The diffusion of molecules through and between organic layers is a serious stability concern in organic electronic devices. In this work, the temperature-dependent diffusion of molecular dopants through small molecule hole transport layers is observed. Specifically we investigate bilayer stacks of small molecules used for hole transport (MeO-TPD) and p-type dopants (F4TCNQ and C60F36) used in hole injection layers for organic light emitting diodes and hole collection electrodes for organic photovoltaics. With the use of absorbance spectroscopy, photoluminescence spectroscopy, neutron reflectometry, and near-edge X-ray absorption fine structure spectroscopy, we are able to obtain a comprehensive picture of the diffusion of fluorinated small molecules through MeO-TPD layers. F4TCNQ spontaneously diffuses into the MeO-TPD material even at room temperature, while C60F36, a much bulkier molecule, is shown to have a substantially higher morphological stability. This study highlights that the differences in size/geometry and thermal properties of small molecular dopants can have a significant impact on their diffusion in organic device architectures.

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulphonate) (PEDOT:PSS) is the most used organic hole injecting or hole transporting material. The hole carrying matrix PEDOT is highly doped by the acidic dopant PSS. When coated onto a substrate, PEDOT:PSS makes a highly uniform conductive layer and a thin (<5 nm) overlayer of PSS covers the air interface. Semiconducting polymer layers for organic photovoltaics or light emitting diodes are coated on top. In this article, we demonstrate that the PSS layer will mix with almost all conjugated polymers upon thermal annealing. Depending on the Fermi energy of the polymer an electrochemical reaction can take place, p-type doping the polymer at the interface between the PEDOT:PSS and the semiconducting polymer. We use chemical and spectroscopic analysis to characterize the polymer/PSS interlayer. We show that the stable and insoluble interlayer has a great effect on the charge injection and extraction from the interface. Finally we demonstrate and electronically model organic photovoltaic devices that are fabricated using these mixed interlayers.