Monte Carlo Simulations of Nanoscale Electrical Inhomogeneity in Organic Light-Emitting Diodes and Its Impact on Their Efficiency and Lifetime

The performance of organic light-emitting diodes (OLEDs) has traditionally been understood on the basis of one-dimensional (1D) models that exploit their planar symmetry. Recently, however, full 3D models have predicted that the current density in these devices is in fact laterally inhomogeneous and highly filamentary on the nanoscale. Here, we implement a 3D kinetic Monte Carlo model to understand the factors that underlie electrical inhomogeneity in OLEDs and explore how it affects their quantum efficiency roll-off and operational lifetime. We find that current filaments initiate at both injecting contacts and internal organic-organic layer interfaces, driven by local injection barrier minima and propagated by percolation paths that naturally occur within the disordered molecular-site distribution. In a classic bilayer OLED, electron and hole filaments are observed to coexist in the same layer and can bypass one another, resulting in substantial efficiency loss due to charge imbalance. In the case of a double-heterostructure phosphorescent OLED, inhomogeneity leads to locally enhanced exciton-polaron annihilation rates that account for an approximately threefold reduction in the operating lifetime and an order-of-magnitude decrease in the critical current density for quantum efficiency roll-off. These results underscore the importance of considering the 3D nature of current transport in OLEDs and point to an unexpected role of organic heterojunctions in exacerbating the degree of inhomogeneity in multilayer devices.