Ultrafast Optical Spectroscopy of Excited States in Conjugated - PowerPoint PPT Presentation

Ultrafast Optical Spectroscopy of Excited States in Conjugated Polymers Frédéric Laquai Max Planck Research Group „Photophysics of Conjugated Materials“ Max Planck Institute for Polymer Research Mainz, Germany – JST-DFG Workshop, Kyoto, Japan, January 2009 – Frédéric Laquai – MPIP Mainz

Outline • Introduction – Photophysics • Photophysical properties of step-ladder polymers • Light amplification in thin films of poly(ladder-type phenylene)s • First results on polymer / fullerene organic solar cells • Summary and Outlook Frédéric Laquai – MPIP Mainz

Excited states in conjugated materials Energy τ Ph τ Fl t Abs = fs = ps - ns = µs – s Frédéric Laquai – MPIP Mainz

Poly(ladder-type phenylene)s Wavelength [nm] 700 600 500 400 R R PL Emission Intensity (normalised) R = 1.0 P1 Polymer P1 Polymer P2 n Polymer P3 0.8 Polymer P4 Ar Ar Polymer P5 0.6 Ar = C 8 H 17 P2 0.4 Ar Ar n Ar Ar Ar Ar 0.2 P3 0.0 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Ar Ar Energy [eV] n Ar Ar Ar Ar • chemically well-defined P4 • no keto defects Ar Ar Ar Ar n • improved stability Me R 2 Me R 2 R 1 R 1 • good solubility and film forming properties P5 R 1 R 1 Me R 2 Me R 2 n Frédéric Laquai – MPIP Mainz

Poly(ladder-type phenylene)s 3.8 R R 3.6 R = P1 3.4 n 3.2 Energy [eV] N = 11 PF2/6 Ar Ar 3.0 Ar = C 8 H 17 P2 2.8 Polymers 2.6 Ar Ar n Monomers 2.4 Kuhn fit MeLPPP Ar Ar Ar Ar 2.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 P3 1 / N Ar Ar n Ar Ar Ar Ar π ' k = + 1 2 cos E E P4 0 + 1 k N 0 Ar Ar Ar Ar n Me R 2 Me R 2 R 1 Kuhn model R 1 P5 R 1 R 1 Me R 2 Me R 2 J. Gierschner, J. Cornil, H.-J. Egelhaaf, Adv. Mater. 2007 , 19 , 173. n Frédéric Laquai – MPIP Mainz

Ultrafast Fluorescence Spectroscopy (Streak Camera) Wavelength [nm] 650 600 550 500 450 400 Ar = C 8 H 17 1.0 PL Intensity (normalised) 0.8 Ar Ar Ar Ar 0.6 0.4 Ar Ar n 0.2 0.0 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 0 Energy [eV] 10 solution 296 K solution 80 K PL Intensity / normalized film 296 K -1 10 -2 10 0 200 400 600 800 1000 1200 Time / ps Frédéric Laquai – MPIP Mainz

Delayed photoluminescence and photoinduced absorption (PIA) Wavelength [nm] Wavelength / nm 1000 900 800 700 600 700 600 500 400 1.2 1.2 Ar Ar Ar Ar 2.90 eV 2.78 eV Polymer P1 1.0 1.0 Polymer P2 PL intensity / normalized Δ T/T (normalised) Polymer P3 Ar Ar n 0.8 0.8 Polymer P4 2.12 eV 3.06 eV 0.6 0.6 2.61 eV 0.4 0.4 2.42 eV 1.94 eV 0.2 0.2 0.0 0.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 Energy [eV] Energy / eV Delayed photoluminescence (solution) Steady-state PIA spectra (film) Frédéric Laquai – MPIP Mainz

Comparison of photophysical properties τ Fl τ Ph Polymer S 1 � S 0 T 1 � S 0 T 1 � T n [eV] [ps] [eV] [s] [eV] P1 (N=2) 2.97 422 2.18 1.0 1.51 P2 (N=3) 2.86 379 2.13 1.2 1.40 P3 (N=4) 2.81 390 2.12 1.3 1.37 P4 (N=5) 2.78 332 2.06 1.0 1.37 P5 (MeLPPP) 2.69 390 2.08 1.1 1.35 F. Laquai, A.K. Mishra, M.R. Ribas, A. Petrozza, J. Jacob, L. Akcelrud, K. Müllen, R.H. Friend, G. Wegner, Adv. Funct. Mater. 2007 , 17 , 3231-3240. Frédéric Laquai – MPIP Mainz

