Elsevier

Nano Energy

Volume 12, March 2015, Pages 374-385
Nano Energy

Rapid communication
PEDOT-decorated nitrogen-doped graphene as the transparent composite film for the counter electrode of a dye-sensitized solar cell

https://doi.org/10.1016/j.nanoen.2015.01.010Get rights and content

Highlights

  • Honeycomb NGr/PEDOT composite functions as the catalytic counter electrode (CE).

  • Transparent NGr/PEDOT CE is used for bifacial dye-sensitized solar cells (DSSCs).

  • The DSSC with NGr/PEDOT CE gives an η of 8.30±0.03% from front-side illumination.

  • The same cell reaches an η of 6.10±0.02% from back-side illumination.

  • The NGr/PEDOT on flexible Ti foil renders its DSSC the highest η of 8.53±0.02%.

Abstract

A honeycomb-like composite film of nitrogen-doped graphene and poly(3,4-ethylenedioxythiophene) (NGr/PEDOT) was prepared as a highly-efficient catalytic material for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The NGr was intended for increasing the conductivity and electrocatalytic ability of the composite film, and the PEDOT was used for a strong adhesion of the composite film to the substrate and for large surface area. The DSSC with the NGr/PEDOT composite CE exhibited a power conversion efficiency (η) of 8.30±0.03% for front-side illumination and 6.10±0.02% for back-side illumination. The DSSC with a Pt CE showed an η of 8.17±0.01% for front-side illumination and 5.76±0.05% for back-side illumination. The composite catalytic film of NGr/PEDOT is a low-cost alternative for replacing the conventional and expensive Pt film. Another DSSC was fabricated, in which its counter electrode contained a flexible Ti foil as the substrate and the NGr/PEDOT film as the catalytic film. This DSSC with its flexible CE exhibited an η of 8.53±0.02%.

Introduction

Generally, a dye-sensitized solar cell (DSSC) is composed of three main components, i.e., a photoanode with a metal oxide semiconductor adsorbed with dye molecules, a platinum (Pt) counter electrode (CE), and an electrolyte with an I/I3 redox mediator [1]. A DSSC is notable for its capability of working under bifacial illumination [2]. It is well known that the CE is crucial for achieving a high performance for a DSSC. A thin film of Pt is generally used as the catalytic layer on the CE, due to its excellent electrocatalytic ability and high conductivity. The thin film of Pt is usually coated on a transparent conductive oxide glass, e.g., indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) glass. As Pt is very expensive, rarely available, and has poor corrosion-resistance, alternative materials, such as carbon materials or conducting polymers, have been extensively investigated, in recent years, as the catalytic materials for the CEs of DSSCs. Irradiated under sunlight, the dye molecules in a DSSC inject electrons into the conduction band of the semiconductor, which then flow through the semiconductor and then through the external load to the CE to trigger the reduction of triiodide ions to form iodide ions, which in turn regenerate the oxidized dye and thereby complete the circuit. The catalytic material of the CE is the key for I regeneration. Several materials with high electrocatalytic ability and conductivity were widely studied intending to replace Pt; they include carbon-based materials [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], conductive polymers [13], [14], [15], [16], [17], [18], [19], [20], [21], inorganic metal compounds [22], [23], and composite materials [24], [25], [26], [27]. Among them, carbon-based materials, including graphene [4], [5], graphite [6], nanocarbon [7], [8], carbon black [9], activated carbon [10], single-walled carbon nanotube (SWCNT) [11], and multi-walled carbon nanotube (MWCNT) [12] are very promising candidates, because of their abundance in nature, low cost, good catalytic ability, high conductivity, and high chemical stability.

Graphene was first discovered by the Noble prize winners, Novoselov and Geim [28], and is highlighted for its two-dimensional planar structure, high conductivity, good transparency and extraordinarily high specific surface area. In a DSSC, graphene shows interesting catalytic ability for I3 reduction, due to its effective edge defects. However, the lack of inner defects of graphene limits the performance of its DSSC. To solve this problem, nitrogen heteroatom was inserted into the planar sp2 carbon atom network of graphene to obtain the corresponding nitrogen-doped graphene (NGr) [29]. The insertion of nitrogen causes a reorganized electron resonance phenomenon, which enables the enhancement of catalytic ability of the graphene and thereby renders a high conductivity to the NGr. An outstanding performance of 9.05% was reported for a DSSC using NGr counter electrode [30]. On the other hand, conductive polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) [13], [14], poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) [15], [16], polypyrrole (PPy) [17], [18], polyaniline (PANI) [19], [20], and poly(3,3-diethyl-3,4-dihydro-2-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) [21] are attractive materials for use as catalytic materials for the CEs of DSSCs. Among these conductive polymers, PEDOT has been extensively studied and utilized in DSSCs due to its good catalytic ability and high effective surface area. However, owing to its deficiency of orientated charge-transfer pathways, a PEDOT counter electrode exhibits relatively low conductivity, and thereby renders a poor performance to its DSSC.

