- Email: [email protected]
CdS solar cells
Surface & Coatings Technology 360 (2019) 68–72
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Magnetron sputtering deposition and selenization of Sb2Se3 thin film for substrate Sb2Se3/CdS solar cells
T
⁎
Rong Tang, Xing-Ye Chen, Guang-Xing Liang , Zheng-Hua Su, Jing-ting Luo, Ping Fan Shenzhen Key Laboratory of Advanced Thin Films and Applications, Institute of Thin Film Physics and Applications, College of Physics and Energy, Shenzhen University 518060, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sb2Se3 RF magnetron sputtering Selenization Solar cell
Antimony triselenide (Sb2Se3) thin film was fabricated as the absorber layer in photovoltaic solar cell using magnetron sputtering deposition method. An additional selenization process was applied to increase the crystallinity of the film as well as to compensate for selenium loss during the sputtering process. Various analytical techniques were used to characterize the morphology and composition of the film. It was found that the annealing temperature is the key factor for thin film quality and the sample annealed at 350 °C shows the optimum result with a power conversion efficiency of 2.1%.
1. Introduction Thin film solar cells have been investigated and developed rapidly over the last decade, with the best power conversion efficiency (PCE) of copper indium gallium selenide (CIGS) thin film solar cell was reported to be 21.7% [1]. However, due to the high cost of indium and gallium, and given that CIGS is a quaternary compound which requires precise composition control, mass production for CIGS thin film solar cell with high quality would be very challenging. Cadmium telluride (CdTe) is a promising absorber material for thin film solar cells which has achieved 21% conversion efficiency [2]. However, high toxicity of cadmium could hinder the further development of CdTe. Over the last few years, perovskite-based solar cells have become the most conspicuous thin film solar cells as its conversion efficiency has increased up to over 22% for only a couple of years [3]. However, toxicity of lead and low thermal stability of absorber layer would make the commercialization of perovskite-based solar cells problematic. Therefore, it is vital to explore new earth-abundant and nontoxic materials for thin film solar cells. Antimony selenide (Sb2Se3) is a promising alternative absorber material for thin film photovoltaics with an orthorhombic structure. Sb2Se3 is a binary compound with only one stable phase which crystal structure is much simpler than that of CIGS and copper zinc tin sulfide (CZTS). Sb2Se3 shows ideal optical bandgap of 1.15 eV and high absorption coefficient which is greater than 105 cm−1, making Sb2Se3 very suitable for absorber layer of thin film solar cells. According to theoretical calculations, the conversion efficiency of Sb2Se3 can be as
⁎
high as over 30% [4]. In practical, although Sb2Se3-based thin film solar cells have been studied for only a few years [5–8], its conversion efficiency increased rapidly from 1.9% to 7.6%, showing a bright future for the material. General fabrication methods of Sb2Se3 thin films are solution method [9] and evaporation. For solution method, Sb and Se are dissolved and mixed in the first place, spin coating and annealing process would be applied subsequently to form the film. One of the biggest downsides of this method is impurities can be easily induced into the film since the whole process is conducted under a non-vacuum environment. On the other hand, some of the best Sb2Se3-based solar cells with high efficiency were fabricated using evaporation methods, however, drawbacks such as bad film uniformity, difficulty of composition and reaction matching will possibly degrade the device performance. In this work, Sb2Se3 thin film was fabricated using radio-frequency (RF) magnetron sputtering to deposit the amorphous film. Subsequently an annealing process was applied to crystallize the as-deposited film as well as to compensate for selenium loss during the sputtering process. Morphology and composition of the Sb2Se3 absorber layer were characterized using various analytical techniques such as scanning electron microscope (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). At last, a thin film solar cell with substrate configuration of Mo/Sb2Se3/CdS/ITO/Ag was fabricated and an encouraging PCE of 2.1% was achieved.
Corresponding author. E-mail address: [email protected] (G.-X. Liang).
https://doi.org/10.1016/j.surfcoat.2018.12.102 Received 31 August 2018; Received in revised form 20 November 2018; Accepted 25 December 2018 Available online 28 December 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
Fig. 1. SEM images of Sb2Se3 thin films annealed at different temperatures: A) amorphous B) 350 °C C) 400 °C D) 450 °C.
