Transport and Recombination in Polymer:Fullerene Solar Cells

Transcription

1 Max Planck Institut für Polymer Forschung, Mainz Transport and Recombination in Polymer:Fullerene Solar Cells Paul Blom

2 Outline 1. Charge transport in Organic Semiconductors -Hole Transport, Electron Transport < 2 2. Photocurrent Generation in Organic Solar Cells -Space Charge, Recombination 3. Recombination in organic solar cells -Bimolecular Recombination, Trap-assisted`Recombination 4. Origin of the Recombination in Organic Solar Cells -CT electroluminescence, ideality factor, Exciton Diffusion

3 J (A/m 2 ) Current-Voltage characteristic of a PPV Hole-Only Device < J V 9 8 L 2 3 APL 68, 3308 (1996) PPV m 0.3 m Vbias (V) 0.7 m cm 2 Vs Au ITO Hole Current is Space Charge (Bulk) Limited!!

4 SCLC: PLED acts as a Capacitor < 4 V=0 V>> Au Au + PPV V> PPV ITO ITO Au + + PPV + + J=charge velocity CV V/L ITO Charge density and Electric field ~ V

5 Mobility is Density Dependent! < 5 Phys. Rev. Lett. 91, (2003) 10-7 OC 1 C 10 -PPV h, FET (m 2 /Vs) T=295 K LED FET p (m -3 )

6 Effect of Carrier Density? < 6 Low carrier density Higher carrier density 0 Transport level 0 Equilibrium level E f E f ln 1 T 2 ln 1 T

7 Theoretical model for µ(p,t,e) developed < 7 Phys. Rev. Lett. 94, (2005) J (A/m 2 ) T=298 K T=272 K T=252 K T=233 K V (V)

8 Electron Transport in PPV < 8 APL 68, 3308 (1996) Holes 0.34 um 0.3 um Ca Ca Electrons 0.22 um 0.37 um Low Electron Current, Steep J-V: Traps?

9 10 1 Gaussian LUMO and Gaussian Traps? < 9 Current Density (A/m²) OC 1 C 10 -PPV L = 300 nm 295 K 273 K 251 K 230 K 211 K (a) Trap-limited Electron currents in PPV derivatives also described by Gaussian trap distribution Current Density (A/m²) MEH-PPV L = 270 nm 295 K 275 K 255 K 235 K 215 K 195 K (b) N t ~ cm -3 σ t = 0.10 ev Current Density (A/m²) NRS-PPV L = 320 nm 290 K 275 K 255 K 235 K 215 K V-V bi (V) (c) E t ~ ev Phys. Rev. B 83, (2011)

10 Slope vs LUMO position < NRS-PPV OC 1 C 10 -PPV 5 P3HT PF1CVTP Slope 4 3 F8BT PCNEPV PF10TBT PCPDTBT 2 1 Trap-free PCBM P(NDI2OD-T2) LUMO (ev) Explained by change of Gaussian Trap Depth!!!

11 < 11 Electron Trapping in OLEDs: One kind of trap responsible for trapping in all OLEDs!!! NRS-PPV OC 1 C 10 -PPV P3HT N t ~ cm -3 σ t = 0.10 ev F8BT PF10TBT PCPDTBT LUMO -3.8 Trap Nature Materials 11, p.882 (2012) Electron current can be predicted when LUMO is known

12 Origin of Trap? < 12 C. Campbell, C. Risko, J. L. Brédas, Georgia Tech Photo-oxidation? Trap-depth ev Water-polymer complexes? Hydrated-oxygen complexes O 2 (H2O) 2 Potential Deep Trap Peter Ho et al., Adv. Mat. 21, 4747 (2009)

13 Outline 1. Charge transport in Organic Semiconductors -Hole Transport, Electron Transport < Photocurrent Generation in Organic Solar Cells -Space Charge, Recombination 3. Recombination in organic solar cells -Bimolecular Recombination, Trap-assisted`Recombination 4. Origin of the Recombination in Organic Solar Cells -CT electroluminescence, ideality factor, Eciton Diffusion

