Time resolved optical spectroscopy methods for organic photovoltaics Enrico Da Como Department of Physics, University of Bath
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
Energy Why time resolved spectroscopy for OPV Polymer Fullerene LUMO Charge separation absorption LUMO HOMO HOMO Large donor-acceptor interface morphology & mobility
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
How short are laser pulses?
How short are laser pulses? Q-switched lasers Mode-locked lasers Soliton lasers
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
Mode locking Appl. Phys. Lett. 5, 4 (1964); http://dx.doi.org/10.1063/1.1754025 (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
Train of pulses spaced in time by c d n n t t t n n Sep 2 1 2 1) 2( 1 gainbandwidth N Nc d N t t Sep P 1 1 2
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
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
Ti:sapphire laser Wilson Sibbett (St Andrews) Central wavelength: 790 nm, tuneable 690-1040 nm Pulse duration: sub 10 fs to 1 ps Average power: 300 mw to 4 W (oscillator) Repetition rate: 40 MHz - 1GHz
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
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
Need different colors? Optical parametric amplification Intensity (norm.) 1.0 0.5 Photon energy: 1.3 to 0.25 ev Wavelength: 1 to 5 mm 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Energy (ev)
Time resolved spectroscopy methods
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
Time resolved pump probe Femtosecond transient absorption Heterodyne pumpprobe www.fisi.polimi.it - 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
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
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
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
Deschler, EDC et al. Phys. Rev. Lett. 107, 127402 (2011) Probing charge transfer exciton recombination with time resolved PL Norm. PL intensity 0 200 400 600 800 Time (ps) 0% 2% 4% 5%
PL intensity (arb. units) 0% 2% 4% 5% 0 100 200 300 Time (ps)
Phys. Rev. Lett. 107, 127402 (2011) Probing polaron formation with pump-probe E Probe
Increased polaron formation -T/T (x10-4 ) 2 0 2 0 2 0 2 0 0% 2% 4% 5% 0 50 100 150 200 250 300 Time delay (ps) E Probe
Photoinduced polaron pairs 0.1 P 2 5 @ 100 fs Chemically induced OD (arb. u.) 0.0-0.1 0.1 0.0-0.1 0.1 0.0-0.1 0.1 0.0 GB GB GB P 1 P 1 P 1 P 1 Ex Ex Ex P 2 P 2 P 2 0-5 10 0-10 5 0-5 5 0 Optically induced (10-4 ) Nature Comm. 3, 970 (2012) -0.1 GB 500 1000 1500 2000 2500 3000 3500 Wavelength (nm)
Polymer fullerene blends
Example of spectrally resolved pump-probe
Time resolved harmonic generation
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