2. Experimental
Table 1 Chemical composition of Sb2Se3 films annealed at different temperatures. Temperature (°C)
Sb
Se
Sb/Se
RT 350 °C 400 °C 450 °C
44.93 37.57 30.38 16.30
55.07 62.43 69.62 83.70
0.816 0.602 0.436 0.195
2.1. Deposition and selenization of Sb2Se3 film Mo-coated soda-lime glass was used as substrates in this work. Before deposition, the substrate was subsequently cleaned in an ultrasonic bath using detergent, ethanol and deionized water for 10 min. Sb2Se3 amorphous film was deposited onto the substrate by RF magnetron sputtering with a 99.99% pure stoichiometric Sb2Se3 target using a homemade thin film sputtering deposition system (Shenyang Pengcheng Vacuum Technology Co. Ltd). The pressure of the vacuum chamber was pumped to less than 7 × 10−4 Pa before deposition. The
Fig. 2. Chemical composition and element mapping of Sb2Se3 annealed at 350 °C. 69
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
Fig. 3. XRD spectra of Sb2Se3 films annealed at different temperatures.
Fig. 4. Light and dark J-V characteristic of Sb2Se3 thin film solar cell annealed at 350 °C.
working pressure and sputtering power were fixed at 1.5 Pa and 40 W, respectively once the deposition process began. The substrate temperature was not specifically controlled during the sputtering process. The sputtering time was 90 min. It has been reported that selenium vacancies acting as deep recombination centers could occur during the deposition process due to the high vapor pressure of Se, leading to performance deterioration of devices [10]. As a result, in this work a selenization process was applied to compensate for selenium loss during the sputtering process. Once the deposition process was finished, high purity Se powder and the as-deposited Sb2Se3 sample were place into a quartz crucible for selenization treatment. The selenization time was fixed at 20 min for all the samples and the selenization temperatures
were selected as room temperature (RT), 350 °C, 400 °C and 450 °C to investigate the optimum annealing temperature. 2.2. Characterization of Sb2Se3 film The crystal structure of Sb2Se3 films were investigated by X-ray diffraction (Ultima-IV XRD) with CuKα radiation from 10° to 80°. The morphology of Sb2Se3 films were studied by scanning electron microscope (SUPRA 55 SEM). The chemical compositions of the samples were analyzed by energy-dispersive X-ray spectroscopy (BRUKER QUANTAX 200 EDS). Current density–voltage (J-V) curves were measured from −0.1 V to 0.6 V at a step length of 0.01 V using a multi-meter (Keithley, 70
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
Fig. 5. EQE spectrum of Sb2Se3 thin film solar cell.
concentration ratio of Sb/Se is 0.816 which is higher than that of the stoichiometric composition of 0.66, indicating partial Se sublimation during the sputtering process due to high vapor pressure of Se [11]. To compensate for the Se loss, a selenization process was applied and not surprisingly, the ratio of Sb/Se decreases with the increase of the selenization temperature. Fig. 3 shows the XRD patterns of amorphous and annealed Sb2Se3 samples. As expected, the pattern of the amorphous film is nearly featureless apart from two diffraction peaks locate at around 41° and 74° which are attributed to Mo. The sample annealed at 350 °C displays all diffraction peaks that are corresponding to the Sb2Se3 orthorhombic phase. When the selenization temperature increased to 400 °C, apparently dominant crystal face (211), (221), (301) and (321) of Sb2Se3 disappear, indicating the decomposition of the thin film material, which is consistent with the surface morphology results obtained via SEM (Fig. 1). Meanwhile, it is worth noting that the intensity of crystal face (120) which is reported to be harmful for the device performance [12], is directly proportional to the selenization temperature. From the above discussion, it can be concluded that the sample annealed at the selenization temperature of 350 °C shows the best quality in terms of morphology and crystal structure. Therefore, a thin film solar cell based on Sb2Se3 annealed at 350 °C using a substrate configuration of Mo/Sb2Se3/CdS/ITO/Ag was fabricated and the J-V characteristic of the device is given in Fig. 4. An encouraging efficiency of 2.1% has been obtained as there are few works focused on Sb2Se3based thin film solar cells with substrate configuration. Obviously the main limitations of our device are relatively low Voc and FF, which is partially owing to high series resistance of the film caused by the crystal face (120) [7,13,14]. In order to explore the reasons for low Voc and FF further, dark current measurement was also carried out. It is clear that the device presented a poor rectification possibly due to the rough surface of Sb2Se3 film, leading to the poor Sb2Se3/CdS interface [10]. The external quantum efficiency (EQE) spectrum of the device is shown in Fig. 5. The spectrum is quite sharp compared to the high efficiency CdTe solar cell that has an EQE plateau higher than 90% nearly over the whole region [14]. As a result, future work will be focused on the optimization of the Sb2Se3 thin film layer as well as the Sb2Se3/CdS junction.
