X-Shaped donor molecules based on benzo[2,1-b:3,4-b′]dithiophene as organic solar cell materials with PDIs as acceptors

Abstract

Employing a double overlapping wave band strategy based on DFT, five X-shaped anisotropic low energy gap donor compounds (D1–D5) have been designed for solar cell applications. A series of new PDI acceptor molecules are built to match each designed donor in terms of frontier molecular orbital energy levels by tuning substituents at P1 and P2 positions of PDI1. The designed donors consist of a central electron donor fragment benzodithiophene (DF), electron accepting fragments (A1 to A5) and terfuran and ethynyl-terfuran bridges (B1 and B2 respectively). The absorption bands of the designed donors based on TD-DFT not only cover the visible region but also extend to the infrared region of the spectrum. The donor fragment DF and multibranched spacers B1 and B2 are responsible for absorption in short and middle wavelength regions while the acceptor fragments contribute to the middle and long wavelength regions of the spectrum. In addition, PDIs also exhibit complementary absorptions in the visible range of the solar spectrum. Among designed donors, D1 exhibits ideal lowest band gap, FMO energy levels and exclusive broadest absorption because of the strongest electron withdrawing fragment. The lower λ_e values as compared to λ_h illustrate that these five donors would be favorable for electron transfer. The carrier mobility of D1 in the crystalline state has been predicted using the P2₁/c space group. D1 displays higher carrier mobility for μ_e = 2.00 cm² V⁻¹ s⁻¹ and μ_h = 1.7 × 10⁻² cm² V⁻¹ s⁻¹. The calculated V_oc of D1 is 1.02 V. The designed donors and PDI acceptors are suitable and recommended for high performance solar cell devices.