S,N-Heteropentacene based small molecules with A–D–A structure for solution processed organic bulk heterojunction solar cells

Abstract

Two small molecules with acceptor–donor–acceptor (A–D–A) structures, i.e. SN(BTTh₂)₂ and SN(BTAOTh₂)₂, denoted as J and K, respectively, containing an electron rich planar S,N-heteropentacene as the central core flanked with alkoxy substituted and un-substituted benzothiadiazole (BT) and end capped with hexyl-substituted bi-thiophene units, were designed and synthesized. Optical and electrochemical investigation of these two small molecules reveals that these compounds possess low optical bandgaps and broader absorption profiles to harvest solar photons, as well as suitable electrochemical molecular energy levels for application as donors in bulk heterojunction organic solar cells. The photovoltaic properties of these two small molecules as donors and PC₇₁BM as the acceptor in solution processed bulk heterojunction cells were investigated. Devices based on cast J : PC₇₁BM and K : PC₇₁BM active layers showed power conversion efficiencies (PCE) of 1.96% and 1.57%, respectively; these efficiencies improved to 6.02% and 5.06% when the active layers were subjected to two steps annealing (TSA), i.e. combined thermal annealing followed by solvent vapor annealing. The increase in PCE was due to the enhancement of both the short circuit current (J_sc) and fill factor (FF) due to the enhancement of the light harvesting ability of the active layer, better phase separation between the small molecules and PC₇₁BM, and more balanced charge transport induced by the TSA treatment.