Computational study of linear carbon chain based organic dyes for dye sensitized solar cells

Abstract

Spectroscopic, electronic and electron injection properties of a new class of linear carbon chain (LCC) based organic dyes have been investigated, by means of density functional theory (DFT) and time-dependent density functional theory (TDDFT), for application in dye-sensitized solar cells (DSSCs). The photophysical properties of LCC-based dyes are tuned by changing the length of the linear carbon chain; UV/VIS absorption is red-shifted with increasing LCC length whereas oscillator strength and electron injection properties are reduced. Excellent nonlinear optical properties are predicted in particular for PY-N4 and PY-S4 dyes in the planar conformation. Results indicate that a LCC-bridge produces better results compared to benzene and thiophene bridges. Simulations of I^--Dye@(TiO₂)₁₄ and Dye@(TiO₂)₁₄ anatase complexes indicate that designed dyes inject electrons efficiently into the TiO₂ surface and can be regenerated by electron transfer from the electrolyte. Superior properties in terms of efficiency are shown by compounds with a pyrrole ring as the donor group and PY-3N is expected to be a promising candidate for applications, however all the investigated dyes could provide a good performance in solar energy conversion. Our study demonstrates that computational design can provide a significant contribution to experimental work; we expect this study will contribute to future developments to identify new and highly efficient sensitizers.

Fig. 1. Molecular structures of the designed dyes.

Fig. 2. C–N–C–C dihedral angle Φ and partial double bond N–C are highlighted in red (sulphur atoms are colored in yellow).

Fig. 3. Selected MOs energy levels (from HOMO−4 to LUMO+4) for the studied dyes at B3LYP/6-311+G(3df,3dp)/SMD level together with TiO₂ CB and I⁻/I₃⁻ redox potential. p = planar structure.

Fig. 4. Absorption spectra of dyes in acetonitrile at TD-CAM-B3LYP/6-311+G(3df,3pd)/SMD level. p = planar structure. The spectra are Gaussian broaded with 0.3 eV (half width half maximum).

Fig. 5. Frontiers molecular orbitals of HOMO and LUMO of the dyes in acetonitrile at TD-CAM-B3LYP/6-311+G(3df,3dp)/SMD level.

Fig. 6. LHE curves of all dyes (Γ = 20 nmol cm⁻² is taken for PY-4N, PY-4S, PY-3N, PY-3S, Γ = 30 nmol cm⁻² is considered for the other dyes). AM 1.5G solar spectrum is reported in green.

Fig. 7. Optimized geometries of bidentate binding mode of PY-4N, PY-4S, PY-3N and PY-3S on (TiO₂)₁₄.

Fig. 8. Electronic transitions of PY-4N@(TiO₂)₁₄ (up left), PY-4S@(TiO₂)₁₄ (up right), PY-3N@(TiO₂)₁₄ (down left) PY-3S@(TiO₂)₁₄ (down right) calculated at TD-M062X/6-31G(d)/LANL2DZ level.

Fig. 9. Optimized geometries of I⁻-PY-4N@(TiO₂)₁₄ (a), I⁻-PY-4S@(TiO₂)₁₄ (b), I⁻-PY-3N@(TiO₂)₁₄ (c), I⁻-PY-3S@(TiO₂)₁₄ (d) at B3LYP-D3/6-31G(d)/LANL2DZ/SDDALL level.

Fig. 10. Electronic transitions of I⁻-PY-4N@(TiO₂)₁₄ (up left), I⁻-PY-4S@(TiO₂)₁₄ (up right), I⁻-PY-3N@(TiO₂)₁₄ (down left) I⁻-PY-3S@(TiO₂)₁₄ (down right) calculated at TD-M062X/6-31G(d)/LANL2DZ/SDDALL level.