Solution-processed molecular functional materials for organic optoelectronics

Electroluminescent devices—for instance, the common LED—produce light through radiative electron and hole recombination under the influence of an electric field. To date, most such devices have been based on solid-state semiconductor materials and technologies. Now, the demand for low-cost and large-area flexible organic optoelectronics has stimulated tremendous interest and progress in solution-processed optoelectronic materials and devices.

Ideally, a light-emitting organic material would incorporate all its basic elemental functions (electron and hole injection/transport and light emission) into a single molecular architecture, and in fact conjugated polymers have been identified as active materials for light emission and charge generation. But in recent years, many solution-processable small organic molecules with these properties have also emerged. In contrast to polymers, which typically exhibit a wide distribution of molecular weights, small molecules possess the advantage of potentially being readily producible at high purity (to which device performance is sensitive) using simple techniques. They also lend themselves to a much more straightforward analysis of molecular structure/properties relationships.

Highly photoluminescent π-conjugated soluble organic molecular glasses are attractive solid-state emitters for organic (diode) lasers, organic LEDs (OLEDs), and light-emitting field-effect transistors, as well as for sensing and imaging purposes. Through solution processes such as spin coating and ink-jet printing, these molecules can be made to yield smooth and uniform transparent films similar to those produced with polymers. An example is C-545P (see Figure 1), which emits a green light. One disadvantage of this compound is that although it is highly photoluminescent in dilute solution, it shows reduced efficiency in the solid state, due to strong intermolecular interactions.

Figure 1. Molecular structure of the compounds 1, 2, and C-545P.

We have recently studied a number of solution-processable molecular RGB (red, green, blue) emitters,^{1, 2} two of which also appear in Figure 1. The green-emitting compound 1 features an inherent amorphous morphology and high solubility (>100mg mL⁻¹ in p-xylene). It also shows remarkably high photoluminescent efficiency even in the solid state (φ_em>0.7), in contrast with the concentration quenching seen in C-545P. The reason for this may be that compound 1's rigid branched molecular structure effectively maintains a separation between the emissive cores of adjacent molecules.

The characterization of OLEDs consisting of non-doped spin-cast films of compound 1 shows promise. At a current density of ∼20mA cm⁻², it was found that the wavelength at maximum electroluminescence λ_max^EL=520nm and luminous efficiency η_c= 11.7cd A⁻¹ at International Commission on Illumination color space coordinates (0.31, 0.60). Additionally, since in OLEDs hole transport generally predominates over electron transport, the design of molecular emitters with well-balanced electron and hole transport is critical. Compound 1's lower hole mobility yields a smaller efficiency roll-off with increasing current density than that seen in yellow-emitting compound 2.

OLED development can also take advantage of the application of solution-processed organic materials to facilitate electron injection from more air-stable electrodes, such as aluminum. We have studied alcohol-processable amorphous molecular electrolytes, in which monoammonium acts as an electron injection layer.^{3, 4} It is worthy of note that within an OLED, a thin layer of an amorphous molecular electrolyte produces greater luminous efficiency than does a CsF electron-injection layer. An electrolyte with the BF₄⁻ counteranion shows better device stability than does one with Br⁻. Such electrolytes can also act as a cathode interfacial layer for some other types of charge-injection devices, for instance, organic field-effect transistors.

Highly photoluminescent soluble semiconducting molecular glasses represent a unique and promising class of non-doped solution-processable emitters for OLEDs. These organic functional materials consist of small, solution-processable molecules that are easy to synthesize and purify. They incorporate their own hole injection/transport functional unit, in which electron and hole transport can be made more nearly equivalent. We are now investigating alcohol-processable non-ionic molecular glasses to facilitate electron injection and are comparing them with the results achieved with electrolytes. The results may hold promise for organic optoelectronics in general.

The author sincerely thanks those co-workers, colleagues, and collaborators who have contributed to this work. Financial support from South China University of Technology, Ministry of Education, National Natural Science Foundation of China, and Ministry of Science and Technology (grants 2012ZZ0001, 51173051, 2009CB930604, and 2009CB623601) is gratefully acknowledged.