Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

TL;DR

This study reveals that in delayed fluorescence organic semiconductors, spontaneous exciton dissociation into intermolecular charge transfer states enables rapid spin state interconversion, improving LED efficiency despite a large singlet-triplet gap.

Contribution

The paper demonstrates that intermolecular charge transfer states facilitate fast spin interconversion, overcoming limitations of small radiative rates in low singlet-triplet gap materials.

Findings

Intermolecular CT states occur on picosecond timescales in aggregated films.

Hyperfine interactions mediate rISC in inter-CT states on ~24 ns timescale.

Transfer back to singlet excitons enables efficient delayed fluorescence and LED operation.

Abstract

Engineering a low singlet-triplet energy gap ({\Delta}EST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors, but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient =3.8x10^5 cm^-1) and a relatively large {\Delta}EST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (260 {\mu}s), but in aggregated films, BF2 generates intermolecular CT (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of >1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states resolves the conflicting requirements of fast radiative emission and low {\Delta}EST.