How End-Capped Acceptors Regulate the Photovoltaic Performance of the Organic Solar Cells: A Detailed Density Functional Exploration of Their Impact on the A–D−π–D–A Type Small Molecular Electron Donors

Recent investigations on organic solar cells have demonstrated the superior photovoltaic performance of A−π–D−π–A type small molecular electron donors (SMEDs) compared to that of the D–A(π)–A and D–A(π)–D molecular frameworks because of their intense intramolecular charge transfer transitions, narrow band gap, and broad optical profiles at the near-infrared region. These characteristic features mainly originated from their molecular functionalization of the core and end-capped acceptor building blocks, which generate quite a greater impact through the wavefunction overlap of the intra-/intermolecular interactions. Nonetheless, SMEDs with various reported end-capped acceptors, 1,3-indanedione (IND), N-alkyl rhodanine (NAR), and dicyanovinylene (DCV), exhibited excellent photovoltaic performance, the reason behind this phenomenon remains unexplained. To gain better insights in this regard, we have designed a series of SMEDs named DFR, DFM, and DFI by embedding these exceptionally performing NAR, DCV, and IND end-capping units, respectively, into a newly designed A–D−π–D–A molecular framework. A detailed investigation was carried out to understand the influence of end-capped acceptors on the photovoltaic parameters at the molecular level using density functional theory (DFT) and time-dependent DFT methods. Exploration of this study reveals that the NAR unit of the A–D−π–D–A framework (DFR) enabled a bathochromic shift compared to that of the DCV counterpart (DFI), a reverse pattern of absorption to that of the widely reported A−π–D−π–A system. A series of charge transfer parameters related to excited state properties including charge density difference, amount of charge transferred (qCT), charge transfer distance (dCT), dipole variation, H-index, t-index, and hole–electron overlap (S±) and other components such as ionization potential, electron affinity, delocalization, and reorganization energies were computed. In addition, photovoltaic parameters such as exciton binding energy and open-circuit voltage have been systematically evaluated with respect to the fullerene and Y6 electron acceptors. The antiaromatic characteristics of the cyclic NAR and IND acceptors were well-demonstrated using the nucleus-independent chemical shift, 2D isochemical shielding surface, and anisotropy of the induced current density analyses. This study highlights that the performance of each acceptor is distinctively different because it not only can be determined from its electron-withdrawing strength but also depends on its potential to allow the charge density population. The greater heterofunctionalities of NAR and IND acceptors could help increase the Jsc due to its strong accommodating potential of electron density population at the peripherals, but the minimized contribution from the sp-hybridized C≡N unit of the DCV acceptor failed in this regard. The computed results followed an excellent agreement with the experimental observations. The results obtained from this study would be helpful for the researchers to gain a better understanding of the chemistry behind the relationship between the structure of the end-capped acceptors and photovoltaic activity and suggesting the beneficial features of including more heterofunctionalities into large-sized terminal acceptors: (1) band gap narrowing through lowest unoccupied molecular orbital stabilization; (2) efficient ICT transitions; and (3) greater accommodating potential of charge density population at the peripherals, which could help the facilitation of charge transport at the D−A interface.