Can Isotope Effects Enable Organic Solar Cells to Achieve Smaller Non-Radiative Energy Losses and Why?

Abstract

It has been proposed that isotope effects could effectively downshift intramolecular vibrational frequencies of lightharvesting materials, thereby reducing the non-radiative recombination from the charge-transfer (CT) state to the ground state (GS) and achieving a smaller non-radiative energy loss (ΔE loss non-rad ) theoretically in organic solar cells (OSCs). However, there are no systematic experimental studies to address such a crucial issue: can isotope effects enable OSCs to achieve a smaller ΔE loss non-rad and why? Herein, we constructed 29 non-fullerene acceptors (NFAs) by isotope substitution on different functional groups based on four high-performance NFA systems and further investigated their photovoltaic performance systematically. Large-scale statistical experimental and theoretical analyses indicate no significant difference of PCE and ΔE loss non-rad due to the intrinsically very weak electron-vibration coupling between the CT state and GS (EVC CT-GS ) and largely unimpacted coupling strength (t CT-LE ) between the CT and local exciton states. Also based on theoretical results from the Huang−Rhys factor, although different vibration modes could have different influences on the strength of EVC CT-GS , all are quite small. Both experimental and theoretical results suggest that an isotope strategy may not be a feasible way to significantly improve PCEs of high-performance OSCs by reducing ΔE loss non-rad at the current stage. Article pubs.acs.org/cm

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