1PhotovoltaicsPhotovoltaicsand Photodetectors and Photodetectors --part IIpart II@ MITMarch 11, 2003 – Organic Optoelectronics - Lecture 10• Organic Heterojunction Photovoltaic Cell• Organic Multilayer PhotodetectorData on Solar Cells and Photodetectors taken fromPeumans, Bulovic, and Forrest., Appl. Phys. Lett. 76, 2650 (2000) – solar cell Appl. Phys. Lett. 76, 3855 (2000) - photodetector2Organic Heterojunction PVsCBVBLDLDIncidentLightV3PhotovoltaicsPhotovoltaicsPhotodetectorsPhotodetectorsOptical power ⇒ electrical powerFIGURES OF MERRIT:Power conversion efficiencyFull solar intensitiesReliabilityConsume power to detect a signalFIGURES OF MERRIT:High external quantum efficiencyHigh bandwidthLow noise, low power consumption4•high absorbtion in the visible spectrum•have relaxed deposition requirements•can be manufactured in a low cost process (roll-to-roll, web-processing, etc.)•can be grown on thin, flexible substrates → light weight•can add value to existing products (window coatings, etc.)Solid state organic solar cellsCHALLENGE!Current power conversion efficiencies are too low for commercial implementation (especially at full solar intensities)505101520LaboratoryProductionCrystalline-SiPoly-SiAmorphous-SiOrganic[ % ]Solar Cell Power Efficiency6VACUUMCHAMBERTURBOPUMPCOLDTRAPROUGHINGPUMPsubstrateholderthicknessmonitorshutterGNDsubstratePOWERSUPPLIESsourceboatsDevice Preparation and Growth• Glass substrates precoated with ITO- 94% transparent-15 Ω/square• PrecleaningTergitol, TCEAcetone, 2-Propanol• Growth-5 x 10-7Torr-Room T- 20 to 2000 Ålayer thickness7History and Progress ‘86: Heterojunction Solar Cell• first heterojunction for efficient charge generation •~0.95% conversion efficiency• nearly ideal IVs (FF~0.65)• under full solar illumination(1 sun)‘90s: Polymer NetworksG. Yu et al., Science 270, 1789 (1995).• series resistance problem (low FF)•calculatedpower conversion efficiency of ~1.5%• not well matched to solar spectrumShaheen et al. , Appl. Phys. Lett. 78, 841 (2001).• power conversion efficiency ~2.5%• not well matched to solar spectrum• long term stability ?C.W. Tangη = 0.95%ff = 0.65η = 0.95%ff = 0.65-0.4 -0.2 0.0 0.2 0.4-3-2-10123Voltage [V]Current [mA/cm2] ISC= 2.3mA/cm 2VOC= 450mVTang, Appl Phys Lett. 48, 183 (1986).CuPc (300Å)PTCBI (500Å)GlassITOAg8LUMOHOMOD: CuPcA: PTCBI2344Photoinduced Charge-Transfer1231Exciton generation by absorption of light4Exciton diffusion over ~LDExciton dissociation by rapid and efficient charge transferCharge extraction by the internal electric fieldProcesses occuring at a Donor-Acceptor heterojunction123449Exciton Diffusion: Experiment and Theory0 40 80 120 1600.00.20.40.60.81.0experimenttheoryPL efficiency ratio ηLPTCBI[Angstrom]CuPcPTCBIPTCBI540nmPL1PL2()()121exp 211exp 2DDDtLPL LPL ttLη⎡⎤−−⋅⎢⎥⎣⎦==−⋅⎡⎤+−⋅⎢⎥⎣⎦•Photoluminescence (PL) probes the exciton lifetime•Exciton lifetime depends on proximity of donor-acceptor interfaceLD=(30±3)Å10Double HeterojunctionAgProblemSolution•cathode metal diffusion•deposition damage•exciton-plasmon interaction•vanishing optical field•electrical shortsIntroduce ‘Exciton Blocking Layer’ (EBL) to:•confine excitons