Semiconductor-Electrolyte Interactions

Interfacing of semiconductors with electrolytes has provided new ways of operating semiconducting devices (e.g. by electrolyte gating) as well as additional functionality. We study semiconductor/electrolyte interactions and their effects on the structure and electronic properties of active materials including conjugated polymers, 2D transition-metal dichalcogenides, and ALD-grown titanium oxide in photoanodes.

Conjugated Polymers

There is increasing interest in gating devices through electrolytes to enable new functionality. However, the effects of the interaction with the electrolyte on the active layer are not fully known, especially for soft materials such as conjugated polymers, in which ions from the electrolyte can enter and electrochemically dope the semicrystalline film. High-resolution synchrotron grazing incidence X-ray diffraction of polythiophene films electrochemically doped with an ionic liquid reveals changes with applied voltage, cycling, and frequency in lattice spacing, crystallite orientation, and crystallinity in the bulk and at the buried interface. These advanced measurements offer an insight into the interactions between mobile charged species under the action of an external field and conjugated polymers. Work is ongoing to connect the newly characterized structural changes with electronic transport. This fundamental work furthers the understanding of how electrochemical transistors, polymer-based artificial synapses and polymer batteries work.

An illustration of ionic-liquid penetration into a semiconducting polymer thin film. In an ionic-liquid gated polymer thin film transistor, ions penetrate the amorphous domains as they dope the film but only enter the denser, crystalline domains when operating frequency allows enough time and higher voltages supply a sufficient driving force.

Layered Transition-Metal Dichalcogenides

Ionic liquids are room temperature molten salts that are being widely investigated for uses such as electric-field induced superconductivity, green chemistry, carbon capture, and electronic devices. As an electrolyte, the ionic liquid formation of an electric double layer allows it to induce extremely high surface charge density, surpassing that obtained with conventionally gated oxide dielectrics. We are using ionic liquids to study the structural reversible phase change in the layered transition metal dichalcogenide, MoTe2. Starting in the semiconducting 2H phase, MoTe2 can be electrostatically switched to a semi-metallic 1T’ phase under bias. The switch in conductivity between the two phases makes MoTe2 an attractive material for next-generation phase change memory. We are using a variety of optical and structural techniques to characterize the phase change and better understand its mechanism.

The formation of an electric double layer using an ionic liquid electrolyte allows us to inject large numbers of charges (~1e14 cm-2) into MoTe2, driving a phase change from the hexagonal semiconducting 2H phase to the monoclinic semi-metallic 1T’ phase.

Atomic-layer-deposited Titanium Oxide in Photoanodes

Atomic-layer-deposited TiO2 thin film coatings have significantly improved the semiconductor stability in solar absorber / photocatalyst type of photoanodes for water oxidation.These nominally insulating TiO2 thin films must be somewhat electrically “leaky” to perform their desired function. The conduction mechanism remains unclear, but previous studies have found defect states related to Ti3+ and oxygen vacancies may play an important role. In collaboration with the McIntyre group at Stanford, we are using electrical and photoelectrochemical methods to probe the charge transport and transfer characteristics in ALD-deposited TiO2 films to identify the conduction mechanism and its correlation to the defects.