14 Energy Photocurrent in a semiconductor: < 14 Assumptions: uniform generation of e-h pairs throughout the volume of the active layer non injection contacts for both electrons and holes one dimensional case diffusion ignored Goodman and Rose: J. Appl. Phys. 42, 1971, 2823 Before light excitation L E C E V field: E=V/L mean carrier drift lengths: w n = n t n E w p = p t p E x

15 Built-in Voltage: < 15 BHJ Solar Cell: V bi V=0 Goodman and Rose: V=0 LiF PEDOT V=V bi LiF V bi PEDOT V eff =V-V bi

16 Small applied voltage: < 16 main carrier drift length w n = n t n E and w p = p t p E <<L, E=V/L=constant. After light excitation L hn _ J + V=V OC -V bias V J-V characteristic (Ohmic regime): eg V J t t L n n p p V

17 Intermediate voltage: < 17 L 1 n t n > p t p, w n >> L, w p < L hn + _ V=V OC -bias J V 1/2 V V Recombination (t Limited regime: J eg 1/ 2 1/ 2 1/ 2 t V ; J GV n n

18 High Voltage Regime: < 18 hn L saturation regime: w n > L, w p > L, E=V/L=constant equal electron and hole current. + _ V=V OC -bias J V V 1/2 Constant V J-V characteristic is: J egl

19 Space-Charge Limited Photocurrent: < 19 hn L 1 n t n > p t p, p <<, t p >> j ph1 1 j 1 SCLC 9 h 8 V d _ V=V OC -bias J V 1/2 Maximum electrostatically allowed current: SCL Photocurrent: J 9 p q 8q 1/ 4 G 3/ 4 V 1/ 2 ; J G 3/ 4 V 1/ 2 V

20 Photocurrent in a Polymer:Fullerene Solar Cell < 20 Light Anode ITO/PEDOT 5.2 ev LUMO PPV HOMO PPV Donor Hole transport Exciton diffusion Charge transfer CT-state Acceptor LUMO PCBM Electron transport HOMO PCBM Cathode LiF/Al 3.8 ev G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, CT-state: If r 0 =1 nm and r =3, then binding energy is 0.5 ev!!

21 Apply GR Model to BHJ Solar Cell: < ev LUMO PPV 3.7 ev LUMO C 60 LiF/Al j M 3.0 ev PEDOT:PSS j M 5.2 ev 5.1 ev 6.1 ev HOMO PPV HOMO C 60 Effective Medium: LUMO=LUMO PCBM HOMO=HOMO PPV

22 Photocurrent in PPV:PCBM (1:4 wt.%) solar cells < J D J [A/m 2 ] J L J ph =J L -J D V [V] V OC V 0 J ph =J L -J D [A/m 2 ] 10 J V diffusion drift V 0 -V [V] J=qGL L=120 nm T=295 K deviation at high (reverse) voltages due to field-dependence of G?

23 MDMO-PPV:PCBM Saturation Regime: Braun: J. Chem. Phys. 80, 1984, 4157 Saturated regime: photocurrent J=e G(E,T) L due to dissociation of bound electron-hole pairs 60% J sc Phys. Rev. Lett., 93, (2004) eg MAX L < 23 J ph [A/m 2 ] K 270 K 250 K 230 K 210 K At J sc only 60% of bound e-h pairs is dissociated!! V OC -V [V]

24 Solar Cell Device Model Inclusion of (Langevin) recombination and G(E,T) requires numerical modeling Phys. Rev. B 72, (2005) MDMO-PPV:PCBM < 24 J light -J dark [A/m 2 ] 10 1 data 295 K data 250 K q G(V) L 295 K q G(V) L 250 K simulation 295 K simulation 250 K Voc-V [V]

25 Transport in a BEH-BMB PPV/ PCBM blend < 25 1:4 wt. % 10-7 [m 2 /Vs] V 1 G * O O 1 O O 3 * ran Electrons L 1 L /T [K -1 ] Holes At T=210 K factor 10 3 difference in e/h mobilities

26 Photocurrent in the BEH-BMB PPV/ PCBM blend J ph =J L -J D [A/m 2 ] K 270 K 250 K 230 K 10 0 J ph V 1/2 210 K V 0 -V [V] Light-intensity (G) dependence?

