Поляризационная спектроскопия

Transcription

1 UNIVERSITY OF CAMBRIDGE Cavendish laboratory Поляризационная спектроскопия органических полупроводников Артем Бакулин

2 Plastic electronics Transistors Displays Solar cells

3 The bulk-heterojunction concept: LUMO HOMO LUMO 1 s donor 45 fs 10 s ps HOMO acceptor h ps 1. Optical excitation 2. Charge separation 3. Charge-transfer state 4. Recombination 5. Or Free charges

4 What happens in such a PV cell at the molecular level?

5 Transient Absorption Spectroscopy pump p probe singlet charge triplet ts) 1.0 (arb.uni T/T probe energy (ev)

6 Transient Absorption Spectroscopy pump p probe singlet charge triplet - T/T T (arb.un nits) Delay (a.u.)

7 IR Signatures of Charges HOMO LUMO LE - T/T (a.u.) Absorption Probe (ev) LE HE 0.0 PIA PPV/C60 VAIR cw Wavenumber (cm -1 ) Photo-Induced Absorption in IR is an excellent probe for the concentration of charges Low-energy (LE) peal allows background-free measurements Bakulin et al., JPC B112, (2008)

8 Vis (35 fs) IR (70 fs) spectroscopy set-up 3um probe Tunable pump

9 Vis (35 fs) IR (70 fs) spectroscopy set-up

10 Samples: donor- acceptor blends Excitation C 60 Acceptors: Weak CTC OD (arb.units s) MEH-PPV MEH-PPV:TNF MEH-PPV:DNAQ MEH-PPV:C (nm) Goris et al, J of Mat. Sci.(2005) Drori et al, PRL (2008) Strong CTCs O 2 N NO 2 O NO 2 O NO 2 O DNAQ TNF Bakulin et al, Synth. Met. (2004) NO 2

11 Watching charge dynamics in solar cell Excitation by theultrafast Laser Charge Concentration Time

12 Spectroscopy vs. cell performance its) 1 Time-resolved PIA C 60 () (a) Good solar cell - T/T T (arb.un 0.1 CTCs Photocurrent 0.01 Bad excitation solar ti 650 cells nm Delay (ps)

13 What do we know about charge transport? Conductivity within chain and through the complete device.

14 One can build a microscopic model Distribution ib ti of states t + hopping rates V ij (E,T, ) Remember Nir s talk

15 Can we measure the distribution of hopping rates? ij

16 E Polarization sensitive technique

17 Polarization TA in CTCs E E HIGH anisotropy

18 Polarization TA in CTCs E E lower anisotropy

19 Polarization TA in CTCs E E lower anisotropy

20 Polarization TA in CTCs E E no anisotropy

21 Measuring transient anisotropy information about hopping (V ij ) Transient Anisotropy Time The distribution of hopping probabilities (V ij ) will depend on: polymer doping external field temperature morphology and modeling is essential

22 Example I. Localization of charges

23 Photophysics in CTCs 1 C 60 (a) - T/ /T (arb b.units s) 0.1 Good solar cell CTCs 0.01 excitation Bad solar 650 cells nm Delay (ps)

24 Transient Anisotropy in CTCs Localization of charges in pronounced CTCs Dynamics do not depend on wavelength same pathway of charge generation Chem. Phys. Lett. 482, 99 (2009)

25 Photophysics of CTC donor acceptor Efficient pathway of geminate recombination Efficient pathway of geminate recombination Charges are localized

26 Photophysics in ternary PPV/DNAQ/C 60 blend 0,4 PPV/DNAQ Aniso otropy 0,3 02 0,2 PPV/DNAQ/C 60 0,1 superposition PPV/C 60 0, Delay (ps)

27 Charge dynamics in ternary blend CTC e ~0.4 ev PPV h DNAQ C O 60 * O n * O NO 2 NO 2 O 1+1+1=

28 Example II. Charge photogeneration pathway.

29 Significance of PCBM excitation MDMO-PPV PCBM extinctio on per mo ol 1.0x x10 3 PCBM MDMO-PPV Sun photon flux (nm) Fullerene provides absorption in the IR region Fullerene absorption in Visible is probably underestimated aggregated dispersed

30 Polarization sensitive technique Muller et al, PRB (2005) Westenhoff et al, JACS (2008) Bakulin et al, JPC B (2008) E Polymer excitation

31 Polarization sensitive technique E E Polymer excitation HIGH anisotropy

32 Polarization sensitive technique E Fullerene excitation

33 Polarization sensitive technique E E Fullerene excitation LOW anisotropy

34 PCBM excitation contrast The contrast of PCBM excitation vs. excitation of polymer/ctc 1% PCBM 70% PCBM ~ 10

35 AFM Images 1x1 m 0.2x0.2 m 1x1 m 5x5 m 1x1 mm 5x55 m

36 Charge generation pathways Evidence of the morphology change

37 Conclusions Ultrafast t spectroscopy is a powerful tool for studying charge dynamics Anisotropy of nonlinear response provides information about delocalization of charges and charge generation pathway Potential for development of chargetransport models

38 Richard Friend Jenny Clark Simon Gelinas Akshay Rao Acknowledgements Maxim Pshenichnikov, Paul van Loosdrecht, Kees Hummelen (University it of Groningen) Dmitry Paraschuk, Dmitry Martianov, Sergey Zapunidy (Moscow State University) J. Piris, T. Dykstra, L. Siebbiles (Technical University of Delft) Group of prof. K. Mullen Funding: University of Cambridge University of Groningen & Zernike Institute for Advanced Materials, EPSRC, NWO, РФФИ

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40 Спасибо за внимание!