19.32% Efficiency Polymer Solar Cells Enabled by Fine‐Tuning Stacking Modes of Y‐Type Molecule Acceptors: Synergistic Bromine and Fluorine Substitution of the End Groups

Abstract

The success of Y6‐type non‐fullerene small molecule acceptors (NF‐SMAs) in polymer solar cells (PSCs) can be attributed to their unique honeycomb stacking style, which leads to favorable thin‐film morphologies. The intermolecular interactions related to the crystallization tendency of these NF‐SMAs is closely governed by their electron accepting end groups. For example, the high performance Y6 derivative L8‐BO (BTP‐4F) presents three types of stacking modes in contrast to two stacking modes of Y6. Hence, it is ultimately interesting to obtain more insight on the packing properties and the preferences influenced by chemical modifications such as end group engineering. In this work, we designed and synthesized asymmetric and symmetric L8‐BO derivatives with brominated end groups and explored the stacking preferences in various modes. The asymmetric BTP‐3FBr displayed an optimized crystallization tendency and thin film morphology, leading to a decent power conversion efficiency (PCE) of 18.34% in binary devices and a top PCE of 19.32% in ternary devices containing 15 wt% IDIC as the second acceptor.

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