Light emitting diodes (LEDs) based on the inorganic semiconductor gallium nitride (GaN) replaced the classical incandescent ("Edison") light bulb and truly enabled a new era in lighting that was honored by the Noble Prize in Physics 2014. Although the epitaxial growth of p-type GaN was an essential part of this innovation that became state-of-the-art in two-dimensional (2D) LED manufacturing today, it has rather poor current spreading properties to enable next-generation LEDs based on three-dimensional (3D) core-shell architecture.

To overcome this obstacle, we decided to find a proper substitute for the inorganic p-type GaN and looked into the materials typically employed in organic light emitting diodes (OLEDs). Although OLEDs have not been honored with the Noble Prize, they employ a very extraordinary class of materials that was worth the Nobel Prize in Chemistry 2000 – the "Conductive Polymers". These materials have a conjugated carbon backbone that forms a delocalized π-electron system and enables efficient charge transport within the polymer chain. Poly(3,4-ethylenedioxythiophene) mixed with poly(styrenesulfonate), better known as "PEDOT:PSS", is an attractive organic p-conductor. The hydrophilic PSS is required because it enables liquid deposition on simple, planar substrates used by the OLED industry.

However, it is indeed very challenging to obtain conformal, pinhole-free coatings of PEDOT:PSS on complex, 3D substrates such as microrods or nanowires. To overcome this challenge, we suggested oxidative chemical vapor deposition (oCVD), a technique originally developed by the group of co-author Professor Karen K. Gleason at MIT. In oCVD, gases of monomeric EDOT and an oxidizing agent pass simultaneously over a temperature-controlled stage, uniformly attach to the substrate surface and polymerize via step-growth mechanism into thin layers of conductive PEDOT.

In our paper, we propose the idea to replace p-GaN with oCVD PEDOT and describe a route toward the design of hybrid inorganic/organic optoelectronics based on three different prototypes (see Figure 1).

First, 2D reference structures consisting of GaN/oCVD-PEDOT were considered to reveal the electrical characteristics at the hybrid interface. We suggest thermionic emission as the main conduction mechanism and obtained high rectification ratios of up to 10⁷ at ±2 V, low ideality factors of ~2.01, mean barrier heights of ~1.42 eV at room temperature and excellent thermal and temporal stability, a prerequisite for reliable applications in optoelectronics. Therefore, we proceeded in a second step with 2D LED structures featuring an InGaN multi-quantum well (MQW) for blue light emission and compared purely inorganic LEDs (based on state-of-the-art p-GaN) with our hybrid design (based on oCVD PEDOT). We found that the hybrid LED offers pronounced electroluminescence and a substantially enlarged light-emitting area attributed to the advanced current spreading property of the conductive polymer. In the third step, we applied our hybrid concept to a cutting-edge 3D core-shell microrod LED design provided by OSRAM Opto Semiconductors and observed a broad electroluminescence spectrum that indicates light emission from the entire microrod length. From these findings, we conclude that oCVD PEDOT can be a key component in the realization of new hybrid inorganic/organic 3D LEDs based on GaN microstructures.