Abstract
In this paper, we report an efficient alkali metal doping system for organic single crystals. Our system employs an enhanced diffusion method for the introduction of alkali metal into organic single crystals by controlling the sample temperature to induce secondary thermal activation. Using this system, we achieved intercalation of potassium into picene single crystals with closed packed crystal structures. Using optical microscopy and Raman spectroscopy, we confirmed that the resulting samples were uniformly doped and became K2picene single crystal, while only parts of the crystal are doped and transformed into K2picene without secondary thermal activation. Moreover, using a customized electrical measurement system, the insulator-to-semiconductor transition of picene single crystals upon doping was confirmed by in situ electrical conductivity and ex situ temperature-dependent resistivity measurements. X-ray diffraction studies showed that potassium atoms were intercalated between molecular layers of picene, and doped samples did not show any KH- nor KOH-related peaks, indicating that picene molecules are retained without structural decomposition. During recent decades, tremendous efforts have been exerted to develop high-performance organic semiconductors and superconductors, whereas as little attention has been devoted to doped organic crystals. Our method will enable efficient alkali metal doping of organic crystals and will be a resource for future systematic studies on the electrical property changes of these organic crystals upon doping.
Highlights
Organic molecular systems have gained much attention in recent years as a potential core component for various electronic devices such as organic light emitting diodes (OLED)[1], organic field-effect transistors (OFET)[2], and organic solar cells (OSC)[3]
As seen in the schematic of the doping system shown in Fig. 1a, the doping system basically consists of two parts: the left part is the source chamber from which the potassium vapor is generated, and the right part is the sample chamber where the organic single-crystal devices are placed on top of the temperature-controllable stage
We developed a thermally enhanced diffusion method for alkali metal doping into organic single crystals by applying secondary thermal activation to target crystals
Summary
Organic molecular systems have gained much attention in recent years as a potential core component for various electronic devices such as organic light emitting diodes (OLED)[1], organic field-effect transistors (OFET)[2], and organic solar cells (OSC)[3]. The traditional method is designed to induce spontaneous diffusion of vaporized alkali metal from a stoichiometric mixture of alkali metal and target organic systems, mostly in powder form This process is relatively fast in granular specimen because the grain boundaries and defects that are highly populated in the granular specimen lower the activation energy of the diffusion process[13]; this cannot be achieved for organic single crystals by increasing the source heating temperature because of another important obstacle to the study of organic single-crystal systems. A new doping method that can promote diffusion more efficiently should be developed for soft PAH single crystals
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