Amplified spontaneous emission of conjugated polymers Poly(9,9‘-dioctyl-fluorene) / PFO n G. Heliotis et al., Appl. Phys. Lett. 2002 , 81 , 415. Methyl-substituted ladder-type PPP Me R 2 Me R 2 R 1 R 1 R 1 R 1 Me R 2 Me R 2 n C. Zenz et al., Appl. Phys. Lett. 1997 , 18 , 2566. Frédéric Laquai – MPIP Mainz

Amplified spontaneous emission (ASE) in polymer waveguides Pump laser beam scattered light n air n polymer n substrate scattered light n air < n polymer > n substrate Frédéric Laquai – MPIP Mainz

Experimental setup Beam profile after homogenizer Lens arrays Raw laser beam profile Frédéric Laquai – MPIP Mainz

Characterisation of ASE parameters 1. Gain coefficient g( λ ): sample AI λ λ = − ( ) ( ) p ( 1 ) g l I e λ ( ) g laser stripe razor blade 2. Absorption coefficient α : sample − α = x I I e 0 out laser stripe Frédéric Laquai – MPIP Mainz

Amplified spontaneous emission (ASE) / slab waveguide Wavelength / nm 650 600 550 500 450 400 1.2 2 x pulse 11.3 μ J/cm PL Emission Intensity / a.u. 1.0 2 x pulse 28.3 μ J/cm 2 x pulse 283 μ J/cm 0.8 0.6 0.4 0.2 0.0 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 4-level laser system Energy / eV Ar Ar I th = 3 µJ/cm 2 ~ 375 W/cm 2 Ar Ar n F. Laquai, P.E. Keivanidis, S. Baluschev, J. Jacob, K. Müllen, G. Wegner, Appl. Phys. Lett. 87 , 261917 (2005). Frédéric Laquai – MPIP Mainz

Amplified spontaneous emission (ASE) / thresholds 6 10 m = 2 ASE Peak Intensity (a.u.) 5 10 ASE onset 15 Width of Gauss Fit (nm) 4 10 10 3.0 µJ/cm² 5 3 10 1 10 100 2 x pulse) Pump Energy Density (µJ/cm 1 10 100 1000 2 x pulse)) Pump Energy Density (µJ/(cm Frédéric Laquai – MPIP Mainz

Amplified spontaneous emission (ASE) / gain coefficient 5 10 ASE Peak Intensity (a.u.) ASE Peak Intensity (a.u.) 25 4 10 20 -1 15 g = 21 cm 10 5 3 10 0 0.0 0.1 0.2 Stripe Length (cm) 0.0 0.1 0.2 0.3 0.4 0.5 Excitation Stripe Length (cm) sample AI λ λ = ( ) − ( ) p ( 1 ) g l I e λ ( ) g laser stripe g( λ ) = σ SE ( λ ) × razor blade N exc Frédéric Laquai – MPIP Mainz

ASE of Poly(ladder-type phenylene)s Ar Ar Wavelength [nm] Aryl-PF 520 500 480 460 440 1.2 n P5 P3 Aryl-PF Ar Ar C2 P4 P2 1.0 PL Intensity (normalised) Ar = C 8 H 17 P2 0.8 Ar Ar n Ar Ar 0.6 Ar Ar P3 0.4 Ar Ar n 0.2 Ar Ar Ar Ar 0.0 2.3 2.4 2.5 2.6 2.7 2.8 2.9 P4 Energy [eV] Ar Ar Ar Ar n 1000 carbon-bridged Me R 2 Me R 2 R 1 R 1 -1 ] carbazole-containing -2 pulse C2 P5 C1 R 1 R 1 Me R 2 Me R 2 100 ASE threshold [ μ J cm n P4 Aryl-PF Ar Ar Ar Ar R = P3 C1 10 n N R P2 R Ar Ar N C2 1 2 3 4 5 6 N n N (number of bridged phenyl rings) Frédéric Laquai – MPIP Mainz Ar Ar R

