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Molecular structure impacts polymer opto-electronics
- 1. The role of molecular structure and conformation in polymer opto-electronics Charge separation: Molecular structure Enrico Da Como
- 3. Conjugated polymers polythiophene Bottom – up design for electronics Optical, electrical & ordering properties arise at the molecular scale
- 4. Structure-function relationships Polymer solar cells: transport, recombination & efficiency -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 -12 -8 -4 0 4 8 Currentdensity(mA/cm 2 ) Voltage (V)
- 5. Solar cells 1. Absorption of light and photogeneration of excitons Mott-Wannier ~ 0.1 eV Large radius Charge transfer excitons ~ 0.1 – 1.0 eV Localised between molecules Frenkel excitons ~ 0.5 – 1.0 eV Localised on molecule
- 6. Solar cells E e - e - e - h + h + h + h + h + DriftDiffusion e - e - eV Built-in Potential pn junction, heterojunction 2. Exciton dissociation & 3. Transport of charge
- 7. Solar cells 2. Exciton dissociation & 3. Transport of charge Donor-Acceptor system S - +
- 8. G. Yu, … A. J. Heeger, Science 207, 1789 (1995) Y. Yang & Solarmer Nature Photonics 3, 649 (2009) Polymer/fullerene photovoltaics > 8% efficiency on lab cells Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact
- 13. The race for 10 % Konarka's Power Plastic Achieves World Record 8.3%
- 14. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Physics on different length scales Efficiency Layers & interfaces
- 15. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Mesoscopic scale bulk heterojunction Physics on different length scales Charge transport Morphology & molecular ordering
- 16. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Molecular scale Donor-Acceptor Mesoscopic scale bulk heterojunction Physics on different length scales Exciton generation & dissociation molecular ordering & mobility
- 17. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Molecular scale Donor-Acceptor Charge transport Charge separation Physics on different length scales Mesoscopic scale bulk heterojunction Exciton generation & dissociation molecular ordering & mobility
- 18. Photovoltaic action: competing mechanisms? Energy Charge separation absorption Polymer Fullerene HOMO HOMO LUMO LUMO
- 19. Energy Charge separation absorption Large donor-acceptor interface Polymer Fullerene morphology & mobility HOMO LUMO HOMO LUMO Photovoltaic action: competing mechanisms?
- 20. Efficiency = JscVocFF Pin Polmyer solar cell: device parameters -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 -12 -8 -4 0 4 8 Currentdensity(mA/cm 2 ) Voltage (V) = 2.9 % P3HT:PCBM Short circuit current Jsc light absorption transport Fill factor FF charge collection Open circuit voltage Voc HOMO-LUMO offset
- 21. Charge transfer @ polymer:fullerene interface structure conformation ordering Donor-acceptor distance
- 22. Excitons in polymers Frenkel exciton (~ 0.5 eV – 1 eV) Intra (inter) chain excitation Lifetime ~ns, diffusion length ~ 10 – 20 nm Polymer structure, conformation & excitons
- 23. Excitons in polymers Frenkel exciton (~ 0.5 eV – 1 eV) Inter or intrachain excitation Lifetime ~ns, diffusion length ~ 10 – 20 nm Chemical structure, excitons, long range ordering
- 25. Excitons in Polymer:fullerene systems Charge transfer exciton Coulomb bound electron-hole pair @ the donor-acceptor interface
- 26. Excitons in Polymer:fullerene systems Polymer Fullerene HOMO HOMO LUMO LUMO Energy S0 S1 S2 T1 CTE?
- 27. Excitons in Polymer:fullerene systems Polymer Fullerene HOMO HOMO LUMO LUMO Energy S0 S1 S2 T1 CTE? Where is the CTE energetically? What role does it play in charge transfer/recombination? CTE vs molecular structure, conformation and ordering?
