prajwal-graphene-proposal 26 may 2011

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MS Thesis Proposal

Fabrication and Characterization of Organic Solar Cell using Graphene as Transparent

Conducting Electrode.

Prajwal Thapa, 26 May 2011

Advisor: Dr. Venkat Bommisetty

Introduction:

Solar energy harvesting using organic photovoltaic (OPV) cells has been proposed as a means to

achieve lowcost energy due to their ease of manufacture, light weight, and compatibility with

flexible substrates. [1] A critical aspect of any OPV device is a transparent conductive electrode

through which light can couple with active material to create electricity. Recent studies indicate

that Graphene, a highly conductive and highly transparent form of carbon made up of atom-thick

sheets of carbon has high potential to fill this role. [1]

OPVs require the use of transparent conducting electrode materials in order to admit light into

the active region as well as to collect charge. The cost of transparent conductive films is thus a

critical component of the total OPV price. Indium tin oxide (ITO) is currently the de facto

standard transparent conductive oxide (TCO) used in OPV technology due its high optical

transparency, metallic conductivity, and high work function (discuss how these properties help

OPV). Moreover, transparent conductor (TC) layer in thin film solar cell modules has a

significant impact on the power conversion efficiency. Reflection, absorption, resistive losses and

lost active area either from the scribed interconnect region in monolithically integrated modules

or from the shadow losses of a metal grid in standard modules typically reduce the efficiency by

1025%.

Indium tin oxide (ITO) and fluorine tin oxide (FTO) have been widely used as window

electrodes in optoelectronic devices. Its success can be attributed to its ability to function as a

transparent conductor with high transparency and low resistance. Having both of these qualities

is highly desirable in the field of OPV. These metal oxides appear to be increasingly problematic

due to shortage of the element indium on earth and their instability in the presence of acid or


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base. Moreover, their susceptibility to ion diffusion into polymer layers, limited transparency in

the near infrared region and the current leakage of FTO devices caused by FTO structure defects

are other problems that surface in use of such electrodes.

Graphene is an allotrope of Carbon like Diamond, Graphite and Carbon nanotube. Tough carbon

as in coal is non-conductive; it does conduct when it is crystallized. Graphite, crystalline-form of

carbon is being used as electrode in zinc cells for decades. Allotropy is property of certain

substances to remain in multiple molecular forms. It is the structure bonding atoms which

decides what form and characteristics will an allotrope have.

Transparent, conductive, and ultrathin Graphene films could be alternative to the ubiquitously

employed metal oxides window electrodes for solid-state dye-sensitized solar cells. Compared to

indium tin oxide (ITO) electrodes, which have a typical sheet resistance of 560 /sq and 85% transmittance in the visible range (400900 nm) , the Chemical Vapor Deposition (CVD) -

grown Graphene electrodes have a higher/flatter transmittance in the visible to IR region and a re

more robust under bending. Recently, Li et al have reported that four layers of Graphene has the

sheet resistance of 350 /sq at about 90%; these were prepared on glass with the PMMA

transfer method after growing Graphene on Cu. Nevertheless, the lowest sheet resistance of the

currently available CVD Graphene electrodes is higher than that of ITO. [4]

The search for novel electrode materials with good stability, high transparency and excellent

conductivity is therefore a crucial goal for optoelectronics. Planar sheet of Graphene absorbs

wavelengths between 200 to 900 nm. Graphene, exhibits remarkable electronic properties that

qualify it for applications in future optoelectronic devices. Recently, transparent and conductive

Graphene based composites have been prepared by incorporation of Graphene sheets into

polystyrene or silica.

Problem Identification:

One of the major challenges in synthesizing large area uniform Graphene layer with high

electrical conductivity and desired optical transparency is to develop is to develop an efficient


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reduction technique. The chemical reduction of exfoliated graphite oxide may be a candidate for

large-scale preparation of graphene. However, it might be difficult to fully recover the electrical

properties of Graphene from oxidized Graphene sheets. This is possibly due to a combination of

factors, such as damage of the Graphene lattice during the formation of graphite oxide into a

colloidal suspension (particularly the use of ultrasonication to disperse the graphite oxide) or the

creation of point defects during the reduction of graphite oxide. [10].

