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High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films
Journal Pre-proof High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films Xian-Gang Hu, Peng-Xiang Hou, Jin-Bo Wu, Xin Li, Jian Luan, Chang Liu, Gang Liu, Hui-Ming Cheng PII:
S2211-2855(19)31159-0
DOI:
https://doi.org/10.1016/j.nanoen.2019.104442
Reference:
NANOEN 104442
To appear in:
Nano Energy
Received Date: 18 June 2019 Revised Date:
22 December 2019
Accepted Date: 27 December 2019
Please cite this article as: X.-G. Hu, P.-X. Hou, J.-B. Wu, X. Li, J. Luan, C. Liu, G. Liu, H.-M. Cheng, High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104442. 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. © 2019 Published by Elsevier Ltd.
High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films Xian-Gang Hua,b, Peng-Xiang Houa,b,*, Jin-Bo Wua,b, Xin Lia,b, Jian Luana,b, Chang Liua,b,*, Gang Liua,b, Hui-Ming Chenga,c
a
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, PR China. b
School of Materials Science and Engineering, University of Science and Technology
of China, Hefei 230026, PR China. c
Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute, Tsinghua
University, Shenzhen 518055, PR China *
Corresponding Authors: [email protected] (P.X. Hou) or [email protected] (C. Liu).
ABSTRACT High-quality single-wall carbon nanotube (SWCNT) films were lightly fluorinated by treatment with xenon difluoride at room temperature, which led to the formation of ionic C-F bonds on tube walls and controllable p-type doping of the nanotubes. The fluorinated SWCNT films showed improved electronic conductivity and a higher work function. In addition, the fluorination process increased the areal density of SWCNT films and decreased their surface roughness, leading to better interface contact between them and silicon. As a result, a heterojunction solar cell constructed using the lightly fluorinated SWCNT film has a high power conversion efficiency of 13.6% and excellent stability.
Keywords: fluorination, carbon nanotubes, transparent conductive films, high work function, heterojunction, solar cell
1. Introduction Single-wall carbon nanotube (SWCNT) films show great promise as flexible and transparent conductive electrodes owing to their outstanding optical and electrical properties.[1-3] Because of this, SWCNT transparent conductive films (TCFs) have been used to fabricate various solar cells, including silicon heterojunction solar cells,[4, 5] dye-sensitized solar cells,[6] polymer solar cells[7] and perovskite solar cells.[8-10] SWCNT/silicon (Si) heterojunction solar cells have attracted considerable interest due to their simple structure, easy fabrication and low cost.[5, 11-14] In this type of device, the SWCNT films mainly serve as transparent electrodes to transfer carriers and to form a built-in potential with Si to separate photo-generated carriers.[15] Therefore, the power conversion efficiency (PCE) of SWCNT/Si heterojunction solar cells largely depends on the structure and performance of the SWCNT films used. Compared with poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) films, although the SWCNT film possesses higher stability than PEDOT:PSS film, the work function of SWCNT films (~4.8 eV) is relatively low,[16] which is a serious drawback in constructing high-performance SWCNT/Si heterojunction solar cells. Because of this, SWCNT films with a high conductivity and a large work function are required to improve the efficiency of the cells. Chemical functionalization is generally used to tune the physical and chemical properties of SWCNTs.[17] Halogen atoms, such as fluorine (F), chlorine (Cl) and bromine (Br), have been recognized as efficient dopants because of their high electronegativity.[18] Fluorination is a commonly used functionalization technique because fluorine is the most electronegative element. Fluorinated SWCNTs indeed exhibit many intriguing properties and have been used in supercapacitors,[19] lithium batteries[20] and sensors.[21] A number of methods have been used to fluorinate SWCNTs, including treatment with F2 gas at a temperature of ~400
[22] and
treatment with a CF4 plasma.[23] However, these strong treatments usually lead to the formation of covalent C-F bonds and, at the same time, the SWCNTs are changed from conducting to insulating due to the increased band gap with high fluorine doping
concentrations (F/C ratio > 0.5).[24] As a result this kind of fluorinated SWCNT film cannot be used as a transparent electrode in photovoltaic devices. In contrast, Nakajima et al. reported that the conductivity of highly oriented pyrolytic graphite (HOPG) was improved greatly after fluorination at a low temperature and with a low fluorine content.[25, 26] They attributed this to the formation of ionic C-F bonds, in which F atoms act as electron acceptors, and the π electrons in HOPG are strongly delocalized and contribute to the C-F bond.[27] We therefore expect that the controlled introduction of ionic C-F bonds to SWCNTs would lead to increased electrical conductivity, but there are few reports on the preparation and properties of lightly fluorinated SWCNT films. In this paper, we report the fluorination of SWCNT (F-SWCNT) films by directly exposing as-prepared free-standing SWCNT films to xenon difluoride (XeF2) gas in an airtight chamber at room temperature. The degree of fluorination is controlled by changing the exposure time and chamber temperature. The type of C-F bonds in the lightly fluorinated F-SWCNT is confirmed to be ionic. The fluorinated films showed a higher conductivity, higher work function and higher areal density than the pristine SWCNT films. As a result, a fabricated F-SWCNT/Si heterojunction solar cell demonstrated a high PCE of 13.6% and excellent stability in air.
