. We can put Na atoms into Cu2O structure directly when this structure is forming in vacuum chamber with helping of plasma energy. The resistivity of Cu2O:Na thin films get the lowest value at 6.8 Ω.cm associated with a hole concentration of 2.1x1018 cm-3. The role of Na impurity in Cu2O structure is proposed that based on the changing of structure and alignment of elements. Characteristics of IGZO/ZnO/Cu2O:Na thin film solar cell fabricated by DC magnetron sputtering have been investigated.
Abstract: In this work, the optical and electrical properties of absorber layers based on cuprous oxide material were investigated. The CuO and Cu2O thin films have been fabricated by reactive DC magnetron sputtering method. All films are p-type semiconductors and have high absorbance in visible range. A solution to improve the electrical properties of the films was also mentioned by incorporating of Sodium (Na) impurities. The Na-doped Cu2O thin films exhibit the hole concentration in magnitude of 1018 cm-3 associated with low resistivity of 6.8 Ω.cm which are suitable for photovoltaic applications. Copper
vacancies produced when Na atoms were incorporated in the Cu2O lattice have caused the increase of hole concentration. The solar cell with structure of IGZO/ZnO/Cu2O:Na was fabricated and investigated for the first time. Remarkable results including open circuit–voltage, fill factor, and conversion efficiency were 0.68 V, 0.42, and 1.68%. Keywords: Cu2O:Na thin film, P-type semiconductor, Na doping, Solar cell, Cu2O material
I. INTRODUCTION Research and improvement of solar cell performance have become urgent when the requirement for clean and renewable energy sources is worldwide nowadays. During the last decade, the development of low cost photovoltaic devices got the considerable attentions of scientific community [1]. In order to replace the expensive Si crystalline, many absorber materials such as Cu2ZnSn(S, Se), Cu2S, CuInS2, CuSbS2, perovskite… have been investigated and developed widely. The efficiency of solar cells based on these materials was reported in range of 7-15% [2, 3]. In recent years, one of the most interesting photovoltaic materials, the cuprous oxide semiconductors are being studied as a potential candidate for photovoltaic applications because of their reasonable electrical and optical properties. Cu2O is a p type semiconductor having a bandgap of 2.1 eV while CuO is about 1.5-1.9 eV [4]. The bandgap of both oxides is well
matched as an absorber for photovoltaic applications and theoretical efficiency of solar cell based on these materials is about 20%. It is very important to realize that the solar cells made from metal oxide materials have some advantages as simple fabrication process, non-toxic, and large scale manufacturing. Therefore, thin film solar cell having heterostructure of Cu2O/ZnO has become an attractive topic in spite of its low efficiency [5]. One of the most important reasons caused the limited performance of this kind of cells is that the hole concentration of pure Cu2O or CuO is low led to high resistivity, respectively [6]. The resistivity of Cu2O thin films could be significantly decreased by doping various impurities. There have been a number of studies aimed at increasing the hole concentration and decreasing the resistivity of films [7, 8, 9]. With doping Ni by pulsed laser deposition method, N. Kikuchi et al has observed the fact that the hole density of the films was found to be constant (1x1015–2x1015cm-3) while the resistivity of as-deposited films remained at 2x102–8x102 Ω.cm and the decreasing of mobility from 22 cm2V-1s-1 to 3.5 cm2V-1s-1 with increasing Ni impurity concentration [8]. Several other impurities such as Be [9], Si [10], In [11], Co and Mn [12] were clearly investigated but none of them gave a reduction of resistivity. In this work, different phases of copper oxides are prepared by varying the substrate temperature via DC reactive magnetron sputtering method. The benefit of this vacuum-based technique is potential for large scale solar cell manufacture. An improvement in the electrical properties of the Cu2O thin films by doping Na impurities was carried out. This kind of films has showed the high hole concentration and low resistivity which is potential for photovoltaic applications. It has been demonstrated that sodium has worked as a surfactant and suppressor of non-radiative recombination center at surface of Cu2ZnSn(S, Se) absorber layers [22]. Na impurities have been also shown to enhance the crystalline and electrical properties in Cu2O structure using hightemperature annealing [19]. And in prior works, Na was doped into Cu2O lattices by the same way in which Na atoms diffused from Cu2O surface by the support of thermal energy [5, 14]. An open question remains whether we can put Na atoms into Cu2O structure directly when this structure is forming in vacuum chamber with helping of plasma energy and whether ZnO/Cu2O:Na thin film solar cell can be fabricated layer by layer in which the thickness of every layer can be controlled easily. As shown in this paper, the Cu2O:Na thin film and ZnO/Cu2O:Na heterojunction were fabricated and investigated for the first time. The front electrode IGZO was also used to replace ITO or FTO which have often appeared in other reports [2, 3, 6]. We hope that these replacement will lead to low mismatch at interface of front electrode and ZnO layer. And from our overview, characteristics of IGZO/ZnO/Cu2O:Na thin film solar cell fabricated by DC magnetron sputtering haven’t been reported so far. Therefore, contribution of p-type absorber Cu2O:Na layer to metal oxide thin film solar cell is really significant.
