Molecular Design of Organic Dyes. for Hybrid Solar Cells. Institute of Molecular Sciences - University of Bordeaux -

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1 Molecular Design of Organic Dyes for Hybrid Solar Cells Céline OLIVIER Institute of Molecular Sciences - University of Bordeaux - Symposium on Quantum Modeling of Electronic Processes in Optoelectronic Devices November, 7 th -8 th BORDEAUX

2 The Photovoltaic Issue Introduction SOLAR RADIATION ELECTRICAL POWER Definition : method of generating electrical power by converting solar radiation into current electricity using semiconductors that exhibit the photovoltaic effect. ENERGY = A Major Environmental Concern 1. ENERGY 2. Water 3. Food 4. Environment 5. Poverty 6. War 7. Disease Humanity s Top 10 Preoccupations for next 50 years RENEWABLE ENERGY SOURCES Cheap ; Abounding; Green SOLAR CELLS Classical solar cells based on crystalline silicon = high cost / limited production Dye-Sensitized Solar Cells (M. Grätzel 1991) = genuine alternative

3 The Photovoltaic Issue Introduction

4 Dye-Sensitized Solar Cells Introduction Prof. Michael Grätzel B. O Regan, M. Grätzel Nature 1991, 353, Many advantages : Transparency due to TiO 2 nanoparticles Aesthetic coloration due to the dyes Low cost fabrication Diffuse light Flexibility

5 First applications Introduction Konarka Technologies Became First Company to Receive EPFL License for DSCs Solar panels and modules DSSC-solar bags, Cell phone chargers Flexible DSC-based solar module for Australian Army camouflage (Wales, (Australia,

6 Analogy with photosynthesis Introduction Leaves vs. DSCs Electrons generated by chlorophyll Chemical energy formation Electrons generated by the dye Electrical power formation

7 Schematic diagram of the electron flow in DSCs Introduction The principle Semiconductor = nanoporous metal oxide Photoexcitation of the dye is followed by electron injection into the conduction band of an oxide semiconductor thin film. The dye molecule is regenerated by the redox system contained in the electrolyte, which itself is regenerated at the counter-electrode by electrons passed through the load. Highest efficiency : 12.3 % of power conversion (M. Grätzel et al. Science 2011)

8 Energy Levels vs. vacuum (Potentials vs. NHE) Schematic diagram of the electron flow in DSCs Introduction Energy diagram (-1.0 V) (-0.5 V) (0.0 V) (+0.5V ) (+1.0V )

9 Absorption Molaire (10-4 Mol -1.cm -1 ) Original studies Reference dyes State of the Art Ruthenium-based dyes yielding up to 11% of conversion efficiency N719 in EtOH l (nm) B. O Regan, M. Grätzel Nature 1991, 353, 737; M. Grätzel Nature 2001, 414, 338.

10 Best liquid-electrolyte based system reported so far State of the Art E vs. NHE (V) CB S + /S* V oc I - /I V Dyes C 6 H 1 3 C 6 H 1 3 N N N Z n N N C O O H C 6 H 1 3 C 6 H 1 3 C 8 H 17 O O C 8 H 17 N N N Z n N N O O C 8 H 17 C 8 H 17 C O O H VB TiO 2 S + /S 0 Dye Co II /Co III 0.53 V Redox Shuttle N N N C o N N N Y D 2 C 6 H 13 Y D 2 - o - C 8 I I I / I I C 6 H 1 3 C 6 H 1 3 N Y S S C 6 H 13 C 6 H 13 C N C O O H C 6 H 1 3 Dye DSCs performances under AM mw.cm -2 J sc [ma.cm -2 ] V oc [mv] ff h [%] YD YD2-o-C YD2-o-C8 / Y M. Grätzel, E. W.-G. Diau et al. Science 2011, 334, 629.

