Modeling Dye Sensitized Solar Cells Filippo De Angelis

Transcription

1 Modeling Dye Sensitized Solar Cells Filippo De Angelis Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO) Istituto CNR di Scienze e Tecnologie Molecolari (ISTM), I Perugia, Italy

2 Computational Laboratory for Hybrid/Organic Photovoltaics Collaborations: Md. K. Nazeeruddin, M. Grätzel, A. Selloni Financial support: EU-FP7: NMP-2009 SANS ENERGY-2010 ESCORT ICT-2010 SUNFLOWER

3 CONVENTIONAL PHOTOVOLTAICS: SILICON P JUNCTION N CONVENTIONAL PHOTOVOLTAICS: SILICON HIGH EFFICIENCY / HIGH PRICE

4 WHY SILICON? BECAUSE IT S BLACK!

5 CAN I USE A DIFFERENT SEMICONDUCTOR? Ti O TITANIUM DIOXIDE IS A HIGHLY STABLE SEMICONDUCTOR WITH EXCELLENT ELECTRON MOBILITY BUT IT S WHITE! NANOCRYSTALLINE TiO 2

6 Part I Dye-sensitized Solar Cells

7 The problem is to gather solar light at an effective cost! Solar Emission CONDUCTION SILICON 1100 nm 1.12 ev TiO 2 VALENCE SILICON CONDUCTION TiO nm 3.20 ev 300 Wavelength (nm) Energy (ev) 0.50 VALENCE

8 DYE SENSITIZED SOLAR CELLS CONDUCTION 380 nm 3.20 ev 1 e - D* 680 nm 1.82 ev DYE (Chlorophyll) VALENCE NATURAL DYES TiO 2

9 I-V curve Efficiency = Jsc x Voc x FF/ Is

10 Dye-Sensitized Solar Cells: Flexible, colorful, transparent PVs DYESOL-TATA / FUJIKURA / LOGITECH

11 Large dimensions : realistic models usually require dealing with a few hundred atoms (oversimplified models are often inaccurate) Complexity : complex potential energy surfaces with several minima; dealing with transition metals (electronic correlation) Optical properties: need an accurate description of the excited states Dynamical aspects : need to perform ab initio molecular dynamics simulations Geometry optimizations of extended systems, in condensed phase, for both ground and excited states (DFT + DFTB) Ab initio molecular dynamics (Car-Parrinello) UV-vis absorption and emission spectra (TDDFT-ab initio) Inclusion of solvation effects (explicit or PCM) F. De Angelis, S. Fantacci, A. Sgamellotti, Theor. Chem. Acc. 2007, 17, 1093.

12 Part II Dyes

13 Ru-dyes: Absorption spectra π π* Exp. Theor. Intensity (arb. units) MLCT (II) MLCT (I) HOMO-3 HOMO Energy (ev) LUMO S. Fantacci, F. De Angelis, A. Selloni J. Am. Chem. Soc. 2003, 125, F. De Angelis, S. Fantacci, A. Selloni Chem. Phys. Lett. 2004, 389, 204. F. De Angelis, S. Fantacci, M.K. Nazeeruddin Chem. Phys. Lett. 2005, 415, 115. F. De Angelis,. S. Fantacci, M. Grätzel et al. J. Am. Chem. Soc. 2005, 127,

14 TDDFT prediction of the ground and excited state oxidation potential of organic dyes ESOP=GSOP + E 0-0 E 0-0 Exp. Cal. Energy (vs. NHE) CALCULATE THE GSOP AND THE EXCITED STATE GEOMETRY M. Pastore, F. De Angelis J. Phys. Chem. C 2011.

