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CARBON NANOTUBE TRANSPARENT ELECTRODES: A CASE FORPHOTOVOLTAICSPaul Glatkowski.' Evgeniya Turevskaya.' David Britz," David Rich,1 Matt DiCologero,1 Timothy Kelllher.' John Sennott,'David Landis,' Robert Braden,' Patrick Mack,1Joseph Piche11EikosInc, 2 Master Drive, Franklin, MA 02038

ABSTRACT

INTRODUCTION

Because of their spz hybridization, carbon nanotubes areoptically and electrically stable. Additionally, the covalentbonding of SWNTs eliminates electromigration. Becauseof their conductance and carrier mobil ity, SWNTs can bespread thinly over a surface to create a virtuallytransparent, highly conductive film. Carbon nanotubeelectrodes are unlike doped wide band gap TCOs (e.g.ITO and AlO) used in solar cells. The transparency of

SWNTs (Fig. 2) are one dimensional conductors andnarrow bandgap semiconductors with a diameter of about1 nanometer and a length of several micrometers. Carbonnanotubes are made of hexagonally bonded carbon, likeplanar graphite. Physically, carbon nanotubes can beviewed as rolled sheets of graphite with a hollow core.SWNTs exhibit exceptional electronic conductionproperties, including electron and hole mobil ities of ca.80,000 cmzNs, substantially higher than all other knownsemiconductors.[9] Long metallic nanotubes have beenfound to have volume conductivities of ca. 700,000 Stemwhich is almost as conductive as pure copper.

We have integrated carbon nanotube coatings into a widerange of devices, including anti-static protection, touchscreens, e-paper, photovoltaics, and EMI shielding.Fabricated photovoltaic devices with SWNT TCs haveincluded organic photovoltaics, CulnzSeJ!CdS (CIGS), andCdTetCdS devices. [6, 7, 8] This early work hasdemonstrated that SWNT TCs are capable of producinghigh-efficiency devices at the laboratory scale. Furtherimprovements are made by incorporating a refractiveindex-tuned binder to reduce surface reflections.

Fig. 2. Artist's rendition of a SWNT on a surface. Inset:TEM of bundles of carbon nanotubes.

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A wide range of thin film photovoltaics (PVs) usetransparent conductors (TCs). ZnO:AI, Sn02:F, andIn203:Sn (ITO) are the most commonly used TCs in PVcells. However, these materials are not ideal for all solarapplications. As an alternative, single walled carbonnanotube (SWNT) coatings can be used as a TC. TheSWCNT thin films are essentially hole conducting andhighly transparent, which is a desirable and previouslyunobtainable combination of properties for use inphotovoltaics. SWNT coatings are solution depositedusing standard wet coating processes. Optically, nanotubecoatings exhibit high transparency, especially in the nearinfrared. Additionally we form multifunctional compositesby infiltrating the CNT network with materials suited for awide range applications and device structures.

Single walled carbon nanotube (SWNT) networks havebeen used successfully in several devices as areplacement for more traditional vacuum depositedtransparent conductive oxides (TCOs) (see Fig. 1).[1-4]These groups have been motivated to replace transparentconductors both because of the difficulty and cost ofmanufacturing high quality TCOs and because carbonnanotube networks are expected to be both more versatileand more electronically compatible than TCOs. As anexample, SWNT coatings have been found to be apreferentially hole-conducting contact, whereas mostTCOs are electron conducting contacts.[5]

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Sheet Resistancen 'sqFig. 1. Applications for SWNTTCs based on conductivity.

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...................... (3) CureI Substrate I E SubstrateFig. 3. Schematic diagram of the solution coating processto make lnvlsicon" on solar cells.Binders impart adhesion and abrasion resistance foraircraft canopies, heat and humidity stability for touchpanels, or UV resistance for solar cells. These binders areeither organic or inorganic, depending on accessibleprocessing conditions and application needs. Bothnanotube inks and binders can be printed roll-to-roll onplastic sheets or conformally on complex surfaces usinggravure, inkjet printing, or spray coating.

TCOs is fundamentally limited by band-edge absorption oflight in the UV and free carrier plasma absorption in theNIR. Because a SWNT TC is a nanoscopically thinconductive grid, the large majority of light penetratesthrough the pores and through the thickness of the film,maintaining conductivityand uniformsurface appearance.Eikos has branded our SWNT TC as Invisicon".lnvlsicon" is a composite coating made using a two-stepcoating process, followed by curing (see Fig. 3). In the firststep, an ink containing only carbon nanotubes, water, andalcohol is printed onto a substrate. Once the solvent dries,a uniform film of nanotubes with 50% void space is left. Inthe second step, a solution of a polymer or metal oxideprecursor is deposited into the SWNT film. The binderpenetrates through the pores of the SWNT layer down tothe substrate, providing adhesion, as well as changing theoptical characteristics of composite. The curing step isused to densify and cross-link the binder. The conditionsof curing are dictated by the binder and underlyingmaterials.

