1PhotovoltaicsPhotovoltaicsand Photodetectors and Photodetectors --part Ipart I@ MITMarch 6, 2003 – Organic Optoelectronics - Lecture 9•Photogeneration•Organic Heterojunction Photovoltaic Cell•Organic Multilayer PhotodetectorRecitation Handout: Yu et al., Science 270, 1789 (1995),Shaheen et al. , Appl. Phys. Lett. 78, 841 (2001).Anouncement: Lab #1 write-upis due in class on Tuesday, March 112Single Layer Organic PV CellsIncidentLightPTCDAInITOGlassVxyz 11.96 Å 17.34 Å3.21ÅPTCDAPTCDA3Photocurrent GenerationELIGHT ABSORPTIONEXCITON GENERATIONEXCITON DIFFUSIONEXCITON DISSOCIATIONCARRIER SEPARATIONCARRIER TRANSPORTDELIVERY TOEXTERNAL CIRCUIT42.0 2.5 3.010-310-210-1positive field (In +, ITO -)0.040.011.00 0.400.200.10Yield [%]Energy [eV]2.0 2.5 3.010-310-210-1negative field (In -, ITO +)0.040.01 0.00 1.00 0.40 0.10 Yield [%]Energy [eV]Photocurrent Dependence on Electric FieldV/µmV/µmDifferent photocurrent response for positive and negative applied biasIncidentLightPTCDAInITOGlassVV50V-V+VIncidentLight(a)(c)(b)ITO PTCDA In"NEAR" "FAR"Electric Field inside the Organic FilmPTCDA is assumed to be a depleted hole-transporting organic semiconductorPTCDA carrier density isn = 5 x 1014cm-3Æ band bending of 0.1 V at each interface would be sufficient to completely deplete 500 nm thick sample6Exciton Distribution0 100 200 300 400 5000123dxdnJdiff(E,z) = -D ∝ΙIncidentLightExciton Density,n(z)ITOInPTCDA2.10 2.10 eVeV2.23 2.23 eVeV2.60 2.60 eVeVDistance, z [nm]excitondiffusion currentphotocurrentDIFFUSION EQUATIONn =n =where, β = 1/LDLD= exciton diffusion length72.0 2.5 3.010-310-210-1positive field (In +, ITO -)0.040.011.00 0.400.200.10Yield [%]Energy [eV]2.0 2.5 3.010-310-210-1negative field (In -, ITO +)0.040.01 0.00 1.00 0.40 0.10 Yield [%]Energy [eV]Photocurrent Dependence on Electric FieldV/µmV/µmDifferent photocurrent response for positive and negative applied bias+V+V-V-V2.102.10eVeV2.232.23eVeV2.602.60eVeV2.102.10eVeV2.232.23eVeV2.602.60eVeV810-410-310-2(b)0.00 V/µm0.01 V/µm0.02 V/µm0.05 V/µm0.10 V/µm0.20 V/µm0.50 V/µm1µm PTCDAYield [%]Energy [eV]Wavelength [nm]800 700 600 500 4002.0 2.5 3.010-210-1100(a)0.33 V/µm0.67 V/µm0.17 V/µm0.07 V/µm0.03 V/µm0.00 V/µm3000 Å PTCDAYield [%]800 700 600 500 400Wavelength [nm]Energy [eV]2.0 2.5 3.0Photocurrent response changes with device thicknessFor the 1µm PTCDA device red light (2.1) eV has the lowest absorption constant hence 2.1 eV excitons can be generated next to the far, In interface(positive bias = far interface, In, active) +VIncidentLight2.102.10eVeV2.232.23eVeV2.602.60eVeV9ΙIn∝ ηc(V) α