MRSEC Seminar Series

Neither Crystalline Nor Amorphous: How a Quantitative Assessment
of Microstructure Helps Us Design Better Organic Semiconductors

Alberto Salleo

Materials Science and Engineering
Stanford University

Wednesday, February 8, 2012, 2:30 PM- 3:30 PM

Cook 2058

Organic semiconductors have been proposed as fundamental building blocks for electronic devices such as transistors, LEDs and solar cells fabricated using low-cost techniques such as printing enabling electronic systems on flexible plastic foils. The promise of organic electronics is to dial in desirable properties (emission wavelength, mobility, chemical sensitivity) and use the power of organic chemistry to rationally design new synthetic semiconductors without being limited by Nature and the periodic table.
From the fundamental standpoint, organic semiconductors are fascinating as they are neither crystalline nor amorphous and their microstructure plays a central role in governing charge transport. I will show that measuring disorder is the key to understanding the optoelectronic properties of these materials. We are particularly interested in cumulative disorder (paracrystallinity), where the lattice parameter takes on a Gaussian distribution about its mean value. The disorder parameter g allows us to rank materials quantitatively on a continuous scale, from a perfectly crystalline material (g<1%) to an amorphous one (g>10%). Surprisingly, even the polymers that are conventionally considered to be very ordered have a g in the pi-stacking direction close to that of an amorphous material. Using first principle calculations and tight binding methods, I will show that paracrystallinity provides a fundamental mechanism for the existence of an exponential distribution of localized tail states in the gap.
Using disorder as a ranking parameter, I will discuss the differences in transport between small molecule and polymer films as well as their respective inherent limitations and bottlenecks. This work may help devising design rules for new materials with desirable transport properties for polarons and excitons.

Biography:

Alberto Salleo graduated as a Fulbright Fellow with a PhD in Materials Science from UC Berkeley in 2001 working at Lawrence Livermore National Laboratory on laser-induced optical breakdown in fused silica. From 2001 to 2005 Salleo was first post-doctoral research fellow and successively member of research staff at Xerox Palo Alto Research Center, where he worked with Bob Street on device and materials physics of disordered and polymeric semiconductors. In 2005 Salleo joined the Materials Science and Engineering Department at Stanford as an Assistant Professor. While at Stanford, Salleo won the NSF Career Award, the 3M Untenured Faculty Award and the SPIE Early Career Award. Salleo is an Associate Editor of the Journal of Electronic Materials and a Principal Editor of MRS Communications.