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After an extensive literature survey, we believe that SWNTelectrodes demonstrate the best figures of merit of anyknown p-type TC in terms of transparency, conductivity,and environmental stability, making it an ideal electrodefor many types of PVs (Fig. 5). Carbon nanotubeelectrodesexhibit a volume conductivity of ca. 5,000 S/cm,>90% transparency, are deposited at less than 150C,and are thermally stable to at least 550 C. p-Typetransparent conductive oxides were first reported in 1997,when a Japanese group found that CuAI02 selectivelyconducted holes with a volume conductivity of 0.01 S/cmat room temperature.[10] The field has continued toprogress, achieving conductivities as high as 220 S/cm inCuCrMg02 films, though at the expense oftransparency.[11] Many p-type TCOs that exceed 10S/cm have visible transmission less than 50%, makingthem unsuitable for solar applications. Additionally, manyp-type TCs are deposited at high temperatures and reactwith moisture and air, making them unsuitable forprocessing and long product life.

conductivity that is appropriate for the specific deviceapplication.

Fig. 4. UV-Vis-NIR transparency of SWNT films, ITO, andp-typeCuAI02.

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This process has several general advantages over ITO fordesigning an application-tailored TC. Because the SWNTand binder layers are deposited separately, the twosystems can be engineered independently to have optimalelectronic and optical properties. The process is lowtemperature and atmospheric, which reduces cost ofprocessing and the potential for device damage. Becausethe layers are printed sequentially, in-line testing andrefinishing of coatings is possible to achieve highermanufacturing yields. As a result of the depositionprocess, lnvisicon is a particularly versatile transparentelectrode.OPTOELECTRONIC PROPERTIES: SWNTS

A unique advantage of the lnvisicon" SWNT coating is itsability to tailor sheet resistance (Rs) over a broad rangefrom 1 010 to 100 MOlD. SWNT films have a flattransmittance profile, compared to PEDOT:PSS, ZnO:AI,FTO and ITO, giving the film more uniform coloration (Fig.4). This ability also allows the device designer to choose a

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Fig. 5. Comparison of volume conductivities of holeconducting electrodes.Based on the work function of carbon nanotubes (ca. 4.8eV) and their selective hole transport, we believe that

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nanotube electrodes could offer performanceimprovements in devices, such as amorphous silicon PVs,organic photovoltaics, and as a back contact or atransparent back contact for CdTe or wide bandgap CISdevices.OPTICAL PROPERTIES: BINDERS

Nanotube coatings are expected to be excellent lighttrapping TCs because they are inherently rough,compared to sputter deposited TCOs. AFM images (Fig.6, left) of CNT layers show an RMS roughness typically of-50 nm, with high feature spacing of approximately 500nm. The lnvisicon roughness and feature spacing isconsidered ideal for light trapping TCOs.

Fig. 6. AFM image of Invisicon nanotube film (left)with nobinder and (right) with AR binder.Fig 6 shows typical atomic force microscope images of anlnvisicon" layer with and without binder. The left imageshows a wispy network of nanotubes with a significantfraction of void space. The right image shows a nanotubenetwork with a solution-deposited anti-reflective binder.The binder adheres to the substrate and protects carbonnanotube films from abrasion, but is thin enough to allowsurface conductivity. The CNT-binder composite inheritsthe optical properties of the binder, reducing surfacereflections down to 0.1% when using appropriate refractiveindex binder. The binder coating does not appreciablyaffect the CNT surface roughness (Fig. 6, right). UnlikeITO, CNT-binder composites can have their thicknesscontinuously tuned to reach quarterwave thickness.