27 Observation of SCL photocurrent Light-intensity dependence: < 27 J ph [A/m 2 ] L=275 nm T=210 K 6 mw/cm 2 80 mw/cm 2 V sat V 0 -V [V] J ph [A/m 2 ] V sat [V] J V 0 -V=10 V J V 0 -V=0.1 V S = 0.95 S = 0.76 S = Incident Light Power [mw/cm 2 ] 10 1 Phys. Rev. Lett. 94, (2005) At J sc losses due to bimolecular recombination weak (4%)

28 Low Bandgap Polymer PCPDTBT (Konarka) < 28 Mühlbacher et al, Adv. Mater., 18, (2006) Poly [2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-b;3,4-b ]dithiophene)-alt-4,7- (2,1,3-benzothiadiazole)] (PCPDTBT) V oc = 0.65 V J sc = 90 A/m 2 (PC 61 BM) = 110 A/m 2 (PC 71 BM) FF 47%?? PCE = 2.67 % (PC 61 BM) = 3.16 % (PC 71 BM) 28

29 PCPDTBT:PCBM Solar Cells < J L [A/m 2 ] V [V] J ph [A/m 2 ] Low Fill Factor (~40-45%) combined with square root regime in photocurrent: Space-Charge Limited??? 10 V 0 -V [V] T [K]

30 Single Carrier Devices < 30 Polymer:Fullerene Blend J [A/m 2 ] LUMO HOMO J [A/m 2 ] LUMO HOMO 1 h =3x10-8 m 2 /Vs 10 e =7x10-8 m 2 /Vs V-V bi -V rs V-V res -V bi [V] Hole/Electron mobility almost balanced: SC Limit Unlikely!!! 30

31 Intensity dependence of Photocurrent: < 31 Adv. Funct. Mater. 2009, 19, J ph [A/m 2 ] J ph α V 1/2 J ph α G V sat = constant V sat V 0 -V [V] Fingerprint of recombination limited current!!!

32 Square Root Dependence; μτ vs sc limited < 32 Two different origins for a square root dependence of J ph Space Charge Limited: e >> h J ph ( qg) 0 r h 8 J ph α V 1/2 J ph α G 3/ V sat α G 1/2 V. D. Mihailetchi et al., Phys. Rev. Lett. 94, (2005) A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823 (1971) V μτ-limited: w n,p = te<l ; p t p < n t n 100 J ph qg t h J ph α V 1/2 h J ph α G V sat = constant V V 1 L 1 G L J ph [A/m 2 ] 10 V sat V 0 -V [V] 32

33 Outline 1. Charge transport in Organic Semiconductors -Hole Transport, Electron Transport < Photocurrent Generation in Organic Solar Cells -Space Charge, Recombination 3. Recombination in organic solar cells -Bimolecular Recombination, Trap-assisted`Recombination 4. Origin of the Recombination in Organic Solar Cells -CT electroluminescence, ideality factor

34 Bimolecular Langevin recombination Limited by Diffusion of Electrons and Holes towards each other Critical Coulomb Radius: binding energy hole-electron = kt q 2 /4kT (20 nm) >> mean free path in PPV (1-3 nm) < 34 P. Langevin, Ann. Chem. Phys. 28, 289 (1903) 1-3 nm 20nm U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 (1995)

35 How to characterize recombination? < 35 Study Recombination at V oc!! Solar cell with bimolecular recombination: V oc E gap q kt q ln 1 P N PG 2 c APL 86, (2005) E. A. Schiff, Sol. Eng. Mater. Sol. Cells 2003, 78, 567. Measure V oc ~ Light Intensity!!

36 Light intensity dependence of V oc < 36 Only bimolecular Recombination!!!!! 0.90 MDMO-PPV:PCBM V oc [V] APL 86, (2005) Ln (intensity) [a.u.] 295 K 250 K 210 K Linear dependence of V oc on ln(i) with slope kt/q, n=1!