II. EXPERIMENTAL The Cu2O, CuO and Cu2O:Na thin films were deposited by the reactive DC magnetron sputtering method on glass substrates (Marienfeld, Germany) using a Cu target with 99.99% purity. The base pressure of the sputtering chamber was 10-6 Torr and the pressure during deposition was about 3x10-3 Torr. The Argon and Oxygen were injected into the chamber through a nozzle
whose end was placed near the substrate. The glass substrates were cleaned by HCl solution followed by rinsing in pure water. Then they were cleaned again by acetone solution before putting into vacuum chamber. The substrate temperatures were various from 200oC to 500oC and the target-substrate distance 5 cm was constant during deposition. The pressure ratio between oxygen and argon in sputtering chamber was maintained at 2:3. The sputtering power was about 150W during 30 minutes. The formation of CuO and Cu2O phases depends on substrate temperature and pressure ratio between Oxygen and Argon [13]. In this study, the Cu2O films were fabricated at temperature 200oC while CuO films could be obtained at 500oC. For Na doping process, the Na particle with mass of about 0.005g was cut from high purity Na metal. On the surface of Cu target, a hole with diameter of 2 mm was drilled at the center in order to immobilize this Na particle in plasma regions. The purpose of this process was to create many Cu, O, and Na atoms using plasma energy. These atoms help build the crystal structure and substitute by Na atom at Cu site or vacancy location in lattice easily. The thicknesses of films were determined by using a stylus profiler (Dektak 6M). The crystalline structure of the samples was determined from X-ray diffraction (XRD) patterns which were obtained by using a D8 Advance-Bruker system with source X-rays of Cu Kα = 1.54184 Å. The morphology of the products was analyzed by using scanning electron microscopy (SEM). The UV-Vis Jasco V-530 in the wavelength range of 200-1100 nm was used to record optical spectra of thin films. Moreover, the electrical properties of films were characterized using Hall measurements (Ecopia HMS-3000). The concentration of elements in Cu2O:Na film was detected by Energy dispersive X-ray spectroscopy (EDX) and the X-ray photoelectron spectroscopy (XPS, ThermoVG 350, with the X-ray source of Mg Kα = 1253.6 eV, 150W) was also applied to determine the Na impurity concentration and alignment of elements. In addition, the I–V characteristics of the IGZO/Cu2O:Na and IGZO/ZnO/Cu2O:Na heterojunctions were defined by The Keithley K2612A source and Agilent 4294 Precision Impedance Analyser. Finally, by using a solar simulator (XES-40S1, San-Ei) equipped with AM 1.5 G filters used at 100 mW/cm2, the photovoltaic properties of the solar cell were evaluated exactly. III.
RESULTS AND DISCUSSION
3.1 Crystal structure and Morphology The crystal structural features of Cu2O, CuO and Cu2O:Na thin films are shown at Fig 1. XRD results were taken by D8 Advance-Bruker system with Cu Kα = 1.54184 Å X-rays, step angle: 0.030°, step time: 0.7 s and room temperature. The thin films deposited at 200oC have showed the clearly peak at 2θ=36.5o which characterizes the (111) orientation of Cu2O structure [2, 3, 6]. Otherwise, the films prepared at over 500oC include a main peak at 2θ=38o and an extra peak at 2θ=36.8o according to (111) orientation of CuO and Cu2O [7]. This implies that both of CuO and Cu2O structures also exist in the same film but the properties of it seem to depend on main CuO structures. The effect of higher substrate temperature for the formation of CuO phase can be understood by the following reaction: 2Cu2O + O2 = 4CuO (1)
At lower temperature, first Cu2O phase is formed. When the temperature rises and reaches 500oC and above, Cu2O starts reacting with O and form CuO phase [17]. However, when Na impurities were doped into Cu2O structure, only the (111) peak of Cu2O structure at 2θ=36.9o is clearly observed in this figure. In addition, none special XRD peak of Na compound is detected on patterns proves the fact that Na impurity has been dissolved in the Cu2O crystalline structure [15, 18]. Especially, the full-widths-at-half-maximum (FWHM) of (111) Cu2O:Na structure also decreases respectively. This reveals the fact that Na doping has enhanced the crystal quality and grain boundaries in Cu2O structure. The group I element as Na seem to be relevant to passive the dangling bonds that have worked as point defects in the Cu2O lattice. Surface morphology of the Cu2O, CuO and Cu2O:Na thin film is displayed on Fig 2. The cubic structure of Cu2O with size of 100 nm is observed on Fig 2a obviously. When substrate temperature reaches the 500oC, the crystalline structure of CuO is strongly grown with size of 200 nm as Fig 2b. However, the smooth and uniformity on a large area of Cu2O:Na thin films are found immediately. Unlike the cubic structure often observed on Cu2O thin films, the orderly structure with latitude of 100 nm is explored on the surface of Cu2O:Na thin films as Fig 2c. It is attributed to the influence of Na impurity on morphology immediately [14]. The thickness of Cu2O:Na thin film is about 1µm as Fig 2d. The smooth surface of Cu2O:Na thin film is potential for solar cell application which needs the low defect density at interface. 3.2 Optical properties Fig 3a shows absorption spectra of Cu2O, CuO and Cu2O:Na thin films. Absorption peaks of samples are clearly defined at 500 nm and absorption range is widely in visible from 400-600nm according to Cu2O and Cu2O:Na. In case of CuO thin films, intensity of absorption peaks increases sharply and absorption range also extends in range of 400-700 nm. The bandgap energy of Cu2O, CuO and Cu2O:Na thin films are about 2.16 eV, 1.9 eV and 2.04 eV respectively. Especially, bandgap energy has been shrunk 0.12 eV with contribution of Na impurities. It reveals the truth that Na atoms have caused many defect levels occurred in bandgap [5, 15]. It seem to be that the dissolving of Na impurities in the Cu2O crystalline structure led to form acceptor levels near valence band. As the concentration of Na impurities is large enough, the acceptor levels form a band which in turn reduces the bandgap energy of material.
3.3 Electrical properties The electrical properties of Cu2O, CuO and Cu2O:Na samples including carrier concentration, mobility, and resistivity are shown in Table 1. All of samples are p-type semiconductors obviously. For CuO thin films, the hole concentration and resistivity are about 9.31x1014 cm-3 and 2.43x103 Ω.cm respectively. The Cu2O thin films showed the higher hole concentration at 2.83x1015 cm-3 and lower resistivity at 4.14x102 Ω.cm. It seem to be the main reason why solar cells based on CuO/ZnO structure often have lower efficiency than Cu2O/ZnO structure [6, 8, 10]. In addition, the highest hole concentration on the order of 1018 cm-3 and the lowest resistivity of 6.8 Ω.cm can be observed clearly with Cu2O:Na thin films. The increasing of hole concentration is due to the substitution of Na impurities for copper sites and acted as acceptors.
In p-type non-doped Cu2O, an oxygen vacancy (VO) which acts as a donor is self-compensated by two VCu which act as an acceptor; that is, a hole in the valence band can be excited from a VCu-, and two electrons in the conduction band can be excited from VO+2 [14]. An increase of hole concentration in the p-type Cu2O, caused by incorporating Na, may be achieved by a further formation of an acceptor, or VCu. As we discuss above, the formation of acceptor levels is possible. Obviously, the increasing of hole concentration has led to the decreasing of resistivity which expected to improve the performance of solar cells based on Cu2O material. Finally, a further understanding about the alignment of impurities in the Cu2O lattice is necessary to establish and propose the Na incorporating mechanisms.
