Abstract:
Based on a non-planar fused perylene diimide unit (FPDI-Th), a new class of polymers, PPDIBT-Th and PPDIBT-Th-C6, were designed and synthesized, and they were served as acceptors for application in all-polymer solar cells. Compared to PPDIBT-Th, hexyl side chain was introduced into PPDIBT-Th-C6 to tune the molecular conformation. These two polymers as well as the fabrication and measurement of all-polymer solar cell devices were systematically studied by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), UV-Vis absorption and cyclic voltammetry (CV). Both of devices blending PPDIBT-Th or PPDIBT-Th-C6 with polymer donor PTB7-Th show good photovoltaic performances, indicating that the polymers contained FPDI-Th are promising acceptors for all-polymer solar cells. In addition, the introduction of side chains into the polymers decreases the planarity of the backbones, leading to a weaker molecular packing and absorption intensity. However, the side chains not only affect the properties of acceptor itself but also tune the molecular packing of polymer donor that is another component in bulk heterojunction (BHJ). In the blend film, PPDIBT-Th with strong intermolecular interaction inhibits the aggregation of donor PTB7-Th, leading to an inferior morphology which has been verified by the device performances and atomic force microscopy (AFM) characterization. Due to the balance between miscibility and crystallinity, all-polymer solar cells based on PPDIBT-Th-C6 and PTB7-Th display a higher short-circuit current density (
Jsc) of 12.15 mA/cm
2, eventually achieving a superior power conversion efficiency (PCE) of 4.95%.