- Email: [email protected]
p-type Cu2O sheet heterojunction solar cells
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Photovoltaic properties of low-damage magnetron-sputtered n-type ZnO thin film/p-type Cu2 O sheet heterojunction solar cells Toshihiro Miyata , Hiroki Tokunaga , Kyosuke Watanabe , Noriaki Ikenaga , Tadatsugu Minami PII: DOI: Reference:
S0040-6090(20)30041-9 https://doi.org/10.1016/j.tsf.2020.137825 TSF 137825
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
4 August 2019 18 December 2019 26 January 2020
Please cite this article as: Toshihiro Miyata , Hiroki Tokunaga , Kyosuke Watanabe , Noriaki Ikenaga , Tadatsugu Minami , Photovoltaic properties of low-damage magnetron-sputtered n-type ZnO thin film/p-type Cu2 O sheet heterojunction solar cells, Thin Solid Films (2020), doi: https://doi.org/10.1016/j.tsf.2020.137825
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Highlights
Highest conversion efficiency of 3.22% was achieved.
Optimum conditions found for target-substrate distance and r.f.:d.c. power ratio.
Efficiency further improved by r.f. power superimposed DC magnetron sputtering.
1
Photovoltaic properties of low-damage magnetron-sputtered n-type ZnO thin film/p-type Cu2O sheet heterojunction solar cells Toshihiro Miyata†, Hiroki Tokunaga, Kyosuke Watanabe, Noriaki Ikenaga and Tadatsugu Minami Optoelectronic Device System R&D Center, Kanazawa Institute of Technology, 7-1 Nonoichi, Ishikawa 921-8501, Japan
Abstract: The photovoltaic properties of Cu2O-based heterojunction solar cells were improved using an n-type layer composed of thin films in a binary oxide semiconductor. This layer was prepared by a low-damage deposition method that applies a system for multi-chamber radio frequency (r.f.) power superimposed direct current (d.c.) magnetron sputtering. For an Al-doped ZnO (AZO)/n-ZnO/p-Cu2O heterojunction solar cell prepared using r.f. power superimposed d.c magnetron sputtering, we achieved the highest efficiency yet reported (3.22%) by optimizing sputtering conditions such as the substrate-target distance and the r.f.:d.c. power ratio. This value represents characteristics that exceed those of AZO/Cu2O solar cells having a similar structure based on r.f. power superimposed d.c magnetron sputtering.
Key words: Magnetron sputtering, Oxide thin film, Cu2O, ZnO, AZO, Solar cells
PACS: 80.84; 80.85
EEACC: 2560Z; 4250
* The research reported here was supported in part through a Grant-in-Aid for Scientific Research on Innovative Areas (No.18K0494500) provided by Japan’s Ministry of Education, Culture, Sports, Science and Technology.
†Corresponding author: email:[email protected] 2
1. Introduction Researchers have long given attention to p-type cuprous oxide (p-Cu2O) as a material for solar cells due to its remarkable theoretical conversion efficiency (approx. 20%)[1-9]. Furthermore, this material has attracted great interest for use in solar cells because it is nontoxic, suitable for building sustainable semiconductors, and promising for cost-effective manufacturing[10–17]. Consequently, the use of Cu2O for solar cells has been extensively studied over the past few years. In previous work, we significantly enhanced the efficiencies of n-type Al-doped ZnO (AZO)/p-Cu2O heterojunction solar cells, which were produced through pulsed-laser deposition (PLD) of AZO thin film on thermally oxidized p-Cu2O using techniques involving low damage and low temperatures[18–20]. By taking this approach at room temperature (RT), it was possible to fabricate AZO thin films not only into n-type semiconductor window layers but also transparent electrodes for heterojunction solar cells with efficiencies exceeding 3%[21]. However, PLD methods pose technical challenges to fabricating solar cells, including low deposition rates and complications arising from deposition over large areas. Magnetron sputtering deposition (MSD), however, can readily handle large areas and achieve high deposition rates. In this work, we improve the photovoltaic properties of Cu2O-based heterojunction solar cells that adopt a binary oxide n-type layer composed of non-doped ZnO thin films. This layer is prepared by a based on low-damage radio frequency (r.f.) power superimposed direct current (d.c.) magnetron sputtering deposition (MSD) and a multi-chamber system.
