Organic Photovoltaic Devices. Hole Transfer Dynamics in. Maxim S. Pshenichnikov. Jan C. Hummelen. Paul H.M. van Loosdrecht. Dmitry Paraschuk (MSU)

Transcription

1 Federal Agency for Science and Innovations, Russia (grant ) NGC211, Moscow, Sep 211 Artem A. Bakulin (Cambridge U) Almis Serbenta Jan C. Hummelen Vlad Pavelyev Paul H.M. van Loosdrecht Dmitry Paraschuk (MSU) Maxim S. Pshenichnikov Hole Transfer Dynamics in Organic Photovoltaic Devices Zernike Institute for Advanced Materials, University of Groningen, the Netherlands International Laser Center, MSU, Russia

2 Polymer Solar Cells Mixture of polymer and fullerene

3 Polymer Solar Cells Advantages: Inexpensive Lightweight Flexible Tunable properties Printing-processable

4 Niche Market Products Advantages: Inexpensive Lightweight Flexible Tunable properties Printing-processable

5 Bulk-Heterojunction Concept Anode (Al) p-type / n-type MDMO-PPV/PCBM polymer: MDMO-PPV Light absorption Cathode (ITO) glass e - e - e - hω fullerene: PCBM Charge separation e - e - e - e - Brabec, Sariciftci, Hummelen, AFM 11, 15 (21) e - e -

6 Bulk-Heterojunction Concept Anode (Al) p-type / n-type MDMO-PPV/PCBM polymer: MDMO-PPV Light absorption Phase separation Cathode (ITO) glass e - e - e - - Interpenetrating network - Typical domain size 1 s nm hω fullerene: PCBM - Increased interface area - Electron and hole transport Charge separation e - e - e - e - Brabec, Sariciftci, Hummelen, AFM 11, 15 (21) e - e -

7 LUMO Spectroscopic Viewpoint e - hω HOMO + donor polymer: MDMO-PPV - LUMO HOMO acceptor fullerene: PCBM e - hω h + acceptor donor

8 LUMO Why Going Ultrafast? e - hω HOMO + donor polymer: MDMO-PPV - LUMO HOMO acceptor fullerene: PCBM e - 1 fs = 1-15 s 1 ps = 1-12 s 1 µs 45 fs 1 s ps hω 1 fs h + acceptor donor

9 Charge dynamics studies on polymer/fullerene mixtures Electron transfer Vardeny (ps-fs broad band probe studies ) Heeger (1 fs VAIR, sub-ps NIR spectroscopy) Sariciftci (5 fs resolution ET 45 fs ET time) Friend (1 fs exciton and charge dynamics) Sundstrom (low flux spectroscopy) Janssen (ET in diads, copolymers & ) Many, many more.. Dynamics of charges after the polymer excitation: Exciton diffusion in polymer (up to 2 ps, ~1 nm) Photoinduced electron transfer 45 fs Geminate recombination of charges (1 s ps)

10 Fullerene Excitation and Hole Transfer LUMO HOMO donor polymer: MDMO-PPV e - LUMO + hω - HOMO acceptor fullerene: PCBM e - hω acceptor donor h +

11 Fullerene Excitation and Hole Transfer LUMO HOMO donor polymer: MDMO-PPV - LUMO + hω h + HOMO acceptor fullerene: PCBM e - hω acceptor donor Fullerene accounts for 5-8% of the solar cell active layer h +

12 Significance of Fullerene Excitation MDMO-PPV [C6]PCBM [C6]PCBM MDMO-PPV Sun photon flux Fullerene provides absorption in the red and IR region Wavelength λ (nm) extinction per mol

13 Absorption (a. u.) 1. Significance of Fullerene Excitation [6]PCBM [7]PCBM P3HT [C7]PCBM P3HT.5 Fullerene provides absorption in the red and IR region Wavelength (nm) Fullerene absorption in Visible is probably underestimated Cook et al., CPL 445, 276 (27) aggregated dispersed For modern materials, fullerene absorption amounts up to 3%!