2. Results and Discussion The pristine SWCNT films were synthesized by a floating catalyst chemical vapor deposition (FCCVD) method (Details in the Experimental Section).[3, 28] The processes of the preparation of fluorinated SWCNT films and the construction of F-SWCNT/Si solar cells are schematically shown in Figure 1a. Briefly, an as-prepared free-standing SWCNT film was exposed to XeF2 gas sublimated from XeF2 powder in an airtight chamber at room temperature, during which the XeF2 decomposed to produce atomic fluorine that uniformly fluorinated the SWCNTs [17, 29] (For details, see the Supporting Information). After fluorination, the film was cut into the size required to fabricate the solar cells. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed
to characterize the fluorinated SWCNT films. Raman spectra of the pristine and fluorinated SWCNT films excited with 633 nm laser are shown in Figures 1b and c. A decrease in the intensity is observed for both the radial breathing mode (RBM, Figure. 1b) and the G mode (Figure. 1c) peaks after fluorination. In particular, the G/D ratio of the F-SWCNT films decreased to 52 from the original 110. In addition, the blue shift of the G and G` bands indicates p-type doping. Figure 1d shows UPS spectra of the pristine and fluorinated SWCNT films on a copper substrate. After fluorination, the work function increased from 4.83 eV to 5.04 eV, which can be ascribed to the high electronegativity of the doped fluorine atoms.[18, 30, 31] To confirm the existence of fluorine in SWCNTs, the samples were further characterized using XPS and spectra before and after fluorination are shown in Figure S1. It can be clearly seen that a new F 1s peak has appeared. The elemental contents (atomic percent) of C, O, and F are 90.3%, 8.61% and 1.08%, respectively, indicating light F doping. To understand the changes in chemical bonding, we analyzed the C1s spectra of the pristine and fluorinated SWCNT films. As shown in Figure S2, five main peaks located at 284.6, 285.3, 286.2, 287.6 and 289.5eV were observed for the pristine SWCNT film, which can be assigned to C-C (sp2), C=C (sp3), C-O, C=O and O-C=O bonds, respectively.[32-35]. A new peak was observed at ~287 eV for the F-SWCNT film (Figure 1e), which indicates the presence of C-F bonds.[22, 32] At the same time, the position of C 1s peak of the F-SWCNT film shifted by 0.2 eV towards lower binding energy compared with that of the pristine SWCNT film (Figure 1f). This shift indicates that the C-F bond is ionic[25] and F is incorporated by p-type doping.[36, 37] In addition, a clear F 1s peak was observed at 686.5 eV (Figure 1g), close to that of LiF.[35] These results suggest that the C-F bonds in the F-SWCNT film are mainly ionic. The electrical conductivity of fluorinated carbon materials is closely related to the type of C-F bonding.[27, 38] In the case of ionic C-F bonds, F plays the role of an electron acceptor and contributes to the conductivity.[27] Therefore, the electrical conductivity of our F-SWCNT films is expected to increase.
Figure 1 a) Schematic showing the fabrication of fluorinated SWCNT films and the construction of F-SWCNT/Si heterojunction solar cells. Raman spectra of b) pristine SWCNT film and c) a F-SWCNT film excited by 633 nm laser excitation. d) UPS spectra of SWCNT and F-SWCNT. e-g) XPS spectra of the SWCNTs. e) C 1s curve of the F-SWCNT film. f) Enlarged C1s curve of the pristine and F-SWCNT films. g) F 1s peak of the pristine and fluorinated SWCNT films.
Figure 2. SEM images of a) pristine and b) fluorinated SWCNT films on a SiOx/Si substrate. c) Optical image of the F-SWCNT film transferred to a SiOx/Si substrate for step profiler testing. The measured thicknesses of d) pristine and e) fluorinated SWCNT films. TEM images of f) pristine and g) fluorinated SWCNT films.
To understand the effect of fluorination on the structure of the SWCNT films, we characterized the fluorinated samples using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2a, the SEM image of the pristine SWCNT film shows a high-purity, uniform, randomly entangled SWCNT network. After fluorination, the packing density of the SWCNTs increases greatly as can be seen in Figure 2b. It seems that the surface of the F-SWCNT film is smoother than that of the pristine SWCNT film. To confirm this, we measured the surface roughness of the samples using atomic force microscopy (AFM). As shown in Figure S3, the F-SWCNT film has a surface root-mean-square roughness of 5.89 nm, which is smaller than that of the pristine SWCNT film (7.79 nm). This lower surface roughness would lead to denser packing and improved contacts between nanotubes in the F-SWCNT film. At the same time, this lower surface roughness produces more
contact points between F-SWCNT film and the Si, which increases the junction area of the heterojunction solar cell.[39] The thickness of the two films transferred onto SiOx/Si substrates was measured using a step profiler (Figures 2c). As shown in Figures 2d and 2e, the film thickness was reduced from ~60 nm to ~28 nm after the fluorination, which is consistent with the above results. The thickness reduction should result from SWCNT surface modification during the fluorination process. It was reported that light fluorination treatment could enhance the hydrophilicity and surface adhesive property of nanotubes,[40] which leads to better contact and denser packing of F-SWCNTs. In consequence, the total film thickness decreased evidently after the fluorination. A typical TEM image of the SWCNT film is shown in Figure S4 from which it can be seen that the SWCNT network mainly consists of isolated and small-bundle SWCNTs, which is needed to achieve outstanding photoelectric properties, since large bundles contribute little to the electrical conductivity but lower the light transmittance of the films.[41] A high resolution TEM image (Figure 2f) shows that the pristine SWCNTs have straight walls with good crystallinity. Furthermore, no obvious difference is observed between the two SWCNTs (Figure 2g), which suggests that this fluorination process has not damaged their structure.
Figure 3. a) Optical transmittance of the pristine and fluorinated SWCNT films on quartz substrates. b) I-V curves of the pristine SWCNT film with a transmittance of 86% and a F-SWCNT film with a transmittance of 88% (inset shows a schematic of
the I-V measurement).
We then measured the optoelectronic performance of the two films and their UV-Vis-NIR transmittance spectra are shown in Figure 3a. After fluorination, the F-SWCNT film shows a higher transmittance in the wavelength range 200 to 800 nm, for example, the transmittance for 550 nm light is increased from 86% to 88%. This is attributed to the lower optical loss for the F-SWCNT film because of smaller film thickness and lower light absorption (Figure S5).[42] The I-V curves of these films are shown in Figure 3b. The sheet resistance of the fluorinated SWCNT film is decreased to 54 Ω sq.-1 from the original 86 Ω sq.-1. In other words, the electrical conductivity of the F-SWCNT film is increased by a factor of 1.6 through the fluorination treatment. We attribute this improvement in electrical conductivity to two factors. One is the presence of ionic C-F bonds in the F-SWCNT film, in which F acts as an electron acceptor to increase the conductivity.[27, 35, 43] The other is that the lower surface roughness and denser packing improve inter-tube contacts and decrease total contact resistances.[44] These improved optoelectronic performance and higher work function of F-SWCNT films resulted from lightly fluorination treatment are highly desirable for constructing high-performance SWCNT/Si solar cells.
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Figure 4. Current density-voltage (J-V) characteristics of the pristine and fluorinated SWCNT films. a) Under illumination. b) In the dark. c) I (dV/dI) as a function of I (A). d) EQE spectra of the heterojunction solar cells using both types of film.
