Theoretical study on the electronic properties of different types of the donor:acceptor complexes

Non-fullerene organic solar cells can be classified into four forms in line with the different types of donor (D) and acceptor (A) in the active layer: all-polymer (P_D:P_A), polymer D:small-molecule A (P_D:M_A), small-molecule D:polymer A (M_D:P_A), and all-small-molecule (M_D:M_A). On the basis of having studied the electronic properties of a large number of related monomer molecules and D:A complexes, this work constructed four groups of D:A molecular pairs as described above as examples to investigate their electronic properties with first-principles density functional theory. The results show that the absolute value of the average binding energy of the P_D:P_Acomplex D18:P(NDI2HD-T) is larger than others, indicating the structure is relatively more stable. In accordance of the Bader charge analysis, the intra-molecular charge transfer of small-molecule is greater than polymers. For these blends, the intermolecular charge transfer of the all-polymer pair D18:P(NDI2HD-T) is larger, revealing that the P_D:P_Apair may result in a stronger intermolecular dipole electric field, which is beneficial to facilitate the separation of excitons. In addition, the M_D:M_Apair DRTB-T:FDICTF-2Cl and the P_D:M_Acomplex D18:FDICTF-2Cl all exhibit a larger amount of intra-molecular charge transfer, which indicates that the small-molecule acceptors in D:A complexes are conducive to promoting intra-molecular charge transfer.