Time resolved optical spectroscopy methods for organic photovoltaics. Enrico Da Como. Department of Physics, University of Bath

Transcription

1 Time resolved optical spectroscopy methods for organic photovoltaics Enrico Da Como Department of Physics, University of Bath

2 Outline Introduction Why do we need time resolved spectroscopy in OPV? Short laser pulses and methods How short? Mode-locked lasers and amplified pulses Time resolved spectroscopy methods Applications to organic photovoltaics Ultrafast spectroscopy of polymers and polymer:fullerene blends Nonlinear optical methods

3 Energy Why time resolved spectroscopy for OPV Polymer Fullerene LUMO Charge separation absorption LUMO HOMO HOMO Large donor-acceptor interface morphology & mobility

4 Why time resolved spectroscopy for OPV Measure exciton diffusion length Time resolve electron transfer, thus efficiency of charge separation Measure the lifetime of charge carriers and recombination

5 How short are laser pulses?

6 How short are laser pulses? Q-switched lasers Mode-locked lasers Soliton lasers

7 Q-switched laser Pulse duration: 1-20 ns Peak power: 10 9 W Principle: optical switch that control cavity loss Intracavity acousto- or electro-optical modulators

8 Mode locking Appl. Phys. Lett. 5, 4 (1964); (2 pages) LOCKING OF He-Ne LASER MODES INDUCED BY SYNCHRONOUS INTRACAVITY MODULATION L. E. Hargrove, R. L. Fork, and M. A. Pollack Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey pulsetrain 1/f1 rep time

9 Train of pulses spaced in time by c d n n t t t n n Sep ) 2( 1 gainbandwidth N Nc d N t t Sep P 1 1 2

10 How to get mode-locking Obtained with devices placed inside the cavity or via Self mode-locking Active mode-locking Electronic devices externally controlled Acustoptic modulator (AOM) Crystal that modulates phase of modes through an acoustic wave launched by a trasducer Passive mode-locking Based on optical effects which are self-established upon arrival of high intensity perturbations Kerr-Lens Mode Locking Based on the nonlinear effect of self-focusing in which a variation of the refractive index induces focusing of the beam

11 Dye lasers 4 level system arising from vibrational levels Broad profile for multimode operation Different dyes for covering from UV to NIR C. Shank E. Ippen

12 Ti:sapphire laser Wilson Sibbett (St Andrews) Central wavelength: 790 nm, tuneable nm Pulse duration: sub 10 fs to 1 ps Average power: 300 mw to 4 W (oscillator) Repetition rate: 40 MHz - 1GHz

13 Ti:sapphire: what s in the box Pump laser Nd:YVO 4 Diode lasers at 810 nm Output 1064 nm SHG 532 nm Power: 3 to 15 Watts Ti:sapphire oscillator

14 Central wavelength: 790 nm, limited tunability Pulse duration: 15 fs to 1 ps Average power: 1 W to 3 TW pulse energy (mj to mj) Repetition rate: 1 MHz to 10 Hz Need peak power? Amplifier system The concept of chirped pulse amplification The realization

15 Need different colors? Optical parametric amplification Intensity (norm.) Photon energy: 1.3 to 0.25 ev Wavelength: 1 to 5 mm Energy (ev)

16 Time resolved spectroscopy methods

17 Time resolved photoluminescence Time correlated single photon counting Streak Camera - High sensitivity - High dynamic range - Time resolution >20 ps - High sensitivity - Poor dynamic range - Time resolution >2 ps

18 Time resolved pump probe Femtosecond transient absorption Heterodyne pumpprobe - Possibility to probe different excitations - Time resolution: depends on laser source ~100 fs (can be down to 10 fs in visible) - Maximum delay determined by laser rep rate - Can be interfaced with electrical experiments - Possibility to probe collinearly (ideal for microscopy) - Time resolution: depends on laser source ~100 fs (can be down to 10 fs in visible) - Maximum delay determined by laser rep rate - Possibility to monitor Four Wave mixing

19 Nonlinear optical techniques Second harmonic generation IPHT (Jena) - Probe directly electric field at interfaces - Can monitor charge transfer at buried interfaces - Time resolution limited by the laser pulse

20 Outline Introduction Why do we need time resolved spectroscopy in OPV? Short laser pulses and methods How short? Mode-locked lasers and amplified pulses Time resolved spectroscopy methods Applications to organic photovoltaics Ultrafast spectroscopy of polymers and polymer:fullerene blends Nonlinear optical methods

21 A Fundamental excitations Ground state S n S 1 Exciton Ex S n S 1 S 0 Polaron Polaron S 0 pump-probe spectroscopy Detector Sample Delay Time probe LUMO HOMO P 1P2 0 LUMO HOMO Absorption P 1 P 2 P 1 How to probe short living polarons? Probe-beam: 1.31eV-0.25eV Ex Bleaching Wavelength P 2

22 Deschler, EDC et al. Phys. Rev. Lett. 107, (2011) Probing charge transfer exciton recombination with time resolved PL Norm. PL intensity Time (ps) 0% 2% 4% 5%

23 PL intensity (arb. units) 0% 2% 4% 5% Time (ps)

24 Phys. Rev. Lett. 107, (2011) Probing polaron formation with pump-probe E Probe

25 Increased polaron formation -T/T (x10-4 ) % 2% 4% 5% Time delay (ps) E Probe

26 Photoinduced polaron pairs 0.1 P fs Chemically induced OD (arb. u.) GB GB GB P 1 P 1 P 1 P 1 Ex Ex Ex P 2 P 2 P Optically induced (10-4 ) Nature Comm. 3, 970 (2012) -0.1 GB Wavelength (nm)

27 Polymer fullerene blends

28 Example of spectrally resolved pump-probe

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30 Time resolved harmonic generation

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33 Conclusions Time resolved optical methods unravel important phenomena in photovoltaics: charge transfer, recombination, build up of electric fields, exciton diffusion, etc. They can be used on a variety of materials: polymer and small molecule OPV, perovskites, nanocrystal, dye sensitized, etc. Offer important validation tools for theoretical modelling and predictions Non destructive characterization techniques