Benzobisthiazole unit in 4,8-connection mode to build D–A polymer donors achieving high short-circuit current density for organic solar cells†

In this work, a series of D–A conjugated polymer donors (namely PBDT-BBTH, PBDT-BBTF and PBDT-BBTCl) was designed based on the benzobisthiazole (BBT) unit in the 4,8-connection mode with the benzodithiophene (BDT) unit linked by the thiophene π-bridge. At the same time, an α-alkyl-thiophene ring with different β-atoms (H, F and Cl) was introduced at 2,6-positions of the BBT unit as a side-chain to extend the conjugation and regulate the energy levels. In 4,8-connection mode, the aromatic fused-thiazole ring of the BBT unit can stabilize the quinoid configuration of the main chain to strengthen the intramolecular charge transfer (ICT) and improve the π-electron delocalization of the conjugated backbone. The DFT calculations indicate that there exists the N⋯S noncovalent interaction between the BBT unit and the adjacent thiophene π-bridge that can lock the main-chain conformation to enhance the rigidity of the conjugated backbone. Thus, these polymers exhibit strong absorption in the range of 300–650 nm, which is favorable for light-harvesting to improve the short-circuit current density (J_SC) of the organic solar cells (OSCs). In addition, the introduction of a strongly electronegative F or Cl atom at the β-position of the α-alkyl-thiophene side-chain can reduce the HOMO energy level, which is beneficial for the enhancement of the open-circuit voltage (V_OC). Finally, the optimized OSC devices based on these polymers with an L8-BO acceptor exhibit good J_SC over 22 mA cm⁻², and the devices based on PBDT-BBTF and PBDT-BBTCl show higher V_OC than the device based on PBDT-BBTH. Among these polymers, the PBDT-BBTF-based device achieves more balanced parameters (J_SC = 23.43 mA cm⁻², V_OC = 0.831 V, and FF = 73.36%) to lead to the best PCE of 14.27%.