Objectives:

My objective is to synthesize Graphene films with high chemical and thermal stabilities as well

as an ultra-smooth surface with tunable wettability. My current research is focused on

synthesizing Graphene oxide using modified Hummers method. Hydrogen plasma treatment is

used as a novel reduction technique as efficient reduction is the key to achieving highly

conductive films, while the ability to controllably deposit films with nanometer thickness allows

high optical transparencies. [9] The obtained Graphene films with a thickness of ca. 10 nm will

exhibit a high conductivity of 550 S/cm and a transparency of more than 70% over 1000-3000

nm. The method may provide a pathway to controlled scalable production of Graphene and thus

large-scale fabrication of Graphene devices.

Methodology (Work Plan):

These Graphene films are fabricated from exfoliated graphite oxide, followed by a reduction

process. Graphite oxide is prepared via Hummers method which is then dispersed in water,

deposited onto a Si/SiO2 substrate, and subsequently reduced by hydrogen plasma treatment.

Graphite flakes are oxidized by a mixture of H2SO4 and HNO3 in the presence of KClO3. The

GO is dispersed in 4L of deionized water and filtered. It is then redispersed and washed with a

5% HCl solution and repeated until the pH of the filtrate becomes neutral. This forms GO slurry

that is dried in a vacuum oven at 80

o

C. The Modified Hummers Method is a similar process, butwithout HNO3 and uses KMnO4 as an oxidizer. [8]

Reduction of Graphene Oxide(GO) is then performed using hydrogen plasma.


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Using an atomic force microscope (AFM), the spring constant of suspended raphene sheets is

measured. Graphene sheets- held together by van der Waals forces- are suspended over SiO2

cavities where an AFM tip is probed to test its properties.

Expected Results:

The unusual electronic band structure of Graphene allows it to exhibit a strong ambipolar electric

field effect with high mobility. Each Graphene layer has a very low opacity of 2.3%. These

properties lead to the possibility of its application in high-performance transparent conducting

films (TCFs). [4] The synthesized Graphene will offers two potential advantages over

conventional photovoltaic electrode materials; work function matching and increased power

conversion efficiency due to transparency. These findings indicate that flexible, light-weight all

carbon solar cells can be constructed using Graphene. [6]

Cross section of a monolithically

integrated thin film module [2]

Morphology of GO films. (A) SEM image of

exfoliated graphite oxide (GO). (B) SEM

image of GO film prepared from dip

coating. (C) AFM height image (D) AFM

phase image of the obtained GO film [3]


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References:

[1] Peumans, P.; Yakimov, A.; Forrest, S. R. Small Molecular Weight Organic Thin-Film Photodetectors

and Solar Cells. J. Appl. Phys. 2003, 93, 36933723.

[2]Transparent electrode requirements for thin film solar cell modules Michael W. Rowell and Michael

D. McGehee* Received 20th August 2010, Accepted 20th October 2010 DOI: 10.1039/c0ee00373e

[3] Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells Xuan Wang, Linjie

Zhi,* and Klaus Mu2llen Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128Mainz, Germany

[4] Enhancing the conductivity of transparent graphene films via doping Ki Kang Kim et al Received 21

January 2010, in final form 7 April 2010 Published 28 June 2010

[5] Chemical route to the formation of graphene Farman Ali et al Tata Institute of Fundamental Research,

Homi Bhabha Road, Colaba, Mumbai 400 005, India

[6] Single-layer graphene cathodes for organic photovoltaics, Marshall Cox, Alon Gorodetsky, Bumjung

Kim, Keun Soo Kim, Zhang Jia, Philip Kim, Colin Nuckolls and Ioannis Kymissis

[7] Ibarra L, Jorda C. Effect of a diazide as adhesive agent in elastomeric matrix-short polyamide fiberscomposite. J Appl Polym Sci 1993; 48(3):37581.

[8] Bulk Synthesis and Characterization of High-Purity Graphene Patricia Johnson, Rose Pesce-

Rodriguez, Mark H. Griep, Kris Behler, Govind Mallick, Wendy Sarney and Shashi P. Karna, US Army

Research Laboratory

[9] Chemically Derived Graphene Oxide : Towards Large-Area Thin-Film Electronics and Optoelectronis

Goki Eda Manish Chowalla

[10] Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes

Xuesong Li et al, Department of Mechanical Engineering and the Texas Materials Institute, The

University of Texas at Austin

prajwal-graphene-proposal 26 may 2011

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