3.4 Binding properties X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of elements in the prepared Cu2O films. Fig 4a is the XPS survey spectrum after calibration using the C 1s peak at 284.8 eV. Fig 4b, c shows the detection of the O 1s core level peak at 530.3 ± 0.2 eV, Cu 2p1/2 peaks at 932.7 ± 0.2 and 953.6 ± 0.2 eV respectively. The position of the Cu 2p1/2 peaks indicates the presence of Cu+1 which confirms the growth of Cu2O. Moreover, a sodium core level Na 1s was observed at a binding energy of 1071.8 ± 0.2 eV as Fig 4d. This value of binding energy corresponds to the presence of Na-O binding in Na2O structures [13]. This is a testament to the fact that Na impurities have substituted for a Cu sites in lattice and formed Na-O alignments which leads to the development of Cu2O crystalline [18]. Na2O compounds were expected to form in Cu2O more than CuO structure due to two reasons. Firstly, Na2O forms in the cuprous structure, the same structure as Cu2O that tends to enhance the crystalline structure and improve hole concentration as well as minor carrier lifetime of Cu2O host material. Secondly, the Gibbs free energy of Na-O is about -377.1 kJ/mol and lower than -73.9 kJ/mol of Cu-O [19, 20]. Thus Na atoms tend to attract and form bonds with the O atoms. This mean that oxygen vacancy (VO) which acts as a donor would be decreased whereas VCu which act as an acceptor would be increased. This suggests that a considerable fraction of sodium in the prepared Cu2O films bonds with oxygen as a substitution for copper. The concentration of Cu, O, Na elements defined from XPS spectra are about 72 %, 22 % and 6 % respectively. This values have a little change when compared to the result from EDX spectrum: 70 %, 25 % and 5 % as Fig 5c. Due to the XPS values refer to the immediate surface layer of the samples therefore the concentration estimated for the Na doping is only for the surface region. An estimation of the bulk concentration of the Na would be essential and there is reason why EDX spectra were tried in this situation. As deposited Cu2O:Na sample with orange color is shown on Fig 5a. We have cut this sample to four small samples as G, I, H, K. The EDX spectra of G, I, H, K samples are also displayed on Fig 5c. The Na concentration in Cu2O structure remains the stability at about 5%. In addition, line-scan EDX image of Cu2O:Na surface is used to evaluated the change of element concentration. By scanning the electron beam and collecting the intensity of a selected X-ray line, element distribution image can be produced. With the constant intensity of Cu Lα, O Kα, Na Kα via scanning distance about 1300 µm which is clearly observed on Fig 5b reveals the truth that the homogeneous dopant distribution in sample is possible. Na is a chemical
element with atomic number 11 which easily migrates within the crystal lattice via support of thermal energy and then passive the dangling bonds in the Cu2O lattice. And thus, the Na incorporating plays an important role not only in reducing defects but also in enhancing crystalline and hole concentration of Cu2O material.
3.5 I-V Characterization To confirm p-type conduction of the CuO, Cu2O and Cu2O:Na films, the simple p-n heterojunction diodes were fabricated by growing these p-type films on conductive indium gallium zinc oxide (IGZO) thin film as Fig 6a. The n-type IGZO thin films were deposited on glass substrates by using DC magnetron sputtering. The electrical properties of IGZO thin films, including electron concentration, mobility, and resistivity were corresponding to 8×1020 cm−3, 25 cm2/V.s and 7.5×10−4 Ω.cm. The detailed fabrication conditions of IGZO were described elsewhere [21]. Fig 6b showed the dark I–V characteristic curve of CuO/IGZO. Unfortunately, ohmic behavior has been observed obviously. The constant resistance in reverse and forward bias exhibited the fact that barrier potential region couldn’t be formed at interface of junction. Perhaps preparing CuO layer at high temperatures at 500oC has caused diffusion of metal ions through the interface. This has created the large leakage current across the junction. Otherwise, I–V characteristics of the Cu2O/IGZO and Cu2O:Na/IGZO heterojunctions have exhibited significant diode behaviors as Fig 6c, d. The forward current which corresponds to the positive potential of the p-n junction can be described as follows: 1 (2) Where Is is the reverse bias saturation current, V is the bias voltage, n is the ideality factor, q is the electron charge, k is the Boltzmann’s constant, and T is the absolute temperature (at room temperature T = 300 K). The values of Is and n of the heterojunctions can be determined from Eq. (2) using the data in Fig 6c, d. The ideality factors of Cu2O/IGZO and Cu2O:Na/IGZO heterojunctions were defined at 8 and 7 respectively. The calculated ideality factors were quite larger than the expected values of 1 - 2, which could be attributed to various effects including large series resistance, trap-assisted tunneling, and carrier leakage because of the interface defect states between two layers [22]. The series resistance Rs and the parasitic shunt resistance Rshunt which played an important role on the transport characteristics at low forward and reverse bias (< 0.3V) were also calculated and shown on Fig 6c, d. Both Rs and Rshunt of Cu2O:Na/IGZO were lower than Cu2O/IGZO heterojunctions. This indicated that the electrical conductivity across interface has been increased by Na incorporating. Especially, this proved the fact that increasing hole concentration and reducing resistivity of Cu2O material can be completely achieved by doping Na impurities into Cu2O lattice. Using the fabricating condition of p type Cu2O:Na thin films, we have constructed two structures of heterojunction solar cells: IGZO/Cu2O:Na and IGZO/ZnO/Cu2O:Na as shown on Fig 7a. The cell area equals to the area of the gold electrode which is about 0.1cm2 (0.2cm x 0.5cm). The
thickness of undoped ZnO layer is about 320-380nm which clearly defined by cross section SEM image as Fig 7b. From dark I–V characteristic curves of both structures as Fig 7c, the remarkable diode behaviors have been observed obviously. In particular, the leakage current has been limited significantly according to the presence of ZnO layer in structure. Under the illuminated conditions, the photocurrents which appeared on J-V characteristic as Fig 7d revealed the fact that both cells have photosensitive ability. With IGZO/Cu2O:Na structure, short circuit–current density, open circuit–voltage, fill factor, and conversion efficiency were defined as 2.5 mA/cm2, 0.25 V, 0.28, and 0.34% while these values of IGZO/ZnO/Cu2O:Na structure were 2.4 mA/cm2, 0.68 V, 0.42 and 1.68% respectively. The charge transport mechanisms in both structure was described via energy band diagrams as Fig 8. The surface defect density at interface which have worked as recombination centers and caused tunneling current through junction as Fig 8a was the main reason for ineffective action of IGZO/Cu2O:Na solar cell. The presence of pure ZnO layer was correlated with prevention of parasitic current across barrier [21]. The improvement of open circuit–voltage and efficiency could be attributed to the influence of ZnO layer which increased potential barrier in depletion region and explained by proposed energy band diagram as Fig 8b. Passivation of surface defects could be necessary solution to enhance performance of this solar cells.
