Electronic and Crystallographic Examinations of the Homoepitaxially Grown Rubrene Single Crystals

Abstract

Homoepitaxial growth of organic semiconductor single crystals is a promising methodology toward the establishment of doping technology for organic opto-electronic applications. In this study, both electronic and crystallographic properties of homoepitaxially grown single crystals of rubrene were accurately examined. Undistorted lattice structures of homoepitaxial rubrene were confirmed by high-resolution analyses of grazing-incidence X-ray diffraction (GIXD) using synchrotron radiation. Upon bulk doping of acceptor molecules into the homoepitaxial single crystals of rubrene, highly sensitive photoelectron yield spectroscopy (PYS) measurements unveiled a transition of the electronic states, from induction of hole states at the valence band maximum at an adequate doping ratio (10 ppm), to disturbance of the valence band itself for excessive ratios (≥ 1000 ppm), probably due to the lattice distortion.

Photoelectron spectroscopy on single crystals of organic semiconductors: experimental electronic band structure for optoelectronic properties

“…Generally, the crystals were individually taken out in the ambient atmosphere for further processing to be prepared as specimens for the photoemission experiments as described in the previous paragraph. For maintaining the crystals ''intrinsic'' by avoidance of the exposure to the ambient conditions, 119,120 sample treatment in a glovebox directly connected with the PVT equipment, as illustrated in Fig. 12, is an efficient measure as discussed in Section III.1.…”

Quasi-Homoepitaxial Junction of Organic Semiconductors: A Structurally Seamless but Electronically Abrupt Interface between Rubrene and Bis(trifluoromethyl)dimethylrubrene

“…In previous studies, highly ordered heteroepitaxial interfaces of uniformly aligned n-type molecular materials formed on single crystals of p-type organic semiconductors have been attained. 16 − 21 Moreover, it has been confirmed that single-crystalline thin films with extremely good crystallinity can be obtained by homoepitaxy, 22 − 24 where molecules are deposited on single-crystal substrates of the same molecular species. This methodology enables the realization of p–n homojunctions of a monolithic organic semiconductor single crystal by accurately controlled impurity doping.…”

“… 21 , 48 The VB of RubSC also shifted slightly upward after the fmRub deposition; in other words, the E F moved toward the VB, implying the occurrence of acceptor doping to RubSC. 23 , 24 The suppression of the VB-derived peak of RubSC suggests the complete covering of the RubSC surface by the fmRub overlayer, at least at an fmRub thickness of 3 nm. It is noteworthy that this thickness was much greater than the probing depth (<1 nm) for UPS in the present measurement conditions 49 and was equivalent to the c -axis lattice constant (3.44 nm), which consists of two molecular layers of fmRub.…”

“…Although the expected position for the lowest-unoccupied molecular orbital (LUMO) of fmRub was far above E F , the present results suggest the occurrence of electron transfer from RubSC to fmRub to some extent presumably via midgap states distributed around E F . 24 …”

Research progress of rubrene as an excellent multifunctional organic semiconductor

“…In the solution method, the types of solvents, concentration of rubrene and the temperature of crystal growth may all play an important role in which polymorph is formed. In addition to the above common methods for preparing rubrene single crystals, many novel methods have been developed in recent years, including epitaxy [36,37], microspating in air sublimation [38] and so on.…”