Suppressing Molecular Thermal Motion and Electron‐Phonon Coupling of Fused Ring Electron Acceptors to Prolong Charge Carrier Diffusion Length for High‐Efficiency Organic Solar Cells

ABSTRACT The limited charge carrier diffusion length of organic photovoltaic materials cannot afford separated charge carriers to efficiently diffuse to be collected at electrodes, which impedes the further enhancement of power conversion efficiencies of organic solar cells. Herein, a giant‐molecule electron acceptor with heavy atoms, viz. GMA‐Se‐Cl, is developed and incorporated into L8‐BO to prolong charge carrier diffusion length and improve power conversion efficiencies via taking advantage of giant molecules and heavy‐atom effects. Introducing GMA‐Se‐Cl endows L8‐BO:GMA‐Se‐Cl with a lower molecular diffusion coefficient, suppressed molecular thermal motion, and higher crystallinity, compared with the pristine L8‐BO. Such enables L8‐BO:GMA‐Se‐Cl to afford weaker electron‐phonon coupling, reduced Huang‐Rhys factor, and lower trap density, thus contributing to enhanced charge carrier mobilities and lifetime for the prolonged charge carrier diffusion length. It helps to inhibit charge carrier recombination and facilitate charge transport in devices. Therefore, D18:L8‐BO:GMA‐Se‐Cl based organic solar cells achieve a higher power conversion efficiency of 20.24%, compared with D18:L8‐BO based ones (18.50%). It indicates that introducing giant‐molecule acceptors with heavy atoms is an efficient strategy to restrain molecular thermal motion and electron‐phonon coupling for prolonged charge carrier diffusion length and thus boost power conversion efficiencies of organic solar cells.