IV.
CONCLUSION
In summary, we have demonstrated the ability to obtain CuO, Cu2O and Cu2O:Na thin films by reactive DC magnetron sputtering method. All films were p-type semiconductor and showed high absorbance in visible range which considered as potential materials for solar cell application. Especially, the electrical property of Cu2O thin films could be improved dramatically by incorporating Na into Cu2O crystalline. It was proved that the resistivity of Cu2O:Na thin films get the lowest value at 6.8 Ω.cm associated with a hole concentration of 2.1x1018 cm-3. The I–V characteristics of the Cu2O/IGZO and Cu2O:Na/IGZO heterojunctions have exhibited significant diode behaviors. The series resistance has been reduced significantly according to contribution of Na incorporating. Copper vacancies produced when Na atoms were incorporated in the Cu2O lattice have caused the increase of hole concentration. The solar cells based on ZnO and Cu2O:Na materials were fabricated in vacuum chamber. Remarkable results including open circuit–voltage, fill factor, and conversion efficiency were 0.68 V, 0.42, and 1.68% according to IGZO/ZnO/Cu2O:Na structure.
Acknowledgement The authors wish to acknowledge Nguyen Le Dang Khoa, Dang Thi Bich and Phuoc Huu Le for their technical assistance in the experiments. This research was supported by Vietnam National University (VNU-HCM), Ho Chi Minh City, [grant number B2016-76-01.].
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Figure captions Fig 1: XRD patterns of Cu2O, CuO and Cu2O:Na thin films Fig 2: SEM images of a) Cu2O, b) CuO, c) Cu2O:Na thin films, d) Cross section SEM image of Cu2O:Na thin film. Fig 3: a) Absorption spectra of Cu2O, CuO and Cu2O:Na thin films, b) Tauc plots of Cu2O, CuO and Cu2O:Na thin films. Fig 4: X-ray photoelectron spectroscopy (XPS) spectra of a) Cu2O:Na sample survey, b) O 1s, c) Cu 2p and d) Na 1s. Fig 5: a) Cu2O:Na sample with orange color and four samples G, I, H, K are divided from that, b) line-scan EDX image of Cu2O:Na surface (green, red and turquoise blue according to Cu Lα, O Kα and Na Kα), c) EDX spectra of G, I, H, K samples. Fig 6: a) Schematic diagram of the p-Cu2O:Na/n-IGZO heterojunction, b) The dark I–V characteristic curve of CuO/IGZO, c) The typical semilog I-V curve of Cu2O/IGZO, d) The typical semilog I-V curve of Cu2O:Na/IGZO Fig 7: a) Two structures of solar cell based on Cu2O:Na and ZnO materials, b) Cross section SEM image of IGZO/ZnO junction, c) Dark I–V characteristic curves of both structures, d) J-V characteristic curves of both structures. Fig 8: a) Energy band diagram of the IGZO/Cu2O:Na structure, b) Energy band diagram of the IGZO/ZnO/Cu2O:Na structure.
Table captions Table 1: The Hole concentration, Mobility, and Resistivity of the CuO, Cu2O and Cu2O:Na thin films.
Fig 1: Ke at al
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Cu2O:Na
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Fig 2: Ke at al
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Fig 3: Ke at al
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Fig 4: Ke at al
G
H
I
K
Cu: 69.1%
Sample G
O: 26.4% Na: 4.5%
10 mm
Sample I 24 mm
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Cu: 71.2% O: 24.1% Na: 4.7%
1300 µm
b) Cu: 70.5%
Sample H
O: 24.7% Na: 4.8%
Sample K
Cu: 70.3% O: 25.1% Na: 4.6%
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Fig 5: Ke at al
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Fig 6: Ke at al
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b) ZnO IGZO
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c)
d)
Fig 7: Ke at al
a)
Fig 8: Ke at al
b)
Table 1: Ke at al Sample
Thickness (nm)
Temperature (oC)
Hole concentration (cm-3)
Mobility (cm2.V-1.s-1)
Resistivity (Ω.cm)
CuO
920
500
9.31x1014
31.5
2.43x103
Cu2O
950
200
2.83x1015
48.9
4.14x102
Cu2O:Na
1000
200
2.11x1018
4.18
6.8
S1010-6030(17)30586-5 http://dx.doi.org/10.1016/j.jphotochem.2017.09.016 JPC 10861
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
27-4-2017 29-8-2017 5-9-2017
Please cite this article as: Nguyen Huu Ke, Phan Thi Kieu Loan, Dao Anh Tuan, Huynh Thanh Dat, Cao Vinh Tran, Le Vu Tuan Hung, THE CHARACTERISTICS OF IGZO/ZnO/Cu2O:Na THIN FILM SOLAR CELLS FABRICATED BY DC MAGNETRON SPUTTERING METHOD, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.09.016 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.
