[Paper Review] Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells

Xiankai Chen, Christopher C. S. Chan+9 more|arXiv (Cornell University)|Apr 19, 2023

Organic Electronics and Photovoltaics4 citations

TL;DR

This study reveals that intermolecular charge-transfer (iCT) excitons in Y6-type non-fullerene acceptors enable nanosecond excited-state lifetimes despite low optical gaps (~1.4 eV), by reducing non-adiabatic electron-vibration couplings through strong electronic coupling in densely packed films. This mechanism suppresses non-radiative recombination, explaining the high efficiency of Y6-based organic solar cells with near-infrared absorption up to 1000 nm.

ABSTRACT

State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar cells (OSCs) to achieve highly efficient charge generation with extended near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise molecular-level mechanism that enables low-energy excited states in Y6-type acceptors to achieve nanosecond lifetimes has remained elusive. Here, we demonstrate that the distinct packing of Y6 molecules in film leads to a strong intermolecular charge-transfer (iCT) character of the lowest excited state in Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton energies are greatly reduced by up to ~0.25 eV with respect to excitons formed in separated molecules. Importantly, despite their low energies, the iCT excitons have reduced non-adiabatic electron-vibration couplings with the electronic ground state, thus suppressing non-radiative recombination and allowing Y6 to overcome the well-known energy gap law. Our results reveal the fundamental relationship between molecular packing and nanosecond excited-state lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding performance of Y6-based OSCs.

Motivation & Objective

• To identify the molecular-level mechanism enabling nanosecond excited-state lifetimes in low-bandgap Y6-type non-fullerene acceptors.
• To understand why Y6-type acceptors exhibit long-lived excited states despite their small optical bandgap (~1.4 eV), unlike other low-gap acceptors such as ITIC.
• To elucidate the role of molecular packing in stabilizing low-energy excited states with suppressed non-radiative decay.
• To establish the connection between intermolecular charge-transfer (iCT) character and enhanced excited-state lifetime in Y6 aggregates.

Proposed method

• Employed quantum chemical calculations, including time-dependent density functional theory (TD-DFT), to analyze the electronic structure of Y6 and ITIC in isolated and aggregated forms.
• Quantified intermolecular electronic couplings between adjacent Y6 molecules in crystalline-like packing to assess their impact on exciton energy levels.
• Computed non-adiabatic electron-vibration coupling strengths to evaluate the rate of non-radiative relaxation in iCT states versus localized excitons.
• Compared the excited-state properties of Y6 and ITIC in both isolated and aggregated configurations to isolate the role of intermolecular interactions.
• Used the energy gap law to interpret the observed suppression of non-radiative decay in iCT states despite low excitation energy.
• Analyzed the spatial distribution of the lowest excited state to confirm the delocalized, charge-transfer character in Y6 aggregates.

Experimental results

Research questions

• RQ1 What molecular and electronic factors enable nanosecond excited-state lifetimes in Y6-type non-fullerene acceptors with low optical gaps (~1.4 eV)?
• RQ2 How does molecular packing in Y6 films influence the nature and energy of the lowest excited state compared to isolated molecules or other acceptors like ITIC?
• RQ3 To what extent do intermolecular charge-transfer (iCT) excitons reduce non-adiabatic electron-vibration coupling, thereby suppressing non-radiative recombination?
• RQ4 Why do Y6-based organic solar cells achieve high efficiency with near-infrared absorption up to 1000 nm, despite the energy gap law predicting fast non-radiative decay?
• RQ5 What is the role of strong electronic coupling between adjacent Y6 molecules in stabilizing low-energy iCT excitons and extending excited-state lifetime?

Key findings

• The lowest excited state in Y6 aggregates exhibits strong intermolecular charge-transfer (iCT) character, absent in isolated Y6 molecules or in ITIC-based systems.
• Strong electronic coupling between adjacent Y6 molecules reduces iCT exciton energies by up to ~0.25 eV compared to isolated molecules, enabling low-energy absorption in the near-infrared region.
• Despite their low energy, iCT excitons in Y6 films exhibit significantly reduced non-adiabatic electron-vibration coupling with the ground state, suppressing non-radiative decay pathways.
• This suppression of non-radiative recombination allows Y6 to overcome the energy gap law, achieving nanosecond excited-state lifetimes essential for efficient charge generation.
• The combination of iCT character and strong intermolecular coupling explains the superior performance of Y6-based organic solar cells with extended near-infrared response.
• In contrast, ITIC-based systems lack this iCT character and exhibit faster non-radiative decay, consistent with shorter excited-state lifetimes and lower device efficiency.