37 All-polymer solar cells: Electron traps < 37 - Recombination 1. Langevin Shockley-Read-Hall Parameters: Nt, Tt, Cn, Cp R R SRH Langevin C C n ( np p N t n 2 i ) q ( ) pn p n C n n C p / n 1 p p1 r n p

38 V oc light intensity dependence Only Langevin recombination included 1.50 < 38 V oc (V) S[kT/q]= MDMO-PPV:PCNEPV Light intensity (W/m 2 ) At V oc only losses via Recombination!!!!!

39 All-polymer: SRH recombination effects on Voc < V oc (V) C n,p = m 3 s m 3 s m 3 s Light intensity (W/m 2 )

40 Photocurrent of MDMO-PPV:PCNEPV < 40 J (A/m 2 ) Both Langevin and SRH recombination included J D J L G max = m -3 s -1 K f = s -1 a = 0.62 nm <ε r > = 2.6 N t = m -3 T t = 2500 K C n,p = m 3 s Voltage (V) What does it mean? Adv. Funct. Mat. 17, 2167 (2007)

41 Measure PLED as a solar cell: < 41 kt/q M.M. Mandoc et al. App. Phys. Lett. 91, (2007) M.M. Mandoc et al. Adv. Funct. Mater. 17, (2007) MEH-PPV C n =C p = m 3 /s

42 Origin of SRH Capture Coefficient: q k C pnt p pnt p SRH < 42 N t =electron trap Capture Coefficients (m 3 /s) C = solar cell C = q p - hole-only device T -1 (10-3 K) Phys. Rev. Lett. 107, (2011)

43 Origin of SRH Capture Coefficient: < 43 Trapping 1-3 nm 20nm r k k S R H S R H N q ( 0 p t p ) Idem as Langevin with immobile electron!

44 Diffusion Current V<Vbi < 44 V=0 V=Vbi V>Vbi Vbi Ca Vbi Ca Ca ITO ITO V Diffusion Diffusion J ~ exp( qv / nkt) Drift Diffusion J ep E dp ed dx Drift v=μe ITO

45 OLED Current-voltage characteristics: < 45 Three regimes: 1. Leakage current I V R leakage V bi Diffusion regime J 3. Drift regime qv J0 exp 1 kt J [A/m 2 ] V [V] J 9 V 8 L 2 3 Ideality Factor

46 Origin of Ideality Factor? < 46 Ideality factor equals 2 in the case of trap-assisted recombination in a classical p-n junction J qv J 0 exp 1 2kT C. T. Sah et al., Proc. IRE 45, 1228 (1957)

47 Super Yellow PPV LED < 47 The ideality factor for a Super Yellow LED was determined to have a value of 2 at room temperature. This corresponds to SRH recombination from trapping sites: J qv J 0 exp 1 2kT kt q ln J V 1

48 White OLEDs: Emissive SRH recombination? < 48 Trap-assisted recombination in conventional polymers appears to be non-radiative. In a white emitting polymer, red dyes in the blue backbone function as emissive traps. LUMO HOMO

49 Langevin & SRH recombination! < 49 Luminance of red dyes follows SRH recombination, whereas the blue light follows Langevin recombination nm Longpass Filter Blue Dichroic Filter 4 Current Light 550 nm Longpass Filter Light Blue Dichroic Filter Luminance [a.u.] EL Intensity [a.u.] (kt/q lnj/v) Wavelength [nm] V [V] V-V bi [V]

50 Outline 1. Charge transport in Organic Semiconductors -Hole Transport, Electron Transport < Photocurrent Generation in Organic Solar Cells -Space Charge, Recombination 3. Recombination in organic solar cells -Bimolecular Recombination, Trap-assisted Recombination 4. Origin of the Recombination in Organic Solar Cells -CT electroluminescence, ideality factor

51 Charge transport in BJH Solar Cell < 51 Electron transport in PCBM and Hole transport in Donor Polymers are trap-free: No SRH recombination expected J [A/m 2 ] 10 4 T = 294 K nm 170 nm J 9 V 0 r e 8 L e 2.0x10-7 m 2 /Vs 10 0 V-V Rs -V bi [V] Adv. Funct. Mater. 2003, 13,

52 Other Polymer:fullerene solar cells: < 52 Slope=kT/q: Only Bimolecular Recombination

53 CT state electroluminescence in OPV < 53 Weak electroluminescence from the charge-transfer state is observed in organic solar cells Cathode Donor Anode Acceptor

54 EL Ideality factor? < 54 Ideality factor of 1 is measured for the CT electroluminescence Emission originates from a free-carrier bimolecular recombination process!

