Donor-acceptor molecular interfaces are nothing but p-n junctions for organic optoelectronic devices such as organic light emitting diodes and organic solar cells where an exciton forms or dissociates. Understanding about dominating factors determining the intermolecular assembly and charge carrier exchange processes are highly anticipated for the establishment of smart design strategies of practically efficient devices. Molecular heterojunctions built on single crystal organic semiconductors provide well-ordered model systems disentangling complex intermolecular contacts in real devices. Moreover, the p-n junctions of highly ordered molecular assembly lead to the enhancement of the mobility of charge carriers and excitons through the inter-molecular delocalization of the electronic states, which potentially opens a new route for efficient optoelectronic applications of molecular semiconductors. In this contribution, crystallographic and electronic structures of well-ordered molecular semiconductor homo- and hetero-epitaxial junctions are overviewed.

One representative system is a “complementary” p-n junction of perfluoropentacene (C₂₂F₁₄) crystalline ad-layers formed on the single crystal pentacene (C₂₂H₁₄) [1]. Grazing-incidence X-ray diffraction (GIXD) analyses revealed hetero-epitaxial growth of perfluoropentacene in a uniform crystallographic orientation with respect to the surface lattice of the single crystal pentacene. The energy-momentum dispersions of inter-molecular electronic bands were successfully demonstrated by angle-resolved photoelectron spectroscopy (ARPES) for the hetero-epitaxial perfluoropentacene as well as for the single crystal pentacene, which strongly suggests realization of the delocalization of electrons and holes across this well-ordered molecular p-n junction.

To further improve the crystallinity of the epitaxial molecular heterojunctions, the authors’ group proposed a new concept of “quasi-homoepitaxial organic semiconductor junctions”, at which a molecular species with nearly identical in-plane lattice constants to those of the substrate molecular single crystal surface is stacked [2]. Actually, quasi-homoepitaxial crystallites of bis(trifluoromethyl)dimethylrubrene was revealed to form highly-ordered interface of much improved mean crystallite size on the single crystal surface of (unsubstituted) rubrene by means of high-resolution GIXD. The electronic structures exhibiting spontaneous electron transfer at this inter-molecular contact and possible opto-electronic applications of this quasi-homoepitaxial junction are also discussed in this presentation.

This contribution is supported by Grant-in Aids for Transformative Research Areas (A) “Dynamic Exciton: Emerging Science and Innovation” (JSPS-KAKENHI Grant Number JP21H05405).

[1] Y. Nakayama, et al., J. Phys. Chem. Lett. 10 (2019) 1312.

[2] K. Takahashi, et al., J. Phys. Chem. Lett. 12 (2021) 11430.