THE CHARACTERISTICS OF IGZO/ZnO/Cu2O:Na THIN FILM SOLAR CELLS FABRICATED BY DC MAGNETRON SPUTTERING METHOD Nguyen Huu Ke1*, Phan Thi Kieu Loan1, Dao Anh Tuan1, Huynh Thanh Dat3, Cao Vinh Tran2, Le Vu Tuan Hung1 1
Department of Applied Physics, University of Science, VNU-HCM Laboratory of Advanced Materials, University of Science, VNU-HCM 3 Vietnam National University - Ho Chi Minh City 227 Nguyen Van Cu Street, Award 4, District 5, Ho Chi Minh city, Viet Nam 2
Email: [email protected]
Graphical Abstract
a)
b) ZnO IGZO
Glass
c)
d)
Highlights -
The Cu2O:Na thin films are successfully deposited by the reactive DC magnetron sputtering method on glass substrates. We can put Na atoms into Cu2O structure directly when this structure is forming in vacuum chamber with helping of plasma energy. The resistivity of Cu2O:Na thin films get the lowest value at 6.8 Ω.cm associated with a hole concentration of 2.1x1018 cm-3. The role of Na impurity in Cu2O structure is proposed that based on the changing of structure and alignment of elements. Characteristics of IGZO/ZnO/Cu2O:Na thin film solar cell fabricated by DC magnetron sputtering have been investigated.
Abstract: In this work, the optical and electrical properties of absorber layers based on cuprous oxide material were investigated. The CuO and Cu2O thin films have been fabricated by reactive DC magnetron sputtering method. All films are p-type semiconductors and have high absorbance in visible range. A solution to improve the electrical properties of the films was also mentioned by incorporating of Sodium (Na) impurities. The Na-doped Cu2O thin films exhibit the hole concentration in magnitude of 1018 cm-3 associated with low resistivity of 6.8 Ω.cm which are suitable for photovoltaic applications. Copper
vacancies produced when Na atoms were incorporated in the Cu2O lattice have caused the increase of hole concentration. The solar cell with structure of IGZO/ZnO/Cu2O:Na was fabricated and investigated for the first time. Remarkable results including open circuit–voltage, fill factor, and conversion efficiency were 0.68 V, 0.42, and 1.68%. Keywords: Cu2O:Na thin film, P-type semiconductor, Na doping, Solar cell, Cu2O material
I. INTRODUCTION Research and improvement of solar cell performance have become urgent when the requirement for clean and renewable energy sources is worldwide nowadays. During the last decade, the development of low cost photovoltaic devices got the considerable attentions of scientific community [1]. In order to replace the expensive Si crystalline, many absorber materials such as Cu2ZnSn(S, Se), Cu2S, CuInS2, CuSbS2, perovskite… have been investigated and developed widely. The efficiency of solar cells based on these materials was reported in range of 7-15% [2, 3]. In recent years, one of the most interesting photovoltaic materials, the cuprous oxide semiconductors are being studied as a potential candidate for photovoltaic applications because of their reasonable electrical and optical properties. Cu2O is a p type semiconductor having a bandgap of 2.1 eV while CuO is about 1.5-1.9 eV [4]. The bandgap of both oxides is well
matched as an absorber for photovoltaic applications and theoretical efficiency of solar cell based on these materials is about 20%. It is very important to realize that the solar cells made from metal oxide materials have some advantages as simple fabrication process, non-toxic, and large scale manufacturing. Therefore, thin film solar cell having heterostructure of Cu2O/ZnO has become an attractive topic in spite of its low efficiency [5]. One of the most important reasons caused the limited performance of this kind of cells is that the hole concentration of pure Cu2O or CuO is low led to high resistivity, respectively [6]. The resistivity of Cu2O thin films could be significantly decreased by doping various impurities. There have been a number of studies aimed at increasing the hole concentration and decreasing the resistivity of films [7, 8, 9]. With doping Ni by pulsed laser deposition method, N. Kikuchi et al has observed the fact that the hole density of the films was found to be constant (1x1015–2x1015cm-3) while the resistivity of as-deposited films remained at 2x102–8x102 Ω.cm and the decreasing of mobility from 22 cm2V-1s-1 to 3.5 cm2V-1s-1 with increasing Ni impurity concentration [8]. Several other impurities such as Be [9], Si [10], In [11], Co and Mn [12] were clearly investigated but none of them gave a reduction of resistivity. In this work, different phases of copper oxides are prepared by varying the substrate temperature via DC reactive magnetron sputtering method. The benefit of this vacuum-based technique is potential for large scale solar cell manufacture. An improvement in the electrical properties of the Cu2O thin films by doping Na impurities was carried out. This kind of films has showed the high hole concentration and low resistivity which is potential for photovoltaic applications. It has been demonstrated that sodium has worked as a surfactant and suppressor of non-radiative recombination center at surface of Cu2ZnSn(S, Se) absorber layers [22]. Na impurities have been also shown to enhance the crystalline and electrical properties in Cu2O structure using hightemperature annealing [19]. And in prior works, Na was doped into Cu2O lattices by the same way in which Na atoms diffused from Cu2O surface by the support of thermal energy [5, 14]. An open question remains whether we can put Na atoms into Cu2O structure directly when this structure is forming in vacuum chamber with helping of plasma energy and whether ZnO/Cu2O:Na thin film solar cell can be fabricated layer by layer in which the thickness of every layer can be controlled easily. As shown in this paper, the Cu2O:Na thin film and ZnO/Cu2O:Na heterojunction were fabricated and investigated for the first time. The front electrode IGZO was also used to replace ITO or FTO which have often appeared in other reports [2, 3, 6]. We hope that these replacement will lead to low mismatch at interface of front electrode and ZnO layer. And from our overview, characteristics of IGZO/ZnO/Cu2O:Na thin film solar cell fabricated by DC magnetron sputtering haven’t been reported so far. Therefore, contribution of p-type absorber Cu2O:Na layer to metal oxide thin film solar cell is really significant.
