Graphene as Transparent Electrodes for Solar Cells

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10 Graphene as Transparent Electrodes for Solar Cells Khaled Parvez, Rongjin Li, and Klaus Müllen

10.1 Introduction

Since its discovery in 2004, graphene has attracted tremendous interest due to its exceptional properties, including high charge carrier mobility, mechanical strength, flexibility, and transparency [1–4]. Therefore, graphene is expected to play an important role as electrode materials in electronics and optoelectronic devices [5–7]. Transparent electrodes are an essential element for numerous devices, such as liquid crystal displays (LCDs), cellular phones, light-emitting diodes (LEDs), and photovoltaic devices (PVs). It is estimated that over 260million displays will be produced in 2016 [8], and thus the demand for transparent electrodes will significantly increase in the near future. Nowadays, indium tin oxide (ITO) is by far the most dominant material used in transparent electrodes ($3 billion worth in 2010 with a 20% growth through 2013) [7, 9]. However, ITO suffers from severe limitations: an ever-increasing cost due to indium scarcity [10], processing requirements, difficulties in patterning [10, 11], and sensitivity to both acidic and basic environments. Moreover, it is brittle and can easily wear out or crack when used in applications involving bending, such as touch screen and flexible displays [12]. These issues have stimulated numerous developments with aim at searching for alternative transparent electrode materials. Several types of new transparent electrode materials, including carbon nanotubes (CNTs) [13, 14], metal nanowires [15], metal grids [16], or other metal oxides [11], conducting polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [17, 18] and graphene films [6, 19, 20], have been explored as alternatives. Among them, graphene is particularly attractive given that it has been successfully synthesized on a large scale as a good conducting and transferable film by chemical vapor deposition (CVD) [21–24]. Moreover, graphene films have a higher transmittance over a wider wavelength range than single-walled carbon nanotube (SWNT) films [14, 25], thin metallic nanowire films [16], and ITO [10, 26] (Figure 10.1a). Many interesting works on graphene-based transparent, conducting electrodes have been reported, such as roll-to-roll production of 30-in. graphene films (Figure 10.1b) that are superior to common transparent Nanocarbons for Advanced Energy Conversion, First Edition. Edited by Xinliang Feng. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.

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Transmittance (%)

100 80

1st 30

2nd

Graphene ITO ZnO/Ag/ZnO

40 20

TiO2/Ag/TiO2 Arc discharge SWNTs

200 (a)

inc

h

60

400 600 Wavelength (nm)

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Figure 10.1 (a) Transmittance for different transparent conductors: graphene, SWNTs, ZnO/Ag/ZnO, TiO2 /Ag/TiO2 , and ITO. (Reproduced from Ref. [27] with permission from Nature Publishing Group.) (b) A transparent 35-in. flexible polyethylene terephthalate

(b) (PET) sheet-supported ultralarge-area graphene film, synthesized by chemical vapor deposition (CVD). (Reproduced from Ref. [28] with permission from Nature Publishing Group.)

electrodes and other alternatives in terms of resistance and transparency [28]. The advantageous properties of using graphene as a transparent electrode are essential when considering realistic applications. To this end, applications of graphene as electrodes in a wide range of devices include the photodetectors [29], field-effect transistors [30], touch screens [28], LCDs [31], organic light-emitting diodes (OLEDs)s [32, 33], solar cells [34–36], and so on. However, to ensure these potential applications, the electronic properties of graphene films require the substantial improvements through their synthesis [37–39], transfer [23, 40], doping [28, 30], and work function tuning [41, 42]. In this chapter, we will summarize the state-of-the-art preparation methods of graphene, their optoelectronic properties, and the fabrication of graphene-based transparent conductive films (TCFs). We will also provide an overview on the applications of graphene-based TCFs in the field of solar cells. It is important to note that, this field is moving rapidly and at best, this chapter will serve as a reasonable snapshot of graphene TCFs for solar cells.

10.2 Production of Graphene

Several approaches have been developed to prepare graphene, including micromechanical cleavage, epitaxial growth from silicon carbide (SiC) [43], liquid-phase exfoliation [44], CVD [28], electrochemical exfoliation [30, 45], reduction of graphene oxide, and so on. The extreme interests in graphene have made it possible to develop the preparation of graphene in near mass production.

10.2 Production of Graphene

10.2.1 Micromechanical Cleavage

The “micromechanical cleavage” also known as Scotch tape method developed by A. Geim and K. Novoselov, involves peeling off a piece of graphite by means of an adhesive tape [1]. The process has been optimized to produce single-layer graphene (SLG) with high structural quality and can be more than 100 μm2 in size [46]. In fact, this approach still provides the best graphene in terms of purity, defects, and electronic and optoelectronic properties [4, 47]. So far, an ultrahigh carrier mobility of 200 000 cm2 V−1 ⋅s for a single-layer-exfoliated graphene suspended between gold contacts has been reported [48]. Unfortunately, this protocol is impractical for large-scale applications and thus is limited to fundamental research only. 10.2.2 Liquid-Phase Exfoliation

