Molecular engineering of A-D-C-D-A configured small molecular acceptors (SMAs) with promising photovoltaic properties for high-efficiency fullerene-free organic solar cells

Non fullerene small acceptor molecules in organic photovoltaics are proven beneficial than the traditional fullerene based acceptors for their fine contribution in organic solar cells. Researchers are constantly doing efforts for designing novel acceptor materials with promising photovoltaic properties. Designing of novel molecules by end-capped modifications is a convenient strategy to obtain high efficiency acceptor molecules for OSCs. Herein, we studied optoelectronic characteristics of five novel acceptor-donor-core-donor-acceptor configured small acceptor molecules (S1-S5) after end-capped modifications of recently synthesized DF-PCIC molecule. Designed molecules S1-S5 consist of 1,4-difluorobenzene (as central core), 4,4-bis(2-ethylhexyl)-2,6-dimethyl-4H-cyclopenta[1,2-b:5,4-b]dithiophene as donor which directly attached with different end-capped acceptors. The electronic and optical properties of newly designed (S1-S5) molecules are examined and compared with reference molecule with the aid of DFT and TD-DFT. Certain key parameters like frontier molecular orbitals analysis, density of states, dipole moment, binding energy along with transition density matrix, excitation energy, charge mobility, absorption maxima and charge transfer analysis have been performed in order to explore the photo-physical, optoelectronic and photovoltaic properties of designed and reference molecule. Out of newly designed structures, S4 displayed lowest energy band-gap (2.25 eV) with red-shifting in absorption spectrum (lambda (max)=712.43 nm) in chloroform which disclosed the perfect relationship between end-capped acceptor with large electron withdrawing character through extended conjugation. Similarly, S1 exhibited highest value (1.53 V) of open circuit voltage (V-oc) with respect to PTB7-Th donor material owing to lower values of lambda (e). Designed molecules exhibit better electron and hole mobility as compared to reference molecule R. All molecules express better absorption maximum, open circuit voltage, low excitation energies, comparable binding energies, large dipole moment and efficient electron and hole transport as compared to reference molecules. So, results these parameters suggest that end-capped modification is a convenient strategy in order to enhance the efficiency of OSCs. Therefore, conceptualized molecules are recommended to experimentalist for out-looking future developments of highly efficient solar cells.