14 Charge dynamics studies on polymer/fullerene mixtures Electron transfer Hole transfer Vardeny (ps-fs broad band probe studies ) Heeger (1 fs VAIR, sub-ps NIR spectroscopy) Sariciftci (5 fs resolution ET 45 fs ET time) Friend (1 fs exciton and charge dynamics) Sundstrom (low flux spectroscopy) Janssen (ET in diads, copolymers & ) Many, many more.. Wienk et al, 23 (C 7 PCBM/PPV,.5 ps resolution) Cook et al, 29 (PCBM/P3HT,.2 ps resolution) Bakulin et al., AFM 2, 1653 (21) PCBM/PPV, 5 fs resolution Dynamics of charges after the polymer excitation: Exciton diffusion in polymer (up to 2 ps, ~1 nm) Photoinduced electron transfer 45 fs Geminate recombination of charges (1 s ps)??

15 Questions to Answer In the blends of polymer and fullerene: What is the Hole-transfer time? What are dynamics of PCBM exciton-tocharge conversion?

16 Cover story: The active layer of the currently most efficient plastic photovoltaic cells is a blend of polymer and methanofullerene molecules. In their article M.S. Pshenichnikov et al. show that hole transfer upon methanofullerene excitation operates simultaneously with electron transfer as the charge generation process in plastic photovoltaics, at a staggering timescale of 3 fs. AFM 2, (21) Laser light Fullerene domain Polymer Exciton diffusion Charges on PPV Polyme Polymer

17 How to Excite Selectively PCBM? Spectral selectivity! Linear absorption experiments CW PIA photoexcitation Goris et al., J of Mat. Sci. (25) Drori et al, PRL (28) Maximum contrast of PCBM excitation is provided by nm pump

18 Linear Absorption Spectra % PCBM 1 % 15 % 3 % 5 % 7 % MDMO-PPV Increase in absorption [C6]PCBM Optical Density Wavelength (nm) Sample preparation: Drop-casting from chlorobenzene solution of PCBM and MDMO-PPV on 18-µm thick fused silica microscope cover slides sample thickness ~2 nm

19 Linear Absorption Spectra % PCBM 1 % 15 % 3 % 5 % 7 % nm PCBM content (%) Linear increase.1. Optical Density Wavelength (nm) Contrast: σ = PCBM extinction PPV extinction 1 (at 63 nm)

20 How Can We See Charges on Polymer? PPV Upon polaron formation in polymer, new optical transitions are formed LUMO LUMO Band gap HOMO + HOMO High-energy (HE) polarons Low-energy (LE) polarons Ground State Charged excitations Polarons Heeger 92, 99; Vardeny 96, 3

21 Optical Signatures of Charges LUMO.8 (ev) HE LE T/T (a.u.) HOMO. -.2 VAIR LE PIA PPV/C6 cw Wavenumber (cm -1 ) HE Photo-Induced Absorption (PIA) is an excellent probe for concentration of charges Bakulin et al., JPC B112, 1373 (28)

22 Optical Signatures of Charges 1. LUMO (ev) Absorption Probe HE LE T/T (a.u.) HOMO. -.2 VAIR LE PIA PPV/C6 cw Wavenumber (cm -1 ) HE Photo-Induced Absorption (PIA) is an excellent probe for concentration of charges Low-energy (LE) peal allows background-free measurements Bakulin et al., JPC B112, 1373 (28)

23 3-fs visible pulses 7-fs IR pulses The Instrument Ultrafast Photocamera

24 Delayed IR probe pulse ~3 µm 8 fs Concept of Experiment hω Pump pulse ~63 nm 3 fs Chopper Sample T/T T/T (arb.units) Excite spectral-selectively fullerene Watch for arrival of the hole to polymer Hole arrival should be delayed by: hole-transfer time (low %fullerene ) exciton diffusion time (high %fullerene) Delay (ps)