In this work, we intended to combine the advantages of both NGr and PEDOT. A transparent composite film of PEDOT and nitrogen-doped graphene (NGr/PEDOT) was obtained through a simple two-step procedure. Highly conductive NGr microsheets form here the matrix for the composite film, and thereby facilitate the charge transfer. The highly porous PEDOT was decorated on the NGr matrix for enlarging the effective surface area of the composite film; this was intended to enhance the electrocatalytic ability of the composite film, and also to improve the adhesion of the composite film to the substrate. The composite NGr/PEDOT film is demonstrated to be highly transparent; owing to this property of the counter electrode material, its DSSC could be illuminated from both front and back sides (bifacial illumination).

Section snippets

Materials

Guanidinium thiocyanate (GuSCN, 99%), ethanol (EtOH, 99.5%), titanium (IV) tetraisopropoxide (TTIP, >98%), isopropyl alcohol (IPA, 99.5%), lithium perchlorate (LiClO4, ≥8.0%), 2-methoxyethanol (≥99.5%), 3,4-ethylenedioxythiophene (EDOT, 99%), Nafion® solution, tetrabutylammonium triiodide (TBAI3, >97%) and nitrogen-doped graphene (NGr) were obtained from Sigma Aldrich. Acetonitrile (ACN, 99.99%), nitric acid (HNO3, ca. 65% solution in water), and acetone (99%) were purchased from J.T. Baker.

Morphology

Surface structures of different types of counter electrode films were observed by FE-SEM images, as shown in Figure 1. Figure 1a shows the structure of an ultra-thin Pt film covered on an FTO substrate. In the inset of Figure 1a, the Pt film is well covered on the FTO glass and is composed of uniformly distributed Pt nanoparticles. The bare NGr film in Figure 1b is composed of two-dimensional (2D) NGr microsheets; it has a compact and non-porous structure and shows flakes and wrinkles, as seen

Conclusion

A PEDOT-decorated NGr architecture was successfully prepared as the composite film for the counter electrode of a dye-sensitized solar cell by a simple two-step procedure. SEM images reveal the decoration of the NGr film with a porous structure of PEDOT. Elemental mapping analyses confirm the uniform distribution of sulfur, nitrogen, and carbon in the composite film. The coexistence of PEDOT and NGr in the composite film is also revealed in the EDS spectrum. X-ray photoelectron spectroscopy

Acknowledgment

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan under the Grant number of NSC-102-2221-E-002-186-MY3.

Pei-Yu Chen, studying under the guidance of Prof. Kuo-Chuan Ho, is currently a senior undergraduate student in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan. Her research interests surround on synthesizing the novel compounds applied as the new catalysts for the counter electrode of dye-sensitized solar cells.

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    Pei-Yu Chen, studying under the guidance of Prof. Kuo-Chuan Ho, is currently a senior undergraduate student in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan. Her research interests surround on synthesizing the novel compounds applied as the new catalysts for the counter electrode of dye-sensitized solar cells.

    Chun-Ting Li received her BS degree and MS degrees in Department of Chemical and Material Engineering at Chang Gung University, Taiwan, in 2010 and in 2011, respectively. Now she is in the third year in her PhD program in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan. Her research interests principally focus on synthesizing and developing electro-catalyst mat      erials in electrochemical devices, including dye-sensitized solar cells as well as energy conversion & storage materials/systems with particular attention to transition metal compounds. Besides, her specialty is material and electrochemical analyses techniques.

    Chuan-Pei Lee received his PhD degree in Chemical Engineering at National Taiwan University in 2012. Currently, he is a postdoctoral research fellow in Center for Condensed Matter Sciences at National Taiwan University. His research interests include solar energy and electrochemical energy materials/systems. Besides, he is also familiar with electrochemical-analysis and micr      oemulsions-synthesis techniques.

    R. Vittal is a Senior Researcher at National Taiwan University. He was a Research Assistant Professor at Korea University, Seoul during 2002–2007. He worked as a Scientist at Central Electrochemical Research Institute, Karaikudi, India during 1982–2002. Previous to this he was a Chemist at Nuclear Fuel Complex, Hyderabad, India. He has published 102 papers with an average IF of 5.65. Addit      ionally he has 14 conference papers and 44 presentations in national and international seminars/symposia. His research interests include photovoltaic devices (dye-sensitized solar cells, quantum dot-sensitized solar cells, organic/plastic/polymer solar cells), electrochemical sensors, and electrochromic devices.

    Kuo-Chuan Ho received his BS and MS degrees in Department of Chemical Engineering from National Cheng Kung University, Taiwan in 1978 and 1980, respectively. He received his PhD in Chemical Engineering at the University of Rochester, USA in 1986. Currently he is a Distinguished Professor jointly appointed by the Department of Chemical Engineering and Institute of Poly       mer Science and Engineering at National Taiwan University. His research interests mainly surround applications of chemically modified electrodes to sensing and electro-optical devices, including dye-sensitized solar cells and electrochromic devices.

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