Recent developments on organic single crystal-based light-emitting devices

Organic single crystals with low impurity content and long-range periodic order usually present high carrier mobility, which have attracted increasing interest in the field of optoelectronic devices, such as organic light-emitting transistors, organic field-effect transistors (OFETS), optically pumped lasers, and organic light-emitting devices (OLEDs). They are recognized as ideal materials for OLEDs because of their better thermal stability and higher carrier mobility compared to their amorphous counterparts. This review focuses on the recent developments of organic single crystal-based OLEDs from four aspects, including a summary of widely used organic single crystals, crystal growth methods, fabrication strategies for organic single-crystal OLEDs, and structure optimizations of the devices. Before the device fabrication, it is very important to choose suitable crystal materials with prominent optoelectronic properties. The crystal materials can be classified into three different subjects according to their charge transporting properties, including P-type, N-type, and ambipolar. These crystal materials are summarized with three major properties of charge mobility, emission peak position, and energy level structure. The second is the growth methods for growing organic single crystals with low impurity and defects. Based on the traditional vapor-processing and solution-processing method, some novel crystal growth strategies have been developed for large-area organic single crystals with controllable thickness down to one molecular layer. In view of the fabrication of organic single crystal-based OLED devices, the template stripping method has been introduced to fabricate organic single crystal-based OLEDs with improved contact between metallic electrodes and crystals. Based on this method, organic single crystal-based OLEDs can be realized with bright surface emission and polarized EL behaviors. Molecular doping technique is also employed for the fabrication of crystal-based OLEDs with three primary colors. In addition, WOLEDs based on double-doped organic single crystals are also discovered with high color rendering index, indicating strong potential in the display and lighting field. Later, ambipolar crystals prepared by doping N-type molecules into a P-type crystal matrix for equal mobility are achieved for OLEDs with a recorded brightness and efficiency. Organic single crystal-based OLEDs have made great progress, however, the device performance still lags far behind the traditional amorphous thin-film OLEDs and faces enormous challenges. This review then introduced several potential strategies that can further improve the performance of organic single crystal-based OLEDs in four aspects: Designing new organic single crystal materials with high solid-state luminous efficiency and balanced carrier transport, exploring new growth strategies for precise control of organic single crystal thickness in combination with the semiconductor manufacturing technology, developing various host-guest doping systems for effectively promoting the efficiency of organic single crystal-based OLEDs, and further optimizing the device structure with improved energy levels. In conclusion, organic single crystals have made remarkable progress in the development of electronic and optoelectronic devices, especially for organic single crystal-based OLEDs, which can be ascribed to the development of new material designs, innovative crystal growth methods, and the revolution of device fabrication. Many efforts are required to seek novel strategies for the improvement of organic single crystal-based OLEDs which are expected to be great potential in optoelectronic devices.