3
2. Experimental Copper sheets (0.2-mm thickness, 99.96% purity) were oxidized to form Cu2O sheets by heat treatment in a furnace while controlling the ambient atmosphere, as explained in previous work[18-20]. These Cu2O sheets were impregnated with NaCl powder (purity: 99.9%, KANTO KAGAKU Co. Ltd.) to incorporate Na into them and heat-treated in an Ar gas atmosphere at 700°C for 1 h[22]. After the sheets were cooled to 500°C, they were exposed to air at RT. Consequently, these sodium-doped Cu2O (Cu2O:Na) sheets became polycrystalline p-type semiconductors having a hole concentration on the order of 1015 cm−3 and a Hall mobility reaching 100 cm2V−1 s−1. Given the ability of Na doping to control carrier concentration,[15] we used it to optimize that of the Cu2O sheet. A multi-chamber MSD apparatus was used to deposit non-doped ZnO thin film and transparent-conducting AZO thin films onto p-Cu2O sheets. Here, the non-doped ZnO thin film formed an n-type semiconductor layer, and the AZO thin film provided a transparent electrode. In this multi-chamber apparatus, equipped with loading and deposition chambers, a direct current (DC) and a radio frequency (r.f.; 13.56 MHz) power supply are applied either separately or together. Deposition conditions were as follows: RT, 10–40 mm distance between target and substrate, non-doped ZnO powder as target for n-type layer, sintered AZO (Al2O3 content 2 wt%, Tosoh Specialty Materials Corp.) as target for transparent electrode, and pure Ar gas atmosphere of 0.6 Pa. Thicknesses of n-ZnO and AZO thin films were 50 and 200 nm, respectively. Simultaneous and/or additional depositions were carried out on glass substrates to analyze the electrical and optical properties of the formed ZnO and AZO thin films. To fabricate solar cells, an AZO/n-ZnO thin film/p-Cu2O sheet structure was formed on the front surface and an Au ohmic electrode was formed on the back surface (Fig. 1). Such a solar cell had a heterojunction structure based on the X-Ray Photoelectron Spectroscopy (XPS, ULVAC-PHI, model 1600) measurement 4
results for the work functions of AZO, ZnO, and Cu2O. By exposing only the AZO transparent electrode area to AM1.5G solar illumination (100 mW/cm2, Asahi Spectra, model HAL320) at 25°C, we evaluated the photovoltaic properties of these Cu2O-based solar cells (electrode area: 3.14 mm2).
3. Results and Discussion 3.1 Dependence of photovoltaic properties on ZnO target-Cu2O sheet distance and sputtering voltage For an n-type ZnO layer formed on a p-Cu2O sheet by low-damage magnetron sputtering, two factors are believed to cause the photovoltaic properties to deteriorate: 1) physical damage on the surface of Cu2O sheets due to bombardment of sputtered particles; 2) excessive oxidation of this surface by oxygen ions. In the r.f. magnetron sputtering apparatus (Fig. 2.), we used a parallel and/or a vertically positioned substrate. The vertical substrate can suppress the physical bombardment caused by the sputtered particles. Typical current-voltage (J-V) characteristics of the AZO/n-ZnO thin film/pCu2O sheet heterojunction solar cells were measured with different ZnO target-Cu2O sheet distances (Fig. 3). A vertically positioned substrate was used to prepare the n-ZnO thin films. Here, the J-V characteristics of the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells were found to be dependent on the ZnO target-Cu2O sheet distance. Conversion efficiency (η), fill factor (FF), VOC, and JSC are plotted in Fig. 4 as functions of the ZnO target-Cu2O sheet distance, ranging from 0 to 20 mm in fabrication, for the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells. Increasing the distance between the ZnO target and the Cu2O sheets to about 5 mm was found to improve the photovoltaic properties. However, the photovoltaic properties gradually worsened as this distance continued to increase. We expected to minimize the physical damage caused by bombardment on the 5
Cu2O sheet’s surface by enlarging the distance between the ZnO target and the Cu2O sheets. However, our results suggest that the deteriorated photovoltaic properties that accompany the increased distance between the target and substrate are not primarily caused by such bombardment damage. Moreover, typical current-voltage (J-V) characteristics were measured for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells (Fig. 5) prepared with a parallel and a vertically placed substrate at the optimized target-substrate distance (5 mm). Here, we can see that the J-V characteristics of AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells were more greatly improved when prepared with a vertically placed substrate than when prepared with a parallel placed substrate. However, the J-V characteristics of the AZO/n-ZnO thin film/p-Cu2O sheet solar cells prepared with vertical substrates were still lower than those of AZO/p-Cu2O sheet Schottky-type solar cells. These results suggest that the condition of the interface between the n-ZnO thin film and the p-Cu2O sheet remained worse than that between the AZO thin film and the p-Cu2O sheet. This is probably due to the excessive oxidation of the p-Cu2O surface in depositing the n-ZnO thin film. Therefore, to improve the photovoltaic characteristics, using a deposition technique to further suppress the oxidation of the p-Cu2O sheet’s surface is crucial. We described the photovoltaic characteristics of a Cu2O heterojunction solar cell with an n-ZnO thin film formed through r.f. superimposed d.c. magnetron sputtering, which can suppress oxygen ions during film formation by sputtering.
