Triazine-core-containing star-shaped compounds as cathode interlayers for efficient inverted polymer solar cells

Two novel star-shaped compounds with 1,3,5-triazine as the core and fluorene as the arms, denoted as Tf1 and Tf2, were designed and synthesized. Compared with Tf1, Tf2 was found to have a pi-extended structure with its additional inner fluorene arms. Owing to the diethylamino end groups, both compounds reduced the work function of ITO effectively. Enhanced device performances were obtained by incorporating these compounds as the cathode interlayers in inverted polymer solar cells with PTB7:PC71BM as the active layer. As revealed by Kelvin probe measurements, the work function of Tf1-modified ITO was lower than that of Tf2-modified ITO. As shown by atomic force microscopy (AFM), the morphology of the Tf1-coated surface was relatively smooth and homogenous, which provided a relatively good contact between the active layer and the cathode. As a result, a power conversion efficiency (PCE) of 8.97% was achieved with Tf1 as the cathode interlayer in an inverted polymer solar cell.