DFT and TD-DFT calculation of new thienopyrazine-based small molecules for organic solar cells

Abstract

Background: Novel six organic donor-π-acceptor molecules (D-π-A) used for Bulk Heterojunction organic solar cells (BHJ), based on thienopyrazine were studied by density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches, to shed light on how the π-conjugation order influence the performance of the solar cells. The electron acceptor group was 2-cyanoacrylic for all compounds, whereas the electron donor unit was varied and the influence was investigated.

Methods: The TD-DFT method, combined with a hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP) in conjunction with a polarizable continuum model of salvation (PCM) together with a 6-31G(d,p) basis set, was used to predict the excitation energies, the absorption and the emission spectra of all molecules.

Results: The trend of the calculated HOMO-LUMO gaps nicely compares with the spectral data. In addition, the estimated values of the open-circuit photovoltage (V_oc) for these compounds were presented in two cases/PC₆₀BM and/PC₇₁BM.

Conclusion: The study of structural, electronics and optical properties for these compounds could help to design more efficient functional photovoltaic organic materials.

Keywords: Optoelectronic properties; Organic solar cells; TD-DFT; Thienopyrazine derivatives; Voc (open circuit voltage); π-conjugated molecules.

Fig. 1

Chemical structure of study compounds Pi (i = 1–6)

Fig. 2

Optimized geometries obtained by B3LYP/6-31G(d,p) of the studied molecules

Fig. 3

The contour plots of HOMO and LUMO orbitals of the studied compounds Pi

Fig. 4

Sketch of B3LYP/6-31G(d,p) calculated energies of the HOMO, LUMO level of study molecules

Fig. 5

Calculated efficiency under AM1.5G illumination for single junction devices based on composites that consist of a donor with a variable band gap and LUMO level and an acceptor with a variable LUMO level [34]

Fig. 6

Simulated UV–visible optical absorption spectra of the title compounds with the calculated data at the TD-DFT/CAM-B3LYP/6-31G(d,p) level in chloroform solvent