Boosting hole-blocking and electron-transport properties in indacenodithiophene-based molecules

Organic solar cells (OSCs) using non-fullerene small-molecule acceptors have shown great promise for next-generation photovoltaic applications. The indacenodithiophene-core reference compound IDIC-C8 (AR) was used in this work to design two series of seven new A-D-A type molecules: a cyano-based series and a nitro-based series. This was done by substituting a benzothiadiazole (BT) moiety for one terminal IC acceptor and methodically changing the other end-capped acceptor with various electron-withdrawing groups. The D3-B3LYP/6-311G(d,p) level of theory was selected due to its validation against experimental absorption data, and hence used for geometric optimizations. For comparison, M06-2X functionals were also used for optical property calculations in three phases: gas phase, toluene, and acetonitrile. All studied compounds exhibit deeper HOMO levels (-5.58 to -5.93 eV) than P3HT (-5.10 eV), indicating effective hole-blocking barriers of 0.48 to 0.83 eV. For effective electron transfer from the donor, the LUMO levels (-3.35 to -4.02 eV) exhibit advantageous offsets (0.25-0.92 eV). Notably, molecule A7-NO₂, due to four nitro groups, actively withdrew electrons with the smallest band gap (1.91 eV). On optical analysis at the B3LYP level, a nitro compound (A6-NO₂) had a maximum absorption wavelength (λ _max) of 661.4 nm in the gas phase, which redshifted to 763.8 nm in acetonitrile from all compounds. In contrast, A1-CN had λ _max values of 606.1 nm (gas), 656.0 nm (toluene), and 661.8 nm (acetonitrile). Efficient intramolecular charge transfer (ICT) was demonstrated by transition density matrix (TDM), charge density difference (CDD), and hole-electron distribution analyses, with D index values for designed compounds ranging from 5.94-9.95 Å as compared to the reference (D = 0.00 Å). Theoretical photovoltaic parameters confirm that all designed molecules, with the exception of A7-NO₂, have better power conversion efficiencies than the reference (13.3%). A1-CN has the highest open-circuit voltage (1.45 eV), fill factor (91.15%), and theoretical power conversion efficiency (18.6%). Our findings show that precisely modifying end-capped acceptors is an effective way to produce high-performance non-fullerene acceptors for bulk-heterojunction organic solar cells.