We fabricated SWCNT/Si heterojunction solar cells following the process shown in Figure 1a. Optical images of the pristine and fluorinated SWCNT films on silicon substrates are shown in Figure S6. The current density-voltage characteristics of these cells under illumination with simulated AM 1.5G (100 mW/cm2) light are shown in Figure 4a. It can be seen that the F-SWCNT/Si solar cell shows the better performance than the pristine SWCNT/Si solar cell. The values of open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) are all improved by using the fluorinated SWCNT film. In detail, VOC increased from 0.586 V to 0.593,
FF increased from 60% to 73.2%, and JSC increased from 30.2 to 31.3 mA cm-2. As a result, the PCE of the solar cell is increased from 10.6% to 13.6%. This improvement in the VOC and FF can be attributed to the higher work function and lower sheet resistance of the F-SWCNT films. To evaluate the diode behavior of heterojunction solar cells, the ideality factor (n) and the dark saturation current density (J0) are derived from the J-V characteristics in the dark condition (Figure 4b). The n of the heterojunction solar cell fabricated with a F-SWCNT film is 1.41, much lower than that (1.94) fabricated using a pristine SWCNT film. In addition, the device based on a F-SWCNT film showed a smaller J0. The lower n and J0 of the F-SWCNT/Si solar cell indicate less charge recombination and more efficient carrier transport, thanks to the higher work function and lower surface roughness of the F-SWCNT film. Figure 4c shows the I(dV/dI) of the two solar cells as a function of I. The series resistance (Rs) of the devices is obtained from the slope of the plots. It is found that the Rs of the F-SWCNT/Si solar cell (10.7 Ω) is much lower than that of the SWCNT/Si solar cell (14.9 Ω), mainly due to the decreased sheet resistance of the F-SWCNT film. As mentioned above, the fluorinated film has higher transmittance in the wavelength range from 200 nm to 800 nm compared to the pristine SWCNT film. This means that more photons can penetrate the film and reach the heterojunction, yielding more photo-generated carriers and an increased JSC value. We also measured the external quantum efficiency (EQE) of both cells (Figure 4d) and found that the one with a F-SWCNT film shows a higher EQE in the wavelength range 350 nm to 800 nm, which is consistent with the optical transmittance results shown in Figure 3b. In addition, we calculated the integrated current density from the EQE spectra of the devices. The calculated current density difference between F-SWCNT/Si and SWCNT/Si solar cells obtained from the EQE spectra is ~1 mA/cm2 (Figure S7), which is consistent with the photocurrent densities obtained from the J-V curves shown in Figure 4a. Based on the above discussion and analysis, we further presented the different energy band diagrams for heterojunction solar cells constructed with two type films (Figure 5a). The work functions (WF) of SWCNT film and F-SWCNT film measured by UPS were 4.83 eV and 5.04 eV, respectively. The band gap of silicon is
1.12 eV and the Fermi level is 4.31 eV for the doping concentration of ~1015 cm-3 [45, 46]. Therefore, the built-in potential (Vbi) for the F-SWCNT/Si heterojunction is 0.73 V, which is higher than that of SWCNT/Si (0.52 V). A higher Vbi can improve the capacity of the junction to collect photo-generated carriers.[5] However, the Voc difference between the SWCNT/Si and F-SWCNT/Si solar cells is only ~0.007V. This might be caused by the different thickness of silicon oxide passivation layer formed at the nanotube/Si interfaces (For details see Figure S8, Table S1, and discussions in supporting information). The schematic of the work mechanism for the heterojunction between the p-type F-SWCNT and the n-type Si is shown in Figure 5b. Silicon absorbs the incident light passed through the high transparency of the F-SWCNT film and generates electron-hole pairs, which is separated by the built-in potential at its interface with the F-SWCNT. Holes were collected by the F-SWCNT film and electrons were injected into the n-type silicon. Due to the high work function, high conductivity and low surface roughness of the fluorinated SWCNT film, the F-SWCNT/Si solar cell demonstrates excellent performance.
Figure 5. a) Energy band diagram for the SWCNT/Si and F-SWCNT/Si solar cells. b) Schematic of the work mechanism of the F-SWCNT/Si solar cell.
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Figure 6. a) A comparison of the J-V characteristics of the F-SWCNT/Si solar cell and HNO3-doped SWCNT/Si solar cell placed in air for 0 and 1 hour. b) Normalized photovoltaic efficiency degradation of the F-SWCNT/Si and HNO3-doped SWCNT/Si solar cells.
HNO3 doping is one of the most commonly used methods to improve the performance of SWCNT/Si solar cells, however, it is generally not stable.[47-49] We tested and compared the stability of a F-SWCNT/Si solar cell and a HNO3-doped SWCNT/Si solar cell in ambient atmosphere. As shown in Figure 6a, the calculated PCE of the HNO3-doped SWCNT/Si solar cell reached 13.7%, which is almost the same as that of the F-SWCNT/Si device (13.6%). Nevertheless, the stabilities of these two devices are quite different. After exposure to air for 1 h, the J-V characteristics of the F-SWCNT/Si solar cell remained almost unchanged, while the PCE of the HNO3-doped SWCNT/Si device decreased quickly to 8.5%. This drop primarily results from the decreased FF and VOC, which are attributed to the evaporation of HNO3 molecules and loss of doping effects for the HNO3-doped SWCNTs in an ambient environment.[49, 50] We further tested the stabilities of the F-SWCNT/Si and HNO3-doped SWCNT/Si cells left in ambient conditions without any encapsulation, and the normalized efficiency is shown in Figure 6b. The F-SWCNT/Si solar cell has a modest decay in PCE from 13.6% to 12.5% over 30 days. This ~8% decrease is
much lower than the ~54% for the HNO3-doped SWCNT/Si solar cell. Therefore, the solar cell fabricated using the F-SWCNT film not only has an excellent photovoltaic performance but also good stability in ambient atmosphere. In addition, the high work function and low surface roughness of the F-SWCNT films mean that they could also be used as a hole transport material in organic or perovskite solar cells.