2400 Series) under AM 1.5 G light illumination from a 3A solar simulator with intensity calibrated to 100 mW/cm2 by using a Si reference cell. The external quantum efficiency (EQE) spectra were measured using a power source (Zolix SCS101) with a monochromator and a source meter (Keithley 2400). 2.3. Device fabrication CdS buffer layer was deposited onto the annealed Sb2Se3 absorber layer using chemical bath deposition method. Indium tin oxide (ITO) window layer was deposited subsequently by DC magnetron sputtering at room temperature. The cell surface was scribed into individual small squares with identical area by knife and Ag colloids were painted onto the cell surface to form metallic contact. Thus, a thin film solar cell with substrate configuration of Mo/Sb2Se3/CdS/ITO/Ag was assembled. 3. Results and discussion Surface morphology of Sb2Se3 thin films fabricated under different selenization temperatures by SEM are demonstrated in Fig. 1. The amorphous as-deposited Sb2Se3 film was composed of small grains and crystallized into larger grains when the annealing temperature increased. For the sample annealed at 350 °C, large Sb2Se3 grains with diameter of several hundred nanometers were formed on the sample surface. It can also be seen from the figure that the distribution and arrangement of the Sb2Se3 grains is close and compact. No distinct micro-void can be found on the sample surface. When the selenization temperature increased to 400 °C, rod-shaped crystals appeared on the sample surface which can be attributed to the formation of Sb2Se3 nanorods due to high selenization temperature. However, micro-voids are observed on the sample surface as well probably due to partial decomposition of Sb2Se3 under high temperature. Large Sb2Se3 grains can no longer be seen on the sample surface when the selenization temperature reached 450 °C, instead, several tens of nanometer voids are observed all over the surface, suggesting severe decomposition of the thin film material. Chemical compositions and element mapping of the Sb2Se3 thin films annealed at various selenization temperatures are shown in Table 1 and Fig. 2, respectively. For the as-deposited sample, the atomic 71
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
4. Conclusion
[3] W.S. Yang, J.H. Noh, N.J. Jeon, et al., High-performance photovoltaic perovskite layers fabricated through intramolecular exchange[J], Science 348 (6240) (2015) 1234–1237. [4] X. Liu, C. Chen, L. Wang, et al., Improving the performance of Sb2Se3 thin film solar cells over 4% by controlled addition of oxygen during film deposition[J], Prog. Photovolt. Res. Appl. 23 (12) (2015) 1828–1836. [5] M. Luo, M. Leng, X. Liu, et al., Thermal evaporation and characterization of superstrate CdS/Sb2Se3 solar cells[J], Appl. Phys. Lett. 104 (17) (2014) 173904. [6] M. Leng, M. Luo, C. Chen, et al., Selenization of Sb2Se3 absorber layer: an efficient step to improve device performance of CdS/Sb2Se3 solar cells[J], Appl. Phys. Lett. 105 (8) (2014) 083905. [7] L. Wang, D.B. Li, K. Li, et al., Stable 6%-efficient Sb 2 Se 3 solar cells with a ZnO buffer layer[J], Nat. Energy 2 (4) (2017) 17046. [8] X. Wen, C. Chen, S. Lu, et al., Vapor transport deposition of antimony selenide thin film solar cells with 7.6% efficiency[J], Nat. Commun. 9 (1) (2018) 2179. [9] J. Yang, Y. Lai, Y. Fan, et al., Photoelectrochemically deposited Sb2Se3 thin films: deposition mechanism and characterization[J], RSC Adv. 5 (104) (2015) 85592–85597. [10] X. Liu, J. Chen, M. Luo, et al., Thermal evaporation and characterization of Sb2Se3 thin film for substrate Sb2Se3/CdS solar cells[J], ACS Appl. Mater. Interfaces 6 (13) (2014) 10687–10695. [11] G.X. Liang, X.H. Zhang, H.L. Ma, et al., Facile preparation and enhanced photoelectrical performance of Sb2Se3 nano-rods by magnetron sputtering deposition[J], Sol. Energy Mater. Sol. Cells 160 (2017) 257–262. [12] C.C. Yuan, L.J. Zhang, W.F. Liu, et al., Rapid thermal process to fabricate Sb2Se3 thin film for solar cell application[J], Sol. Energy 137 (2016) 160–256. [13] Y. Zhou, L. Wang, S. Chen, et al., Thin-film Sb 2 Se 3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries[J], Nat. Photonics 9 (6) (2015) 409. [14] M. Gloeckler, I. Sankin, Z. Zhao, CdTe solar cells at the threshold to 20% efficiency, IEEE J. Photovoltaics 3 (4) (2013) 1389–1393.