- 28. Excitons in Polymer:fullerene systems Why do CTEs dissociate? Field dependence Only 60 % of CTEs dissociate in polymer fullerene solar cells at room temperature V. Mihailetchi, L. Koster, J. Hummelen, P. Blom, Phys. Rev. Lett. 93, 216601 (2004)
- 29. Excitons in Polymer:fullerene systems Are CTEs a necessary step for charge separation? Voc limited by CTE Polymer Fullerene HOMO LUMO LUMO
- 30. Excitons in Polymer:fullerene systems Are CTEs a necessary step for charge separation? Polymer Fullerene HOMO LUMO LUMO Veldman et al., JACS 2008 Change molecular ordering, interface states
- 31. Excitons in Polymer:fullerene systems Mixed amorphous & crystalline polymer regions enhance charge separation Higher charge separation efficiency with engineered heterojunctions Bulk properties influence CTE dissociation
- 32. Charge transfer @ polymer:fullerene interface Acceptor concentration
- 33. Energy(eV) -6.1 -5.4 -3.2 -4.2 HOMO LUMO HOMO LUMO MDMO-PPV/PCBM 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.0 0.4 0.8 1.2 PCBM PL(a.u.) 0.0 0.4 0.8 1.2 MDMO-PPV pristine PL(a.u.) Probing recombination with PL spectroscopy Energy (eV) Adv. Funct. Mater. 19, 3662 (2009)
- 34. Energy(eV) -6.1 -5.4 -3.2 -4.2 HOMO LUMO HOMO LUMO MDMO-PPV/PCBM 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.0 0.4 0.8 1.2 MDMO-PPV/PCBM blend PL(a.u.) 0.0 0.4 0.8 1.2 PCBM pristine PL(a.u.) 0.0 0.4 0.8 1.2 MDMO-PPV pristine PL(a.u.) CTE Energy (eV) Probing recombination with PL spectroscopy Adv. Funct. Mater. 19, 3662 (2009)
- 35. 0.8 1.2 1.6 2.0 0 1x10 5 2x10 5 3x10 5 4x10 5 PL(a.u.) Energy (eV) 0.8 1.2 1.6 2.0 Energy (eV) 80 wt % PCBM60 wt % PCBM 0.8 1.2 1.6 2.0 Energy (eV) 20 wt % PCBM Vary the donor-acceptor interface Adv. Funct. Mater. 19, 3662 (2009)
- 36. 0.8 1.2 1.6 2.0 0 1x10 5 2x10 5 3x10 5 4x10 5 PL(a.u.) Energy (eV) 0.8 1.2 1.6 2.0 Energy (eV) 80 wt % PCBM60 wt % PCBM 0.8 1.2 1.6 2.0 Energy (eV) 20 wt % PCBM Vary the donor-acceptor interface Adv. Funct. Mater. 19, 3662 (2009) CTE dissociation depends on acceptor concentration Increased probability of exciton dissociation Arkhipov et al., Appl. Phys. Lett. 2003 82, 4605.
- 37. Charge transfer @ polymer:fullerene interface Donor/Acceptor structure
- 38. The role of the fullerene acceptor Energy(eV) HOMO LUMO HOMO LUMO Donor/acceptor PCBM bis-PCBM DPM MDMO-PPV VOC Appl. Phys. Lett . 97 023301 (2010)
- 39. CTE recombination Appl. Phys. Lett . 97 023301 (2010)
- 40. Anti-Correlation of PLCTE intensity and JSC 0 20 40 60 80 100 120 140 160 180 200 0.0 0.2 0.4 0.6 0.8 1.0 1.2 PLCTE (arb.u.) JSC µA/cm 2 Appl. Phys. Lett . 97 023301 (2010) Anticorrelation Jsc and CTE
- 41. Morphology and transport bis-PCBM PCBM me=2 10-4 cm2/Vs me=8 10-3 cm2/Vsme= 1 10-3cm2/Vs Appl. Phys. Lett . 97 023301 (2010) Long range ordering? Transport?
- 42. Changing morphology with chain regioregularity Regiorandom P3HT Regioregular P3HT Amorphous vs. Polycrystalline
- 43. Adv. Funct. Mater. 19, 3662 (2009) 1.0 1.5 2.0 ra-P3HT PCBM Energy (eV) X10 RE-P3HT RE-P3HT/PCBM Changing morphology with chain regioregularity 100 nm = 2.1% PLIntensity PLIntensity = 0.9% 1.0 1.5 2.0 ra-P3HT ra-P3HT/PCBM Energy (eV) EnergyEnergy 100 nm Regiorandom P3HT Regioregular P3HT What is the role of donor-acceptor distance?
