Universal electrode for ambipolar charge injection in organic electronic devices.

Ambipolar transistors, i.e. transistors with symmetrical n- and p-type performances, open new avenues for the design and integration of high-density, efficient and versatile circuits for advanced technologies. Their performance requires two processes: efficient injection of holes and electrons from the metal electrodes into the semiconductor; and transport of both carriers through the semiconductor. Organic semiconductors (OSCs) support ambipolar transport, but charge injection is strongly asymmetric due to inherent misalignment of the electrode work function with both conducting levels of the OSC. Here we introduce a new electrode concept capable of efficiently injecting both types of charge carriers into OSCs. The electrode has a mosaic-like structure composed of islands of two metals with high and low work functions, in this case Al and Au, respectively. Under suitable applied bias the Au (Al) domains in direct contact with the OSC allow efficient hole (electron) injection into the HOMO (LUMO) level. Implementing this electrode as both the source and drain in an organic field effect transistor (OFET) led to fully balanced ambipolar performance while maintaining high ON/OFF ratios. We then used the ambipolar OFETs to significantly simplify the circuit design and fabricate digital and analogue elements, i.e. a digital inverter and an analogue phase shifter using one type of transistor only. Finally, we demonstrate that a single ambipolar OFET can replace several unipolar transistors to fabricate digital transmission gate circuits. The new electrode design concept can include other metal combinations and compositions to balance ambipolar injection, and the use of the mosaic electrodes can be extended to other electronic devices that require ambipolar charge injection such as light emitting transistors, memory devices etc.

Interlayers Self-Generated by Additive-Metal Interactions in Organic Electronic Devices.

The fundamental structure of all organic electronic devices is a stack of thin layers sandwiched between electrodes, with precise intralayer morphology and interlayer interactions. Solution processing multilayers with little to no intermixing is, however, technically challenging and often incompatible with continuous roll-to-roll, high-speed manufacturing. Here, an overview of a recently developed methodology for self-generation of interlayers positioned between the active layer and metal contact is presented. The interlayer material is blended as an additive in the active layer and migrates to the organic/metal interface during metal deposition. The driving force for this migration is additive-metal interactions. The generated interlayer positions an interfacial dipole that reduces barriers for charge transfer across the organic/metal interface. This methodology is generic and, as reported here, the self-generated interlayers significantly improve the performance of many devices. Importantly, this approach is compatible with printing and reel-to-reel processing. Directives toward additive selection, processing conditions and integration in future applications are also discussed.

Dynamics of Additive Migration to Form Cathodic Interlayers in Organic Solar Cells.

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC71BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating process, i.e., the majority of PEG accumulates at the bottom substrate, while the minority resides in the film. During electrode evaporation, PEG molecules kinetically "trapped" near the film surface migrate to the organic/Al interface to reduce the interfacial energy. This diffusion-limited process is enhanced with the initial concentration of PEG in the solution and with thermal annealing after metal deposition. In contrast, annealing the film before metal deposition stalls PEG migration. This mechanism is supported by corresponding OSC devices showing that Voc increases with PEG content at the interface, up to a saturation value associated with the formation of a continuous PEG interlayer. Presence of a continuous interlayer excludes the driving force for further migration of PEG to the interface. Revealing this mechanism provides practical insight for judicious selection of additives and processing conditions for interfacial engineering of spontaneously generated interlayers.