55 V oc vs Light intensity < 55 A contribution of trap-assisted recombination is observed for P3HT:PCBM Recombination is bimolecular for other solar cells

56 Nonradiative SRH recombination? < 56 Can be exposed by looking at the voltage dependence of the EL quantum efficiency P3HT:PCBM Competition SRH and Bimolecular!

57 P3HT:PCBM solar cells < 57 Hole traps in P3HT? P t < cm -3? SRH small

58 P3HT:PCBM solar cells < 58 In P3HT:PCBM solar cells the Langevin recombination is strongly reduced ~10 3 (CELIV) Pivrikas, Osterbacka, Juska et al., Phys. Rev. Lett. 94, (2005) Two dimensional Langevin recombination in the lamellar structure of RR-P3HT Juska, Osterbacka et al., Appl. Phys. Lett. 95, (2009)

59 P3HT:PCBM solar cells < V oc [V] Langevin + SRH kt/q 0.52 Reduced Langevin + SRH Light Intensity [W/m 2 ] Advanced Energy Materials, accepted

60 Exciton Transport? Ca Photo-excitation Langevin Recombination, Trap-assisted Recombination ITO

61 Neat Polymer PL decay: < 61 Luminescence (a.u.) MEH-PPV exp. calc. t=300ps Time (ps) Intrinsic Exciton Lifetime ~ 300 ps?

62 Exciton diffusion Change Energetic disorder in PPV derivatives 7 NRS-PPV O O 0.5 O 0.5 E E D D O O n MEH-PPV Polymer σ, mev D, cm 2 /s μ(300k), m 2 /Vs NRS- PPV O MEH- PPV O n BEH-PPV BEH- PPV PHYSICAL REVIEW B 72, Reduced energetic disorder enhances exciton diffusion!!

63 Neat Polymer PL decay: D= cm 2 /s D= cm 2 /s < 63 Luminescence (a.u.) NRS-PPV exp. calc. t=800ps Time (ps) Luminescence (a.u.) MEH-PPV exp. calc. t=300ps Time (ps) D= cm 2 /s Luminescence (a.u.) BEH-PPV exp. calc. t=200ps Time (ps) -Less disorder, shorter PL decay -Less disorder, monoexponential decay

64 Exciton diffusion Energetic disorder in PPV derivatives 8 Quenching efficiency NRS-PPV; L D =5 nm MEH-PPV; L D =6.3 nm BEH-PPV; L D =6 nm Polymer film thickness [nm] D and t const Exciton diffusion length 5-7 nm is independent on the amount of energetic disorder!!!! L L D D Dt - quenching centers?

65 Are electron traps also exciton quenchers? < 65 Universal electron trap density ~ cm -3 Distance between traps 1/( ) 1/3 = 12.6 nm Exciton has to travel 6 nm to reach a trap Measure electron transport and exciton diffusion independently in a model system with single exponential PL decay!!

