1. Optical absorption spectra of chromophores in solution: the role of the solvent Ralph Gebauer Monday, July 7 th, 2014 Mastani Summer School IISER – Pune (India) June 30 – July 12, 2014

2. Dye sensitized solar cells

3. Various dyes Source: M. Grätzel, Prog. Photovolt. Res. Appl. 8, 171-185 (2000) Squaraine dye Cyanidin-3-glucoside ("Cyanin")

8. The colour we perceive

11. Optical spectra in the gas phase

12. Spectra computed with various codes:

13. Spectra computed with various functionals:

14. More realistic model of solvent

17. Including Molecular Dynamics

18. Analyzing configurational snapshots

20. Response charge density at selected frequencies

21. Influence of various geometrical distortions

22. Absorption spectrum of alizarin

26. Model of a photovoltaic device: Squaraine on TiO 2 slab Slab geometry: 1x4 TiO2 anatase slab, Exposing (101) surface PBE functional, PW basis set (Quantum-ESPRESSO code) Shown here: minimum energy configuration

27. TiO2 slab with squaraine dye

28. Experimental and TDDFT absorption spectra Experiment Computation

29. A more realistic system: Including the solvent

30. TDDFT calculation of optical spectra and related quantities Various challenges: System is large (429 atoms, 1.666 electrons, 181.581 PWs, resp. 717.690 PWs) Broad spectral region of interest Many excited states in spectral region Computational tool: Recursive Lanczos algorithm for TDDFT

31. TDDFT optical spectrum: dry system Energy [eV]

32. TDDFT optical spectrum including solvent Energy [eV]

39. Energy level fluctuations and electron injection driving force

42. Dye desorption steps:

44. Conclusions TD-DFT study of naural dye in vacuum and with PCM Poor agreement with experiment in absence of solvent in computation Including the solvent explicitly Improvement of computed spectrum, but important features are not reproduced (shoulders, etc.) Very efficient implementation of TD-DFT for large systems/basis sets Recursive Lanczos TDDFT based on time-dependent DFPT for a system composed of 429 atoms and described by ≈ 200,000 PWs Molecular dynamics of dye sensitized slab with explicit solvent Averaging of optical properties over many configurations leads to a description of optical properties in good agreement with experiment

45. Thanks to: Filippo De Angelis (Perugia) Stefano Baroni (SISSA & DEMOCRITOS, Trieste) Brent Walker (University College, London) Dario Rocca (UC Davis) O. Baris Malcioglu (Univ. Liège) Arrigo Calzolari (Modena) Quantum ESPRESSO and its community To know more: Theory & Method: Phys. Rev. Lett. 96, 113001 (2006) J. Chem. Phys. 127, 164106 (2007) J. Chem. Phys. 128, 154105 (2008) Applications to DSSCs: New J. Phys. 13, 085013 (2011) Phys. Status Solidi RRL 5, 259 (2011) J. Phys. Chem. Lett. 2, 813 (2011)