11 The Semi-Conducting Oxide The Main Components TiO 2 : the most efficient semi-conductor for DSCs E vs. NHE (V) Semiconductor Dye Electrolyte - 2,4 1 MLCT - 0,8-0,5 CB e- 3 MLCT - Transparent in the visible range - Wide band-gap E g TiO 2 = 3.2 ev 0,0 V oc max. - Low-lying conduction band edge + 0,4 + 0,5 e- E CB TiO 2 = -0.5 V + 1,0 +1,3 + 2,7 VB HOMO - High surface area / Mesoporous structure S = m 2 /g + 3,7 TiO 2 N719 I 3- /I -

12 Absorption Molaire (10-4 Mol -1.cm -1 ) The Photosensitizer The photosensitizer s requirements - Light harvester in the visible range E vs. NHE (V) The Main Components TiO 2 Dye Electrolyte E Fermi e- S + /S* - Wide absorption spectrum - HOMO/LUMO levels matching with those of MO 2 and redox system - Grafting function onto the metal oxide hn e- Maximum Voltage (V oc max.) E 0 (I 3- /I - ) - Stability towards light and temperature S + /S 0 Ruthenium-based dyes : advantages/drawbacks - High efficiencies with metal-complexes N719 in EtOH - High-cost and scarcity of Ru - Moderate molar absorption coefficients - Limited engineering of the dyes l (nm)

13 Efficiency (%) The Photosensitizer The Main Components Organic dyes Large variety of organic motifs / functions High molar absorption coefficients Good power conversion efficiency rates Wide range of photosensitizers Long syntheses and purification methods Examples: Ru dyes Triarylamines 11.7% 10.1% C212 8 Coumarines Polyenes Year Remarkable advances in the field of organic dyes over the past 10 years NKX-2883

14 The Electrolyte The Main Components Iodine/Iodide-based Liquid Electrolytes Importance of the electrolyte composition Increased V oc and Fill Factor DiMethylImidazolium Iodide Tetrabutyl Ammonium tert-butyl Pyridine Triiodide Lithium Iodide Dye H + Example of liquid electrolyte : Z960 Solvent : Acétonitrile / Valéronitrile (85 / 15) DMII : 1.0 M ; LiI : 0.05 M ; I 2 : 0.03 M Guanidinium Thiocyanate : 0.1 M tert -Butylpyridine : 0.5 M DiMethylImidazolium Iodide (DMII) Guanidinium Thiocyanate (GTC) Performances up to 11% but Tert-ButylPyridine (TBP) corrosion phenomena due to iodine unadapted redox potential => important loss in potential

15 DSCs Research at ISM Molecular Design of Organic Dyes Synthesis of organic dyes showing «push-pull» effect - electron-rich donating group (i.e. triarylamine) - p-conjugated linker providing the electronic pathway - electron-withdrawing group and anchoring function New organic dyes including Naphthylamine units - X-ray crystal structure of dye 1 20 Å

16 DSCs Research at ISM Molecular Design of Organic Dyes Photophysical properties l 1 in DCM 1@TiO 2 l C212 in DCM C212@TiO 2 Dye Absorption (a) l max / nm (e/m -1 cm -1 ) Absorption grafted (b) l max / nm Emission (a) l max / nm Potentials and Energy Levels E ox (c) /V DE (d) /V E ox -DE (e) /V (42 700) (36 100) (33 400) C (30 000) (a) Abs. and Em. in CH 2 Cl 2. (b) Abs. on TiO 2 film. (c) Oxidation potential vs. NHE (CH 2 Cl 2 ). (d) DE OPT. : intersection between Abs. and Em. spectra. (e) Ground-state energy E HOMO = E OX ; First-excited-state energy E LUMO = E OX DE.

17 DSCs Research at ISM Molecular Design of Organic Dyes DFT calculations DE = kcal.mol -1 E = 0 X-ray crystal structure of dye 1 : cis/trans/trans/cis The three conformers may exist in solution at room temperature DE = kcal.mol -1

18 DSCs Research at ISM Molecular Design of Organic Dyes DFT calculations

19 DSCs Research at ISM Molecular Design of Organic Dyes DFT calculations HOMO : located on the electron-donating triarylamine unit LUMO : located on the electron-withdrawing part Isodensity surface plots of the molecular frontier orbitals of 1 Spatial separation favouring intramolecular charge transfer and electron injection into TiO 2

