Computational design of new organics dyes with improved solar absorbance for dye-sensitized solar cells
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Computational design of new organics dyes with improved solar absorbance for dye-sensitized solar cells Sergei Manzhos, Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore Address all correspondence to Sergei Manzhos at [email protected] (Received 18 December 2012; accepted 27 December 2012)
Abstract Two new organic dyes, WS-2.1 and WS-2.2—derivatives of the known dye WS-2—are computationally designed using a recently developed approach with a broad absorption peak at around 775 nm in acetonitrile for WS-2.2 versus 610 nm for WS-2. The red shift includes a significant contribution due to vibrations and is not reproduced by standard computational methods. The oxidation and reduction potentials of the dye render it well suited for use in dye-sensitized solar cells.
Dye design through changing the conjugation order has emerged as a strategy for improving the solar absorbance of simple, easy-to-make molecules.[1–5] Specifically, the position of the methine unit has been shown to influence the spectral peak position of thiophene-containing molecules by controlling the quinoidization.[1,2,4,5] However, the standard density functional theory (DFT) + time-dependent density functional theory (TDDFT) approach for simulating molecular electronic spectra fails when the order of conjugation rather than its extent is changed, thwarting a computational molecular design.[1–4] In Refs. 4,5, we have introduced a computational procedure capable of simulating the main spectral peak’s position and width for organic molecules differing in conjugation order. The indoline dye WS-2 (Fig. 1) has been shown to provide one of the highest efficiencies in organic dye-based dyesensitized solar cells (DSSCs)—up to 9%.[6,7] Its peak absorption is in the green part of the spectrum (500–600 nm). There is therefore potential for improving DSSC current and efficiency if peak absorption is shifted to the red part of the spectrum. The new derivative dyes WS-2.1 and WS-2.2 shown in Fig. 1 are expected to have a significant red shift compared with WS-2 by inducing the thiophene unit’s quinoidization by interaction with the electron-withdrawing cyano-acrylic group.[4] As happened in the case of Ref. 4, calculations using DFT + TDDFT with both hybrid (B3LYP) and range-separated (CAM-B3LYP) functionals fail to show a significant red shift of the visible band of WS-2.1 and WS-2.2 compared with WS-2, see Tables I and II; in fact, a blue shift is predicted in vacuum with CAM-B3LYP. These calculations do predict higher absorption intensities for the new dyes, from about 20% to 65% higher compared with WS-2 depending on the
Figure 1. The structures of the dyes (top to bottom) WS-2, WS-2.1, and WS-2.2. Atom color scheme: C—cyan, S—yellow, N—blue, O—red, H—grey.
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Table 1. Transition energies (Eexc, nm) and oscillator strengths (f ) of the strongest peak in the electro
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