Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-based D -π - A - A type small molecular donors with near-infrared absorption and their photovoltaic applications

Two new asymmetric D-pi-A1-A2 configured small molecules named PTP and TTP bearing thieno[3,4-b] pyrazine (TP) unit as the A1 group have been designed and synthesized for solution-processed bulk-heterojunction organic solar cells (OSCs). These new dipolar molecular donors not only exhibit strong near-IR absorption but pack into antiparallel dimers, in which PTP shows a coexistence of coplanar and cofacial antiparallel dimers configuration, while TTP reveals a coplanar centrosymmetric dimer structure. Solution-processed OSCs for PTP blended with pristine C-70 achieves a promising power conversion efficiency (PCE) of 3.40% along with an open-circuit voltage (V-OC) of 0.64 V, a short-circuit current density (J(SC)) of 9.83 mA cm(-2) and a fill factor (FF) of 54.0%, which is the highest efficiency to date among the TP-based materials. Conversely, the optimized TTP:C-70 solar cell delivers a V-OC of 0.48 V, a J(SC) of 6.02 mA cm(-2) and a FF of 43.3%, corresponding to a moderate PCE of 1.25%.