Dye Sensitized Solar Cells

Transcription

1 Large Scale simulations with CP2K Dye Sensitized Solar Cells Florian Schiffmann 2nd CP2K tutorial University College London

2 Dye sensitized solar cells Grätzel, Nature (1991,2001) Ultrafast forward ET, slow back ET => charge separation at interface Cells : ~10% efficiency Modules: ~7% Silicon (13-24 / 7-15%) DSSCs can be produced cheaply and with relatively little energy and even produce energy under low light conditions.

3 A prototypical high performance device In practice Mesoporous TiO2 (Anatase) Grätzel, Nature (2001) Sensitizing dye In theory Liquid electrolyte

4 Structure of the dye and the surface SCN-groups Ti5 c Ti6 c Ti5 c Ti6 c Ti5 c Ti6 c Ti5 c cis-cooh trans-cooh cis-cooh trans-cooh N3: fully protonated N719: two acidic protons replaced by TBA+ N712: all acidic protons replaced by TBA+ Ti5 c: undercoordinated Ti-atoms anchoring points for COO- groups Ti6 c: fully coordinated Ti-atoms Ti6 c

5 N3 binding on anatase(101) 2.2 x 2.0 x 3.5 nm3 cell Slab geometry, fully relaxed 489 atoms (Ru Ti O296C26N6S2H14) 6min / geometry step (64CPUs) PBE+U But, no defects & no solvent Relative binding energies are indicative: 2-3 carboxylate groups interact 2 bpy ligands involved Monodendate and bridged binding possible Stability increases with spacing along [101] Trans-COO are energectically favorable Hydrogen bonds influence relative stability

6 Stability of the configurations Larger spacing along the [101] more stable configurations Monodentate groups can form H-bonds stabilization of mixed binding on protonated surfaces otherwise double bidentate more stable Energies to close to be conclusive more information needed Relative stabilities on a protonated surface Relative stabilities on neat anatase

7 ATR-IR experiments Sequence 1: N3 Sequence 2: N712 Sequence 3: N719 Sequence 4: N719 => deprotonation with ammonia => reprotonation with TFA => protonation with TFA => deprotonation with ammonia => deprotonation with ammonia => reprotonation with TFA => protonation with TFA => deprotonation with ammonia

8 Vibrational Analysis Using mode tracking to compute normal modes of all configurations Difference in spectra of the protonated and deprotonated dye Promising agreement, but spectra of different configurations very similar Almost perfect match for the computed difference spectrum of I1 and I2configuration Direct comparison not conclusive Consistent with relative stabilities Experiments in collaboration with R. Wirz, A. Urakawa, A. Baiker

9 Conclusions on dye binding Equilibrium between several binding modes Change in protonation state leads to change in binding mode Most important configurations I1 and I2

10 Packing of the dye Dyes in close contact intermolecular hydrogen bonds Deprotonation with base strong charge interaction Experimental sequence shows partial dye desorption after deprotonation F. Schiffmann, J. VandeVondele, J.Hutter, R. Wirz, A. Urakawa, A.Baiker JPC C

11 TiO2 / MeCN interface: DFT & MM enhance DFT results with MM simulations QM single molecule: 14.2 kcal/mol parallel monolayer: 4.88 kcal/mol anti-parallel monolayer: 4.56 kcal/mol MM kcal/mol 6.68 kcal/mol 6.26 kcal/mol Full QM reference MD Long term simulations using newly parametrised classical force fields Excellent agreement between QM and MM Strong binding of a single ACN Clear layering

12 Structure at the interface Distribution function of nitrogen(blue), carbon(black), hydrogen (red) with respect to the distance to the surface MeCN close to the surface color code according to the angle. - layered structure - 3 different structural motifs Angular distribution function of the MeCN dipole vector and the surafce normal with respect to the distance of the MeCN center of mass to the surface F. Schiffmann, J. Hutter, J. VandeVondele; Phys.: Condens. Matter 20 (2008)

13 Structure at the interface Experimental spectrum of liquid MeCN and MeCN close to anatase (ATR-IR) F. Schiffmann, J. Hutter, J. VandeVondele; Phys.: Condens. Matter 20 (2008)

14 Interfacial solvent is not bulk solvent Structure Dynamics Method: Liu, Harder, & Berne JPCB 108, 6595 (2004) - 1nm region around the surface shows very different properties. - 1nm region is exactly where the sensitizer is located. - In mesoporous materials the interfacial region plays an important role Dielectric constant Method: Stern & Feller JCP 118, 3401 (2003)

15 Electrolyte near the interface Availability of the redox couple in the active region Why is iodide triiodide superior to others? Debye-Hueckel Gouy-Chapman Atomistic vs. Continuum

16 Ion concentration profiles from thermodynamic integration Strong deviation from simple Poisson Boltzmann description... at low concentration of the ion Particularly strong enhancement for I- at about 10A from the interface

17 Ion concentration vs liquid structure I- at the location of the first peak The ion can more favorably be accommodated in the liquid structure for certain locations F. Schiffmann, J. VandeVondele, J.Hutter, A. Urakawa, R. Wirz, A.Baiker PNAS, 2010

18 Regeneration of the dye N MeCN + I/I2 2x2x2 nm states x 4210 BF

19 Intermediate complexes SCN-I SCN-I2 Oxidized dye can react with I- and I2In gas phase SCN bound complex and pyridyl bound complex stable In solution only SCN bound complex stable ET from iodide/diiodide to Ru-center when complex is formed

20 The structure of the intermediate complex Free energy profile from TI: Experimental evidence: Weak binding Barrierless process expected to be fast Dye adsorbed to the surface Split of the SCN signal at 2100cm-1 after addition of I2 Reversible by flowing neat MeCN

21 The regeneration mechanism MD simulations of the intermediate complexes in presence on an additional iodide => instantaneous formation of I2-, resp. I3Barrierless formation of the complexes allows for fast regeneration No biradicalic reaction to from I3F. Schiffmann, J. VandeVondele, J.Hutter, A. Urakawa, R. Wirz, A.Baiker PNAS, to be published

22 Explanation of the superior performance I- ClF-

23 Acknowledgment Joost VandeVondele Jürg Hutter Atsushi Urakawa Ronny Wirz Alfons Baiker Swiss National Science foundation