3.2 Photovoltaic properties of AZO/n-ZnO thin film/p-Cu2O sheet solar cell prepared using r.f. superimposed DC magnetron sputtering
6
As described earlier, oxygen ions must be used to reduce excessive oxidation on the Cu2O sheet’s surface and thus improve the photovoltaic properties of AZO/n-ZnO thin film/p-Cu2O sheet solar cells fabricated through forming the n-ZnO thin film by sputtering. In the r.f. superimposed d.c. magnetron sputtering method, it is necessary to optimize the ratio of high-frequency power to direct current power as a way to suppress oxygen ion particles incident on the p-Cu2O sheet surface. Typical current-voltage (J-V) characteristics were measured for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared using different d.c. powers (Fig. 6). Here, the r.f. power was fixed at 100 W. From the results, we see that the J-V characteristics of the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells are dependent on d.c. power. The conversion efficiency (η), fill factor (FF), VOC, and JSC are shown in Fig. 7 as functions of d.c. power, from 0 to 40 W, for AZO/ZnO thin film/p-Cu2O sheet heterojunction solar cells. The photovoltaic properties were improved by increasing d.c. power to about 20 W. However, as the d.c. power increased further, the photovoltaic properties gradually decreased. We found that the optimum rf:d.c. power ratio was 5:1. Comparing Figs. 3 and 6, Voc greatly changes when d.c. power is changed as a film-preparation condition. In addition, when applying reverse bias voltage in the dark, the leakage current strongly depends on the d.c. power. Therefore, the reason why Voc varies greatly when d.c. power is changed as a film-formation condition is that this change strongly affects the interface condition between ZnO and Cu2O. Figure 8 shows typical J-V characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with the r.f. superimposed d.c. MSD method under optimized deposition conditions such as the ZnO target-p-Cu2O sheet distance and the r.f.:d.c. power ratio. We obtained the highest efficiency of 3.22% for an AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cell prepared with a ZnO target-p-Cu2O sheet distance of 5 mm and an r.f.:d.c. power ratio of 5:1. Figure 8 also shows 7
typical J-V characteristics of cells prepared using r.f. MSD, which exhibited inferior properties to those prepared by r.f. superimposed d.c. MDS. The J-V characteristics of these cells were then measured under dark conditions (Fig. 9), and by applying reverse bias voltage, the leakage current drastically decreased in the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared by r.f. superimposed d.c. MSD. In addition, Fig. 10 shows a cross-sectional TEM image of the n-ZnO/pCu2O sheet interface. Figures 10 (a) and (b) show the interface of the n-ZnO/p-Cu2O sheet formed by the conventional r.f. magnetron sputtering method in which the substrate is arranged parallel to the target and by the r.f. superimposed d.c. magnetron sputtering method under optimum film formation conditions in which the substrate is arranged perpendicular to the target, respectively. As can be seen by comparing the two figures, a very rough ZnO thin film interface is formed on the Cu2O sheet placed in parallel with the target using the conventional r.f. magnetron sputtering method. On the other hand, a very flat and good ZnO thin film interface is formed on the Cu2O sheet placed perpendicular to the target using the r.f. superimposed d.c. magnetron sputtering method. These results imply that the p-n junction can be further improved, as seen in this heterojunction, by r.f. superimposed d.c. magnetron sputtering methods. Typical J-V characteristics are shown in Fig. 11 for AZO/n-ZnO thin film/p-Cu2O sheet and AZO/p-Cu2O sheet Schottky-type solar cells prepared under optimum deposition conditions, where the former structure showed the greatest improvement in efficiency.
4. Conclusion Non-doped ZnO and AZO thin films were prepared as an n-type semiconductor layer and a transparent electrode, respectively, by the proposed r.f. superimposed d.c. magnetron sputtering 8
method adopting a multi-chamber sputtering apparatus. We achieved greater improvement in photovoltaic properties by using AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells than using AZO/p-Cu2O sheet heterojunction solar cells. The optimum target-substrate distance and r.f.:d.c. power ratio were confirmed as 5 mm and 5:1, respectively. We assume the high efficiency reached in the heterojunction solar cells is due to a lower level of defects at the interface of the n-ZnO thin film and the p-Cu2O sheet. The highest efficiency, at 3.22%, was achieved in an AZO/n-ZnO thin film/pCu2O sheet heterojunction solar cell. This structure achieved higher characteristics than an AZO/pCu2O sheet Schottky-type solar cell. Therefore, our new r.f. superimposed d.c. magnetron sputtering method, implemented with a multi-chamber apparatus, is a promising approach to using practical binary oxide n-type semiconductor thin film, such as ZnO, as the fundamental technology in producing Cu2O-based solar cells.
Acknowledgments The authors thank R. Takahashi and N. Ogawa for their technical assistance in our experiments.
CREDIT AUTHOR STATEMENT Toshihiro Miyata: Supervision, Investigation. Hiroki Tokunaga: Investigation. Kyosuke Watanabe: Investigation. Noriaki Ikenaga: Investigation. Tadatsugu Minami: Reviewing and Editing,
References 9
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[12] T. Minami, Y. Nishi, and T. Miyata. High-Efficiency Cu2O-based heterojunction solar cells fabricated using a Ga2O3 thin film as n-type layer. Appl. Phys. Express, 2013, 6: 044101. [13] S. W. Lee, Y. S. Lee, J. Heo, S. C. Siah, D. Chua, R. E. Brandt, S. B. Kim, J. P. Mailoa, T. Buonassisi, and R. G. Gordon. Improved Cu2O-based solar cells using atomic layer deposition to control the Cu oxidation state at the p-n junction. Adv. Energy Mater., 2014, 4: 1301916. [14] Y. S. Lee, D. Chua, R. E. Brandt, S. C. Siah, J. V. Li, J. P. Mailoa, S. W. Lee, R. G. Gordon, and T. Buonassisi. Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells. Adv. Mater., 2014, 26: 4704. [15] T. Minami, Y. Nishi, and T. Miyata. Heterojunction solar cell with 6% efficiency based on an n-type aluminum-gallium-oxide thin film and p-type sodium-doped Cu2O sheet. Appl. Phys. Express, 2015, 8: 022301. [16] Y. Ievskaya, R. L. Z. Hoye, A. Sadhanala, K. P. Musselman, and J. L. MacManus-Driscoll. Fabrication of ZnO/Cu2O heterojunctions in atmospheric conditions: Improved interface quality and solar cell performanceSol. Energy Mater. Sol. Cells, 2015, 135: 43. [17] R. L. Z. Hoye, R. E. Brandt, Y. Ievskaya, S. Heffernan, K. P. Musselman, T. Buonassisi, and J. L. MacManus-Driscoll. Perspective: Maintaining surface-phase purity is key to efficient open air fabricated cuprous oxide solar cells. APL Mater., 2015, 3: 020901. [18] T. Minami, T. Miyata, and Y. Nishi. Efficiency improvement of Cu2O-based heterojunction solar cells fabricated using thermally oxidized copper sheets. Thin Solid Films, 2014, 559: 105. [19] T. Minami, T. Miyata, and Y. Nishi. Cu2O-based heterojunction solar cells with an Al-doped ZnO/oxide semiconductor/thermally oxidized Cu2O sheet structure. Sol. Energy, 2014, 105: 206. [20] T. Minami, Y. Nishi, and T. Miyata. Cu2O-based solar cells using oxide semiconductors. J. Semicond., 2016, 37: 014002. 11
[21] Y. Nishi, T. Miyata, J. Nomoto, and T. Minami. High-efficiency Cu2O-based heterojunction solar cells fabricated on thermally oxidized copper sheet. Conf. Rec. 37th IEEE Photovoltaic Specialists Conf., 2011, :266. [22] T. Minami, Y. Nishi, and T. Miyata. Impact of incorporating sodium into polycrystalline p-type Cu2O for heterojunction solar cell applications. Appl. Phys. Lett., 2014, 105: 212104.