Graphene flakes can be produced by ultrasonication of natural graphite in a range of organic solvents, such as N-methyl-pyrrolidone (NMP), N,Ndimethylformamide (DMF), and ortho-dichlorobenzene (o-DCB). Exfoliation of graphite layer occurs because of the strong interaction between graphitic basal planes and the solvent. The solvent that can minimize the interfacial tension (γ) between liquid and graphene flakes (i.e., the force that minimizes the area of the surfaces in contact) is considered to be ideal. Interfacial tension plays a crucial role when a solid is immersed in a liquid medium. Solvents with a surface tension of ∼40 mJ m−2 , such as NMP, DMF, and γ-butyrolactone (GBL), are the best medium for graphite exfoliation. Even though the concentration at which graphene can be dispersed has increased from ∼0.01 to 1.2 mg ml−1 in NMP with low power sonication, the average flake size decreases significantly and thereby increasing the overall interjunction resistance of thin films prepared from these dispersions [44, 49]. Unfortunately, majority of the organic solvents with γ ∼ 40 mJ m−2 , such as NMP (40 mJ m−2 , b.p. 203 ∘ C), DMF (37.1 mJ m−2 , b.p. 154 ∘ C), and o-DCB (37 mJ m−2 , 181 ∘ C), are toxic and have high boiling point, which limit their viability for processing, in particular for thin-film fabrications. Therefore, stable dispersions of graphene in low-boiling point solvents remain highly desirable for the scientific community. Several attempts of producing graphene by liquid-phase exfoliation in low-boiling point solvents have been reported. One approach is the solvothermalassisted exfoliation of expanded graphite in polar organic solvent, that is, acetonitrile [50]. It has been proposed that the dipole-induced interactions between graphene and acetonitrile facilitate the exfoliation and dispersion of graphene. The solvothermal-assisted exfoliation resulted in a yield of ∼10 wt% for graphene. Recently, dispersion of graphene in ethanol was reported by solvent exchange from NMP [51]. The exfoliated graphene in NMP was first filtered through a polytetrafluoroethylene (PTFE) membrane followed by redispersing

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the filtered cake into ethanol. After several times of centrifugation and washing steps, a stable dispersion of graphene in ethanol (0.04 mg ml−1 ) was obtained. However, the dispersion showed 20% sedimentation after 1 week. Water is a natural choice of solvents because of its nontoxicity, low boiling point, and so on. However, the exfoliation of graphene in water is challenging because of its high “γ” value (∼72 mJ m−2 ) and hydrophobic nature of graphene sheets. This can be overcome by using anionic surfactants, such as sodium dodecyl benzene sulfonate (SDBS) [52], sodium deoxycholate (SDC) [53], and sodium cholate (SC) [54, 55]. A concentration up to 0.3 mg ml−1 for exfoliated graphene in water was achieved by sonication of graphite into these surfactant solutions. Nevertheless, depending on the application purpose, the presence of surfactant on graphene might be a critical issue, for example, compromising or decreasing the conductivity of graphene. In addition to organic solvent and surfactant-based liquid-phase exfoliation, ionic liquids have emerged as promising solvents to aid sonication-based graphite exfoliation. Ionic liquids are salts in liquid state below 100 ∘ C and often have surface energies close to graphene. One of the first ionic liquid used for graphite exfoliation was 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, which led to 0.95 mg ml−1 stable dispersion of graphene nanosheets with only 1 h of sonication [56]. The majority of sheets were less than five atomic layers; however no rigorous analysis was carried out in the report. Recently, prolonged sonication (24 h) of graphite flakes in 1-hexyl-3-methylimidazolium hexafluorophosphate (HMIH) yielded a stable graphene dispersion with a concentration as high as 5.33 mg ml−1 and an average graphene thickness of 2 nm [57]. In other examples, graphene sheets have been successfully exfoliated with pyrene derivatives such as 1,3,6,8-pyrene-tetrasulfonic acid tetrasodium salt (Py–(SO3 )4 ) and amino methylpyrene (Py–Me–NH2 ), and the fabrication of TCFs was performed [58]. Besides direct sonication of graphite into solvents, other approaches, such as intercalation and expansion of graphite with highly volatile agents, followed by simultaneous exfoliation in organic solvent have been studied in detail [59, 60]. Nevertheless, the intercalation and exfoliation method also suffers from low yield of thin layers and small lateral sizes of graphene. 10.2.3 Chemical Vapor Deposition

The first attempt to produce graphene can be traced back to 1975 via the thermal decomposition of carbon on single-crystal platinum (Pt) surface. Unfortunately, the process was not studied extensively due to the lack of characterization and application for such kind of graphitic materials. Nevertheless, the formation of few-layered graphene on transition metal surfaces has been known for nearly 50 years [61]. Layers of graphite were first observed on Ni [61, 62] surfaces that were exposed to carbon sources in the form of hydrocarbons at high temperatures (i.e., 1000–1050 ∘ C). The interest in graphene has led to the revaluation of this

10.2 Production of Graphene

method for controllable growth of graphene layers. Graphene growth has been demonstrated on a variety of transition metals, such as Ru [63], Ir [64], Co [65], and Pt [66]. Recent results of growth on relatively inexpensive polycrystalline Ni [67, 68] and Cu [22, 69] substrates have triggered remarkable interests in optimizing CVD conditions for the large-area synthesis of high-quality graphene. Graphene deposited on polycrystalline Ni substrate exhibits a sheet resistance (Rs ) of ∼280 Ω sq−1 with ∼80% transmittance [21]. However, the fundamental limitation on utilizing Ni as the catalyst is that single- and few-layered graphene are obtained over few tens of microns region, rather than over the entire substrate [70]. The lack of control over the number of layers is partially attributed to the fact that the segregation of carbon from the metal carbide upon cooling occurs at different rate within the Ni grains and at the grain boundaries [71]. By contrast, uniform growth of high-quality SLG over large areas has been recently achieved on polycrystalline Cu foils. The CVD growth of graphene on Cu is generally attributed to the thermal decomposition of hydrocarbons on the surface and subsequent surface diffusion of carbon atoms due to the low solubility of carbon atoms (80%) and low sheet resistance (