Efficient and Stable Quasiplanar Heterojunction Solar Cells with an Acetoxy-Substituted Wide-Bandgap Polymer

Abstract

Intermolecular interactions have fundamental importance in the control of active layer morphology, exciton generation, charge transport, and, thus, the overall photovoltaic performance. This is especially true for quasiplanar heterojunction (Q-PHJ) polymer solar cells, because the bilayer device structure requires larger exciton diffusion lengths. However, little effort has been made to design polymer donors with additional organic functional groups intended to control intermolecular hydrogen-bonding interactions. Herein, we report two new copolymers for Q-PHJ solar cells synthesized by the addition of hydroxy (PNTB-OH) and acetoxy groups (PNTB-OAc) onto electron-deficient units. We have systematically investigated the influence of the hydrogen bond on electro-optical behaviors, crystallinity, photovoltaic properties, energy losses, photostability, and storage stability in both types of polymers. The single-crystal data reveals more regular stacking and order orientation driven by hydrogen bonding, of the acetoxy-substituted electron-deficient units. Q-PHJ organic solar cells (OSCs) were fabricated for both polymers with a high-performance nonfullerene acceptor N3. PNTB-OAc-based Q-PHJ OSCs realized the highest photovoltaic performance of 16.53%, which is ∼2.4 times higher than 6.79% obtained from the PNTB-OH-based Q-PHJ OSCs. This high performance is attributable to low nonradiative energy losses, high and balanced electron/hole mobility, and better crystallinity. In contrast, the PNTB-OAc film has a longer crystal coherence length, which is calculated from grazing-incidence wide-angle X-ray scattering (GIWAXS). Furthermore, the PNTB-OAc device demonstrated superior photostability and storage stability, retained more than 85% of the initial PCE after illumination for 1050 h, and 90% of the initial PCE under nitrogen for 1600 h. This work highlights the importance of the acetoxy group to significantly control packing and crystallinity by hydrogen bonding, thus realizing efficient OSCs with durable device stability.