The side chain effects on TPD-based copolymers: the linear chain leads to a higher <i>j</i>_sc

Alkyl substituents appended to polymers play the determining role on self-assembly and film-forming properties, and on device performance. In this work, we highlight the effects of the linear and branched flexible chains appended to the acceptor moiety (A) in D-A type copolymers. Two thieno[3,4-c]-pyrrole-4,6-dione (TPD) based copolymers PT1 and PT2 with different alkyl chains, were designed and synthesized. By comparison their UV-vis absorptions, HOMO/LUMO energy levels, as well as the characters in polymer solar cells, the influences of alkyl chains were investigated. Both copolymers showed molecular weights of 21 kDa and similar optical properties with a medium band gap of 1.93 eV, while PT2 with the branched chain exhibited a lower HOMO than that of PT1 (−5.43 <i>vs</i> − 5.37 eV). In bulk heterojunction (BHJ) solar cells, PT1 with a linear chain presented a short circuit current (<i>J</i>_sc) of 6.76 mA cm^‒2, open circuit voltage (<i>V</i>_oc) of 0.89 V and power conversion efficiency (PCE) of 2.92%. To the contrary, PT2 showed a <i>J</i>_sc of 3.53 mA cm^‒2, <i>V</i>_oc of 0.99 V, delivering a relatively lower PCE of 2.05%. The result indicates that appending a linear alkyl chain to the TPD unit could sufficient enhance the <i>J</i>_sc value of the related polymer.