In this work, RF magnetron sputtering method was effectively utilized to fabricate Sb2Se3 thin film for solar cell application. A selenization process with different annealing temperatures was applied to improve the Sb2Se3 thin film quality. It was found that the selenization temperature is crucial for the film quality and 350 °C was found to be optimum annealing temperature. Higher temperatures will lead to decomposition of the film and thus deteriorate the device performance. At last, a Sb2Se3-based thin film solar cell with substrate configuration was assembled and an encouraging efficiency of 2.1% has been achieved. Acknowledgements Tang Rong and Chen Xing-Ye contributed equally. This work was supported by National Natural Science Foundation of China (Grant no. 61404086) and Shenzhen Key Lab Fund (ZDSYS 20170228105421966). References [1] P. Jackson, D. Hariskos, R. Wuerz, et al., Properties of Cu (In, Ga) Se2 solar cells with new record efficiencies up to 21.7%[J]. Physica status solidi (RRL)–rapid, Res. Lett. 9 (1) (2015) 28–31. [2] Martin A. Green, H. Yoshihiro, et al., Solar cell efficiency tables (version 52), Prog. Photovolt. 26 (7) (2018) 427–436.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Magnetron sputtering deposition and selenization of Sb2Se3 thin film for substrate Sb2Se3/CdS solar cells
T
⁎
Rong Tang, Xing-Ye Chen, Guang-Xing Liang , Zheng-Hua Su, Jing-ting Luo, Ping Fan Shenzhen Key Laboratory of Advanced Thin Films and Applications, Institute of Thin Film Physics and Applications, College of Physics and Energy, Shenzhen University 518060, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sb2Se3 RF magnetron sputtering Selenization Solar cell
Antimony triselenide (Sb2Se3) thin film was fabricated as the absorber layer in photovoltaic solar cell using magnetron sputtering deposition method. An additional selenization process was applied to increase the crystallinity of the film as well as to compensate for selenium loss during the sputtering process. Various analytical techniques were used to characterize the morphology and composition of the film. It was found that the annealing temperature is the key factor for thin film quality and the sample annealed at 350 °C shows the optimum result with a power conversion efficiency of 2.1%.