- 44. Model system: „low band gap“ polymers PCPDT-BT M. Svensson, F. Zhang, O. Inganas, & M. R. Andersson, Synth. Met. 135, 137 (2003) N. Blouin, A. Michaud, M. & Leclerc Adv. Mater. 19, (2007) Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, C. Brabec, Macromolecules 40, 1981 (2007). „Low bandgap“ co-polymers for better light absorption dithiophene benzodiathiazole LUMO HOMO
- 45. Increasing solar cell efficiency PCPDT-BT M. Svensson, F. Zhang, O. Inganas, & M. R. Andersson, Synth. Met. 135, 137 (2003) N. Blouin, A. Michaud, M. & Leclerc Adv. Mater. 19, (2007) Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, C. Brabec, Macromolecules 40, 1981 (2007). „Low bandgap“ co-polymers for better light absorption dithiophene benzodiathiazole
- 47. 500 1000 1500 2000 Absorption(arb.units) Wavelength (nm) 800 nm PCPDT-2TBT PCPDT-BDT PCPDT-2TTP PCPDT-BT 800 nm 660 nm 800 nm Low-bandgap copolymers Tautz et al submitted Stronger vs weaker acceptor Shifting the donor-acceptor centre of mass
- 48. IR Absorption HOMO -1 LUMO +1 Measuring IR absorption of chemically induced polarons HOMO LUMO e- P1 Where are the polarons? -0.1 0.0 0.1 500 1000 1500 2000 2500 3000 3500 -0.1 0.0 0.1 -0.1 0.0 0.1 P1 ChemicallyinducedOD(arb.u.) GB P1 GB Probe Wavelength [nm] Probe P2 P1 GB Ex Ex P2 GB P1 P2 -5 0 5 0 5 -10 0 10 -0.1 0.0 0.1 -5 0 5 P2 Probe P2
- 49. IR Absorption HOMO -1 LUMO +1 Measuring IR absorption of chemically induced polarons HOMO LUMO e- P1 Where are the polarons? -0.1 0.0 0.1 500 1000 1500 2000 2500 3000 3500 -0.1 0.0 0.1 -0.1 0.0 0.1 P1 ChemicallyinducedOD(arb.u.) GB P1 GB Wavelength [nm] P2 P1 GB P2 GB P1 P2 -0.1 0.0 0.1 P2
- 50. IR Absorption HOMO -1 LUMO +1 Measuring IR absorption of chemically induced polarons HOMO LUMO e- P1 Where are the polarons? -0.1 0.0 0.1 500 1000 1500 2000 2500 3000 3500 -0.1 0.0 0.1 -0.1 0.0 0.1 P1 ChemicallyinducedOD(arb.u.) GB P1 GB Probe Probe Wavelength [nm] Probe Ex P2 P1 GB Ex Ex P2 GB P1 P2 -5 0 5 0 5 -10 0 10 Opticallyinduced(10 -4 ) -0.1 0.0 0.1 -5 0 5 P2 Probe
- 51. Polaron formation in realtime 10 20 30 10 20 10 20 Time delay (fs) PCPDT-BT Polaronpairyield(%) P3HT PCPDT-BDT PCPDT-2TBT PCPDT-2TTP IRF -1000 -750 -500 -250 0 250 500 750 0 10 20 10 20 D A D A D A D A UU U U - = 15.9% = 21.4% = 13.9% = 7.9% = 23.6%
- 52. Polaron formation in realtime 10 20 30 10 20 10 20 Time delay (fs) PCPDT-BT Polaronpairyield(%) P3HT PCPDT-BDT PCPDT-2TBT PCPDT-2TTP IRF -1000 -750 -500 -250 0 250 500 750 0 10 20 10 20 D A D A D A D A UU U U - = 15.9% = 21.4% = 13.9% = 7.9% = 23.6%
- 53. Polaron formation in realtime 10 20 30 10 20 10 20 Time delay (fs) PCPDT-BT Polaronpairyield(%) P3HT PCPDT-BDT PCPDT-2TBT PCPDT-2TTP IRF -1000 -750 -500 -250 0 250 500 750 0 10 20 10 20 D A D A D A D A UU U U - = 15.9% = 21.4% = 13.9% = 7.9% = 23.6% Acceptor strength only slightly influencing efficiency Important role of spatial separation
- 54. Charge transfer @ polymer:fullerene interface structure conformation ordering Donor-acceptor distance
- 55. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Mesoscopic scale bulk heterojunction Physics on different length scales Charge transport Morphology & molecular ordering
- 56. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Molecular scale Donor-Acceptor Mesoscopic scale bulk heterojunction Physics on different length scales Exciton generation & dissociation molecular ordering & mobility How to improve efficiency at every length scale?
- 57. Conformation & structure Long range ordering Charge transfer
- 58. Adv. Funct. Mater. 19, 3662 (2009) 1.0 1.5 2.0 ra-P3HT PCBM Energy (eV) X10 RE-P3HT RE-P3HT/PCBM Changing morphology with chain regioregularity 100 nm = 2.1% PLIntensity PLIntensity = 0.9% 1.0 1.5 2.0 ra-P3HT ra-P3HT/PCBM Energy (eV) EnergyEnergy 100 nm Regiorandom P3HT Regioregular P3HT
- 59. 100 nm The effect of long range ordering AnnealedNot Annealed = 2.1% = 4.0%
- 60. 100 nm AnnealedNot Annealed = 2.1% = 4.0% 1.0 1.5 2.0 PLintensity X10 RE-P3HT/PCBM RE-P3HT/PCBM (annealed) Energy (eV)Adv. Funct. Mater. 19, 3662 (2009) The effect of long range ordering
- 61. 100 nm AnnealedNot Annealed = 2.1% = 4.0% Adv. Funct. Mater. 19, 3662 (2009) J. App. Phys.100, 043702 (2006) Ambipolar transportUnipolar (hole) transport The effect of long range ordering How to induce long range ordering?