Figure captions Figure 1 Cross-sectional structure of solar cell using AZO/n-ZnO thin film/p-Cu2O sheet Fig. 2 Schematic diagram of r.f. magnetron sputtering apparatus Fig. 3 Typical current-voltage (J-V) characteristics of AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with different ZnO target-Cu2O sheet distances Fig. 4 Conversion efficiency (η), fill factor (FF), VOC, and JSC as functions of ZnO target-Cu2O sheet distance for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells Fig. 5 Typical current-voltage (J-V) characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with parallel and vertically placed substrates having the optimized target-substrate distance Fig. 6 Typical current-voltage (J-V) characteristics of AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with different DC powers Fig. 7 Conversion efficiency (η), fill factor (FF), VOC, and JSC as functions of DC power for AZO/ZnO thin film/p-Cu2O sheet heterojunction solar cells
Fig. 8 Typical J-V characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared using r.f. superimposed DC MSD and r.f. MSD Fig. 9 J-V characteristics measured under dark conditions obtained in AZO/n-ZnO thin film/p-Cu2O sheet and AZO/p-Cu2O sheet heterojunction solar cell prepared using r.f. superimposed DC MSD and r.f. MSD Fig. 10 Cross-sectional TEM image of n-ZnO/p-Cu2O sheet interface 12
(a) n-ZnO/p-Cu2O sheet formed by conventional r.f. magnetron sputtering method (b) n-ZnO/p-Cu2O sheet formed by r.f. superimposed d.c. magnetron sputtering method
Fig. 11 Typical J-V characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells and AZO/p-Cu2O sheet solar cell prepared by r.f. superimposed DC MSD
light AZO thin film
n-type ZnO thin film
P-Cu2O
Fig. 1
Fig.2
12 Current Density [mA/cm2]
AZO/ZnO/Cu2O 10 8 6
0 mm
20 mm
4
5 mm
2
0
0.1
0.2
0.3
0.4
0.5
Voltage [V]
Fig. 3 13
0.6
0.7
0.8
0.9
0.4
Jsc [mA/cm2]
0.2 10 8 6 4 2 0
Voc [V]
0.6
0
0.8
0.4
FF
0.6
0.2
η [％]
3
0
2 1 0
0
5
10
20
15
x direction [mm]
Fig. 4
Current density [mA/cm2]
12 Vertical substrate
9
AZO/p-Cu2O
6 3
0
Parallel substrate
0.1
0.2
0.3
0.4
Voltage [V]
Fig. 5
14
0.5
0.6
0.7
Current Density [mA/cm2]
12
AZO/ZnO/Cu2O
r.f.-100[W]
10 8 DC:20W
6
DC:0W DC:30W
4 2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Voltage [V]
Fig. 6
0.6 0.4 0.2 0
12 Jsc [mA/cm2]
Voc [V]
0.8
8 4 0.8
0
0.4 0.2
η [％]
4
0
3 2 1 0
0
10
20
30
dc electric power [W]
Fig. 7
15
40
FF
0.6
Current Density [mA/cm2]
12 AZO/n-ZnO/p-Cu2O
10
r.f+dc-MSD
8 6 r.f-MSD
4 2 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Current Density [mA/cm2]
Fig.8
3
10 AZO/n-ZnO/p-Cu2O 2
10
1
10
r.f-MSD
0
10
-1
10
-2
10 -2
r.f+dc-MSD -1
0 1 Voltage [V] Fig. 9
16
2
Evaporated carbon
Evaporated carbon
ZnO
ZnO
Cu2O
200 nm
Cu2O
200 nm
(a)
(b)
Current Density [mA/cm2]
Fig.10
12 AZO/ ZnO/p-Cu2O
10 8 6
AZO/ p-Cu2O
4
η[%] 3.12 AZO/p-Cu2O AZO/ZnO/p-Cu2O 3.22
2 0
0.1
0.2
0.3 0.4 0.5 Voltage [V]
Fig. 11
17
0.6
0.7
Declaration of interests ☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
18
Photovoltaic properties of low-damage magnetron-sputtered n-type ZnO thin film/p-type Cu2 O sheet heterojunction solar cells Toshihiro Miyata , Hiroki Tokunaga , Kyosuke Watanabe , Noriaki Ikenaga , Tadatsugu Minami PII: DOI: Reference:
S0040-6090(20)30041-9 https://doi.org/10.1016/j.tsf.2020.137825 TSF 137825
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
4 August 2019 18 December 2019 26 January 2020
Please cite this article as: Toshihiro Miyata , Hiroki Tokunaga , Kyosuke Watanabe , Noriaki Ikenaga , Tadatsugu Minami , Photovoltaic properties of low-damage magnetron-sputtered n-type ZnO thin film/p-type Cu2 O sheet heterojunction solar cells, Thin Solid Films (2020), doi: https://doi.org/10.1016/j.tsf.2020.137825
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Highlights
Highest conversion efficiency of 3.22% was achieved.
