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1120-M01-03

Charge-Transfer Properties of Dye-Sensitized Solar Cells via Long-Range-Corrected Density Functional Theory Bryan M. Wong Materials Chemistry Department, Sandia National Laboratories, Livermore, CA 94551, U.S.A. ABSTRACT The excited-state properties in a series of solar cell dyes are investigated with a longrange-corrected (LC) functional which provides a more accurate description of charge-transfer states. Using time-dependent density functional theory (TDDFT), the LC formalism correctly predicts a large increase in the excited-state electric dipole moment of the dyes with respect to that of the ground state, indicating a sizable charge separation associated with the S1 ← S0 excitation. The performance of the LC-TDDFT formalism, illustrated by computing excitation energies, oscillator strengths, and excited-state dipole moments, demonstrates that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size. In contrast, the widely-used B3LYP functional severely overestimates excited-state dipole moments and underestimates the experimentally observed excitations, especially for larger dye molecules. The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the charge-transfer dynamics in solar cell dyes. INTRODUCTION Dye-sensitized solar cells (DSCs) have gained immense interest in the last few years due to their potential for converting clean solar energy to electricity at low cost. Originally introduced by Grätzel and co-workers in 1991 [1], several researchers have studied their use in

Figure 1. (left) Molecular structures of C343, NKX-2388, NKX-2311, NKX-2586, and NKX2677. (right) Molecular orbitals for the NKX-2677 S0 ground state and the S1 first excited state.

fabricating nanostructured films, improving photovoltaic devices, and to gain a general scientific understanding of charge-transfer between material interfaces. Although the origins of their work had focused on ruthenium dye complexes, current research is now directed towards organic dye sensitizers which are less expensive and easier to synthesize [2]. Recognizing this need for lowcost alternatives to conventional photovoltaics, Hara and co-workers [3-5] recently synthesized a series of coumarin dye molecules comprised of donor, electron-conducting, and anchoring groups for use in solar cell applications. As shown in figure 1, these organic sensitizers consist of an aniline group acting as an electron donor and a cyanoacrylic acid group functioning as an acceptor. These two functional groups are spatially separated from each other by one or more conjugated units which serve as an electron conductor. In particular, the NKX-2xxx series of courmarin derivatives have been reported as promising candidates for DSCs, and the NKX-2677 dye alone has a conversion efficiency of 7.4% which is comparable to ruthenium-based photosensitizers [6]. In order to develop highly efficient sensitizers, the S1 excited state must possess a significant