3. Conclusion We prepared a lightly-fluorinated SWCNT film (with a F content of ~1%) by simply exposing free-standing SWCNT films to XeF2 gas at room temperature. It was found that stable C-F ionic bonds with p-type doping effect were formed, and as a result the films had better optoelectronic properties and a higher work function than the pristine SWCNT film. Furthermore, fluorination improved the areal density of the films and decreased their surface roughness. Because of the improved optoelectronic properties, the efficiency of the F-SWCNT/Si solar cell with an active area of ~ 9 mm2 reached 13.6%. It also showed much better long-term stability in air than a conventional HNO3-doped SWCNT/Si device, which can be attributed to the formation of ionic C-F bonds. 4. Experimental Section Preparation of SWCNT and F-SWCNT films: The SWCNT films were synthesized by an injection FCCVD method using hydrogen as a carrier gas and toluene and ethylene as the carbon sources at 1100 °C.[3, 28] A liquid carbon source (toluene), catalyst precursor (ferrocene) and growth promoter (thiophene) were mixed with a weight ratios of 10: 0.3: 0.045, and the mixture was then injected into a quartz tube reactor by a syringe pump at a rate of 0.12 mL min-1 for SWCNT growth. A membrane filter (0.45 µm pore diameter) was installed on the gas outlet to collect SWCNT film at room temperature. The collected SWCNT film was fluorinated in an airtight chamber made of teflon by exposing it to XeF2 at room temperature. After fluorination, the samples were stored in a fume hood overnight to remove residual XeF2. The films were transferred onto target substrates by simple dry transfer.
Fabrication of heterojunction solar cells: A n-type silicon wafer (2-4 Ω cm) covered with 300 nm thermal oxide was patterned with square window (3 mm×3 mm). The SiO2 was then etched away by a buffered oxide etchant (6:1 of 40% NH4F and 49% HF) and rinsed with isopropanol. The films were transferred onto the top surface of the silicon substrate. Silver paste was painted around the active area to serve as a front electrode, while the back electrode was a gallium−indium eutectic, which forms ohmic contact with the silicon. A schematic showing the detailed fabrication process is shown in Figure 1a. Characterization: Raman spectra of the SWCNTs were obtained using a Jobin-Yvon Labram HR800 instrument, excited by a 633 nm laser. XPS and UPS were measured by Esclab-250. The optical transmission spectra of the films were measured utilizing a UV–Vis–NIR spectrophotometer (AGILENT CARY 5000). The structure of the films was characterized by SEM (Nova Nano SEM 430) and TEM (Tecnai F20, operated at 200 kV). The sample thickness was measured by a step profiler. AFM was used to characterize the surface roughness. The sheet resistance was measured by a Four Probes Tech (4-probe Tech.) instrument. Solar cell characteristics were determined by a solar simulator (PEC-L01 from Peccell Technologies, Inc.) under AM 1.5G (100 mW/cm2) light and a source meter (Keithley 2450). The irradiation intensity was calibrated by a standard Si solar cell (PECSI 02). EQE measurements were conducted by a solar cell IPCE test system model (QTEST STATION 1000AD from CROWNTECH, INC).
5. Acknowledgements This work was supported by the Ministry of Science and Technology of China (Grant 2016YFA0200102), the National Natural Science Foundation of China (Grants 51625203, 51532008, 51772303, 51761135122, 51872293)
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1.
Lightly fluorinated SWCNT (F-SWCNT) film was prepared by simply exposing the free-standing SWCNT film to XeF2 gas at room temperature.
2.
Stable C-F ionic bonds in F-SWCNT film enhanced film conductivity and caused p-type doping effect.
3.
Fluorination improved the areal density of the SWCNT film and decreased its surface roughness.
4.
Owing to excellent photovoltaic properties, the efficiency of the F-SWCNT/Si solar cell with an active area of ~ 9 mm2 reached 13.6%.
Xian-Gang Hu is currently a Ph.D student in Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. degree in Powder Materials Science and Engineering from Central South University in 2015. His research focuses on synthesis and application of carbon nanotube for photovoltaic devices.
Dr. Peng-Xiang Hou is a professor in materials science and engineering at the Institute of Metal Research, Chinese Academy of Sciences (IMR-CAS). She received her BSc and PhD degrees, both in materials science from IMR in 1999 and 2003, respectively. Her research focuses on CNT-controlled synthesis, properties and their applications.
Jin-Bo Wu is currently a Ph.D student in Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. degree in Materials Physics from Jilin University in 2015. His research focuses on fabrication and application of perovskite solar cells.
Xin Li is currently a Ph.D student in Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. degree in Material Science and Engineering from Xi’an University of Technology in 2009. His research focuses on selective synthesis of carbon nanotubes.
Jian Luan is a research assistant working at the Advanced Carbon Division of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Bachelor's degree in 2011 and Master's degree in 2014 from the College of Chemistry and Chemical Engineering, Bohai University. His research interests mainly focus on the synthesis and application of carbon nanotube.
Dr. Chang Liu is a professor of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Ph.D. in materials science at IMR, CAS in 2000. He mainly works on the preparation and application of carbon nanotubes and their hybrids.
Dr. Gang Liu received his Bachelor degree in Materials Physics in Jilin University in 2003. He obtained his PhD degree in Materials Science at Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS) in 2009. During his Ph. D study, he worked at Prof. G. Q. Max Lu’s laboratory for one and half years in Australia. He was the recipient of the T.S. Kê RESEARCH FELLOPSHIP founded by Shenyang National Laboratory for Materials Science, IMR CAS. Now he is a professor of materials science in IMR. His main research interests focus on solar-driven photocatalyitc materials for renewable energy.
Dr. Hui-Ming Cheng is Professor and Director of the Advanced Carbon Research Division of Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, and the Low-Dimensional Material and Device Laboratory of the Tsinghua-Berkeley Shenzhen Institute, Tsinghua University. His research focuses on carbon nanotubes, graphene, two-dimensional materials, energy storage materials, photocatalytic semiconducting materials, and bulk carbon materials. He is a Highly Cited Researcher in materials science and chemistry fields. He is now the founding Editor-in-Chief of Energy Storage Materials and Associate Editor of Science China Materials. He was elected a member of CAS and a fellow of TWAS.
Declaration of Interest Statement The authors declare no conflict of interest.