1. Introduction Thin film solar cells have been investigated and developed rapidly over the last decade, with the best power conversion efficiency (PCE) of copper indium gallium selenide (CIGS) thin film solar cell was reported to be 21.7% [1]. However, due to the high cost of indium and gallium, and given that CIGS is a quaternary compound which requires precise composition control, mass production for CIGS thin film solar cell with high quality would be very challenging. Cadmium telluride (CdTe) is a promising absorber material for thin film solar cells which has achieved 21% conversion efficiency [2]. However, high toxicity of cadmium could hinder the further development of CdTe. Over the last few years, perovskite-based solar cells have become the most conspicuous thin film solar cells as its conversion efficiency has increased up to over 22% for only a couple of years [3]. However, toxicity of lead and low thermal stability of absorber layer would make the commercialization of perovskite-based solar cells problematic. Therefore, it is vital to explore new earth-abundant and nontoxic materials for thin film solar cells. Antimony selenide (Sb2Se3) is a promising alternative absorber material for thin film photovoltaics with an orthorhombic structure. Sb2Se3 is a binary compound with only one stable phase which crystal structure is much simpler than that of CIGS and copper zinc tin sulfide (CZTS). Sb2Se3 shows ideal optical bandgap of 1.15 eV and high absorption coefficient which is greater than 105 cm−1, making Sb2Se3 very suitable for absorber layer of thin film solar cells. According to theoretical calculations, the conversion efficiency of Sb2Se3 can be as
⁎
high as over 30% [4]. In practical, although Sb2Se3-based thin film solar cells have been studied for only a few years [5–8], its conversion efficiency increased rapidly from 1.9% to 7.6%, showing a bright future for the material. General fabrication methods of Sb2Se3 thin films are solution method [9] and evaporation. For solution method, Sb and Se are dissolved and mixed in the first place, spin coating and annealing process would be applied subsequently to form the film. One of the biggest downsides of this method is impurities can be easily induced into the film since the whole process is conducted under a non-vacuum environment. On the other hand, some of the best Sb2Se3-based solar cells with high efficiency were fabricated using evaporation methods, however, drawbacks such as bad film uniformity, difficulty of composition and reaction matching will possibly degrade the device performance. In this work, Sb2Se3 thin film was fabricated using radio-frequency (RF) magnetron sputtering to deposit the amorphous film. Subsequently an annealing process was applied to crystallize the as-deposited film as well as to compensate for selenium loss during the sputtering process. Morphology and composition of the Sb2Se3 absorber layer were characterized using various analytical techniques such as scanning electron microscope (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). At last, a thin film solar cell with substrate configuration of Mo/Sb2Se3/CdS/ITO/Ag was fabricated and an encouraging PCE of 2.1% was achieved.
Corresponding author. E-mail address: [email protected] (G.-X. Liang).
https://doi.org/10.1016/j.surfcoat.2018.12.102 Received 31 August 2018; Received in revised form 20 November 2018; Accepted 25 December 2018 Available online 28 December 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
Fig. 1. SEM images of Sb2Se3 thin films annealed at different temperatures: A) amorphous B) 350 °C C) 400 °C D) 450 °C.
2. Experimental
Table 1 Chemical composition of Sb2Se3 films annealed at different temperatures. Temperature (°C)
Sb
Se
Sb/Se
RT 350 °C 400 °C 450 °C
44.93 37.57 30.38 16.30
55.07 62.43 69.62 83.70
0.816 0.602 0.436 0.195
2.1. Deposition and selenization of Sb2Se3 film Mo-coated soda-lime glass was used as substrates in this work. Before deposition, the substrate was subsequently cleaned in an ultrasonic bath using detergent, ethanol and deionized water for 10 min. Sb2Se3 amorphous film was deposited onto the substrate by RF magnetron sputtering with a 99.99% pure stoichiometric Sb2Se3 target using a homemade thin film sputtering deposition system (Shenyang Pengcheng Vacuum Technology Co. Ltd). The pressure of the vacuum chamber was pumped to less than 7 × 10−4 Pa before deposition. The
Fig. 2. Chemical composition and element mapping of Sb2Se3 annealed at 350 °C. 69
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
Fig. 3. XRD spectra of Sb2Se3 films annealed at different temperatures.
Fig. 4. Light and dark J-V characteristic of Sb2Se3 thin film solar cell annealed at 350 °C.
working pressure and sputtering power were fixed at 1.5 Pa and 40 W, respectively once the deposition process began. The substrate temperature was not specifically controlled during the sputtering process. The sputtering time was 90 min. It has been reported that selenium vacancies acting as deep recombination centers could occur during the deposition process due to the high vapor pressure of Se, leading to performance deterioration of devices [10]. As a result, in this work a selenization process was applied to compensate for selenium loss during the sputtering process. Once the deposition process was finished, high purity Se powder and the as-deposited Sb2Se3 sample were place into a quartz crucible for selenization treatment. The selenization time was fixed at 20 min for all the samples and the selenization temperatures
were selected as room temperature (RT), 350 °C, 400 °C and 450 °C to investigate the optimum annealing temperature. 2.2. Characterization of Sb2Se3 film The crystal structure of Sb2Se3 films were investigated by X-ray diffraction (Ultima-IV XRD) with CuKα radiation from 10° to 80°. The morphology of Sb2Se3 films were studied by scanning electron microscope (SUPRA 55 SEM). The chemical compositions of the samples were analyzed by energy-dispersive X-ray spectroscopy (BRUKER QUANTAX 200 EDS). Current density–voltage (J-V) curves were measured from −0.1 V to 0.6 V at a step length of 0.01 V using a multi-meter (Keithley, 70
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
Fig. 5. EQE spectrum of Sb2Se3 thin film solar cell.