Comparison of ASE properties τ Fl τ R Polymer I th g( λ ASE ) α ( λ ASE ) Φ F λ ASE [ μ J/cm 2 ] [nm] [ps] [cm -1 ] [cm -1 ] [ps] [%] Aryl-PF 449 318 24 18 <1 558 57 P2 (N=3) 468 240 3 21 7 571 45 P3 (N=4) 475 170 10 15 1 630 23 P4 (N=5) 479 122 100 6 5 349 35 P5 489 130 20 16 2 500 26 (MeLPPP) C1 (N=4) 480 93 110 - - 620 15 C2 (N=5) 492 110 150 - - 786 14 Frédéric Laquai – MPIP Mainz

Excited state absorption (ESA) U. Scherf, S. Riechel, U. Lemmer, R.F. Mahrt, Current Opinion in Solid State and Materials Science 5 (2001) 143. Frédéric Laquai – MPIP Mainz

Ultrafast transient absorption spectroscopy Supercontinuum Probe Pulse Pump Pulse (fs) Change of transmission ( Δ T) of sample Δ T = T pump - T pump on off in the presence of pump beam: Frédéric Laquai – MPIP Mainz

Excited states in conjugated materials PA + SE Δ T/T 0 - PA SE Wavelength absorption � negative Δ T/T PA: Photoinduced SE: Stimulated Emission � positive Δ T/T Frédéric Laquai – MPIP Mainz

ASE of Poly(ladder-type phenylene)s What influences the threshold of light amplification in conjugated polymers? Energy [eV] Energy [eV] 2.6 2.4 2.2 2 2.6 2.4 2.2 2 1.2 1.2 -2 pulse -1 10 μ Jcm 0-1 -2 pulse -1 44 μ Jcm 0-1 -2 pulse -1 1.0 31 μ Jcm PL Intensity (normalised) -2 pulse -1 351 μ Jcm 1.0 0-0 -2 pulse -1 100 μ Jcm -2 pulse -1 3500 μ Jcm PL Intensity (normalised) 0-0 0.8 0.8 0.6 0.6 0-2 0.4 0-2 0.4 0.2 0.2 0.0 450 500 550 600 650 0.0 450 500 550 600 650 Wavelength [nm] Wavelength [nm] R Me R 2 Me R 2 Ar Ar R 1 R 1 N n N R 1 R 1 Me R 2 Ar Ar Me R 2 R n I th = 20 µJcm -2 pulse -1 I th = 150 µJcm -2 pulse -1 � I th = Minimum pump pulse intensity for amplification of light to occur Frédéric Laquai – MPIP Mainz

ASE of Poly(ladder-type phenylene)s What influences the threshold for light amplification in conjugated polymers? Energy [eV] Energy [eV] 2.6 2.5 2.4 2.3 2.2 2.1 2.6 2.5 2.4 2.3 2.2 2.1 2 0.10 0.06 0-1 0.08 0-1 0.06 0.04 0.04 0-2 Δ T/T Δ T/T 0.02 0.02 0-2 0.00 -0.02 SE 0.00 -0.04 Photoinduced Absorption Stimulated Emission PA -0.06 -0.02 460 480 500 520 540 560 580 600 480 500 520 540 560 580 600 620 Wavelength [nm] Wavelength [nm] I th = 20 µJcm -2 pulse -1 I th = 150 µJcm -2 pulse -1 � Strong overlap of SE region and PI absorption increases threshold ! Frédéric Laquai – MPIP Mainz

Transient absorption experiments 1.0 1.0 SE (d~50 nm) SE (0-1) PA (d~50 nm) SE (0-2) SE (d~120 nm) PA 0.5 0.5 Δ T/T (normalised) Δ T/T (normalised) 0.0 0.0 -0.5 -0.5 -1.0 -1.0 1 10 100 1000 0.1 1 10 100 1000 time [ps] time [ps] R Me R 2 Me R 2 R 1 R 1 Ar Ar N n N R 1 R 1 Ar Ar Me R 2 Me R 2 R n � ASE leads to rapid depopulation of excited singlet excitons Frédéric Laquai – MPIP Mainz

Photovoltaic blends – Operating principle LUMO η ~ 5 % (power conversion) HOMO cathode anode Electron Donor Electron Acceptor Frédéric Laquai – MPIP Mainz