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Fig. 7. Transmission and reflection spectra for AR coatingsdesigned for PET and Si wafers.By tuning the composition, density, thickness, androughness of the binder materials, we also can obtain

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effective anti-reflective (AR) coatings on varied substrates,including silicon wafers and polyethylene terephthalate(PET) (Fig. 7).To create an AR coating for PET (RI = 1.48), we targetedsolution processed materials with refractive indices -1 .24.We infiltrated this material into the carbon nanotubenetwork on commercial PET and found that it produced asingle layer AR coating with 0.45% single side reflection atpeak. When both sides of a PET film were coated, weobserved peak transmission of 99.1% at 710 nm. TypicalPET is approximately 88-90% transparent in the visibleregion. Such coatings also are applicable to glass andgive similar results in terms of maximum transparency andreflection.A different material was used as an AR coating for asilicon wafer. Silicon wafer surface reflection varies fromas much as 75% in the UV to 33% in the NIR. By solutiondepositing a single layer coating onto silicon, we found areduction in surface reflection by as much as 62%. For anon-optimized coating, minimum reflection was found tobe 1% at 332 nm. By carefully controlling thickness and byadding layers of other materials, the window of lowreflection can be extended significantly.

CONCLUSIONSSingle walled carbon nanotube electrodes demonstratesignificant benefits for usage in photovoltaics. Nanotubesform the highest performing hole conducting electrodecurrently known, making them suitable as a direct contactto p-type semiconductors. Additionally, nanotubeconductivity is tunable to suit the specific devicerequirements. Bindershave been shown to reduce surfacereflections to less than 1%, which significantly aids in lightcapture for photovoltaics and other devices.

ACKNOWLEDGEMENTSThis work was supported by the U.S. Department ofEnergy under Contract No. DE-FG36-08G088005. Theauthorswould like to acknowledge helpful discussions anddata collection with our collaborators at the NationalRenewable Energy Lab, especially Drs. Teresa Barnesand Tim Coutts.

REFERENCES[1] Z. Wu, C. Zhihong, D. Xu, J. M. Logan, J. Sippel, M.Nikolou, K. Kamaras, J. R. Reynolds, D. B. Tanner, A. F.Hebard, and A. G. Rinzler, "Transparent, ConductiveCarbon Nanotube Films," Science, 305, 2004, 1273-1276.[2] G. Fanchini, H. E. Unalan, and M. Chhowalla,"Optoelectronic Properties of Transparent and ConductingSingle-Wall Carbon Nanotube Thin Films," AppliedPhysics Letters, 88, 2006, 191919.[3] H. Haiping, Z. Fei, Y. Zhizhen, Z. Liping, Z. Binghui,and H. Jingyun, "Defect-Related Vibrational andPhotoluminescence Spectroscopy of a CodopedZno:AI: N

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Film," Journal of Physics D (Applied Physics), 39, 2006,2339-2342.[4] G. Gruner, "Carbon Nanotube Films for Transparentand Plastic Electronics," Journal of Materials Chemistry,16, 2006,3533-3539.[5] K. Lee, Z. Wu, Z. Chen, F. Ren, S. J. Pearton, and A.G. Rinzler, "Single Wall Carbon Nanotubes for p-TypeOhmic Contacts to Gan Light-Emitting Diodes," NanoLetters, 4, 2004, 911-914.[6] J. van de Lagemaat, T. M. Barnes, G. Rumbles, S. E.Shaheen, T. J. Coutts, C. Weeks, I. Levitsky, J. Peltola,and P. Glatkowski, "Organic Solar Cells with CarbonNanotubes Replacing In203 : Sn as the TransparentElectrode,"Applied Physics Letters, 88, 2006, 233503.[7] M. A. Contreras, T. Barnes, J. vandeLagemaat, G.Rumbles, T. J. Coutts, C. Weeks, P. Glatkowski, I.Levitsky, J. Peltola, and D. A. Britz, "Replacement ofTransparent Conductive Oxides by Single-Wall CarbonNanotubes in Cu(in,Ga)Se2-Based Solar Cells," Journal ofPhysical Chemistry C, 111, 2007,14045-14048.[8] T. M. Barnes, X. Wu, J. Zhou, A. Duda, J. van deLagemaat, T. J. Coutts, C. L. Weeks, D. A. Britz, and P.Glatkowski, "Single-Wall Carbon Nanotube Networks as aTransparent Back Contact in CdTe Solar Cells," AppliedPhysics Letters, 90, 2007,243503.[9] T. Durkop, S. A. Getty, E. Cobas, and M. S. Fuhrer, "Extraordinary Mobility in Semiconducting CarbonNanotubes,"Nano Letters, 4, 2004,35-39.[10] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H.Yanagi, H. Hosono, " P-type electrical conduction intransparent thin films of CuAI02," Nature, 389, 1997,939.[11] R. Nagarajana, N. Duana, M. K. Jayarajb, J. Lia, K.A. Vanajaa, A. Yokochia, A. Draesekeb, J. Tateb and A.W. Sleight, " p-Type conductivity in the delafossitestructure," International Journal of Inorganic Materials, 3,2001,265.

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