concentration ratio of Sb/Se is 0.816 which is higher than that of the stoichiometric composition of 0.66, indicating partial Se sublimation during the sputtering process due to high vapor pressure of Se [11]. To compensate for the Se loss, a selenization process was applied and not surprisingly, the ratio of Sb/Se decreases with the increase of the selenization temperature. Fig. 3 shows the XRD patterns of amorphous and annealed Sb2Se3 samples. As expected, the pattern of the amorphous film is nearly featureless apart from two diffraction peaks locate at around 41° and 74° which are attributed to Mo. The sample annealed at 350 °C displays all diffraction peaks that are corresponding to the Sb2Se3 orthorhombic phase. When the selenization temperature increased to 400 °C, apparently dominant crystal face (211), (221), (301) and (321) of Sb2Se3 disappear, indicating the decomposition of the thin film material, which is consistent with the surface morphology results obtained via SEM (Fig. 1). Meanwhile, it is worth noting that the intensity of crystal face (120) which is reported to be harmful for the device performance [12], is directly proportional to the selenization temperature. From the above discussion, it can be concluded that the sample annealed at the selenization temperature of 350 °C shows the best quality in terms of morphology and crystal structure. Therefore, a thin film solar cell based on Sb2Se3 annealed at 350 °C using a substrate configuration of Mo/Sb2Se3/CdS/ITO/Ag was fabricated and the J-V characteristic of the device is given in Fig. 4. An encouraging efficiency of 2.1% has been obtained as there are few works focused on Sb2Se3based thin film solar cells with substrate configuration. Obviously the main limitations of our device are relatively low Voc and FF, which is partially owing to high series resistance of the film caused by the crystal face (120) [7,13,14]. In order to explore the reasons for low Voc and FF further, dark current measurement was also carried out. It is clear that the device presented a poor rectification possibly due to the rough surface of Sb2Se3 film, leading to the poor Sb2Se3/CdS interface [10]. The external quantum efficiency (EQE) spectrum of the device is shown in Fig. 5. The spectrum is quite sharp compared to the high efficiency CdTe solar cell that has an EQE plateau higher than 90% nearly over the whole region [14]. As a result, future work will be focused on the optimization of the Sb2Se3 thin film layer as well as the Sb2Se3/CdS junction.
2400 Series) under AM 1.5 G light illumination from a 3A solar simulator with intensity calibrated to 100 mW/cm2 by using a Si reference cell. The external quantum efficiency (EQE) spectra were measured using a power source (Zolix SCS101) with a monochromator and a source meter (Keithley 2400). 2.3. Device fabrication CdS buffer layer was deposited onto the annealed Sb2Se3 absorber layer using chemical bath deposition method. Indium tin oxide (ITO) window layer was deposited subsequently by DC magnetron sputtering at room temperature. The cell surface was scribed into individual small squares with identical area by knife and Ag colloids were painted onto the cell surface to form metallic contact. Thus, a thin film solar cell with substrate configuration of Mo/Sb2Se3/CdS/ITO/Ag was assembled. 3. Results and discussion Surface morphology of Sb2Se3 thin films fabricated under different selenization temperatures by SEM are demonstrated in Fig. 1. The amorphous as-deposited Sb2Se3 film was composed of small grains and crystallized into larger grains when the annealing temperature increased. For the sample annealed at 350 °C, large Sb2Se3 grains with diameter of several hundred nanometers were formed on the sample surface. It can also be seen from the figure that the distribution and arrangement of the Sb2Se3 grains is close and compact. No distinct micro-void can be found on the sample surface. When the selenization temperature increased to 400 °C, rod-shaped crystals appeared on the sample surface which can be attributed to the formation of Sb2Se3 nanorods due to high selenization temperature. However, micro-voids are observed on the sample surface as well probably due to partial decomposition of Sb2Se3 under high temperature. Large Sb2Se3 grains can no longer be seen on the sample surface when the selenization temperature reached 450 °C, instead, several tens of nanometer voids are observed all over the surface, suggesting severe decomposition of the thin film material. Chemical compositions and element mapping of the Sb2Se3 thin films annealed at various selenization temperatures are shown in Table 1 and Fig. 2, respectively. For the as-deposited sample, the atomic 71