66 Model System PCPDTBT poly[2,6-(4,4-bis-(2-ethylhexyl)-4h-cyclopenta[2,1-b;3,4-b']dithiophene)-alt- 4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) < 66 Hole Transport: μ h = m 2 /Vs σ = ev Electron Transport: N t = cm -3 E t =0.3 ev

67 PL decay PCPDTBT < 67 All Single exponential!!

68 f PL decay Analysis (exp. decay) Stern-Volmer formula: τ= PL decay time of blend with PCBM concentration c 1 1 τ f = PL decay time of pristine PCPDTBT 4 rdc r = Sum of the exciton radii t t D = Exciton diffusion coefficient. < 68 r~ 1 nm D= cm 2 /s

69 Background Quenchers < 69 q c c 0 Background Quenchers PCBM +Stern Volmer: 1 1 4rDc 0 4rDq t t f q=0: Intrinsic Exciton Lifetime τ t t 0 f 4 rdc 0 τ f = PL decay time of pristine PCPDTBT τ 0 = PL decay time of solution

70 Graphical Representation c 0 < 70 c 0 = cm -3, equal to N trap electrons!!!!!

71 t f = 1 G + k nr + k diff What about PL efficiency? Measured lifetime in film < 71 t 0 = 1 G + k nr Measured lifetime in solution k diff = 1 t f - 1 t 0 = 4prDc 0 PL Yield g = G G + k nr + k diff = Stern-Volmer G 1-t = 0 k nr 1 + 4prDc 1+ 4prDc 0 t 0 t 0 0

72 PL from integrating sphere < 72 τ 0 k nr =0.72

73 Conclusions < 73 Imbalanced transport and strong recombination lead to a square-root dependence of the photocurrent, FF~0.4 Nature of recombination can be identified from chargetransfer state electroluminescence CT-state emission is of bimolecular origin Weak trap-assisted recombination is present in P3HT:PCBM solar cells The amount of exciton quenchers is equal to the amount of electron traps. The exciton diffusion length and liftetime are not intrinsic but are determined by extrinsic defects

74 RuG Cristina Tanase Denis Markov Jan Anton Koster Magda Mandoc Irina Craciun Yuan Zhang Herman Nicolai Gert-Jan Wetzelaer Paul de Bruyn Niels van der Kaap Bert de Boer Dago de Leeuw Acknowledgement: UCSB Alex Miknenko Martijn Kuik Jason Lin Quyen Nguyen GeorgiaTech Jean-Luc Bredas Chad Risko Casey Campbell

75 Thank you for your attention!!! < 75

76 Transport and Recombination in a PLED < 76 Burroughes et al., Nature 347, p. 539 (1990) Ca 2.1 ev O H 3 C O n ITO Injection: Barrier Height Transport: Mobility, Traps, Space Charge Recombination

77 Exponential Trap Distribution: Modified Model < 77 E J N c e n Exp. Trap model: en r r1 0 r V C( r 2r1 t( eff ) L n t ) N t n N c T / r=t t /T T t N t ( E) E E ~ exp( kt t t ) N t( eff ) N t E exp tc 2 T t /(2kT) MDMO-PPV: N trap =5*10 17 cm -3 T t =1500 K

78 Trap-limited Electron Transport? < 78 LUMO PPV Deep Traps E tc Shallow Traps Hopping in modified DOS v. Mensfoort et al., PRB 80, (2009) How can we determine μ e, N t, and E tc??

79 n-type doping: < 79 A. Kahn et al., Org. Electr. 9, 575 (2008)

80 n-type doping: < 80 Phys. Rev. B. 81, (2010) After n-doping: Electron current equal to hole current Temperature dependence equal to temperature dependence of hole current μ e = μ h Traps located > 0.4 ev below the LUMO

81 Gaussian LUMO and Exponential Traps? < 81 Use Approximation in Intermediate Voltage Regime: G. Paasch and S. Scheinert, J. Appl. Phys. 107, (2010) Single level Exponential Gaussian n t (m -3 ) LUMO E n (m -3 )

82 Ideality factor solar cells: Dark Current < 82 Polymer:PCBM bulk heterojunction solar cells have an ideality factor of ~1.3 J D qv J s exp 1 kt n>1: Evidence for trap-assisted recombination?