20 IPCE / % Current Density / ma.cm-2 DSCs Research at ISM Molecular Design of Organic Dyes Photovoltaic performances Voltage / V Dye C212 Wavelength / nm IPCE : Incident Photon-to-current Conversion Efficiency High plateau > 80% from 450 to 590 nm J sc [ma.cm -2 ] V oc [mv] FF [%] h [%] J-V curves : photocurrent-voltage curves ( black : in the dark ; red : at 1 sun) Highest efficiency reached with dye 1 : h = 6.6% Best configuration for the series : 1 naphthyl unit in the p-conjugated linker 2 phenyl rings on the external part M e O N C N C N S S C O O H Electrolyte Z960; Light source: AM mw.cm -2 ; Working area: 0.16 cm 2 M e O ChemSusChem. 2011, 4,

21 DSCs Research at ISM Molecular Design of Organic Dyes New organic dyes including Carbazole units High oxidation potential Use of redox mediator with E ox > E ox (I - /I 3- ) Increase the V oc Increase the PCE (a) Abs. and Em. in CH 2 Cl 2. (b) Oxidation potential vs. NHE. (CH 2 Cl 2 ). (c) DE OPT. : intersection between Abs. and Em. spectra. (d) Ground-state energy E HOMO = E OX ; First-excited-state energy E LUMO = E OX DE.

22 DSCs Research at ISM Molecular Design of Organic Dyes X-ray crystal structure High electronic delocalization on the tri-carbazole core DFT calculations 1 HOMO LUMO 4 HOMO LUMO Isodensity surface plots of the molecular frontier orbitals of 1 and 4

23 IPCE (%) DSCs Research at ISM Molecular Design of Organic Dyes Photovoltaic performances 100% 1 (SD30) 1 80% 2 (SD50) 2 60% 3 (SD40) 3 40% 4 (SD106) 4 20% 0% l (nm) IPCE : High plateau > 70% from 400 to 580 nm Conversion efficiency up to 700 nm Dye Film Redox thickness Couple [µm] J sc V oc ff h [ma. cm -2 ] [V] [%] [%] I - /I I - /I Co II/III I - /I I - /I Co II/III I - /I I - /I Co II/III I - /I I - /I Co II/III Light source: AM mw.cm -2 ; Working area: 0.16 cm 2 Good efficiencies even on thin films substrates Promising dyes for ss-dscs S. De Sousa et al., submitted.

24 DSCs Research at ISM 2 Joint experimental and theoretical study of the structure and absorption properties of the C212 dye Top and side views of the C212 dye optimized at the B3LYP/6-31g(d) level. Frontier MOs of C212 calculated at the IEF-PCM/MPW1K/6-31G(d) level using the B3LYP geometry. Normalized experimental and theoretical UV-visible absorption spectra of C212 in CH 2 Cl 2 (MPW1K/6-31g(d)).

25 DSCs Research at ISM 2 Joint experimental and theoretical study of the structure and absorption properties of the C212 dye UV-visible absorption spectra of C212 in DCM and adsorbed on TiO 2 surface (dry, with CH 2 Cl 2 and with electrolyte Z960). Transition energies (ev) and maximum absorption wavelengths (nm) of the neutral and anionic forms of C212 in CH 2 Cl 2 and of the C212@TiO 2 hybrid system. Shifts with respect to the neutral form Blue-shift independent of the experimental conditions employed to record the spectra. The blue-shift partly originates from the decrease of the strength of the acceptor group due to deprotonation and to the anchoring on the TiO 2 surface. L. Ducasse et al., submitted.

26 Acknowledgements Financial Supports CNRS PIE NANODISFLEX Prof. Thierry Toupance Samuel De Sousa, Ludmila Cojocaru ( C2M group, ISM, Bordeaux) ANR Blanc FMOCSOLE ENERMAT - Région Aquitaine - Univ. Bordeaux 1 Dr. Lionel Hirsch (ELORGA, IMS, Bordeaux) Collaborations Theoretical calculations : Dr. Laurent Ducasse & Dr. Frédéric Castet (ISM, Bordeaux) Crystallography : Dr. Brice Kauffmann (IECB, Bordeaux) Dr. Frédéric Sauvage (EPFL, Lausanne) Prof. M. Graëtzel