The Relationship of Excitation Energy Distribution and Emitter Tail State in OLED-A Comparative Study of HLCT and LE States

This paper focuses on the impact of exciton energy distribution in organic light-emitting diodes (OLEDs) on device efficiency. Due to the structural and energetic disorder in organic semiconductors, excitons formed by electron-hole recombination exhibit an energy distribution. In principle, exciton energy has a Gaussian distribution with a peak near the energy gap and a distribution width of tens to several hundreds meV. Using the isomer emitters with hybridized local and charge transfer (HLCT) state and the localized excited (LE) state as case studies, we develop a simple mathematical model to describe exciton energy distribution. By comparing the exciton energy spectrum with the absorption spectrum of emitters, it is observed that some low-energy excitons (especially below the bandgap) are lost during electroluminescence so that emitters with a high state density of band-tail states are highly beneficial for the realization of high exciton utilization.