Optimum conditions found for target-substrate distance and r.f.:d.c. power ratio.
Efficiency further improved by r.f. power superimposed DC magnetron sputtering.
1
Photovoltaic properties of low-damage magnetron-sputtered n-type ZnO thin film/p-type Cu2O sheet heterojunction solar cells Toshihiro Miyata†, Hiroki Tokunaga, Kyosuke Watanabe, Noriaki Ikenaga and Tadatsugu Minami Optoelectronic Device System R&D Center, Kanazawa Institute of Technology, 7-1 Nonoichi, Ishikawa 921-8501, Japan
Abstract: The photovoltaic properties of Cu2O-based heterojunction solar cells were improved using an n-type layer composed of thin films in a binary oxide semiconductor. This layer was prepared by a low-damage deposition method that applies a system for multi-chamber radio frequency (r.f.) power superimposed direct current (d.c.) magnetron sputtering. For an Al-doped ZnO (AZO)/n-ZnO/p-Cu2O heterojunction solar cell prepared using r.f. power superimposed d.c magnetron sputtering, we achieved the highest efficiency yet reported (3.22%) by optimizing sputtering conditions such as the substrate-target distance and the r.f.:d.c. power ratio. This value represents characteristics that exceed those of AZO/Cu2O solar cells having a similar structure based on r.f. power superimposed d.c magnetron sputtering.
Key words: Magnetron sputtering, Oxide thin film, Cu2O, ZnO, AZO, Solar cells
PACS: 80.84; 80.85
EEACC: 2560Z; 4250
* The research reported here was supported in part through a Grant-in-Aid for Scientific Research on Innovative Areas (No.18K0494500) provided by Japan’s Ministry of Education, Culture, Sports, Science and Technology.
†Corresponding author: email:[email protected] 2
1. Introduction Researchers have long given attention to p-type cuprous oxide (p-Cu2O) as a material for solar cells due to its remarkable theoretical conversion efficiency (approx. 20%)[1-9]. Furthermore, this material has attracted great interest for use in solar cells because it is nontoxic, suitable for building sustainable semiconductors, and promising for cost-effective manufacturing[10–17]. Consequently, the use of Cu2O for solar cells has been extensively studied over the past few years. In previous work, we significantly enhanced the efficiencies of n-type Al-doped ZnO (AZO)/p-Cu2O heterojunction solar cells, which were produced through pulsed-laser deposition (PLD) of AZO thin film on thermally oxidized p-Cu2O using techniques involving low damage and low temperatures[18–20]. By taking this approach at room temperature (RT), it was possible to fabricate AZO thin films not only into n-type semiconductor window layers but also transparent electrodes for heterojunction solar cells with efficiencies exceeding 3%[21]. However, PLD methods pose technical challenges to fabricating solar cells, including low deposition rates and complications arising from deposition over large areas. Magnetron sputtering deposition (MSD), however, can readily handle large areas and achieve high deposition rates. In this work, we improve the photovoltaic properties of Cu2O-based heterojunction solar cells that adopt a binary oxide n-type layer composed of non-doped ZnO thin films. This layer is prepared by a based on low-damage radio frequency (r.f.) power superimposed direct current (d.c.) magnetron sputtering deposition (MSD) and a multi-chamber system.