S2211-2855(19)31159-0
DOI:
https://doi.org/10.1016/j.nanoen.2019.104442
Reference:
NANOEN 104442
To appear in:
Nano Energy
Received Date: 18 June 2019 Revised Date:
22 December 2019
Accepted Date: 27 December 2019
Please cite this article as: X.-G. Hu, P.-X. Hou, J.-B. Wu, X. Li, J. Luan, C. Liu, G. Liu, H.-M. Cheng, High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104442. 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. © 2019 Published by Elsevier Ltd.
High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films Xian-Gang Hua,b, Peng-Xiang Houa,b,*, Jin-Bo Wua,b, Xin Lia,b, Jian Luana,b, Chang Liua,b,*, Gang Liua,b, Hui-Ming Chenga,c
a
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, PR China. b
School of Materials Science and Engineering, University of Science and Technology
of China, Hefei 230026, PR China. c
Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute, Tsinghua
University, Shenzhen 518055, PR China *
Corresponding Authors: [email protected] (P.X. Hou) or [email protected] (C. Liu).
ABSTRACT High-quality single-wall carbon nanotube (SWCNT) films were lightly fluorinated by treatment with xenon difluoride at room temperature, which led to the formation of ionic C-F bonds on tube walls and controllable p-type doping of the nanotubes. The fluorinated SWCNT films showed improved electronic conductivity and a higher work function. In addition, the fluorination process increased the areal density of SWCNT films and decreased their surface roughness, leading to better interface contact between them and silicon. As a result, a heterojunction solar cell constructed using the lightly fluorinated SWCNT film has a high power conversion efficiency of 13.6% and excellent stability.
Keywords: fluorination, carbon nanotubes, transparent conductive films, high work function, heterojunction, solar cell
1. Introduction Single-wall carbon nanotube (SWCNT) films show great promise as flexible and transparent conductive electrodes owing to their outstanding optical and electrical properties.[1-3] Because of this, SWCNT transparent conductive films (TCFs) have been used to fabricate various solar cells, including silicon heterojunction solar cells,[4, 5] dye-sensitized solar cells,[6] polymer solar cells[7] and perovskite solar cells.[8-10] SWCNT/silicon (Si) heterojunction solar cells have attracted considerable interest due to their simple structure, easy fabrication and low cost.[5, 11-14] In this type of device, the SWCNT films mainly serve as transparent electrodes to transfer carriers and to form a built-in potential with Si to separate photo-generated carriers.[15] Therefore, the power conversion efficiency (PCE) of SWCNT/Si heterojunction solar cells largely depends on the structure and performance of the SWCNT films used. Compared with poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) films, although the SWCNT film possesses higher stability than PEDOT:PSS film, the work function of SWCNT films (~4.8 eV) is relatively low,[16] which is a serious drawback in constructing high-performance SWCNT/Si heterojunction solar cells. Because of this, SWCNT films with a high conductivity and a large work function are required to improve the efficiency of the cells. Chemical functionalization is generally used to tune the physical and chemical properties of SWCNTs.[17] Halogen atoms, such as fluorine (F), chlorine (Cl) and bromine (Br), have been recognized as efficient dopants because of their high electronegativity.[18] Fluorination is a commonly used functionalization technique because fluorine is the most electronegative element. Fluorinated SWCNTs indeed exhibit many intriguing properties and have been used in supercapacitors,[19] lithium batteries[20] and sensors.[21] A number of methods have been used to fluorinate SWCNTs, including treatment with F2 gas at a temperature of ~400
[22] and
treatment with a CF4 plasma.[23] However, these strong treatments usually lead to the formation of covalent C-F bonds and, at the same time, the SWCNTs are changed from conducting to insulating due to the increased band gap with high fluorine doping
concentrations (F/C ratio > 0.5).[24] As a result this kind of fluorinated SWCNT film cannot be used as a transparent electrode in photovoltaic devices. In contrast, Nakajima et al. reported that the conductivity of highly oriented pyrolytic graphite (HOPG) was improved greatly after fluorination at a low temperature and with a low fluorine content.[25, 26] They attributed this to the formation of ionic C-F bonds, in which F atoms act as electron acceptors, and the π electrons in HOPG are strongly delocalized and contribute to the C-F bond.[27] We therefore expect that the controlled introduction of ionic C-F bonds to SWCNTs would lead to increased electrical conductivity, but there are few reports on the preparation and properties of lightly fluorinated SWCNT films. In this paper, we report the fluorination of SWCNT (F-SWCNT) films by directly exposing as-prepared free-standing SWCNT films to xenon difluoride (XeF2) gas in an airtight chamber at room temperature. The degree of fluorination is controlled by changing the exposure time and chamber temperature. The type of C-F bonds in the lightly fluorinated F-SWCNT is confirmed to be ionic. The fluorinated films showed a higher conductivity, higher work function and higher areal density than the pristine SWCNT films. As a result, a fabricated F-SWCNT/Si heterojunction solar cell demonstrated a high PCE of 13.6% and excellent stability in air.
