Directed migration of additives to form top interlayers in polymer light emitting diodes†
A major advantage of polymer light emitting diodes (PLEDs) over other emitting technologies is solution processing that can offer continuous, roll-to-roll high speed cheap manufacturing. The general structure of efficient PLEDs is a multilayer stack with electrodes and charge injection/transport/blocking layers sandwiching the emissive layer. Solution processing a multilayer structure with little to no intermixing is, however, technically challenging. Here we demonstrate a new methodology to generate discrete interlayers positioned between the emissive layer and the cathode that does not require a discrete processing step, is fully compatible with solution processing and provides significantly enhanced device performances. In this approach, the interlayer material is blended and processed with the emissive layer. Then, when the cathode is deposited, the interlayer molecules spontaneously migrate from the bulk film to the organic/metal interface to form the interlayer. For example, PEG molecules blended with F8BT or SY migrate to the polymer/Al interface during Al deposition, as confirmed by contact angle and XPS measurements. PEG migration is driven by PEG–Al interactions which also modify the interfacial electronic structure. As a result, devices with migrated PEG interlayers exhibit reduced turn on voltage, increased luminance, current density and current efficiency compared to devices without PEG. The dependence of device performance on PEG concentration and thermal treatments revealed that the interfacial composition can be sensitively tuned to yield optimal performance. This methodology offers a simple and efficient interlayer processing protocol that is versatile and compatible with roll-to-roll and printing technologies and can be easily translated to other polymers, contacts and organic-based devices.