Electron transfer optimisation in organic solar cells

Transcription

1 Electron transfer optimisation in organic solar cells James Durrant Centre for Electronic Materials and Devices Departments of Chemistry Imperial College London Introductory remarks Charge recombination vs. charge separation and transport Interface engineering Inhomogeneity

2 Why organic PV now? Political: global warming Commercial: perception that Si based PV may not have the potential for mass PV production Scientific: building up recent advances in Organic electronics LED s and FET s Molecular electronics: supermolecular photochemistry Materials control and measurement on the nanometer scale

3 Organic photovoltaic technologies Glass substrate ITO Mixed Layer e - h + dye sensitised photoelectrochemical + - Molecular thin film Polymer/C60 blend Silver paint Polymer (~50nm) Au electrode PEDOT ITO substrate Porous TiO 2 (~100 nm) Dense TiO 2 (~ 40 nm) (Hole blocking layer) Organic/inorganic hybrid Light Device structure TiO 2 nanoparticles

4 Stability Challenges liquid versus solid state, O 2, water. Processibility low temp processing on flexible substrates Efficiency Improved red spectral response Improved voltage and FF whilst maintaining high IQE Efficiency versus processibility / stability issues Haque et al. Chem Comm 2003

5 Molecular donor/acceptor dyads S S OC 6 H 13 C 8 H 17 S S OC 12 H 25 S S S C 6 H 13 O S S S C 12 H 25 O C 8 H m OD 2.0 τ 50% = 0.8 µs m OD Time (µs) τ 50% = 20 µs Time (µs)

6 Kinetics in organic solar cells Polymer e - Charge separation e - transport e - electron collection hυ Charge recombination hole collection ITO h + h + transport C 60 Al

7 Light driven charge separation hν Electron injection Charge recombination e - TiO Electron injection yield Ultrafast injection m OD Millisecond recombination Time / ps Log 10 time / seconds Tachibana et al. J. Phys Chem 1996

8 Model of Reaction dynamics Injection S* / S + CB Trapping Charge recombination Transport S / S + TiO 2 Dye Charge recombination dynamics controlled upon electron transport and interfacial electron transfer kinetics depending upon metal oxide and sensitiser dye employed.

9 Molecular Control of Recombination O Ti O O Ti O C Ru C r k exp(-βr) where β = 0.95 ± 0.2 Å-1 Clifford et al JACS 2004

10 Signatures of transport in recombination dynamics Relative density of excited dyes S(t)/S mV 300mV 200mV 100mV 0 mv e per Dye Ethanol triflate t 50% n = A t n E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 Time (ns) Dispersive (Stretched exponential) decays Strong dependence on TiO 2 E F Haque et al. J Phys Chem B 1998, 2000, elson et al. Phys Rev B 1999, 2001 t 50% / ns on-linear dependence on electron density: t 50% n -1/α

11 Recombination in MDMO- PPV/PCBM blends π Charge separation T = 220 K T = 298 K 10-5 Charge recombination π polymer PCBM OD time (s) Recombination kinetics dominated by slow, thermally activated power law decay resulting from positive polaron trapping in polymer Montanari et al. APL 2002 ogueira et al. J Phys Chem B 2003

12 Recombination versus Transport in polymer / C60 devices g(e) Absorbance µJ 0.25µJ 75µJ 75µJ data 4µJ data 0.22µJ data Time (s) TAS studies of recombination Current Density / arb. units V 30V Time / s TOF studies of transport Smooth lines from trapping/detrapping model with same dos Same microscopic model explains both recombination and transport Open question of benefit of traps

13 Recombination versus transport in dye sensitised solar cells Transport dynamics Recombination to redox couple Recombination to dye cations TiO 2 200µs 10ms 600µs SnO 2 300ns 9µs ~ 600ns CB Injection S* / S + Trapping Recombination Transport S / S + TiO 2 Dye I - /I 3 - Regeneration J Current Density / Acm -2 sc TiO 2 SnO 2 /MgO SnO V Voltage/V oc

14 Charge separation versus recombination V/2 J av J av J V/2 J J ca Two level system numerical model of organic solar cell Based on assumption that electronic coupling for charge separation and recombination scale proportionally. Monochromatic Efficiency Charge separation rate / s -1 J.elson et al.phys.rev.b 2004, Appl.Phys.A 2004

15 Charge separation in dye sensitised solar cells Dye sensitised film 1.0 (i) Injection Yield 0.5 (i) (ii) Solar cell (ii) 0.0 Haque et al. JACS time / picoseconds

16 Dynamics versus Device function Electrolyte J sc /macm -2 V oc /Volts η / % τ 50% (inj) τ init (rec) +Li ~10 ps 20 ms +Li + /tbp ~150 ps 100 ms +tbp ~ 1 ns 400 ms Optimised device: Injection just sufficient to compete with excited state decay to ground Allows minimisation of recombination losses

