Organic single crystal-based light-emitting devices by template stripping technique

Abstract

Organic single crystals have attracted extensive interests in the field of optoelectronic and electronic devices for their high chemical purity, high charge-carrier mobility and high luminescent quantum efficiency. The motivation for research on organic single crystals is their highly arranged packing structure, which provides an ideal model to investigate the basic interactions between the molecules and stacking modes and optoelectronic performance. Because of advantages of these crystals, they have been employed in much functional optoelectronic devices such as organic field-effect transistors (OFETs), light-emitting organic field-effect transistors (LE-OFETs), optically pumped lasers, photovoltaic cells and so on. However, the reports on organic single crystal based organic light emitting devices (OLEDs) are scarce. There is a big difficulty in the fabrication of organic single crystal-based OLEDs because of their intrinsic properties like soft, fragile and sensitive to organic solvent. In this work, a simple and non-destructive method template stripping has been applied into the single crystal-based OLEDs to improve the contact problem and realize its bright emission. Based on this method, the physical vapour transport grown crystals are firstly transferred to the modified silicon substrate. Then an anode of 100 nm gold film is deposited on the top of the crystal surface by thermal evaporation with metal mask. After dropping a microliter of photoresist polymer on the device, a glass is pressed on top by its own weight. And the polymer will spread to the edge of the glass and totally cover the device. Then it will be cured after exposed to a UV-light source with a power of 500 W. The device will be transferred to the glass with the help of a knife for separation. Finally the cathode of Ca/Ag will be evaporated on the other side of crystal. The sandwiched structure devices can be achieved by this method. Both anodes and cathodes of the crystal-based OLEDs can be deposited onto the opposite surface of the organic single crystals by thermal evaporation on both sides, so that a much improved contact between the crystals and the electrodes can be realized, which enhance the carrier injection into the active layer. Homogenous light emission can be observed from these crystal-based OLEDs. And the molecules of BP2T are arranged layer by layer which are nearly perpendicular to the crystal plane. As we known, the directions of the transition dipole moments lie along the long axis of the molecule. The light in the crystal will transmit to the edge of the crystal which recognized as self-waveguide effect. In the same way, edge-emitting can be seen from the crystal-based OLEDs which attributed to this self-waveguide. In order to clarify the template stripping method can work well on the fabrication of single crystal based OLEDs. We compare the performance of both devices between the lamination method and template stripping method. Based on lamination method, the single crystals are simply laminated onto the top of the anode by van der Waals force which results in an incomplete contact. Compared to the lamination method, the devices show higher carrier injection and low voltage threshold by this compacted contact between crystals and electrodes without any breakages. It is expectable that this technique would support broad applications of the organic single crystals in the crystal-based optoelectronic devices.