Fundamental studies on charge transport in organic semiconductor materials and devices

The organic field-effect transistor (OFET) is the basic building block of electronic applications based on organic semiconductors (OSCs) and an excellent platform for the study of charge transport physics in OSCs under different charge density regimes. Our efforts aim to improve the performance of OFETs by better understanding and control of charge injection and transport, charge trapping and de-trapping, defect generation, migration and lifetime. For instance, at the interface between the semiconductor and the dielectric (the conduction channel), defects may form that trap or scatter charges. At the interface between an electrode (the source or drain) and the semiconductor, an energy mismatch, oxidization, or pinholes can restrict the electrode’s ability to inject and collect charge. Modeling these effects, including non-ideal behaviors, enables advanced device fabrication characterization and guides the experimental efforts.

Read our recent reviews focused on OFET structure and operation, contacts in OFETs and traps in OSCs.

Structure-property relationships in organic semiconductors

The performance of organic semiconductors is directly tied to the solid-state packing and the degree of order at multiple length scales, which, in turn are very sensitive to processing. The structure, processing and performance are thus strongly influenced by each other. The development of high performance OSC is highly dependent on establishing correct molecular structure – crystal packing – processing – property maps. Our studies focus on describing the structure-processing-property relationship that governs the physical processes in organic semiconductors. The results will be providing the frame for comparisons between novel organic semiconducting molecules and will promote the discovery of materials with enhanced electronic properties.

Read here about our work towards accelerating the whole discovery process through computationally guided material design, which involves combining the power of advanced computation with material synthesis and structural and electrical characterization.

Large-area processing for thin-film electronics

The past decades have witnessed a commendable progress in the development of materials for low-cost flexible electronics. New products that are beyond reach with current technologies can soon make the emerging Internet of Everything (IoE) a reality if these materials reach the necessary standards for performance and stability. For this to happen, device manufacturing needs to transition to techniques that circumvent the technological and economical constraints imposed by traditional vacuum processing. Our efforts focus on development of processing techniques that can address the manufacturability‐scalability‐sustainability-performance balance to accelerate the integration of these materials into next-generation electronic devices.

Read more about spray deposition of small molecules and polymers.

This article reports on the first laser printed OFET. Laser printing is a rapid, scalable, environmentally friendly and low-cost manufacturing technique for deposition and patterning of various functional materials on flexible substrates. Here we show how we used it for developing electronics on paper, and here we laser printed metal halide perovskites.

Organic devices for medical applications

Flexible and wearable organic electronic circuits provide opportunities for development of devices that will improve the quality of healthcare. Sensors and radiation detectors which can non-invasively interface and monitor vital signs can be enabled by exploiting the high sensitivity of organic semiconductors and OFETs to environmental stimuli. We study how organic semiconductors can be used in quality control applications, such as real-time dose monitoring during radiation imaging and therapy. Recent projects have focused on the physical mechanism of in-vivo dosimeters, as well as understanding the durability of these sensors when bent to conform to a surface and the response of different materials to radiation.

Metal halide perovskite devices

Hybrid organic-inorganic perovskites exhibit high charge carrier mobilities, long diffusion lengths and tunable energy gaps. These properties, in combination with the low-cost processability, make perovskites a promising material for optoelectronic devices such as photovoltaic cells, light-emitting diodes, photodetectors, and transistors. We study charge transport and photoconductivity in perovskite devices, in relation to material composition and film processing.

Read here about room-temperature operation of hybrid halide perovskite field-effect transistors.

We modified the OFET structure and employed post processing steps to enhance the performance of perovskite FETs.

We pioneered laser printing as an effective and solvent free method for perovskite deposition and patterning.

Molecular electronics

The ultimate goal for the downsizing of electronic devices is the use of a single molecule as an active component, i.e. molecular electronics. We study devices consisting of self-assembled monolayers (SAMs) covalently bonded to a substrate and sandwiched between two electrodes. This molecular diode rectifies current, operating similarly to solid-state diodes. The rectification strength is dependent on the structure of the SAM, its degree of order and interaction with the electrodes.

Read here about the impact of dipole moment on the rectification strength, and here about rectification enhancement by tuning the coupling between the molecule and the electrode.