II. EXPERIMENTAL The Cu2O, CuO and Cu2O:Na thin films were deposited by the reactive DC magnetron sputtering method on glass substrates (Marienfeld, Germany) using a Cu target with 99.99% purity. The base pressure of the sputtering chamber was 10-6 Torr and the pressure during deposition was about 3x10-3 Torr. The Argon and Oxygen were injected into the chamber through a nozzle
whose end was placed near the substrate. The glass substrates were cleaned by HCl solution followed by rinsing in pure water. Then they were cleaned again by acetone solution before putting into vacuum chamber. The substrate temperatures were various from 200oC to 500oC and the target-substrate distance 5 cm was constant during deposition. The pressure ratio between oxygen and argon in sputtering chamber was maintained at 2:3. The sputtering power was about 150W during 30 minutes. The formation of CuO and Cu2O phases depends on substrate temperature and pressure ratio between Oxygen and Argon [13]. In this study, the Cu2O films were fabricated at temperature 200oC while CuO films could be obtained at 500oC. For Na doping process, the Na particle with mass of about 0.005g was cut from high purity Na metal. On the surface of Cu target, a hole with diameter of 2 mm was drilled at the center in order to immobilize this Na particle in plasma regions. The purpose of this process was to create many Cu, O, and Na atoms using plasma energy. These atoms help build the crystal structure and substitute by Na atom at Cu site or vacancy location in lattice easily. The thicknesses of films were determined by using a stylus profiler (Dektak 6M). The crystalline structure of the samples was determined from X-ray diffraction (XRD) patterns which were obtained by using a D8 Advance-Bruker system with source X-rays of Cu Kα = 1.54184 Å. The morphology of the products was analyzed by using scanning electron microscopy (SEM). The UV-Vis Jasco V-530 in the wavelength range of 200-1100 nm was used to record optical spectra of thin films. Moreover, the electrical properties of films were characterized using Hall measurements (Ecopia HMS-3000). The concentration of elements in Cu2O:Na film was detected by Energy dispersive X-ray spectroscopy (EDX) and the X-ray photoelectron spectroscopy (XPS, ThermoVG 350, with the X-ray source of Mg Kα = 1253.6 eV, 150W) was also applied to determine the Na impurity concentration and alignment of elements. In addition, the I–V characteristics of the IGZO/Cu2O:Na and IGZO/ZnO/Cu2O:Na heterojunctions were defined by The Keithley K2612A source and Agilent 4294 Precision Impedance Analyser. Finally, by using a solar simulator (XES-40S1, San-Ei) equipped with AM 1.5 G filters used at 100 mW/cm2, the photovoltaic properties of the solar cell were evaluated exactly. III.
RESULTS AND DISCUSSION
3.1 Crystal structure and Morphology The crystal structural features of Cu2O, CuO and Cu2O:Na thin films are shown at Fig 1. XRD results were taken by D8 Advance-Bruker system with Cu Kα = 1.54184 Å X-rays, step angle: 0.030°, step time: 0.7 s and room temperature. The thin films deposited at 200oC have showed the clearly peak at 2θ=36.5o which characterizes the (111) orientation of Cu2O structure [2, 3, 6]. Otherwise, the films prepared at over 500oC include a main peak at 2θ=38o and an extra peak at 2θ=36.8o according to (111) orientation of CuO and Cu2O [7]. This implies that both of CuO and Cu2O structures also exist in the same film but the properties of it seem to depend on main CuO structures. The effect of higher substrate temperature for the formation of CuO phase can be understood by the following reaction: 2Cu2O + O2 = 4CuO (1)
At lower temperature, first Cu2O phase is formed. When the temperature rises and reaches 500oC and above, Cu2O starts reacting with O and form CuO phase [17]. However, when Na impurities were doped into Cu2O structure, only the (111) peak of Cu2O structure at 2θ=36.9o is clearly observed in this figure. In addition, none special XRD peak of Na compound is detected on patterns proves the fact that Na impurity has been dissolved in the Cu2O crystalline structure [15, 18]. Especially, the full-widths-at-half-maximum (FWHM) of (111) Cu2O:Na structure also decreases respectively. This reveals the fact that Na doping has enhanced the crystal quality and grain boundaries in Cu2O structure. The group I element as Na seem to be relevant to passive the dangling bonds that have worked as point defects in the Cu2O lattice. Surface morphology of the Cu2O, CuO and Cu2O:Na thin film is displayed on Fig 2. The cubic structure of Cu2O with size of 100 nm is observed on Fig 2a obviously. When substrate temperature reaches the 500oC, the crystalline structure of CuO is strongly grown with size of 200 nm as Fig 2b. However, the smooth and uniformity on a large area of Cu2O:Na thin films are found immediately. Unlike the cubic structure often observed on Cu2O thin films, the orderly structure with latitude of 100 nm is explored on the surface of Cu2O:Na thin films as Fig 2c. It is attributed to the influence of Na impurity on morphology immediately [14]. The thickness of Cu2O:Na thin film is about 1µm as Fig 2d. The smooth surface of Cu2O:Na thin film is potential for solar cell application which needs the low defect density at interface. 3.2 Optical properties Fig 3a shows absorption spectra of Cu2O, CuO and Cu2O:Na thin films. Absorption peaks of samples are clearly defined at 500 nm and absorption range is widely in visible from 400-600nm according to Cu2O and Cu2O:Na. In case of CuO thin films, intensity of absorption peaks increases sharply and absorption range also extends in range of 400-700 nm. The bandgap energy of Cu2O, CuO and Cu2O:Na thin films are about 2.16 eV, 1.9 eV and 2.04 eV respectively. Especially, bandgap energy has been shrunk 0.12 eV with contribution of Na impurities. It reveals the truth that Na atoms have caused many defect levels occurred in bandgap [5, 15]. It seem to be that the dissolving of Na impurities in the Cu2O crystalline structure led to form acceptor levels near valence band. As the concentration of Na impurities is large enough, the acceptor levels form a band which in turn reduces the bandgap energy of material.
3.3 Electrical properties The electrical properties of Cu2O, CuO and Cu2O:Na samples including carrier concentration, mobility, and resistivity are shown in Table 1. All of samples are p-type semiconductors obviously. For CuO thin films, the hole concentration and resistivity are about 9.31x1014 cm-3 and 2.43x103 Ω.cm respectively. The Cu2O thin films showed the higher hole concentration at 2.83x1015 cm-3 and lower resistivity at 4.14x102 Ω.cm. It seem to be the main reason why solar cells based on CuO/ZnO structure often have lower efficiency than Cu2O/ZnO structure [6, 8, 10]. In addition, the highest hole concentration on the order of 1018 cm-3 and the lowest resistivity of 6.8 Ω.cm can be observed clearly with Cu2O:Na thin films. The increasing of hole concentration is due to the substitution of Na impurities for copper sites and acted as acceptors.
