Höchst group seeks to disentangle the charge transport mechanisms in organic semiconductors
In any electronic device, charges must be transported through materials. The underlying mechanism of this charge transport was first discussed in the early 1950s, and continues to puzzle theoreticians today.
"In the last couple of years, modeling has tried to explain how the electronic band structure of organic semiconductors, as well as the interaction of carriers with lattice vibrations in the organic crystal, affects the temperature dependent transport properties," explains Hartmut Höchst, senior scientist and research director at University of Wisconsin—Madison’s Synchrotron Radiation Center. "Although these theories stress the importance of both the band structure and electron-vibration interactions, experimental evidence of these effects was missing."
But recent experiments performed at SRC by Höchst’s research group have shed light on a decades-old problem of how charges are transported through a crystal consisting of organic molecules. In a recently published Physical Review Letter (PRL104, 047601 (2010)), Höchst and his colleagues utilized the tunability of synchrotron radiation in their experiments on Pentacene, a prototypical organic semiconductor, to establish a link between the band structure E(K) and how charges are transported through this material.
"This result was surprising in light of recent theory, which predicts the band structure to be destroyed due to the significantly reduced bond strength in organic crystalline solids (also referred to as soft matter). This is in contrast to more common semiconducting materials such as Si or GaAs, which are covalently bonded," Höchst explains.
Furthermore, their experiments on the interaction between charges and lattice vibrations in crystalline Pentacene films demonstrated that vibrations can—rather astoundingly—both aid and hinder charge transport.
As the researchers explain, science into these fascinating materials not only serves to shed light on a previously unlit corner of semiconductor science, it also has vast possibilities for technical applications.
"Understanding charge transport through organic semiconductors is key in explaining why these materials are promising candidates for novel electronic applications such as flexible computer displays, spray-or paint-on solar cells, and ‘printed’ electronics. Organic semiconductors may create a paradigm shift in the fabrication of solar cells and other electronic devices by means of spray-on or paint-on techniques with mass production coming off a rotary printing press rather than by the elaborate and tedious production steps currently involved in the fabrication of electronic circuits," adds Richard Hatch, a doctoral student in Höchst’s group and co-author on the current paper. "Considering the much simpler manufacturing processes and their comparatively low environmental impact, solar cells based on organic molecules can be a key in solving the world’s energy crisis."
And so, this latest research performed at SRC begins to provide some answers to decades old questions with far-reaching future possibilities.