2. Results and Discussion The pristine SWCNT films were synthesized by a floating catalyst chemical vapor deposition (FCCVD) method (Details in the Experimental Section).[3, 28] The processes of the preparation of fluorinated SWCNT films and the construction of F-SWCNT/Si solar cells are schematically shown in Figure 1a. Briefly, an as-prepared free-standing SWCNT film was exposed to XeF2 gas sublimated from XeF2 powder in an airtight chamber at room temperature, during which the XeF2 decomposed to produce atomic fluorine that uniformly fluorinated the SWCNTs [17, 29] (For details, see the Supporting Information). After fluorination, the film was cut into the size required to fabricate the solar cells. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed
to characterize the fluorinated SWCNT films. Raman spectra of the pristine and fluorinated SWCNT films excited with 633 nm laser are shown in Figures 1b and c. A decrease in the intensity is observed for both the radial breathing mode (RBM, Figure. 1b) and the G mode (Figure. 1c) peaks after fluorination. In particular, the G/D ratio of the F-SWCNT films decreased to 52 from the original 110. In addition, the blue shift of the G and G` bands indicates p-type doping. Figure 1d shows UPS spectra of the pristine and fluorinated SWCNT films on a copper substrate. After fluorination, the work function increased from 4.83 eV to 5.04 eV, which can be ascribed to the high electronegativity of the doped fluorine atoms.[18, 30, 31] To confirm the existence of fluorine in SWCNTs, the samples were further characterized using XPS and spectra before and after fluorination are shown in Figure S1. It can be clearly seen that a new F 1s peak has appeared. The elemental contents (atomic percent) of C, O, and F are 90.3%, 8.61% and 1.08%, respectively, indicating light F doping. To understand the changes in chemical bonding, we analyzed the C1s spectra of the pristine and fluorinated SWCNT films. As shown in Figure S2, five main peaks located at 284.6, 285.3, 286.2, 287.6 and 289.5eV were observed for the pristine SWCNT film, which can be assigned to C-C (sp2), C=C (sp3), C-O, C=O and O-C=O bonds, respectively.[32-35]. A new peak was observed at ~287 eV for the F-SWCNT film (Figure 1e), which indicates the presence of C-F bonds.[22, 32] At the same time, the position of C 1s peak of the F-SWCNT film shifted by 0.2 eV towards lower binding energy compared with that of the pristine SWCNT film (Figure 1f). This shift indicates that the C-F bond is ionic[25] and F is incorporated by p-type doping.[36, 37] In addition, a clear F 1s peak was observed at 686.5 eV (Figure 1g), close to that of LiF.[35] These results suggest that the C-F bonds in the F-SWCNT film are mainly ionic. The electrical conductivity of fluorinated carbon materials is closely related to the type of C-F bonding.[27, 38] In the case of ionic C-F bonds, F plays the role of an electron acceptor and contributes to the conductivity.[27] Therefore, the electrical conductivity of our F-SWCNT films is expected to increase.
Figure 1 a) Schematic showing the fabrication of fluorinated SWCNT films and the construction of F-SWCNT/Si heterojunction solar cells. Raman spectra of b) pristine SWCNT film and c) a F-SWCNT film excited by 633 nm laser excitation. d) UPS spectra of SWCNT and F-SWCNT. e-g) XPS spectra of the SWCNTs. e) C 1s curve of the F-SWCNT film. f) Enlarged C1s curve of the pristine and F-SWCNT films. g) F 1s peak of the pristine and fluorinated SWCNT films.
Figure 2. SEM images of a) pristine and b) fluorinated SWCNT films on a SiOx/Si substrate. c) Optical image of the F-SWCNT film transferred to a SiOx/Si substrate for step profiler testing. The measured thicknesses of d) pristine and e) fluorinated SWCNT films. TEM images of f) pristine and g) fluorinated SWCNT films.
To understand the effect of fluorination on the structure of the SWCNT films, we characterized the fluorinated samples using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2a, the SEM image of the pristine SWCNT film shows a high-purity, uniform, randomly entangled SWCNT network. After fluorination, the packing density of the SWCNTs increases greatly as can be seen in Figure 2b. It seems that the surface of the F-SWCNT film is smoother than that of the pristine SWCNT film. To confirm this, we measured the surface roughness of the samples using atomic force microscopy (AFM). As shown in Figure S3, the F-SWCNT film has a surface root-mean-square roughness of 5.89 nm, which is smaller than that of the pristine SWCNT film (7.79 nm). This lower surface roughness would lead to denser packing and improved contacts between nanotubes in the F-SWCNT film. At the same time, this lower surface roughness produces more
contact points between F-SWCNT film and the Si, which increases the junction area of the heterojunction solar cell.[39] The thickness of the two films transferred onto SiOx/Si substrates was measured using a step profiler (Figures 2c). As shown in Figures 2d and 2e, the film thickness was reduced from ~60 nm to ~28 nm after the fluorination, which is consistent with the above results. The thickness reduction should result from SWCNT surface modification during the fluorination process. It was reported that light fluorination treatment could enhance the hydrophilicity and surface adhesive property of nanotubes,[40] which leads to better contact and denser packing of F-SWCNTs. In consequence, the total film thickness decreased evidently after the fluorination. A typical TEM image of the SWCNT film is shown in Figure S4 from which it can be seen that the SWCNT network mainly consists of isolated and small-bundle SWCNTs, which is needed to achieve outstanding photoelectric properties, since large bundles contribute little to the electrical conductivity but lower the light transmittance of the films.[41] A high resolution TEM image (Figure 2f) shows that the pristine SWCNTs have straight walls with good crystallinity. Furthermore, no obvious difference is observed between the two SWCNTs (Figure 2g), which suggests that this fluorination process has not damaged their structure.
Figure 3. a) Optical transmittance of the pristine and fluorinated SWCNT films on quartz substrates. b) I-V curves of the pristine SWCNT film with a transmittance of 86% and a F-SWCNT film with a transmittance of 88% (inset shows a schematic of
the I-V measurement).
We then measured the optoelectronic performance of the two films and their UV-Vis-NIR transmittance spectra are shown in Figure 3a. After fluorination, the F-SWCNT film shows a higher transmittance in the wavelength range 200 to 800 nm, for example, the transmittance for 550 nm light is increased from 86% to 88%. This is attributed to the lower optical loss for the F-SWCNT film because of smaller film thickness and lower light absorption (Figure S5).[42] The I-V curves of these films are shown in Figure 3b. The sheet resistance of the fluorinated SWCNT film is decreased to 54 Ω sq.-1 from the original 86 Ω sq.-1. In other words, the electrical conductivity of the F-SWCNT film is increased by a factor of 1.6 through the fluorination treatment. We attribute this improvement in electrical conductivity to two factors. One is the presence of ionic C-F bonds in the F-SWCNT film, in which F acts as an electron acceptor to increase the conductivity.[27, 35, 43] The other is that the lower surface roughness and denser packing improve inter-tube contacts and decrease total contact resistances.[44] These improved optoelectronic performance and higher work function of F-SWCNT films resulted from lightly fluorination treatment are highly desirable for constructing high-performance SWCNT/Si solar cells.
a
b SWCNT/Si F-SWCNT/Si
Current density (mA/cm2)
Current density (mA/cm2)
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SWCNT/Si F-SWCNT/Si
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■■■■ SWCNT/Si ●●●● F-SWCNT/Si
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Rs=10.7 Ω
20
0.05 0.002
0.004
0.006
I (A)
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Figure 4. Current density-voltage (J-V) characteristics of the pristine and fluorinated SWCNT films. a) Under illumination. b) In the dark. c) I (dV/dI) as a function of I (A). d) EQE spectra of the heterojunction solar cells using both types of film.