15 Part III Semiconductors

16 Anatase TiO 2 nanocrystals [100] 5m m 1µm 1µm H. G. Yang et al. Nature 453, 2008, 29 [101] Catal. Today 85, 2003,932 Mesoporous film of anatase TiO 2 nanocrystals: PSD for 20 nm particles

17 Modeling of TiO 2 nanoparticles: Stoichiometric anatase (TiO 2 ) 38 and (TiO 2 ) 82 clusters of 1 and 2 nm dimensions exposing (101) surfaces! 20.5 Å 18.5 Å Ti 82 O 164! B3LYP/3-21g* B3LYP/DZVP TD-DFT gap in water 3.20/3.41 ev 3.13/3.35 ev! Experimental gap in acqueous solutions: ev!! F. De Angelis, A. Tilocca, A. Selloni J. Am. Chem. Soc. 2004, 126, 15024

18 E (ev) TiO 2 C.B Alignment of excited state potentials: E(S + /S*) E(S + /S*) E(S + /S*) ΔE S =0.62 ΔE T =0.42 ΔE S =0.63 ΔE T =0.44 ΔE S =0.40 ΔE T = (1.75) 2.00 (1.80) 1.77 (1.56) TiO 2 E(S + /S) Exp. AA BB AB N719 Theor. TiO 2 C.B - 4.0/-4.2 ev ev TiO 2 vs. NHE V N719 vs. NHE V (-3.42/-3.22 ev) ΔE =0.48 V ΔE T = ev F. De Angelis, S. Fantacci, A. Selloni, Nanotechnology, 2008, 19,

19 Modeling of ZnO nanostructures: (ZnO) 42 (ZnO) 84 (ZnO) 222 F. De Angelis, L. Armelao, PhysChemChemPhys 2011 J.M. Azpiroz, E, Mosconi, F. De Angelis J. Phys. Chem. 2012

20 Modeling organic dyes adsorbed on ZnO

21 Dye/ZnO excited state mixing changes with semiconductor dimensions (ZnO) 42 (ZnO) 2222 A. Amat, F. De Angelis PCCP 2012.

22 Realistic models of TiO 2 NTs and NCs TiO 2 -NPs: Origin of sub-band gap states? Single and Multi-Wall TiO 2 -NTs: Adsorption mode Work in progress F. Nunzi, F. De Angelis, J. Phys.Chem. C, 2010

23 Trap states in TiO 2 Injected electron

24 DOS FOR A SINGLE TiO 2 NANOCRYSTAL 1 N DOS FIT EXP. Density of states ev Energy (ev) SURFACE STATES OF INDIVIDUAL TiO 2 NANOCRYSTALS INTRODUCE SUB BAND-GAP STATES IN THE DOS

25 EFFECT OF SATURATING LIGANDS: 154 H2O - TiO 2 Energy (ev) CB Edge Water-saturated Density of states Bare TiO 2 NC LUM O LUMO +4 LUMO +70

26 Space /energy distribution in a TiO2 NC

27 THE INTERACTION OF TWO TiO 2 NANOCRYSTALS : 101/ / / /001 DOS DOS DOS DOS NO TRAP STATES AT THE GRAIN BOUNDARIES!

28 Part IV 2 Solvent/Electrolyte

29 N3 (N719) dye adsorption onto TiO 2 : AIMD

30 Tuning the properties of Ru(II) TiO 2 sensitizers Control of protonation/ conuterions N719 N3 N3 Stability/ Charge separation N621/Z907 Ion-coordinating ligands Improved light harvesting N945 Ru-EDOT N866 K68 Quaterpyridil ligands Trans isomers

31 Heteroleptic Ru(II) TiO 2 sensitizers N621 N945 Ru-EDOT Sensitizer Number of protons Current ma/cm 2 Potential ( mv ) Fill Factor Efficiency at 1.5 AM N N K N Ru-EDOT A considerable reduction of the open circuit potential (ca. 180 mv) and therefore of the overall efficiency is observed with heteroleptic sensitizers F. De Angelis, S. Fantacci, A. Selloni, M. Grätzel, M.K. Nazeeruddin Nano Lett. 2007, 7, 3189.

32 The success of N719: Adsorption geometry Right (and tunable) number of protons; three anchoring points; reduced negative dipole compared to heteroleptic dyes F. De Angelis, S. Fantacci, M. Grätzel, J. Phys.Chem. C, 2010.

33 CALC. vs. EXP. ABSORPTION SPECTRA OF 2 The lowest intense transition has a strong excited state delocalization into the TiO2, suggesting a strong coupling and an almost instantaneous electron transfer following light absorption.