17 Influence of electrolyte composition upon density of conduction band / trap states TiO Electrolyte control of interfacial dynamics 2 TiO 2 Dye TiO 2 Dye TiO 2 Dye E CB / trap states 1 hv D * /D + 1 hv D * /D + CB / trap states 1 hv D * /D I - / I 3-2 I - / I 3-2 I - / I 3 - Electrolyte B: o Li + Slow Electron Injection (1) Slow Charge Recombination rates (2) & (3) D/D D/D ++ Electrolyte B + tert-butyl pyridine D/D + Electrolyte A Electrolyte A: Both Li + and 4-tert-butyl pyridine + tert-butyl pyridine and Li + Intermediate Electron Injection rate (1) Intermediate Charge Recombination rates (2) & (3) Electrolyte C: o 4-tert-butyl pyridine Fast Electron Injection rate (1) Fast Charge Recombination rates (2) & (3) Optimum device performance: injection half-time ~ 150 ps D/D + Electrolyte C + Li +

18 Materials approaches to control of interfacial electron transfer dynamics 0.25 a b m O.D H 3 CO A HO H3C H 3 C B CH 3 CH 3 Haque et al. Adv Mat 2004 c d Al 2 O 3 coated Uncoated 0.00 SO 3 a Time / Seconds 3 Li + - DFHTM Palomares et al. JACS 2003 Li + - DFHTM TiO 2 Dye MFHTM OCH 3 OCH3 m OD 2 1 TiO 2 Dye -Li + + Li + DFHTM MFHTM O n O O Li + O O Li + O O O Haque et al. Adv Func Mat Time / Seconds

19 Heterosupramolecular Photochemistry nanoseconds hν ~ 1 s Supramolecular control of recombination dynamics picoseconds TiO 2

20 Distance control: supersensitiser function 719 Pump:550nm, Probe:800nm 845 Pump:516nm, Probe:850nm Hirata et al. Chem. Eur. J O.D. (normalized) 1 HOMO calcs: Increase in distance ~ 4 Å 0 1E-6 1E-5 1E-4 1E Time [s] COOH e - HOOC HOOC Ru C C S COOH HOOC HOOC Ru C C S O OCH 3 S S OCH 3

21 Influence of inhomogeneity Wide bandgap semiconductor Adsorbed Sensitiser Dye Electrolyte Electron injection e - S * / S + inhomo Charge recombination S 0 / S + e - I - Dye re-reduction / I 3 - Inhomogeneous energetics result in non-exponential dynamics and make device optimisation much harder

22 Modelling electron injection energetics g(e) E g 2 g 0 g 1 g(e) exp(e/e 0 ) 1.0 (i) <d i >=0 d 2 d 1 1 D* / D + Injection Yield 0.5 (i) (ii) Excited state decay to ground (ii) k ( d ) = k( 0) i V V TiO 2 2 ( di ) ( 0) 2 Dye 2d i ( 0) exp E0 Monte Carlo Simulation as detailed in: Tachibana et al. (2002) J. Photochem Photobiol A: Chemistry Only fit parameters k(0) and ratio /E 0 = k time / picoseconds Inhomogeneous broadening inhomo ~ 0.15 ev film inhomo ~ 0.3 ev DSSC

23 Hole transfer in solid state DSSC s: Wide bandgap semiconductor Adsorbed Sensitiser Dye Hole transporting material e - S * / S + OCH 3 OCH 3 Conduction Band H 3 CO OCH 3 H 3 CO OCH 3 S 0 / S + e - Dye re-reduction HTM/HTM + OCH 3 OCH 3 Valence Band Hole transfer ~ 300 ps (Another example of kinetic redundency!) Hole transfer controlled by thermodynamics not kinetics

24 Hole transfer yield as function of mean reaction free energy Experimental data Inhomogeneous Model Yield of hole transfer / % Homogeneous Model G (Dye-HTM) / ev Inhomogeneous Model E m (D/D + ) Distribution of D/D + states + Vacuum Level IP E m (HTM/HTM + ) G (Dye-HTM) = E m (HTM + / HTM) E m (D + / D) R 1 R 2 Haque et al. Chem Phys Chem (2003) R 4 R 3

25 Minimisation of energetic inhomogeneity R 1 R 2 Dye regeneration efficiency / % ITO TiO 2 Dye HTM / Li + Li + Li + Li + Li + Li + Li + Li + Li + Li + Li + + Li + -Li G (dye-htm) ITO TiO 2 Dye HTM / ev O R O 4 Li O O O + Li + O O O 2[(CF 3 SO 2 ) 2 ] - R 1 R 4 R 3 R 2 R 3 Ionic screening by Li + ions reduces inhomogeneity of hole transfer energetics

26 Conclusions Exciting times for organic PV Optimisation of electron transfer dynamics in organic PV requires consideration of: Recombination versus transport, and the role of traps Charge separation versus recombination and the potential for interface engineering Energetic inhomogeneities

27 Acknowledgements Colleagues at Imperial College: Jenny elson, Donal Bradley, David Klug Steffan Cook, Ana Flavia ogueira, Ivan Montanari, Samantha Handa Emilio Palomares, Saif Haque, arukuni Hirata, Alex Green, Hari Upadahyaya, John Clifford Collaborations: Michael Gratzel (EPFL), Jan Kroon (EC) Andrew Holmes (Cambridge / ICL), Serdar Sariciftci (Linz) Christoph Brabec (Siemens/Konarka), azario Martin (Madrid), Kees Hummelen (Groningen), Merck Chemicals, Dow Chemicals, Covion GmbH, Johnson Matthey Ltd. Funding: EPSRC, DTI, EU, BP