3
2. Experimental Copper sheets (0.2-mm thickness, 99.96% purity) were oxidized to form Cu2O sheets by heat treatment in a furnace while controlling the ambient atmosphere, as explained in previous work[18-20]. These Cu2O sheets were impregnated with NaCl powder (purity: 99.9%, KANTO KAGAKU Co. Ltd.) to incorporate Na into them and heat-treated in an Ar gas atmosphere at 700°C for 1 h[22]. After the sheets were cooled to 500°C, they were exposed to air at RT. Consequently, these sodium-doped Cu2O (Cu2O:Na) sheets became polycrystalline p-type semiconductors having a hole concentration on the order of 1015 cm−3 and a Hall mobility reaching 100 cm2V−1 s−1. Given the ability of Na doping to control carrier concentration,[15] we used it to optimize that of the Cu2O sheet. A multi-chamber MSD apparatus was used to deposit non-doped ZnO thin film and transparent-conducting AZO thin films onto p-Cu2O sheets. Here, the non-doped ZnO thin film formed an n-type semiconductor layer, and the AZO thin film provided a transparent electrode. In this multi-chamber apparatus, equipped with loading and deposition chambers, a direct current (DC) and a radio frequency (r.f.; 13.56 MHz) power supply are applied either separately or together. Deposition conditions were as follows: RT, 10–40 mm distance between target and substrate, non-doped ZnO powder as target for n-type layer, sintered AZO (Al2O3 content 2 wt%, Tosoh Specialty Materials Corp.) as target for transparent electrode, and pure Ar gas atmosphere of 0.6 Pa. Thicknesses of n-ZnO and AZO thin films were 50 and 200 nm, respectively. Simultaneous and/or additional depositions were carried out on glass substrates to analyze the electrical and optical properties of the formed ZnO and AZO thin films. To fabricate solar cells, an AZO/n-ZnO thin film/p-Cu2O sheet structure was formed on the front surface and an Au ohmic electrode was formed on the back surface (Fig. 1). Such a solar cell had a heterojunction structure based on the X-Ray Photoelectron Spectroscopy (XPS, ULVAC-PHI, model 1600) measurement 4
results for the work functions of AZO, ZnO, and Cu2O. By exposing only the AZO transparent electrode area to AM1.5G solar illumination (100 mW/cm2, Asahi Spectra, model HAL320) at 25°C, we evaluated the photovoltaic properties of these Cu2O-based solar cells (electrode area: 3.14 mm2).
3. Results and Discussion 3.1 Dependence of photovoltaic properties on ZnO target-Cu2O sheet distance and sputtering voltage For an n-type ZnO layer formed on a p-Cu2O sheet by low-damage magnetron sputtering, two factors are believed to cause the photovoltaic properties to deteriorate: 1) physical damage on the surface of Cu2O sheets due to bombardment of sputtered particles; 2) excessive oxidation of this surface by oxygen ions. In the r.f. magnetron sputtering apparatus (Fig. 2.), we used a parallel and/or a vertically positioned substrate. The vertical substrate can suppress the physical bombardment caused by the sputtered particles. Typical current-voltage (J-V) characteristics of the AZO/n-ZnO thin film/pCu2O sheet heterojunction solar cells were measured with different ZnO target-Cu2O sheet distances (Fig. 3). A vertically positioned substrate was used to prepare the n-ZnO thin films. Here, the J-V characteristics of the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells were found to be dependent on the ZnO target-Cu2O sheet distance. Conversion efficiency (η), fill factor (FF), VOC, and JSC are plotted in Fig. 4 as functions of the ZnO target-Cu2O sheet distance, ranging from 0 to 20 mm in fabrication, for the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells. Increasing the distance between the ZnO target and the Cu2O sheets to about 5 mm was found to improve the photovoltaic properties. However, the photovoltaic properties gradually worsened as this distance continued to increase. We expected to minimize the physical damage caused by bombardment on the 5
Cu2O sheet’s surface by enlarging the distance between the ZnO target and the Cu2O sheets. However, our results suggest that the deteriorated photovoltaic properties that accompany the increased distance between the target and substrate are not primarily caused by such bombardment damage. Moreover, typical current-voltage (J-V) characteristics were measured for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells (Fig. 5) prepared with a parallel and a vertically placed substrate at the optimized target-substrate distance (5 mm). Here, we can see that the J-V characteristics of AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells were more greatly improved when prepared with a vertically placed substrate than when prepared with a parallel placed substrate. However, the J-V characteristics of the AZO/n-ZnO thin film/p-Cu2O sheet solar cells prepared with vertical substrates were still lower than those of AZO/p-Cu2O sheet Schottky-type solar cells. These results suggest that the condition of the interface between the n-ZnO thin film and the p-Cu2O sheet remained worse than that between the AZO thin film and the p-Cu2O sheet. This is probably due to the excessive oxidation of the p-Cu2O surface in depositing the n-ZnO thin film. Therefore, to improve the photovoltaic characteristics, using a deposition technique to further suppress the oxidation of the p-Cu2O sheet’s surface is crucial. We described the photovoltaic characteristics of a Cu2O heterojunction solar cell with an n-ZnO thin film formed through r.f. superimposed d.c. magnetron sputtering, which can suppress oxygen ions during film formation by sputtering.