We fabricated SWCNT/Si heterojunction solar cells following the process shown in Figure 1a. Optical images of the pristine and fluorinated SWCNT films on silicon substrates are shown in Figure S6. The current density-voltage characteristics of these cells under illumination with simulated AM 1.5G (100 mW/cm2) light are shown in Figure 4a. It can be seen that the F-SWCNT/Si solar cell shows the better performance than the pristine SWCNT/Si solar cell. The values of open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) are all improved by using the fluorinated SWCNT film. In detail, VOC increased from 0.586 V to 0.593,
FF increased from 60% to 73.2%, and JSC increased from 30.2 to 31.3 mA cm-2. As a result, the PCE of the solar cell is increased from 10.6% to 13.6%. This improvement in the VOC and FF can be attributed to the higher work function and lower sheet resistance of the F-SWCNT films. To evaluate the diode behavior of heterojunction solar cells, the ideality factor (n) and the dark saturation current density (J0) are derived from the J-V characteristics in the dark condition (Figure 4b). The n of the heterojunction solar cell fabricated with a F-SWCNT film is 1.41, much lower than that (1.94) fabricated using a pristine SWCNT film. In addition, the device based on a F-SWCNT film showed a smaller J0. The lower n and J0 of the F-SWCNT/Si solar cell indicate less charge recombination and more efficient carrier transport, thanks to the higher work function and lower surface roughness of the F-SWCNT film. Figure 4c shows the I(dV/dI) of the two solar cells as a function of I. The series resistance (Rs) of the devices is obtained from the slope of the plots. It is found that the Rs of the F-SWCNT/Si solar cell (10.7 Ω) is much lower than that of the SWCNT/Si solar cell (14.9 Ω), mainly due to the decreased sheet resistance of the F-SWCNT film. As mentioned above, the fluorinated film has higher transmittance in the wavelength range from 200 nm to 800 nm compared to the pristine SWCNT film. This means that more photons can penetrate the film and reach the heterojunction, yielding more photo-generated carriers and an increased JSC value. We also measured the external quantum efficiency (EQE) of both cells (Figure 4d) and found that the one with a F-SWCNT film shows a higher EQE in the wavelength range 350 nm to 800 nm, which is consistent with the optical transmittance results shown in Figure 3b. In addition, we calculated the integrated current density from the EQE spectra of the devices. The calculated current density difference between F-SWCNT/Si and SWCNT/Si solar cells obtained from the EQE spectra is ~1 mA/cm2 (Figure S7), which is consistent with the photocurrent densities obtained from the J-V curves shown in Figure 4a. Based on the above discussion and analysis, we further presented the different energy band diagrams for heterojunction solar cells constructed with two type films (Figure 5a). The work functions (WF) of SWCNT film and F-SWCNT film measured by UPS were 4.83 eV and 5.04 eV, respectively. The band gap of silicon is
1.12 eV and the Fermi level is 4.31 eV for the doping concentration of ~1015 cm-3 [45, 46]. Therefore, the built-in potential (Vbi) for the F-SWCNT/Si heterojunction is 0.73 V, which is higher than that of SWCNT/Si (0.52 V). A higher Vbi can improve the capacity of the junction to collect photo-generated carriers.[5] However, the Voc difference between the SWCNT/Si and F-SWCNT/Si solar cells is only ~0.007V. This might be caused by the different thickness of silicon oxide passivation layer formed at the nanotube/Si interfaces (For details see Figure S8, Table S1, and discussions in supporting information). The schematic of the work mechanism for the heterojunction between the p-type F-SWCNT and the n-type Si is shown in Figure 5b. Silicon absorbs the incident light passed through the high transparency of the F-SWCNT film and generates electron-hole pairs, which is separated by the built-in potential at its interface with the F-SWCNT. Holes were collected by the F-SWCNT film and electrons were injected into the n-type silicon. Due to the high work function, high conductivity and low surface roughness of the fluorinated SWCNT film, the F-SWCNT/Si solar cell demonstrates excellent performance.
Figure 5. a) Energy band diagram for the SWCNT/Si and F-SWCNT/Si solar cells. b) Schematic of the work mechanism of the F-SWCNT/Si solar cell.
20
b F-SWCNT/Si HNO3 doped SWCNT/Si ■■■■ F-SWCNT/Si after 1h ■■■■ HNO3 doped SWCNT/Si after 1h
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Figure 6. a) A comparison of the J-V characteristics of the F-SWCNT/Si solar cell and HNO3-doped SWCNT/Si solar cell placed in air for 0 and 1 hour. b) Normalized photovoltaic efficiency degradation of the F-SWCNT/Si and HNO3-doped SWCNT/Si solar cells.
HNO3 doping is one of the most commonly used methods to improve the performance of SWCNT/Si solar cells, however, it is generally not stable.[47-49] We tested and compared the stability of a F-SWCNT/Si solar cell and a HNO3-doped SWCNT/Si solar cell in ambient atmosphere. As shown in Figure 6a, the calculated PCE of the HNO3-doped SWCNT/Si solar cell reached 13.7%, which is almost the same as that of the F-SWCNT/Si device (13.6%). Nevertheless, the stabilities of these two devices are quite different. After exposure to air for 1 h, the J-V characteristics of the F-SWCNT/Si solar cell remained almost unchanged, while the PCE of the HNO3-doped SWCNT/Si device decreased quickly to 8.5%. This drop primarily results from the decreased FF and VOC, which are attributed to the evaporation of HNO3 molecules and loss of doping effects for the HNO3-doped SWCNTs in an ambient environment.[49, 50] We further tested the stabilities of the F-SWCNT/Si and HNO3-doped SWCNT/Si cells left in ambient conditions without any encapsulation, and the normalized efficiency is shown in Figure 6b. The F-SWCNT/Si solar cell has a modest decay in PCE from 13.6% to 12.5% over 30 days. This ~8% decrease is
much lower than the ~54% for the HNO3-doped SWCNT/Si solar cell. Therefore, the solar cell fabricated using the F-SWCNT film not only has an excellent photovoltaic performance but also good stability in ambient atmosphere. In addition, the high work function and low surface roughness of the F-SWCNT films mean that they could also be used as a hole transport material in organic or perovskite solar cells.
