Influences of the backbone randomness on the properties, morphology and performances of the fluorinated benzoselenadiazole–benzothiadiazole based random copolymers

Abstract

To investigate the influences of the 5,6-difluoro-benzoselenadiazole : 5,6-difluoro-benzothiadiazole (FBSe : FBT) ratio on the polymer properties, solid-state morphology and device performances, a series of FBSe : FBT based copolymers were synthesized. Copolymers with higher FBSe ratios were found to have narrower E_g, and higher-lying E_HOMO. Because of the size and electronegativity differences of the selenium and sulfur atoms, the FBSe : FBT ratio further affects the structural regularity of the conjugated chains, and their self-assembly behaviors. DSC results indicated that P1, which has the most irregular FBSe : FBT sequence along the backbone showed the lowest T_m. Interestingly, XRD results showed that the main-chain irregularity degrades the order of the lamellar stacking, but not the order of π-stacking. The random ternary copolymers, P1 and P2, possess more ordered π-stacking than the alternating copolymers, PTh₄FBSe and PTh₄FBT. The highest OFET μ_h of 0.46 cm² V⁻¹ s⁻¹ was delivered by P1, which has the smallest d_π–π among the copolymers. In the polymer : PC₇₁BM blend films, the FBSe containing copolymers have good miscibility in PC₇₁BM. The degree of phase separation of PTh₄FBSe : PC₇₁BM can be enhanced by DIO additive, but it is not effective for the random ternary copolymers. The highest PCE of 6.06% with V_oc of 0.64 V, J_sc of 15.3 mA cm⁻², and FF of 61.8% was delivered by the PTh₄FBSe : PC₇₁BM PSCs.