- 62. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact
- 63. Conformation & structure Long range ordering Doping Charge transfer Increase mobility without changing morphology?
- 64. Increasing mobility by molecular doping P doping by electron transfer in the ground state F4TCNQ Yim et al., Adv Mater, 2008, 20 Zhang et al., Phys Rev B, 2010, 81 Zhang et al., Adv Func Mater, 2009, 19
- 65. Increasing mobility by molecular doping P doping by electron transfer in the ground state PCPDTBT:PCBM F4TCNQ SPP1355
- 66. Fill tail states with excess charge carriers Increase Mobility + Energy(eV) Disordered film Increasing mobility by molecular doping g(E)
- 67. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Charge transport Charge separation Improvement in charge separation, mobility, efficiency Photocurrent & Efficiency?
- 68. 0.9 1.0 1.1 1.2 1.3 1.4 PCBM PCPDTBT 1.0 1.2 1.4 1.6 1.8 PLintensity(arb.units) Energy (eV) PCPDTBT/PCBM Energy (eV) PLintensity(arb.units) Doping & Charge separation + -
- 69. 0.9 1.0 1.1 1.2 1.3 1.4 PCBM PCPDTBT 1.0 1.2 1.4 1.6 1.8 PLintensity(arb.units) Energy (eV) PCPDTBT/PCBM Energy (eV) PLintensity(arb.units) + - 0% 1% 3% 4% Doping & CTE recombination
- 70. 0 200 400 600 800 Time (ps) 0% 2% 4% 5% Norm.PLintensity Doping & CTE recombination
- 71. 0 100 200 300 0% 2% 4% 5% PLintensity(arb.units) Time (ps) Lower density of CTE or very fast dissociation with doping? Doping & CTE recombination
- 72. Doping & Polaron formation Janssen et al. Adv. Mater (2010) -T/Tx104 0 50 100 150 200 250 300 0 2 Time delay (ps) -T/T(x10 -3 ) 0% EProbe
- 73. Doping & Polaron formation Janssen et al. Adv. Mater (2010) -T/Tx104 0 2 0 50 100 150 200 250 300 0 2 2% -T/T(x10 -3 ) 0% Time delay (ps) EProbe
- 74. Doping & Polaron formation Janssen et al. Adv. Mater (2010) -T/Tx104 0 2 0 2 0 50 100 150 200 250 300 0 2 4% 2% -T/T(x10 -3 ) 0% Time delay (ps) EProbe
- 75. Doping & Polaron formation Janssen et al. Adv. Mater (2010) -T/Tx104 0 2 0 2 0 2 0 50 100 150 200 250 300 0 2 5% 4% 2% -T/T(x10 -3 ) 0% Time delay (ps) EProbe
- 76. PCPDTBT PCBM Energy tCT-r tCT-ftFR-r tP-f tP-r 0 100 200 300 0% 2% 4% 5% PLintensity(arb.units) Time (ps) Time Delay (ps) 0 2 0.0 1.6 0 2 0.0 1.6 0 2 0.0 1.6 0 2 0.0 1.6 0 50 100 150 200 250 300 Polarondensity(x10 17 /cm 3 ) 0% -T/T(x10 -4 ) 2% 4% 5% Rate equation model
- 77. PCPDTBT PCBM Energy tCT-r tCT-ftFR-r tP-f tP-r 0 100 200 300 0% 2% 4% 5% PLintensity(arb.units) Time (ps) Time Delay (ps) 0 2 0.0 1.6 0 2 0.0 1.6 0 2 0.0 1.6 0 2 0.0 1.6 0 50 100 150 200 250 300 Polarondensity(x10 17 /cm 3 ) 0% -T/T(x10 -4 ) 2% 4% 5% Doping [%] tFR-r tCT-f tP-f tCT-r tP-r [ps] [ps] [ps] [ps] [ps] 0 125 0.2 0.2 300 1400 2 125 0.5 0.2 300 1000 4 125 0.95 0.2 300 400 5 0.15 0.95 0.2 250 300 Rate equation model
- 78. Decrease in CTE emission and larger density of polarons with doping Conclusion: doping helps! Phys. Rev. Lett. 107, 127402 (2011)
- 79. Substrate Anode Transport layer Active layer: Polymer/fullerene Metal contact Charge transport Charge separation Improvement in charge separation, mobility, efficiency Efficiency?
- 80. 09.03.2015 Präsentationstitel 80 Thank you for your attention