Elsevier

Nano Energy

Volume 69, March 2020, 104442
Nano Energy

Full paper
High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films

https://doi.org/10.1016/j.nanoen.2019.104442Get rights and content

Highlights

  • Lightly fluorinated SWCNT (F-SWCNT) film was prepared by simply exposing the free-standing SWCNT film to XeF2 gas at room temperature.

  • Stable C–F ionic bonds in F-SWCNT film enhanced film conductivity and caused p-type doping effect.

  • Fluorination improved the areal density of the SWCNT film and decreased its surface roughness.

  • Owing to excellent photovoltaic properties, the PCE of the F-SWCNT/Si solar cell with an active area of 9 mm2 reached 13.6%.

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.

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], [2], [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], [9], [10]]. SWCNT/silicon (Si) heterojunction solar cells have attracted considerable interest due to their simple structure, easy fabrication and low cost [5,[11], [12], [13], [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 °C [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.

Section snippets

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 Fig. 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

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

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

Declaration of competing interest

The authors declare no conflict of interest.

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).

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.

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  • Cited by (0)

    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.

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