83 Single Carrier Dark-Current (no rec!!) Ideality factor single-carrier devices of separate materials match ideality factors of the blend => Ideality factor is determined by transport-dominating constituent of the solar cell blend. Appl. Phys. Lett. 99, (2011)

84 Other conjugated polymers? < 84 Current Density (A/m²) nm PCPDTBT 85 nm PF10TBT nm F8BT 300 nm OC 1 C 10 -PPV V (V) Slope of Trap-limited Electron Current varies for different polymers

85 PLED Operation: < 85 Ca ITO Trap-Free SCL Hole Transport Trap-limited Electron Transport Langevin Recombination, Shockley-Read-Hall Recombination

86 Assumption: Diffusion neglected < 86 Hughes and Sokel: J. Appl. Phys. 52, 1981, 6743 No recombination losses: L diffusion drift + hn _ V=V OC -bias J V V J-V characteristic is: J J n J p egl exp( ev exp( ev / kt) 1 / kt) 1 2kT ev

87 Recap: < 87 Low voltage: Ohmic behaviour: J ev egl exp( ev J / kt) 1 / kt) 1 2kT ev exp( L V eg 1 / 2 V 1/ 2 n t n J egl or J eg nt n pt p?? Drift vs diffusion Intermediate voltage: Square root behaviour: 1/ 4 9 3/ 4 1/ 2 p or J q G V 8q?? Saturation regime: Voltage independent

88 Exciton Diffusion Length O C 4 H N O F2D C 4 H 9 PL quenching [a.u.] NRS-PPV/poly(F2D) L D = 5 nm Time-independent!! NRS-PPV film thickness [nm] J. Phys. Chem. A 2005, 109, Poly(F2D) allows formation of a completely immobilized well-defined heterostructure L D =5-7 nm Photovoltaic response: 7 nm J.J.M.Halls et.al., Appl. Phys. Lett., 1996, 68, 3120

89 Exciton Diffusion Coefficient n( x, t) t n( x, t) t ( t) D 2 n( x, t) 2 x S( x) n( x, t) g( x, t) PL intensity [a.u.] D NRS Time [ps] Neat NRS-PPV Film thickness 15.5 nm 8 nm 4 nm cm s 2 Photo-induced defect quenching: D= cm 2 /s M. Yan et. al., Phys. Rev. Let., 1994, 73, 744 PHYSICAL REVIEW B 72, Neat Polymer used as reference

90 Exciton Diffusion Coefficient < 90 Bulk Quencher PCBM C-PCPDTBT

91 Exciton Diffusion Coefficient < 91 Relative quenching efficiency Q 1 PL PL blend pristine dt dt Energy Environ. Sci., 2012, 5, 6960 Intimate mixture: no clusters! MC simulation: D= cm 2 /s L D =10 nm

92 Neat Polymer PL decay < 92 PL decay is not single exponential. -relaxation of excitons in Gaussian DOS Movaghar, B.; Grünewald, M.; Ries, B.; Bässler, H.; Würtz, D. Phys. Rev. B 1986, 33, J. Phys. Chem. B 2008, 112,

93 Recap: < 93 Reduction of disorder leads to increase of exciton diffusion coefficient For exciton diffusion coefficient >10-3 cm 2 /Vs the PL decay is single exponential The exciton diffusion length ~5-7 nm is independent on disorder.

94 Organic BHJ vs. Si-based Solar cell < 94 1 J/J max constant G field dependent G 0.1 JV MDMO-PPV:PCBM Si p/n cell Voc-V [V] The generation rate in blends of MDMO-PPV:PCBM is field dependent!

95 Introduction of TCNQ electron traps: < 95 Can we prove that recombination with trapped electrons is responsible for the enhanced dependence of Voc on light intensity? LUMO MDMO-PPV Exciton diffusion N LUMO PCBM H H N HOMO MDMO-PPV 4.5 ev TCNQ N H H N HOMO PCBM

96 V oc light intensity dependence < No Traps V oc (V) S[kT/q]=1 TCNQ Traps 0.2 S[kT/q]=3 Appl. Phys. Lett. 91, Light intensity (W/m 2 )

97 PL decay PCPDTBT < 97 τ 0 =212 ps τ f = 146 ps

98 Disorder dominated charge transport < 98 Low mobility:~ cm 2 Vs + 3-D Transport by hopping between conjugated parts of the chain Bässler, Phys. Stat. Sol. (b) 175, 15 (1993)