MURATA Laboratory
<Major Research Areas>Organic electronic devices, molecular electronics, vapor-phase growth of conjugated organic thin films

Soft Matter Devices - An Inter
disciplinary Research Field
that Integrates Chemistry and Physics

Research activity

　　Research on the properties of organic materials is one of the fields in which Japan has been leading the world. Organic electroluminescent (EL) displays have been commercialized in Japan and the implications and significance of our success in organic electronic devices have been widely recognized. At Murata Laboratory we are developing organic electronic devices that are expected to play an important role in the ubiquitous society in the future. Our research interests involve synthesis of new materials and development of novel organic devices such as organic light emitting diodes (OLEDs), organic solar cells (OSCs) and organic field effect transistors (OFETs).

1Device physics of OLEDs

　　We study the optical and electrical properties of organic thin films injected with a large amount of electric charge. In particular, we are interested in the photochemical processes that occur in the excited state and the subsequent degradation mechanism of organic molecules when a large amount of current is injected. To investigate the intrinsic degradation processes, we have fabricated OLEDs under ultra-high vacuum conditions. We have reported that the device performance and stability were greatly enhanced in the OLEDs where the emitting layer was composed of fluorescent dye doped into the emitting layer. The enhancement mechanism of the devices is due to the direct recombination of carriers at fluorescent dyes. These studies may lead to achievement of the theoretical limits of device efficiency and stability of OLEDs.

2Design and property evaluation of novel organic electronic devices

　　We are trying to develop highly efficient OLEDs, OSCs and OFETs and organic memories through detailed knowledge of injection, transport and recombination of charge carriers in organic thin films. Regarding OSCs, we have succeeded in controlling device performance (open-circuit voltage) using a novel device structure. As far as the field of organic thin film transistors is concerned, we are interested in developing n-type materials for OFETs.

3Vapor-phase growth of conjugated polymer thin films by the control of ordered architecture

　　We employ vapor-phase epitaxial polymerization for the synthesis of π-conjugated polymers to realize extremely high chemical purity and ordered structure. Vapor-phase epitaxial polymerization can produce conjugated polymers with a precise, ordered structure, uniform molecular chain length or perpendicular orientation to a substrate. These properties cannot be obtained by means of the conventional solution-polymerization method. Such polymer films are expected to exhibit superior electronic and optical properties. By using such materials, we would like to realize electronic and optical devices with extremely high energy conversion efficiency. Furthermore, we aim to create new electronic devices through the integration of organic electronics and silicon technology.

Equipment

Material characterization: Ionization potential measurement system (Riken Keiki AC-2),　UV-vis absorption spectrometer (JASCO V-570), Fluorescence spectrometer (FP-6500), Confocal Raman microspectrometer,　Carrier mobility measurement system (Time-of-flight method)
Device fabrication: Ultra high-vacuum deposition chamber, Glove box, CVD chamber, Spin coater, Dip coater.
Device characterization: Keithley 4200 Semiconductor characterization system, EL characterization system with integrating sphere, Solar light simulator (AM 1.5), Shared facilities:XRD, ESR, AFM (SII), XPS

The main research achievements in the past five years

1. T. Matsushima, M. Takamori, Y. Miyashita, Y. Honma, T. Tanaka, H. Aihara, and H. Murata, High electron mobility layers of triazines for improving driving voltages, power conversion efficiencies, operational stability of organic light-emitting diodes, Organic Electronics, 11, 16-22 (2010).
2. T. Matsushima and H. Murata, Observation of space-charge-limited current due to charge generation at interface of molybdenum dioxide and organic layer, Appl. Phys. Lett., 95, 203306 (2009).
3. M. Campoy-Quiles, Y. Kanai, A. El-Basaty, H. Sakai, and H. Murata, Ternary mixing: A simple method to tailor the morphology of organic solar cells, Org. Electr., 10, 1120-1132 (2009).
4. T. Matsushima, and H. Murata, Marked improvement in electroluminescence characteristics of organic light-emitting diodes using an ultrathin hole-injection layer of molybdenum oxide, J. Appl. Phys., 104, 054501 (2008).
5. Y. Kinoshita, R. Takenaka, and H. Murata, Independent control of open-circuit voltage of organic solar cells by changing film thickness of MoO₃ buffer layer, Appl. Phys. Lett., 92, 243309 (2008).