In p-type non-doped Cu2O, an oxygen vacancy (VO) which acts as a donor is self-compensated by two VCu which act as an acceptor; that is, a hole in the valence band can be excited from a VCu-, and two electrons in the conduction band can be excited from VO+2 [14]. An increase of hole concentration in the p-type Cu2O, caused by incorporating Na, may be achieved by a further formation of an acceptor, or VCu. As we discuss above, the formation of acceptor levels is possible. Obviously, the increasing of hole concentration has led to the decreasing of resistivity which expected to improve the performance of solar cells based on Cu2O material. Finally, a further understanding about the alignment of impurities in the Cu2O lattice is necessary to establish and propose the Na incorporating mechanisms.
3.4 Binding properties X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of elements in the prepared Cu2O films. Fig 4a is the XPS survey spectrum after calibration using the C 1s peak at 284.8 eV. Fig 4b, c shows the detection of the O 1s core level peak at 530.3 ± 0.2 eV, Cu 2p1/2 peaks at 932.7 ± 0.2 and 953.6 ± 0.2 eV respectively. The position of the Cu 2p1/2 peaks indicates the presence of Cu+1 which confirms the growth of Cu2O. Moreover, a sodium core level Na 1s was observed at a binding energy of 1071.8 ± 0.2 eV as Fig 4d. This value of binding energy corresponds to the presence of Na-O binding in Na2O structures [13]. This is a testament to the fact that Na impurities have substituted for a Cu sites in lattice and formed Na-O alignments which leads to the development of Cu2O crystalline [18]. Na2O compounds were expected to form in Cu2O more than CuO structure due to two reasons. Firstly, Na2O forms in the cuprous structure, the same structure as Cu2O that tends to enhance the crystalline structure and improve hole concentration as well as minor carrier lifetime of Cu2O host material. Secondly, the Gibbs free energy of Na-O is about -377.1 kJ/mol and lower than -73.9 kJ/mol of Cu-O [19, 20]. Thus Na atoms tend to attract and form bonds with the O atoms. This mean that oxygen vacancy (VO) which acts as a donor would be decreased whereas VCu which act as an acceptor would be increased. This suggests that a considerable fraction of sodium in the prepared Cu2O films bonds with oxygen as a substitution for copper. The concentration of Cu, O, Na elements defined from XPS spectra are about 72 %, 22 % and 6 % respectively. This values have a little change when compared to the result from EDX spectrum: 70 %, 25 % and 5 % as Fig 5c. Due to the XPS values refer to the immediate surface layer of the samples therefore the concentration estimated for the Na doping is only for the surface region. An estimation of the bulk concentration of the Na would be essential and there is reason why EDX spectra were tried in this situation. As deposited Cu2O:Na sample with orange color is shown on Fig 5a. We have cut this sample to four small samples as G, I, H, K. The EDX spectra of G, I, H, K samples are also displayed on Fig 5c. The Na concentration in Cu2O structure remains the stability at about 5%. In addition, line-scan EDX image of Cu2O:Na surface is used to evaluated the change of element concentration. By scanning the electron beam and collecting the intensity of a selected X-ray line, element distribution image can be produced. With the constant intensity of Cu Lα, O Kα, Na Kα via scanning distance about 1300 µm which is clearly observed on Fig 5b reveals the truth that the homogeneous dopant distribution in sample is possible. Na is a chemical
element with atomic number 11 which easily migrates within the crystal lattice via support of thermal energy and then passive the dangling bonds in the Cu2O lattice. And thus, the Na incorporating plays an important role not only in reducing defects but also in enhancing crystalline and hole concentration of Cu2O material.
3.5 I-V Characterization To confirm p-type conduction of the CuO, Cu2O and Cu2O:Na films, the simple p-n heterojunction diodes were fabricated by growing these p-type films on conductive indium gallium zinc oxide (IGZO) thin film as Fig 6a. The n-type IGZO thin films were deposited on glass substrates by using DC magnetron sputtering. The electrical properties of IGZO thin films, including electron concentration, mobility, and resistivity were corresponding to 8×1020 cm−3, 25 cm2/V.s and 7.5×10−4 Ω.cm. The detailed fabrication conditions of IGZO were described elsewhere [21]. Fig 6b showed the dark I–V characteristic curve of CuO/IGZO. Unfortunately, ohmic behavior has been observed obviously. The constant resistance in reverse and forward bias exhibited the fact that barrier potential region couldn’t be formed at interface of junction. Perhaps preparing CuO layer at high temperatures at 500oC has caused diffusion of metal ions through the interface. This has created the large leakage current across the junction. Otherwise, I–V characteristics of the Cu2O/IGZO and Cu2O:Na/IGZO heterojunctions have exhibited significant diode behaviors as Fig 6c, d. The forward current which corresponds to the positive potential of the p-n junction can be described as follows: 1 (2) Where Is is the reverse bias saturation current, V is the bias voltage, n is the ideality factor, q is the electron charge, k is the Boltzmann’s constant, and T is the absolute temperature (at room temperature T = 300 K). The values of Is and n of the heterojunctions can be determined from Eq. (2) using the data in Fig 6c, d. The ideality factors of Cu2O/IGZO and Cu2O:Na/IGZO heterojunctions were defined at 8 and 7 respectively. The calculated ideality factors were quite larger than the expected values of 1 - 2, which could be attributed to various effects including large series resistance, trap-assisted tunneling, and carrier leakage because of the interface defect states between two layers [22]. The series resistance Rs and the parasitic shunt resistance Rshunt which played an important role on the transport characteristics at low forward and reverse bias (< 0.3V) were also calculated and shown on Fig 6c, d. Both Rs and Rshunt of Cu2O:Na/IGZO were lower than Cu2O/IGZO heterojunctions. This indicated that the electrical conductivity across interface has been increased by Na incorporating. Especially, this proved the fact that increasing hole concentration and reducing resistivity of Cu2O material can be completely achieved by doping Na impurities into Cu2O lattice. Using the fabricating condition of p type Cu2O:Na thin films, we have constructed two structures of heterojunction solar cells: IGZO/Cu2O:Na and IGZO/ZnO/Cu2O:Na as shown on Fig 7a. The cell area equals to the area of the gold electrode which is about 0.1cm2 (0.2cm x 0.5cm). The
thickness of undoped ZnO layer is about 320-380nm which clearly defined by cross section SEM image as Fig 7b. From dark I–V characteristic curves of both structures as Fig 7c, the remarkable diode behaviors have been observed obviously. In particular, the leakage current has been limited significantly according to the presence of ZnO layer in structure. Under the illuminated conditions, the photocurrents which appeared on J-V characteristic as Fig 7d revealed the fact that both cells have photosensitive ability. With IGZO/Cu2O:Na structure, short circuit–current density, open circuit–voltage, fill factor, and conversion efficiency were defined as 2.5 mA/cm2, 0.25 V, 0.28, and 0.34% while these values of IGZO/ZnO/Cu2O:Na structure were 2.4 mA/cm2, 0.68 V, 0.42 and 1.68% respectively. The charge transport mechanisms in both structure was described via energy band diagrams as Fig 8. The surface defect density at interface which have worked as recombination centers and caused tunneling current through junction as Fig 8a was the main reason for ineffective action of IGZO/Cu2O:Na solar cell. The presence of pure ZnO layer was correlated with prevention of parasitic current across barrier [21]. The improvement of open circuit–voltage and efficiency could be attributed to the influence of ZnO layer which increased potential barrier in depletion region and explained by proposed energy band diagram as Fig 8b. Passivation of surface defects could be necessary solution to enhance performance of this solar cells.