25 Experimental Transients PCBM = 1% PCBM = 5% PCBM = 15% PCBM = 3% PCBM = 5% Change in transmission - T/T (x1 3 ) Ge thin film: Instantaneous response δτ PCBM = 7% Delay (fs) MDMO-PPV [C6]PCBM

26 Results I. Hole-Transfer Time PCBM = 1% PCBM = 5% % PCBM 1% PCBM = 15% 15% 3% PCBM = 3% 5% 7% Ge PCBM = 5% Delay (fs) Change in transmission - T/T (x1 3 ) T/T (arb.u.). Ge thin film: Instantaneous response δτ δτ PCBM = 7% Delay (fs)

27 Results I. Hole-Transfer Time High concentration: PCBM excitation Delay δτ (fs) MDMO-PPV/PCBM Ge Low concentration: Polymer excitation PCBM mass content, %

28 Results I. Hole-Transfer Time Delay δτ (fs) High concentration: PCBM excitation Hole-transfer time of 3 fs Hole-transfer is very efficient! MDMO-PPV/PCBM Ge Low concentration: Polymer excitation Two-component model: PCBM mass content, % σ N δτ = σ N + (1 N ) δτ MAX

29 Results I. Hole-Transfer Time MDMO-PPV [C6]PCBM P3HT [C7]PCBM P3HT:[7]PCBM MDMO-PPV:[7]PCBM Delay δτ(fs) ~3 fs [C6]PCBM MDMO-PPV:[6]PCBM PCBM concentration (%) ~6 fs [C7]PCBM

30 Results I. Hole-Transfer Time MDMO-PPV [C6]PCBM P3HT [C7]PCBM P3HT:[7]PCBM MDMO-PPV:[7]PCBM WHY??? Delay δτ(fs) ~3 fs [C6]PCBM MDMO-PPV:[6]PCBM PCBM concentration (%) ~6 fs [C7]PCBM

31 HOMO Energies Argument LUMO HOMO - LUMO + hω h + HOMO Materials HOMO (ev) C6-6.1 PCBM -6.1 [7]PCBM -6.1 P3HT -5.2 MDMO-PPV HOMO level mismatch (ev) P3HT MDMO-PPV C PCBM.9.79 [7]PCBM.9.79 The energy argument does not work!

32 Results II. PIA Amplitudes PCBM = 1% PCBM = 5% PCBM = 15% PCBM = 3% PCBM = 5% PCBM = 7% Change in transmission - T/T (x1 3 ) Delay (fs)

33 Results II. PIA Amplitudes PCBM = 1% Change in transmission - T/T (x1 3 ) PCBM = 5% PCBM content 1 % 9 PCBM = 15% 5 % 15 % 18 3 % PCBM = 3% 5 % 7 % 1 PCBM = 5% % - T/T (x1 3 ) Delay (fs) PCBM = 7% Delay (fs)

34 Charge Generation Efficiency P3HT:[7]PCBM MDMO-PPV:[7]PCBM MDMO-PPV:[6]PCBM [- T/T]/OD PCBM concentration (%).1. PC 71 BM Concentration R Exciton Hole Electron R exciton travel distance P3HT

35 From Cartoons to AFM Images 1x1 µm.2x.2 µm 1x1 µm 5x5 µm 1x1 µm PCBM 5x5 µm PPV Spatial correlation analysis: ϕ( a, b ) f ( x, y ) f ( x + a, y + b ) = dxdy

36 Phase Separation and Charge Generation Phase separation scale (nm) PCBM content (%) 4 2 Peak value of - T/T (x1 3 ) Correlation between the amount of charges and domain size

37 Conclusions Ultrafast Vis IR spectroscopy is a powerful tool for studying early-time dynamics in organic PV Hole transfer time in polymer-fullerene blends is extremely fast so this process is very efficient Blend morphology strongly influences the hole transfer process Charge generation efficiency decreases with the increase of fullerene concentration -- the method is interface-sensitive More experiments on real devices to come

38