3.2 Photovoltaic properties of AZO/n-ZnO thin film/p-Cu2O sheet solar cell prepared using r.f. superimposed DC magnetron sputtering
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As described earlier, oxygen ions must be used to reduce excessive oxidation on the Cu2O sheet’s surface and thus improve the photovoltaic properties of AZO/n-ZnO thin film/p-Cu2O sheet solar cells fabricated through forming the n-ZnO thin film by sputtering. In the r.f. superimposed d.c. magnetron sputtering method, it is necessary to optimize the ratio of high-frequency power to direct current power as a way to suppress oxygen ion particles incident on the p-Cu2O sheet surface. Typical current-voltage (J-V) characteristics were measured for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared using different d.c. powers (Fig. 6). Here, the r.f. power was fixed at 100 W. From the results, we see that the J-V characteristics of the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells are dependent on d.c. power. The conversion efficiency (η), fill factor (FF), VOC, and JSC are shown in Fig. 7 as functions of d.c. power, from 0 to 40 W, for AZO/ZnO thin film/p-Cu2O sheet heterojunction solar cells. The photovoltaic properties were improved by increasing d.c. power to about 20 W. However, as the d.c. power increased further, the photovoltaic properties gradually decreased. We found that the optimum rf:d.c. power ratio was 5:1. Comparing Figs. 3 and 6, Voc greatly changes when d.c. power is changed as a film-preparation condition. In addition, when applying reverse bias voltage in the dark, the leakage current strongly depends on the d.c. power. Therefore, the reason why Voc varies greatly when d.c. power is changed as a film-formation condition is that this change strongly affects the interface condition between ZnO and Cu2O. Figure 8 shows typical J-V characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with the r.f. superimposed d.c. MSD method under optimized deposition conditions such as the ZnO target-p-Cu2O sheet distance and the r.f.:d.c. power ratio. We obtained the highest efficiency of 3.22% for an AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cell prepared with a ZnO target-p-Cu2O sheet distance of 5 mm and an r.f.:d.c. power ratio of 5:1. Figure 8 also shows 7
typical J-V characteristics of cells prepared using r.f. MSD, which exhibited inferior properties to those prepared by r.f. superimposed d.c. MDS. The J-V characteristics of these cells were then measured under dark conditions (Fig. 9), and by applying reverse bias voltage, the leakage current drastically decreased in the AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared by r.f. superimposed d.c. MSD. In addition, Fig. 10 shows a cross-sectional TEM image of the n-ZnO/pCu2O sheet interface. Figures 10 (a) and (b) show the interface of the n-ZnO/p-Cu2O sheet formed by the conventional r.f. magnetron sputtering method in which the substrate is arranged parallel to the target and by the r.f. superimposed d.c. magnetron sputtering method under optimum film formation conditions in which the substrate is arranged perpendicular to the target, respectively. As can be seen by comparing the two figures, a very rough ZnO thin film interface is formed on the Cu2O sheet placed in parallel with the target using the conventional r.f. magnetron sputtering method. On the other hand, a very flat and good ZnO thin film interface is formed on the Cu2O sheet placed perpendicular to the target using the r.f. superimposed d.c. magnetron sputtering method. These results imply that the p-n junction can be further improved, as seen in this heterojunction, by r.f. superimposed d.c. magnetron sputtering methods. Typical J-V characteristics are shown in Fig. 11 for AZO/n-ZnO thin film/p-Cu2O sheet and AZO/p-Cu2O sheet Schottky-type solar cells prepared under optimum deposition conditions, where the former structure showed the greatest improvement in efficiency.
4. Conclusion Non-doped ZnO and AZO thin films were prepared as an n-type semiconductor layer and a transparent electrode, respectively, by the proposed r.f. superimposed d.c. magnetron sputtering 8
method adopting a multi-chamber sputtering apparatus. We achieved greater improvement in photovoltaic properties by using AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells than using AZO/p-Cu2O sheet heterojunction solar cells. The optimum target-substrate distance and r.f.:d.c. power ratio were confirmed as 5 mm and 5:1, respectively. We assume the high efficiency reached in the heterojunction solar cells is due to a lower level of defects at the interface of the n-ZnO thin film and the p-Cu2O sheet. The highest efficiency, at 3.22%, was achieved in an AZO/n-ZnO thin film/pCu2O sheet heterojunction solar cell. This structure achieved higher characteristics than an AZO/pCu2O sheet Schottky-type solar cell. Therefore, our new r.f. superimposed d.c. magnetron sputtering method, implemented with a multi-chamber apparatus, is a promising approach to using practical binary oxide n-type semiconductor thin film, such as ZnO, as the fundamental technology in producing Cu2O-based solar cells.
Acknowledgments The authors thank R. Takahashi and N. Ogawa for their technical assistance in our experiments.
