Efficient post-treatment-free polymer solar cells from indacenodithiophene and fluorinated quinoxaline-based conjugated polymers

Two novel narrow bandgap copolymers PIDTFPQ and PIDTFTQ based on bulky indacenodithiophene (IDT) and mono-fluorophenanthrenequinoxaline (FPQ) or mono-fluorodithienobenzoquinoxaline (FTQ), which possess a large fused phenanthrene or benzodithiophene ring on top of pyrazine unit, are designed and synthesized. The optical properties, electrochemical properties, π-π packing behavior, charge carrier transport, morphology and photovoltaic properties of two copolymers were thoroughly investigated via a series of techniques. Owing to the smaller volume of dithienobenzoquinoxaline, PIDTFTQ shows more planar structure. Furthermore, PIDTFTQ presents slightly red-shifted onset absorption and denser π-π stacking. Besides, PIDTFTQ:PC₇₁BM blend film displays higher hole and electron mobilities of 1.46 × 10⁻⁴ cm²V⁻¹s⁻¹ and 2.64 × 10⁻⁴ cm²V⁻¹s⁻¹ than those of PIDTFPQ/PC₇₁BM blend film. More excitingly, without using any additives or any thermal/solvent annealing, the polymer solar cells based on PIDTFTQ/PC₇₁BM and PIDTFPQ/PC₇₁BM obtained higher V_oc of 0.92 V and 0.93 V and promising PCE of 5.54% and 5.00%, respectively. The results show that PIDTFTQ or PIDTFPQ possess a great potential for large area roll-to-roll manufacturing or printing technology for PSCs.