Many-body Green's function study of coumarins for dye-sensitized solar cells

Carina Faber, Ivan Duchemin, Thierry Deutsch, and Xavier Blase

Phys. Rev. B 86, 155315 – Published 22 October 2012

Abstract

We study within the many-body Green's function $G W$ and Bethe-Salpeter formalisms the excitation energies of several coumarin dyes proposed as an efficient alternative to ruthenium complexes for dye-sensitized solar cells. Due to their internal donor-acceptor structure, these chromophores present low-lying excitations showing a strong intramolecular charge-transfer character. We show that combining $G W$ and Bethe-Salpeter calculations leads to charge-transfer excitation energies and oscillator strengths in excellent agreement with reference range-separated functional studies or coupled-cluster calculations. The present results confirm the ability of this family of approaches to describe accurately Frenkel and charge-transfer photo-excitations in both extended and finite size systems without any system-dependent adjustable parameter, paving the way to the study of dye-sensitized semiconducting surfaces.

Received 7 August 2012

DOI:https://doi.org/10.1103/PhysRevB.86.155315

Authors & Affiliations

Carina Faber¹, Ivan Duchemin², Thierry Deutsch², and Xavier Blase¹

¹Institut Néel, CNRS and Université Joseph Fourier, B.P. 166, 38042 Grenoble Cedex 09, France
²INAC, SP2M/L_Sim, CEA/UJF Cedex 09, 38054 Grenoble, France

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Issue

Vol. 86, Iss. 15 — 15 October 2012

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Images

Figure 1
Symbolic representation of the studied coumarins: (a) parent C343, (b) NKX-2388 (cis), (c) NKX-2311 (cis), NKX-2586 (cis), and NKX-2677. The difference with the corresponding (trans) structures is represented in Fig. 2 for the NKX-2311 case. Black, white, red, blue, and yellow atoms represent carbon, hydrogen, oxygen, nitrogen, and sulfur, respectively.Reuse & Permissions
Figure 2
Symbolic representation of the NKX-2311 (trans) coumarin dye. This structure can be compared to the NKX-2311 (cis) structure in Fig. 1c, which differs from the orientation of the $π$ conjugated polymethine (-HC=CH-) bridge (orange frame), the cyano (-C=N) (blue dot-dashed box), and the carboxylic (-COOH) (full red box) groups. The donor coumarin parent is on the left side in the green dot-dashed frame. The anchoring group to the semiconducting TiO $_{2}$ surface is the carboxylic unit.Reuse & Permissions
Figure 3
Calculated lowest singlet excitation energies (in eV) as a function of the coupled-cluster CC2 reference values. Results in perfect agreement with the CC2 calculations should fall on the first diagonal (black line). The present $G W$ -BSE calculations (empty red squares) are compared to the TD-B3LYP results of Refs. 12 and 13 (filled black circles) and the “adjusted” (see text) TD-LC-BLYP data from Ref. 13 (blue triangles up). The coumarins' names are indicated by their number (removing the NKX prefix) with (c) standing for -cis and (t) for -trans. The axes' physical length is scaled according to their respective energy range.Reuse & Permissions
Figure 4
(a) and (b) Isocontour representation of the C343 HOMO and LUMO Kohn-Sham eigenstates. Different colors indicate different signs of the wave function. In panels (d) and (e), similar plots for NKX-2677. In panels (c) and (f), isocontour representation of the electron (yellow) and hole (light blue) probability distribution for the lowest C343 and NKX-2677 singlet excited states, respectively, as obtained within BSE. The red arrows indicate the average hole (left arrow) and electron (right arrow) positions. The C343 and NKX-2677 molecules are not represented on the same scale.Reuse & Permissions
Figure 5
Theoretical strength as a function of the $G W$ -BSE excitation energies (eV). The BSE values (red empty squares) are compared to the CC2 (black diamond), LC-BLYP (blue triangle up), and the BNL J1/J2 (empty green circles) results. Results for the NKX-2677 and NKX-2586 (s-cis) in the dashed box are reproduced in the upper-right inset.Reuse & Permissions

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