IV.
CONCLUSION
In summary, we have demonstrated the ability to obtain CuO, Cu2O and Cu2O:Na thin films by reactive DC magnetron sputtering method. All films were p-type semiconductor and showed high absorbance in visible range which considered as potential materials for solar cell application. Especially, the electrical property of Cu2O thin films could be improved dramatically by incorporating Na into Cu2O crystalline. It was proved that the resistivity of Cu2O:Na thin films get the lowest value at 6.8 Ω.cm associated with a hole concentration of 2.1x1018 cm-3. The I–V characteristics of the Cu2O/IGZO and Cu2O:Na/IGZO heterojunctions have exhibited significant diode behaviors. The series resistance has been reduced significantly according to contribution of Na incorporating. Copper vacancies produced when Na atoms were incorporated in the Cu2O lattice have caused the increase of hole concentration. The solar cells based on ZnO and Cu2O:Na materials were fabricated in vacuum chamber. Remarkable results including open circuit–voltage, fill factor, and conversion efficiency were 0.68 V, 0.42, and 1.68% according to IGZO/ZnO/Cu2O:Na structure.
Acknowledgement The authors wish to acknowledge Nguyen Le Dang Khoa, Dang Thi Bich and Phuoc Huu Le for their technical assistance in the experiments. This research was supported by Vietnam National University (VNU-HCM), Ho Chi Minh City, [grant number B2016-76-01.].
References
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C. Jayathilaka, V. Kapaklis, W. Siripala, and S. Jayanetti, Improved efficiency of electrodeposited p-CuO/n-Cu2O heterojunction solar cell, Applied Physics Express, 8 (2015) 065503.
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Figure captions Fig 1: XRD patterns of Cu2O, CuO and Cu2O:Na thin films Fig 2: SEM images of a) Cu2O, b) CuO, c) Cu2O:Na thin films, d) Cross section SEM image of Cu2O:Na thin film. Fig 3: a) Absorption spectra of Cu2O, CuO and Cu2O:Na thin films, b) Tauc plots of Cu2O, CuO and Cu2O:Na thin films. Fig 4: X-ray photoelectron spectroscopy (XPS) spectra of a) Cu2O:Na sample survey, b) O 1s, c) Cu 2p and d) Na 1s. Fig 5: a) Cu2O:Na sample with orange color and four samples G, I, H, K are divided from that, b) line-scan EDX image of Cu2O:Na surface (green, red and turquoise blue according to Cu Lα, O Kα and Na Kα), c) EDX spectra of G, I, H, K samples. Fig 6: a) Schematic diagram of the p-Cu2O:Na/n-IGZO heterojunction, b) The dark I–V characteristic curve of CuO/IGZO, c) The typical semilog I-V curve of Cu2O/IGZO, d) The typical semilog I-V curve of Cu2O:Na/IGZO Fig 7: a) Two structures of solar cell based on Cu2O:Na and ZnO materials, b) Cross section SEM image of IGZO/ZnO junction, c) Dark I–V characteristic curves of both structures, d) J-V characteristic curves of both structures. Fig 8: a) Energy band diagram of the IGZO/Cu2O:Na structure, b) Energy band diagram of the IGZO/ZnO/Cu2O:Na structure.
Table captions Table 1: The Hole concentration, Mobility, and Resistivity of the CuO, Cu2O and Cu2O:Na thin films.
Fig 1: Ke at al
a)
b)
Cu2O:Na
c)
Glass
Fig 2: Ke at al
1 µm
d)
a)
Fig 3: Ke at al
b)
a)
b)
c)
d)
Fig 4: Ke at al
G
H
I
K
Cu: 69.1%
Sample G
O: 26.4% Na: 4.5%
10 mm
Sample I 24 mm
a)
Cu: 71.2% O: 24.1% Na: 4.7%
1300 µm
b) Cu: 70.5%
Sample H
O: 24.7% Na: 4.8%
Sample K
Cu: 70.3% O: 25.1% Na: 4.6%
c)
Fig 5: Ke at al
b)
a)
c)
d)
Fig 6: Ke at al
a)
b) ZnO IGZO
Glass
c)
d)
Fig 7: Ke at al
a)
Fig 8: Ke at al
b)
Table 1: Ke at al Sample
Thickness (nm)
Temperature (oC)
Hole concentration (cm-3)
Mobility (cm2.V-1.s-1)
Resistivity (Ω.cm)
CuO
920
500
9.31x1014
31.5
2.43x103
Cu2O
950
200
2.83x1015
48.9
4.14x102
Cu2O:Na
1000
200
2.11x1018
4.18
6.8