(171d) Dopant Diffusion and Implications for Device Stability

Organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs) are typically fabricated from a series of different layers, each of which is designed to carry out a specific electronic or optical function. The P-I-N structure consists of a p-type doped or hole transport layer (HTL), an intrinsic emitting or absorbing layer, and n-type doped or electron transport layer (ETL). HTLs and ETLs are often a mixture of an intrinsic organic semiconductor and a molecular dopant that increases the charge density and adjusts the Fermi energy, just as n- and p-type dopants control charge transport in Silicon. However, in contrast so Silicon, the semiconductor has a glass transition temperature below 100ºC and is almost never a perfect crystal. This means that organic dopants are subject to thermal diffusion and electric field induced drift at typical device operating conditions. Here we measure and model the diffusion of p-type dopants in typical HTL layers used for OLED and OPV devices. We demonstrate several different techniques (including fluorescence quenching, quasi-elastic neutron scattering (QENS), and near edge x-ray absorption fine structure (NEXAFS) spectroscopy) to determine that diffusion occurred between layers and for quantifying the diffusion within a material layers with thickness below 50 nm. Finally we address the negative implications for organic device longevity that can occur when the dopants diffuse from the HTL to the intrinsic layer.