exp(-αL) / (1/LD-α)ΙIn∝ ηc(V) α exp(-αL) / (1/LD-α)ΙITO∝ ηc(V) [α / (α+1/LD)ΙITO∝ ηc(V) [α / (α+1/LD)Ι = ΙITO+ ΙInLIGHTABSORPTIONEXCITONGENERATIONEXCITONDIFFUSIONEXCITONDISSOCIATIONCARRIERSEPARATIONCARRIERTRANSPORTηdηcηgJdiffχASSUME ηg, ηd, χindependent of E-fieldΙ ∝ ηc(V) Jdiff(E,z)⇒at ITOITO interface (z = 0):at InIn interface (z = L):10Exciton Diffusion Length (for E = 1.99 - 2.10 eV exciton)200 400 600 800 10000.02.04.06.08.01.00 V/µm0.40 V/µm0.20 V/µm0.10 V/µm1 / [ηFARexp (αL)] (x 10-3)1 / α[nm]LD~ 225 nmLD~ 225 nm2.02.22.42.62.83.0Energy [eV]11Thin Film Excitation Fluorescence2.0 2.4 2.8 3.2 3.60102030650 nm PTCDAThin FilmIntegrated Fluorescence [a.u.]Excitation Energy [eV]CT [0-ST]CT [0-F]S1[0-0]S1[0-1]S1[0-2]S1[0-3]* Fluorescence energy and shape is not affected by the change in excitation energy* Fluorescence efficiency increases when exciting directly into CT stateS1S0CTT1S1S0CTT112200 400 600 800 10000.02.04.06.08.0LD~ 225 nm1.00 V/µm0.40 V/µm0.20 V/µm0.10 V/µm(a) 1.99 - 2.10 eV1 / [ηFARexp (αL)] (x 10-3)1 / α [nm]200 400 600 800 10000481.00 V/µm0.40 V/µm0.20 V/µm0.10 V/µm1/ηFAR(x 10-3) 1 / α [nm]20 30 400.51.01.52.0-LD= -78 nm-LD= -82 nm-LD= -92 nm-0.10 V/µm-0.40 V/µm-1.00 V/µm(b) 2.36 - 2.60 eV1 /ηNEAR(x 10-3)1 / α [nm]2.0 2.2 2.4 2.6 2.8 3.0Energy [eV]2.0 2.2 2.4 2.6 2.8 3.0Energy [eV]132.0 2.2 2.4 2.6 2.8 3.00V2µV4µV6µV8µV10µVα[cm-1]Photocurrent / PhotovoltageEnergy [eV]01x1052x1053x1054x1055x1050A5pA10pA15pA20pA25pAαPhotocurrentExciton Diffusion LengthsLD=225nmLD=85nmLD=80nm[S0 -S1][S0 -CT]14[S0 -S1]1.5 2.0 2.5 3.0 3.5AbsorptionSpectrumS1[0-3]S1[0-2]S1[0-1]CT [0-ST]S1[0-0]CT [0-F]Energy [eV][S0 -CT]LD=225nmLD=85nmLD=80nmExciton Diffusion LengthsS1S0CTT1S1S0CTT115E-Field Dependent Photocurrent Response-1.0-0.8-0.6-0.4-0.20.00.20.40.60.8η[%]-1 -0.1 -0.01 0.01 0.1 1Voltage [V]ExcitationWavelength 2.10 eVΙ ∝ Jdiffηc= Jdiffexp(-t/τrec)= Jdiffexp(-L2/Vµτrec)one parameter fit !16E-Field Dependent Photocurrent Response-1 -0.1 -0.01-1.0-0.8-0.6-0.4-0.20.00.20.40.60.8η[%]0.01 0.1 1Voltage [V]2.10 eV2.23 eV2.60 eV0.03 0.04 0.05 0.0601020τh[ns]1/Field [µm/V]from the FIT to PHOTOCURRENT RESPONSEµτrec= 10-8cm2/Vfrom the TOFmeasurementsµ = 0.03 cm2/Vsη ∝ Jdiffηc= Jdiffexp(-t/τrec)= Jdiffexp(-L2/Vµτrec)one parameter fit !17Organic Heterojunction PVsCBVBLDLDIncidentLightV18Organic Heterojunction PVsη = 1.80%ff = 0.35η = 1.80%ff = 0.35Current [mA/cm2] Voltage [V]-0.4 -0.2 0.0 0.2 0.4-3-2-10123CuPc (250Å)PTCDA (350Å)QuartzITOInForrest, et al., J. Appl. Phys. 66, 5908 (1989).η = 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