Barnes, T.

Blackburn, J.

Coutts, T.

Heben, M.

Morfa, A.

Tenent, R.

UNT Digital Library

OPTIMIZING CARBON NANOTUBE CONTACTS FOR USE IN ORGANIC PHOTOVOLTAICS Teresa Barnes, Jeff Blackburn, Robert Tenent, Anthony Morfa, Mike Heben, and Tim CouttsPVSC 20085/13/2008NREL/PR-520-43262Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San Diego, CaliforniaTransparent Contacts in PV• Thin Film Devices– CdTe (16.5%) – TC sees 600oC– CIGS (20%) – TC must be grown at room T• TCOs– ZnO:Al, SnO2 :F, In2 O3 :Sn, Cd2 SnO4• TCs must meet different requirements in each technology• Successful demonstration of SWNT electrodes in thin film deviceswww.nrel.gov/ncpv/thin_film/docs/wc4papernoufi__.doc13% w/CNT12% w/CNTCIGS - Contreras, J.Phys.Chem. C, 14045, 2007; CdTe - Barnes, APL, 243503, 2007Carbon Nanotube Basics• Many synthesis methods– CVD, Arc, HipCO, laser vaporization• Extensive post- processing• Impurities• Multi vs. Single Wall• Random chirality distribution– 1/3 metallic, 2/3 semiconductingArmchair (m)Zig-zagChiralMulti-wall tubes Single-wall bundle Baughman, Science 287 2002OPV Device Structures Bulk HeterojunctionsOPV Device Results:J. van de Lagemaat, T. M. Barnes, G. Rumbles et al., Applied Physics Letters 88 (23), 3 (2006).Short diffusions lengths would require impractically thin absorber layers in a “stacked” heterojunction (BHJ ~ 300nm thick)Mix P3HT and PCBM in a 1:1 ratio to create a “bulk” heterojunctionP3HT carries holes to the ITO side, PCBM transports electrons to AlITOEarly SWCNT Production at NREL• All “TCO” work so far uses laser tubes• 1nm < d < 1.4 nm; all SWCNT• PLV tubes are very long with low defect density (Raman D/G ~1/190)• Films formed by vacuum filtration/ membrane transferWu, Science. 305, 1274 (2004)NREL Pulsed Laser VaporizationScalable Production: Ultra-sonic sprayMetrics:  Transparency, Conductivity, StabilityParameter MatrixPurification:  metals, amorphous CSurfactant:  SWCNT dispersion, bundle sizeSonication:  SWCNT length, bundle size, defectsPost Processing:  surfactant removal, dopingSurface Functionalization:  wetting, drop formation1 x 4 inchInk Formulation for Ultrasonic Spray• Aqueous inks• Ink sonication• Tube purification/ starting material • Surfactant choice is key– Morphology– Conductivity– RemovalBest OK BadSprayed vs. Membrane Transferred• Large scale roughness in membrane films• Possible texturing from membrane• Wrinkling• Sprayed films are more uniform• ~70 Ω/sq, 80% T (400-1500 nm)• Smooth and featureless– Potential for fewer shorts in devices– Fewer apparent impuritiesCompare TCOs with SWCNTs• Transparent SWCNT– Spaghetti-like network– ~50 nm thick• Hole conducting• Functional in solar cells (w/ performance drop)• Need to improve Rs /T– Doping, separations,– improved deposition, purification processes3020100Seebeck V (µV)2.01.51.00.50.0Seebeck DT (K)Sample:  50 Ω/sq CNTTemp:  300.4 KSeebeck coeff.:   16.5  ± 1.4 µV/K “p-type”Conductivity in “Bulk” Networks• Mixture of metallic (1/3) and semiconducting (2/3) tubes• Tubes/bundles have high aspect ratio in “good” networks• Measured “p” is low, but networks behave like degenerate semi• Mobility of the network is very different from individual tubesμH, tube ~ 1000-10000 cm2/V-sμΗ, Network ~ 0.02-0.1cm2/V-s Junctions between tubes and bundles dominate conductivitySchottky barriers are thought to exist between M and S tubes and limit μSWCNT Network Conductivity• Networks are soaked in HNO3 after deposition– Network consolidation– Conductivity Improvement– Rs = 60 Ω/sq• Rinsing after acid– Promotes adhesion– Improves stability– Rs increases to 108 Ω/sq806040200Transmission (%)200015001000500W avelength (nm) rinsed as-dopedAbsorption peaks at 650 and 950 are due to fundamental excitonic transitions of SWCNTDevices on NREL SWCNT Networks• Ultrasonic spray deposition• Several ~ 3% devices • Thick active layers - spun at 200 rpm • Reducing electrode roughness is key• PEDOT can be eliminated40200Current Density (mA/cm2)-1.0 -0.5 0.0 0.5 1.0Voltage (V)ITO Referenceη = 3.5%Jsc = 11.09Voc = 570.6FF = 55%6040200Current Density (mA/cm2)-1.0 -0.5 0.0 0.5 1.0Voltage (V)SWCNTη = 3.5%Jsc = 10.65Voc = 571.8FF = 57.6% 151050-5Current Density (mA/cm2)-1.0 -0.5 0.0 0.5 1.0Voltage (V)NO PEDOTη = 1.05%Jsc = 5.4Voc = 491.1FF = 39.2%40302010IPCE800700600500400Wavelength (nm) CNT  ITO refConclusions• SWCNT networks are the best hole-conducting TC available – Enables new device structures using “p” TC• SWCNT is a good flexible TC• Optoelectronic performance is improving – SWCNT processing, deposition, doping, m/s ratio• Improve process compatibility for better device performance, new architectures (no PEDOT, invert)• Device performance likely to improve for sprayed or printed active layers• SWCNT networks could be a model for new TCs– If a completely black material works as a TC, then what else could we use??Acknowledgements• Jao van de Lagemaat, Nikos Kopidakis, Garry Rumbles, Glenn Teeter, Bobby To, Tim Gessert, Joel Duenow, Matt Reese, Xerxes Steirer, Matt White, and many others at NREL• This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337 with the National Renewable Energy Laboratory.• for materials and additional funding

Optimizing Carbon Nanotube Contacts For Use In Organic Photovoltaics

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Optimizing Carbon Nanotube Contacts For Use In Organic Photovoltaics

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