The Salleo group's research focuses on novel materials and processing technologies for large-area and flexible electronic/photonic devices.

Organic Semiconductor Microstructure and Charge Transport

Organic semiconductors have garnered significant interest in recent years for applications in large-area, low-cost, and flexible electronics. These materials can be processed from solution and are compatible with inexpensive and high-throughput printing and coating deposition techniques. We are interested in structure/property relationships in these materials, and use an array of techniques to characterize the structural, optoelectronic, and electrical properties of polymer and small molecule thin films. Our group has special expertise in synchrotron-based X-ray diffraction techniques and we conduct many experiments at the Stanford Linear Accelerator Center (SLAC). Fabrication of nano-scale devices is also used to characterize fundamental transport processes in these materials.

(a) Polarized microscope image and (b) phase mode AFM image of directionally crystallized P3HT.

Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) based on conducting polymers have undergone significant progress in recent years and are poised to become the device of choice for fabricating biosensors using semiconducting polymers. Due to their ability to support both efficient ionic and electronic transport, OECTs are able to transduce biological signals, which typically involve ion flux, into electrical signals with high gain. Our group is currently working on fundamental models to describe the performance of biosensors in which an OECT is integrated with an ion-blocking membrane (e.g., biological layers, including lipid bilayer membranes and confluent cell layers). Some of our developments have provided guidelines on how to optimize biosensors in which OECTs transduce changes in the impedance of different types of membranes.

(a) Schematic of a barrier layer-functionalized OECT and (b) plot of the membrane resistance as a function of device area. For each device area, the orange region indicates the simulated range of membrane resistances that can be properly characterized.

Organic Solar Cells

The efficiency of organic solar cells has improved dramatically in recent years, reaching values in excess of 10%. However, many aspects of the internal workings of these devices are still unclear, and there remains significant potential for increasing the efficiency further. Our group studies the mechanisms of charge generation, separation, and transport in these devices, particularly the relationship between microstructure and the efficiency of opto-electronic processes determining the photovoltaic action.

The internal aspects which affect photovoltaic performance in polymer:fullerene solar cells.

Upconverting Nanoparticles

Although recent innovations in cell architecture and fabrication techniques have yielded substantial improvements, the efficiencies of single-junction solar cells remain fundamentally limited by transmission of sub-band gap photons. Photon upconversion represents a promising approach toward overcoming this efficiency limit; by absorbing sub-band gap light and upconverting it to energies above the solar cell's band gap, an upconverting layer can significantly boost the efficiency of any single-junction cell by enabling it to utilize photons it would otherwise waste. Our group is actively exploring routes to boost the efficiency of upconverting systems and elevate the technology from a largely academic pursuit to a commercial breakthrough.

(a) Schematic depicting proposed device architecture. (b) Dispersion of NaYF4:Er,Yb nanoparticles illuminated with near-infrared light and emitting green light.