Designing Benzodithiophene-Based Small Molecule Donors for Organic Solar Cells by Regulation of Halogenation Effects

The donors are key components of organic solar cells (OSCs) and play crucial roles in their photovoltaic performance. Herein, we designed two new donors (BTR-γ-Cl and BTR-γ-F) by finely optimizing small molecule donors (BTR-Cl and BTR-F) with a high performance. The optoelectronic properties of the four donors and their interfacial properties with the well-known acceptor Y6 were studied by density functional theory and time-dependent density functional theory. Our calculations show that the studied four donors have large hole mobility and strong interactions with Y6, where the BTR-γ-Cl/Y6 has the largest binding energy. Importantly, the proportion of charge transfer (CT) states increases at the BTR-γ-Cl/Y6 (50%) and BTR-γ-F/Y6 (45%) interfaces. The newly designed donors are more likely to achieve CT states through intermolecular electric field (IEF) and hot exciton mechanisms than the parent molecules; meanwhile, donors containing Cl atoms are more inclined to produce CT states through the direct excitation mechanism than those containing F atoms. Our results not only provided two promising donors but also shed light on the halogenation effects on donors in OSCs, which might be important to design efficient photovoltaic materials.

Incorporation of a Boron-Nitrogen Covalent Bond Improves the Charge-Transport and Charge-Transfer Characteristics of Organoboron Small-Molecule Acceptors for Organic Solar Cells

An organoboron small-molecular acceptor (OSMA) M_B←N containing a boron-nitrogen coordination bond (B←N) exhibits good light absorption in organic solar cells (OSCs). In this work, based on M_B←N, OSMA M_B-N, with the incorporation of a boron-nitrogen covalent bond (B-N), was designed. We have systematically investigated the charge-transport properties and interfacial charge-transfer characteristics of M_B-N, along with M_B←N, using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT). Theoretical calculations show that M_B-N can simultaneously boost the open-circuit voltage (from 0.78 V to 0.85 V) and the short-circuit current due to its high-lying lowest unoccupied molecular orbital and the reduced energy gap. Moreover, its large dipole shortens stacking and greatly enhances electron mobility by up to 5.91 × 10^-3 cm²·V^-1·s^-1. Notably, the excellent interfacial properties of PTB7-Th/M_B-N, owing to more charge transfer states generated through the direct excitation process and the intermolecular electric field mechanism, are expected to improve OSCs performance. Together with the excellent properties of M_B-N, we demonstrate a new OSMA and develop a new organoboron building block with B-N units. The computations also shed light on the structure-property relationships and provide in-depth theoretical guidance for the application of organoboron photovoltaic materials.

Rational Design and Characterization of Symmetry-Breaking Organic Semiconductors in Polymer Solar Cells: A Theory Insight of the Asymmetric Advantage

Asymmetric molecule strategy is considered an effective method to achieve high power conversion efficiency (PCE) of polymer solar cells (PSCs). In this paper, nine oligomers are designed by combining three new electron-deficient units (unitA)-n1, n2, and n3-and three electron-donating units (unitD)-D, E, and F-with their π-conjugation area extended. The relationships between symmetric/asymmetric molecule structure and the performance of the oligomers are investigated using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The results indicate that asymmetry molecule PEn2 has the minimum dihedral angle in the angle between two planes of unitD and unitA among all the molecules, which exhibited the advantages of asymmetric structures in molecular stacking. The relationship of the values of ionization potentials (IP) and electron affinities (EA) along with the unitD/unitA π-extend are revealed. The calculated reorganization energy results also demonstrate that the asymmetric molecules PDn2 and PEn2 could better charge the extraction of the PSCs than other molecules for their lower reorganization energy of 0.180 eV and 0.181 eV, respectively.

Theoretical design of asymmetric A-D1A'D2-A type non-fullerene acceptors for organic solar cells

The acceptor in organic solar cells (OSCs) is of paramount importance for achieving a high photovoltaic performance. Based on the well-known non-fullerene acceptor Y6, we designed a set of asymmetric A-D1A'D2-A type new acceptors Y6-C, Y6-N, Y6-O, Y6-Se, and Y6-Si by substituting the two S atoms of one thieno[3,2-b]thiophene unit with C, N, O, Se, and Si atoms, respectively. The electronic, optical, and crystal properties of Y6 and the designed acceptors, as well as the interfacial charge-transfer (CT) mechanisms between the donor PM6 and the investigated acceptors have been systematically studied. It is found that the newly designed asymmetric acceptors possess suitable energy levels and strong interactions with the donor PM6. Importantly, the newly designed acceptors exhibit enhanced light harvesting ability and more CT states with larger oscillator strengths in the 40 lowest excited states. Among the multiple CT mechanisms, the direct excitation of CT states is found to be more favored in the case of PM6/newly designed acceptors than that of PM6/Y6. This work not only offers a set of promising acceptors superior to Y6, but also demonstrates that designing acceptors with asymmetric structure could be an effective strategy to improve the performance of OSCs.

End-capped group manipulation of indacenodithienothiophene-based non-fullerene small molecule acceptors for efficient organic solar cells

As the key component of organic solar cells (OSCs), the acceptor plays key roles in determining the power conversion efficiency (PCE). Based on the famous non-fullerene acceptor ITIC, a series of acceptors (A1-A5) were designed by introducing fused-ring units (phenanthrene, pyrene, benzopyrazine, dibenzo[a,c]phenazine, and phenanthro[4,5-abc]phenazine) as the end groups. Theoretical calculations showed that A1-A5 display improved solubility and redshifted absorption spectra compared with ITIC. More importantly, the newly designed acceptors exhibit much higher electron mobility, where the electron mobility of A5_h (similar to A5 but with the same hexyl side chain as ITIC) is about four orders of magnitude larger than that of ITIC. The computed binding energies of the donor PBDB-TF with the acceptor ITIC and A5_h are -2.52 eV and -3.75 eV, indicating much stronger interface interactions in PBDB-TF/A5_h. In terms of charge-transfer (CT) mechanism, we found that both PBDB-TF/ITIC and PBDB-TF/A5_h can generate CT states through direct excitation and hot excitons, meanwhile there exist more opportunities of producing CT states via the intermolecular electric field (IEF) mechanism in PBDB-TF/A5_h. Our results not only offer a set of promising ITIC-based acceptors, but also provide new insights into the donor/acceptor interface properties, which are closely related to the PCE of OSCs.