Surface & Coatings Technology 360 (2019) 68–72
R. Tang et al.
4. Conclusion
[3] W.S. Yang, J.H. Noh, N.J. Jeon, et al., High-performance photovoltaic perovskite layers fabricated through intramolecular exchange[J], Science 348 (6240) (2015) 1234–1237. [4] X. Liu, C. Chen, L. Wang, et al., Improving the performance of Sb2Se3 thin film solar cells over 4% by controlled addition of oxygen during film deposition[J], Prog. Photovolt. Res. Appl. 23 (12) (2015) 1828–1836. [5] M. Luo, M. Leng, X. Liu, et al., Thermal evaporation and characterization of superstrate CdS/Sb2Se3 solar cells[J], Appl. Phys. Lett. 104 (17) (2014) 173904. [6] M. Leng, M. Luo, C. Chen, et al., Selenization of Sb2Se3 absorber layer: an efficient step to improve device performance of CdS/Sb2Se3 solar cells[J], Appl. Phys. Lett. 105 (8) (2014) 083905. [7] L. Wang, D.B. Li, K. Li, et al., Stable 6%-efficient Sb 2 Se 3 solar cells with a ZnO buffer layer[J], Nat. Energy 2 (4) (2017) 17046. [8] X. Wen, C. Chen, S. Lu, et al., Vapor transport deposition of antimony selenide thin film solar cells with 7.6% efficiency[J], Nat. Commun. 9 (1) (2018) 2179. [9] J. Yang, Y. Lai, Y. Fan, et al., Photoelectrochemically deposited Sb2Se3 thin films: deposition mechanism and characterization[J], RSC Adv. 5 (104) (2015) 85592–85597. [10] X. Liu, J. Chen, M. Luo, et al., Thermal evaporation and characterization of Sb2Se3 thin film for substrate Sb2Se3/CdS solar cells[J], ACS Appl. Mater. Interfaces 6 (13) (2014) 10687–10695. [11] G.X. Liang, X.H. Zhang, H.L. Ma, et al., Facile preparation and enhanced photoelectrical performance of Sb2Se3 nano-rods by magnetron sputtering deposition[J], Sol. Energy Mater. Sol. Cells 160 (2017) 257–262. [12] C.C. Yuan, L.J. Zhang, W.F. Liu, et al., Rapid thermal process to fabricate Sb2Se3 thin film for solar cell application[J], Sol. Energy 137 (2016) 160–256. [13] Y. Zhou, L. Wang, S. Chen, et al., Thin-film Sb 2 Se 3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries[J], Nat. Photonics 9 (6) (2015) 409. [14] M. Gloeckler, I. Sankin, Z. Zhao, CdTe solar cells at the threshold to 20% efficiency, IEEE J. Photovoltaics 3 (4) (2013) 1389–1393.
In this work, RF magnetron sputtering method was effectively utilized to fabricate Sb2Se3 thin film for solar cell application. A selenization process with different annealing temperatures was applied to improve the Sb2Se3 thin film quality. It was found that the selenization temperature is crucial for the film quality and 350 °C was found to be optimum annealing temperature. Higher temperatures will lead to decomposition of the film and thus deteriorate the device performance. At last, a Sb2Se3-based thin film solar cell with substrate configuration was assembled and an encouraging efficiency of 2.1% has been achieved. Acknowledgements Tang Rong and Chen Xing-Ye contributed equally. This work was supported by National Natural Science Foundation of China (Grant no. 61404086) and Shenzhen Key Lab Fund (ZDSYS 20170228105421966). References [1] P. Jackson, D. Hariskos, R. Wuerz, et al., Properties of Cu (In, Ga) Se2 solar cells with new record efficiencies up to 21.7%[J]. Physica status solidi (RRL)–rapid, Res. Lett. 9 (1) (2015) 28–31. [2] Martin A. Green, H. Yoshihiro, et al., Solar cell efficiency tables (version 52), Prog. Photovolt. 26 (7) (2018) 427–436.
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