3. Conclusion We prepared a lightly-fluorinated SWCNT film (with a F content of ~1%) by simply exposing free-standing SWCNT films to XeF2 gas at room temperature. It was found that stable C-F ionic bonds with p-type doping effect were formed, and as a result the films had better optoelectronic properties and a higher work function than the pristine SWCNT film. Furthermore, fluorination improved the areal density of the films and decreased their surface roughness. Because of the improved optoelectronic properties, the efficiency of the F-SWCNT/Si solar cell with an active area of ~ 9 mm2 reached 13.6%. It also showed much better long-term stability in air than a conventional HNO3-doped SWCNT/Si device, which can be attributed to the formation of ionic C-F bonds. 4. Experimental Section Preparation of SWCNT and F-SWCNT films: The SWCNT films were synthesized by an injection FCCVD method using hydrogen as a carrier gas and toluene and ethylene as the carbon sources at 1100 °C.[3, 28] A liquid carbon source (toluene), catalyst precursor (ferrocene) and growth promoter (thiophene) were mixed with a weight ratios of 10: 0.3: 0.045, and the mixture was then injected into a quartz tube reactor by a syringe pump at a rate of 0.12 mL min-1 for SWCNT growth. A membrane filter (0.45 µm pore diameter) was installed on the gas outlet to collect SWCNT film at room temperature. The collected SWCNT film was fluorinated in an airtight chamber made of teflon by exposing it to XeF2 at room temperature. After fluorination, the samples were stored in a fume hood overnight to remove residual XeF2. The films were transferred onto target substrates by simple dry transfer.
Fabrication of heterojunction solar cells: A n-type silicon wafer (2-4 Ω cm) covered with 300 nm thermal oxide was patterned with square window (3 mm×3 mm). The SiO2 was then etched away by a buffered oxide etchant (6:1 of 40% NH4F and 49% HF) and rinsed with isopropanol. The films were transferred onto the top surface of the silicon substrate. Silver paste was painted around the active area to serve as a front electrode, while the back electrode was a gallium−indium eutectic, which forms ohmic contact with the silicon. A schematic showing the detailed fabrication process is shown in Figure 1a. Characterization: Raman spectra of the SWCNTs were obtained using a Jobin-Yvon Labram HR800 instrument, excited by a 633 nm laser. XPS and UPS were measured by Esclab-250. The optical transmission spectra of the films were measured utilizing a UV–Vis–NIR spectrophotometer (AGILENT CARY 5000). The structure of the films was characterized by SEM (Nova Nano SEM 430) and TEM (Tecnai F20, operated at 200 kV). The sample thickness was measured by a step profiler. AFM was used to characterize the surface roughness. The sheet resistance was measured by a Four Probes Tech (4-probe Tech.) instrument. Solar cell characteristics were determined by a solar simulator (PEC-L01 from Peccell Technologies, Inc.) under AM 1.5G (100 mW/cm2) light and a source meter (Keithley 2450). The irradiation intensity was calibrated by a standard Si solar cell (PECSI 02). EQE measurements were conducted by a solar cell IPCE test system model (QTEST STATION 1000AD from CROWNTECH, INC).
5. Acknowledgements This work was supported by the Ministry of Science and Technology of China (Grant 2016YFA0200102), the National Natural Science Foundation of China (Grants 51625203, 51532008, 51772303, 51761135122, 51872293)
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1.
Lightly fluorinated SWCNT (F-SWCNT) film was prepared by simply exposing the free-standing SWCNT film to XeF2 gas at room temperature.
2.
Stable C-F ionic bonds in F-SWCNT film enhanced film conductivity and caused p-type doping effect.
3.
Fluorination improved the areal density of the SWCNT film and decreased its surface roughness.
4.
Owing to excellent photovoltaic properties, the efficiency of the F-SWCNT/Si solar cell with an active area of ~ 9 mm2 reached 13.6%.
Xian-Gang Hu is currently a Ph.D student in Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. degree in Powder Materials Science and Engineering from Central South University in 2015. His research focuses on synthesis and application of carbon nanotube for photovoltaic devices.
Dr. Peng-Xiang Hou is a professor in materials science and engineering at the Institute of Metal Research, Chinese Academy of Sciences (IMR-CAS). She received her BSc and PhD degrees, both in materials science from IMR in 1999 and 2003, respectively. Her research focuses on CNT-controlled synthesis, properties and their applications.
Jin-Bo Wu is currently a Ph.D student in Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. degree in Materials Physics from Jilin University in 2015. His research focuses on fabrication and application of perovskite solar cells.
Xin Li is currently a Ph.D student in Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. degree in Material Science and Engineering from Xi’an University of Technology in 2009. His research focuses on selective synthesis of carbon nanotubes.
Jian Luan is a research assistant working at the Advanced Carbon Division of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Bachelor's degree in 2011 and Master's degree in 2014 from the College of Chemistry and Chemical Engineering, Bohai University. His research interests mainly focus on the synthesis and application of carbon nanotube.
Dr. Chang Liu is a professor of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Ph.D. in materials science at IMR, CAS in 2000. He mainly works on the preparation and application of carbon nanotubes and their hybrids.
Dr. Gang Liu received his Bachelor degree in Materials Physics in Jilin University in 2003. He obtained his PhD degree in Materials Science at Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS) in 2009. During his Ph. D study, he worked at Prof. G. Q. Max Lu’s laboratory for one and half years in Australia. He was the recipient of the T.S. Kê RESEARCH FELLOPSHIP founded by Shenyang National Laboratory for Materials Science, IMR CAS. Now he is a professor of materials science in IMR. His main research interests focus on solar-driven photocatalyitc materials for renewable energy.
Dr. Hui-Ming Cheng is Professor and Director of the Advanced Carbon Research Division of Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, and the Low-Dimensional Material and Device Laboratory of the Tsinghua-Berkeley Shenzhen Institute, Tsinghua University. His research focuses on carbon nanotubes, graphene, two-dimensional materials, energy storage materials, photocatalytic semiconducting materials, and bulk carbon materials. He is a Highly Cited Researcher in materials science and chemistry fields. He is now the founding Editor-in-Chief of Energy Storage Materials and Associate Editor of Science China Materials. He was elected a member of CAS and a fellow of TWAS.
Declaration of Interest Statement The authors declare no conflict of interest.