34 ALIGNMENT OF GROUND/EXCITED STATES ENERGY LEVELS F. De Angelis, S. Fantacci, M.K. Nazeeruddin, M. Graeztel, J. Phys. Chem. C

35 1 O DSSCs based on organic dye-sensitizers: O NEt 3 / EtOH OH HO O 5 N + I - C 8 H 17 3 O C 8 H 17 N + N 4 benzene / n-butanol N 6 O - O OH η=8.1 % η= 4.5% S.Kim, F. De Angelis, S. Fantacci, M. Grätzel, et al. J. Am. Chem. Soc. 2006, 128, J.-H. Yum, F. De Angelis, M. Grätzel, et al. J. Am. Chem. Soc. 2007, 129, D.P. Hagberg, F. De Angelis, M. Grätzel, et al. J. Am. Chem. Soc. 2008, 130, M. Pastore, F. De Angelis, M. Grätzel, et al. J. Phys. Chem. C. 2010, in press.

36 Effect of thermal averaging and of explicit solvation: Exp. Solution Exp. TiO 2 Theor. Solution Theor. TiO 2 X D. Rocca, F. De Angelis et al. Chem. Phys. Lett. 2009, 475, 49.

37 Effect of thermal averaging and of explicit solvation:

38 Solution thermally averaged UV-vis spectrum: Photocurrent Generation! F. F. De De Angelis, S. S. Fantacci, R. R. Gebauer. J. Phys. J. Phys. Chem. Chem. Lett. Lett

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41 Z907: HIGHER PERFORMANCES SLOWER REGENERATION N719: LOWER PERFORMANCES FASTER REGENARATION Z907 Z907 N719 N719 ROLE OF ION-PAIRING?

42 [Co(bpy) 3 ] +2 QUARTET / DOUBLET STATES D Q JT distorted e g e g states t 2g Theor. (ev) Exp (ev) Co(II) Q-D Co(III) S-T 1.43?? G ox (V) Co(II) Q vs. NHE Co(II) D vs. NHE

43 2 and Co(bpy) 3 +3 Redox Mediator Electrolyte: [Co(bpy) 3 ] +3 Adsorbed Dye: N719 (1H+) Periodic TiO 2 Slab 43

44 STARTING STRUCTURE eV eV eV

45 N719 Z ev ION PAIR BINDING ENERGY ev e- e- N719: HIGHER RECOMBINATION Z907: LOWER RECOMBINATION

46 Regenera'on mechanism - N719 - Co(bpy) 3 +3/+2 Doublet Co(II) Singlet Co(III) (0.0 ev) LUMO Doublet Ru(III) HOMO Singlet Ru(II)

47 Quartet Co(II) Triplet Co(III) (+1.43 ev) HOMO Singlet Ru(II)

48 Quartet Co(II) Singlet Co(III) (0.0 ev) LUMO Spin flip HOMO

49 Energy [ev] N719(1H) Q D E # Q Q S λ Q = 1.34 λ D = 0.61 =0.15 ev Reaction Coordinate Ru(III)-Co(II) Q E # D =0.01 ev Ru(III)-Co(II) D D S ΔG 0 = Ru(II)-Co(III)

50 Z907(0H) Ru(III)-Co(II) Q Energy [ev] Ru(III)-Co(II) D Ru(III)-Co(II) Q E # Q Ru(III)-Co(II) D λ Q = 1.45 λ D = 0.71 =0.30 ev Reaction Coordinate Ru(II)-Co(III) Ru(II)-Co(III) E # D =0.08 ev ΔG 0 = -0.24

51 Oxida'on poten'als and reorganiza'on energies of Co complexes ΔE* (Q-D) ΔE OX vs. NHE Ox. Pot. vs. NHE λ D λ Q cmb a dtb a bpy a Cl-bpy phen a Cl-phen a NO 2 -phen a bpy-pz b a Exp. λ=0.8 ev Theor. λ= = 0.72 ev a Feldt, S. M. et al.. J. Phys. Chem. C 2011, 115, b Yum, J.-H. Nat. Commun. 2012, 3, 631.