Factors promoting hot-exciton formation during charge recombination in organic thin films

Abstract

The outcome of excited-state processes is strongly dependent on initial conditions, i.e., the nature of the state arising from excitation. The physics of photoexcitation and the nature of the excited states generated are well established and applied routinely in the exploration of photochemical mechanisms and the interpretation of laser experiments across a broad range of molecules and materials. However, despite its importance in emerging technologies such as organic light-emitting diodes (OLEDs), the nature of excited states formed by electrical excitation is much less understood. In this paper, we use kinetic Monte Carlo (KMC) simulations to reveal that during charge recombination there is a substantial probability of forming higher-lying excited states with >0.5 eV excess energy compared to the lowest excited state. The probability of these hot-exciton pathways depends on the energy gap between the states and the intrinsic energetic disorder in the system. Our simulations also reveal the concept of electrical selection, in that only higher-lying excited states exhibiting HOMO→x or x→LUMO character (where "HOMO"refers to the highest occupied molecular orbital and "LUMO"refers to the lowest unoccupied molecular orbital) can be formed. Importantly, the potential to form these higher-lying excited states is likely to have significant implications for the lifetime of OLEDs, especially those operating in the blue, as the excess energy will push the core components closer to the damage thresholds, increasing the probability of degradation.