Could you explain the significance of your article to the non-specialist?

Since the fabrication of electronic devices such as field effect transistors based on organic semiconductor materials has been demonstrated this technology is expected to provide the basis for future mass production of simple devices e.g. display drivers or RF-id tags. However, a detailed understanding of the relationship between structural and electronic properties in such materials and the preparation of well ordered, possibly crystalline films is often not achieved. In contrast to their inorganic counterparts the molecular semiconductors reveal only weak (mostly van der Waals) interaction forces and the anisotropic shape of the molecular building blocks causes an orientational degree of freedom. In the present work we demonstrate for the case of rubrene that also the molecular conformation is important because it can be different for the gas phase and the crystalline phase. While molecular beam deposition fails to produce crystalline films the activation energy for the conformation change is provided by "hot wall" deposition and results in the formation of long range ordered crystalline 
films.

What has motivated you to conduct this work?

Previous studies have demonstrated a remarkable high charge carrier mobility for single crystals of the molecular semiconductor rubrene [Sundar et al., Science 2004, 303, 1644] whereas the growth of crystalline films have not been possible for this material. This was surprising because for other promising aromatic semiconductors such as pentacene crystalline thin films have been grown by several groups. The apparent impossibility to prepare crystalline thin films of rubrene has encouraged us to unravel the differences in film growth and to overcome this obvious limitation. 

Where do you see this work developing in the future?

Our new approach allows the preparation of crystalline films under ultra-clean i.e. ultra-high vacuum conditions as well as their further processing (e.g. contacted) and thus provide a characterization of the electronic samples properties without any contaminations of the crystal surfaces and/or a controlled modification of the interfaces.

Are there any particular challenges facing future research in this area?

At present the contributions of bulk and surface properties to charge carrier transport are not well understood. With our new approach we believe that model systems with (chemically and structural) well defined interfaces can be prepared which provide an approach to study in details the influence of surface doping on the resulting charge transport mechanism.