to active region•act as a damage-absorberAgITOITOEBL~200Å11ITOCuPcPTCBIBCPAgglass substrateExciton Blocking Layer (EBL)0.70.90.85 eVCuPcEG=1.7PTCBIEG=1.7BCPEG=3.5ITOAgHOMOLUMOEnergy Band Diagramcourtesy of I. Hill & A. Kahn• conducts electrons• transparent• effectively blocks excitons• absorbs damage• separates active layers from metalBCP(2, 9-dimethyl, 4, 7-diphenyl,1, 10-phenanthroline)(aka bathocuproine)12051015202530CuPc and PTCBI Layer Thickness [Å]Quantum Efficiency, η [%]100 400200 800ηINTERNALηEXTERNALw/ EBLno EBLExciton Blocking Layer (EBL)Improves Thin Cell Efficiency>2.5-fold improvementin efficiencyPrevious best devices (Tang, ‘86)13Ag mirror / cathodeorganics + ITO (anode)molded substrate with collector profiles and coated with AgSolarIlluminationCROSS SECTION3-D VIEWPractical Realization:MicroMolded Winston Collectors14400 600 800 10000102030λ [nm]024Photon Flux [1018s-1m-2nm-1]13Quantum Efficiency, η [%]ηINTERNALAM1.5ηEXTERNALSolar SpectrumBroad Spectral ResponseMatches Solar Spectrumlight trapping90Å CuPc/ 90Å PTCBI15ISCVOC96062020064192-0.4 -0.2 0.0 0.2 0.4 0.6-80.0-60.0-40.0-20.00.020.0Voltage [V]Current [mA/cm2]1300 mW/cm2= 17 suns (AM1.5)VMAXIMAXI-V Response under Solar Illumination•Highest illumination levels to date•Record-high short circuit currents90Å CuPc/ 90Å PTCBI16ISC[mA/cm2]1101000.40.50.6VOC[V]10 100 10000.60.81.01.2Optical power [# suns] (AM1.5 spectrum)Optical power [mW/cm2] (AM1.5 spectrum)ηP[%]0.30.40.50.6FF0.1 1 10I-V Response under Varying Solar Illumination Intensitylinearmonotonic increasebroad plateauno drastic decrease171 10 100 10000.00.51.01.52.02.53.0no light trappingw/ light trappingOptical power [mW/cm2]Power Conversion Efficiency [%](2.4 + 0.3)%Ag apertureGlass SubstrateITO / Organic LayersAg cathode~10 suns Light Trapping Improves PV Efficiency ~2.5 Foldno light trappinglight trapping configuration(1.0 + 0.1)%60Å CuPc/ 60Å PTCBI18REFLECTING CATHODECOLLECTORMIRRORREFLECTIVEUNDERCOATTHIN PVGLASS SUBSTRATEPractical Realization:MicroMolded Winston Collectors•Raytracing calculations•Microcavity effects ignored•1D geometry•Mirror/cathode reflectivity: 95%•Cell absorption: 30%•ηEXT= 90% of ηINT•ηP= 2.7% (ideal = 3.0%)•Local intensity never exceeds 3 sunsPeumans et al, US Patents #6440, 769, Aug 27, 200219HEATPRESSUREMOLDSUPPORTPOLYMERSHAPEDPOLYMERIC FILMCollector Fabrication100-200µm100-200µmPeumans et al, US Patents #6440, 769, Aug 27, 200220METAL COATINGANTI-REFLECTION COATINGDEVICE DEPOSITIONTRANSPARENT POLYMERIC FILLERITO ANODEORGANICS: CuPc/PTCBI/BCPAg CATHODE200-400µmENCAPSULATING FILMCollector Fabrication2110 100 10001100CuPc and PTCBI Layer Thickness [Å]Internal Quantum Efficiency, η [%]30603006003060LD=100Å40Å20ÅηP=2.4%SingleHeterojunctionηP~1.0%1036improved materialgrowththinnercellsOutlook – towards future PVsPeumans et al., APL 76 (19), p. 2650 (2000).22Multilayer Organic PVIncidentLightV+V-Bulović and Forrest, patents: US 6,198,091 (2001); US 6,198,092 (2001); US 6,278,055 (2001) .THIN sub-PVs connected in PARALLEL• increased likelihood of exciton dissociation• decreased R• ISCof all sub-PVs add while VOC~constant23Adapted from A. Yakimov and