CREDIT AUTHOR STATEMENT Toshihiro Miyata: Supervision, Investigation. Hiroki Tokunaga: Investigation. Kyosuke Watanabe: Investigation. Noriaki Ikenaga: Investigation. Tadatsugu Minami: Reviewing and Editing,
References 9
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[12] T. Minami, Y. Nishi, and T. Miyata. High-Efficiency Cu2O-based heterojunction solar cells fabricated using a Ga2O3 thin film as n-type layer. Appl. Phys. Express, 2013, 6: 044101. [13] S. W. Lee, Y. S. Lee, J. Heo, S. C. Siah, D. Chua, R. E. Brandt, S. B. Kim, J. P. Mailoa, T. Buonassisi, and R. G. Gordon. Improved Cu2O-based solar cells using atomic layer deposition to control the Cu oxidation state at the p-n junction. Adv. Energy Mater., 2014, 4: 1301916. [14] Y. S. Lee, D. Chua, R. E. Brandt, S. C. Siah, J. V. Li, J. P. Mailoa, S. W. Lee, R. G. Gordon, and T. Buonassisi. Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells. Adv. Mater., 2014, 26: 4704. [15] T. Minami, Y. Nishi, and T. Miyata. Heterojunction solar cell with 6% efficiency based on an n-type aluminum-gallium-oxide thin film and p-type sodium-doped Cu2O sheet. Appl. Phys. Express, 2015, 8: 022301. [16] Y. Ievskaya, R. L. Z. Hoye, A. Sadhanala, K. P. Musselman, and J. L. MacManus-Driscoll. Fabrication of ZnO/Cu2O heterojunctions in atmospheric conditions: Improved interface quality and solar cell performanceSol. Energy Mater. Sol. Cells, 2015, 135: 43. [17] R. L. Z. Hoye, R. E. Brandt, Y. Ievskaya, S. Heffernan, K. P. Musselman, T. Buonassisi, and J. L. MacManus-Driscoll. Perspective: Maintaining surface-phase purity is key to efficient open air fabricated cuprous oxide solar cells. APL Mater., 2015, 3: 020901. [18] T. Minami, T. Miyata, and Y. Nishi. Efficiency improvement of Cu2O-based heterojunction solar cells fabricated using thermally oxidized copper sheets. Thin Solid Films, 2014, 559: 105. [19] T. Minami, T. Miyata, and Y. Nishi. Cu2O-based heterojunction solar cells with an Al-doped ZnO/oxide semiconductor/thermally oxidized Cu2O sheet structure. Sol. Energy, 2014, 105: 206. [20] T. Minami, Y. Nishi, and T. Miyata. Cu2O-based solar cells using oxide semiconductors. J. Semicond., 2016, 37: 014002. 11
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Figure captions Figure 1 Cross-sectional structure of solar cell using AZO/n-ZnO thin film/p-Cu2O sheet Fig. 2 Schematic diagram of r.f. magnetron sputtering apparatus Fig. 3 Typical current-voltage (J-V) characteristics of AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with different ZnO target-Cu2O sheet distances Fig. 4 Conversion efficiency (η), fill factor (FF), VOC, and JSC as functions of ZnO target-Cu2O sheet distance for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells Fig. 5 Typical current-voltage (J-V) characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with parallel and vertically placed substrates having the optimized target-substrate distance Fig. 6 Typical current-voltage (J-V) characteristics of AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared with different DC powers Fig. 7 Conversion efficiency (η), fill factor (FF), VOC, and JSC as functions of DC power for AZO/ZnO thin film/p-Cu2O sheet heterojunction solar cells
Fig. 8 Typical J-V characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells prepared using r.f. superimposed DC MSD and r.f. MSD Fig. 9 J-V characteristics measured under dark conditions obtained in AZO/n-ZnO thin film/p-Cu2O sheet and AZO/p-Cu2O sheet heterojunction solar cell prepared using r.f. superimposed DC MSD and r.f. MSD Fig. 10 Cross-sectional TEM image of n-ZnO/p-Cu2O sheet interface 12
(a) n-ZnO/p-Cu2O sheet formed by conventional r.f. magnetron sputtering method (b) n-ZnO/p-Cu2O sheet formed by r.f. superimposed d.c. magnetron sputtering method
Fig. 11 Typical J-V characteristics for AZO/n-ZnO thin film/p-Cu2O sheet heterojunction solar cells and AZO/p-Cu2O sheet solar cell prepared by r.f. superimposed DC MSD
light AZO thin film
n-type ZnO thin film
P-Cu2O
Fig. 1
Fig.2
12 Current Density [mA/cm2]
AZO/ZnO/Cu2O 10 8 6
0 mm
20 mm
4
5 mm
2
0
0.1
0.2
0.3
0.4
0.5
Voltage [V]
Fig. 3 13
0.6
0.7
0.8
0.9
0.4
Jsc [mA/cm2]
0.2 10 8 6 4 2 0
Voc [V]
0.6
0
0.8
0.4
FF
0.6
0.2
η [％]
3
0
2 1 0
0
5
10
20
15
x direction [mm]
Fig. 4
Current density [mA/cm2]
12 Vertical substrate
9
AZO/p-Cu2O
6 3
0
Parallel substrate
0.1
0.2
0.3
0.4
Voltage [V]
Fig. 5
14
0.5
0.6
0.7
Current Density [mA/cm2]
12
AZO/ZnO/Cu2O
r.f.-100[W]
10 8 DC:20W
6
DC:0W DC:30W
4 2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Voltage [V]
Fig. 6
0.6 0.4 0.2 0
12 Jsc [mA/cm2]
Voc [V]
0.8
8 4 0.8
0
0.4 0.2
η [％]
4
0
3 2 1 0
0
10
20
30
dc electric power [W]
Fig. 7
15
40
FF
0.6
Current Density [mA/cm2]
12 AZO/n-ZnO/p-Cu2O
10
r.f+dc-MSD
8 6 r.f-MSD
4 2 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Current Density [mA/cm2]
Fig.8
3
10 AZO/n-ZnO/p-Cu2O 2
10
1
10
r.f-MSD
0
10
-1
10
-2
10 -2
r.f+dc-MSD -1
0 1 Voltage [V] Fig. 9
16
2
Evaporated carbon
Evaporated carbon
ZnO
ZnO
Cu2O
200 nm
Cu2O
200 nm
(a)
(b)
Current Density [mA/cm2]
Fig.10
12 AZO/ ZnO/p-Cu2O
10 8 6
AZO/ p-Cu2O
4
η[%] 3.12 AZO/p-Cu2O AZO/ZnO/p-Cu2O 3.22
2 0
0.1
0.2
0.3 0.4 0.5 Voltage [V]
Fig. 11
17
0.6
0.7
Declaration of interests ☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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