Earth-abundant Metal Complex Drives OLED Efficiency Gain

The demand for low-cost, environmentally friendly light sources has led to greater use of OLEDs to illuminate digital displays and indoor/outdoor spaces. One of the most popular commercial methods for their fabrication is through solution processing. While this process is simple and low-cost, the raw materials used during solution processing often include materials that drive up costs, such as rare Earth metals.

The challenge for solution processing is to develop efficient transition metal complexes that are inexpensive and readily available to be used in place of expensive metals, thereby reducing the fabrication costs for OLED devices.

Researchers at Dongguk University responded to this need by developing a cost-effective, eco-friendly, bright green light-emitting, manganese (Mn) complex for OLEDs called MnBz. The research team, led by professor Vijaya Gopalan Sree, used MnBz to fabricate a first-of-its-kind, Mn-based, warm-white OLED device and a green OLED device. Both devices demonstrated record-high efficiency.

“Replacing expensive rare Earth metals, like gold and platinum with crystalline, Earth-abundant transition metal complexes can help achieve lighting solutions or displays that are cheaper, yet bright and vibrant,” Sree said.

Mn complexes are highly stable due to the hydrophobic nature of large-sized organic ligands, which protect the emissive Mn center. In addition, manganese is non-toxic and environmentally benign. Neutral Mn(II) complexes exhibit good structural stability, making them suitable for use in electroluminescent devices.

Mn(II) adopts multiple geometries depending on the ligand environment, which results in distinct emission colors. High photoluminescence quantum yield and low production costs make zero-dimensional hybrid Mn(II) complexes an appealing choice for light-emitting applications.

The researchers synthesized MnBz, a zero-dimensional Mn(II) complex, by subjecting manganese bromide (MnBr2) and benzyltriphenylphosphonium bromide (Ph₃BzPBr) to solvent-free grinding, followed by dissolution in acetonitrile solvent. The resulting solution was allowed to slowly evaporate over days to create single crystals of MnBz.

Scientists at Dongguk University have developed a new, environmentally friendly and cost-effective bright green light-emitting manganese (Mn) complex called MnBz for OLEDs. The material was used to fabricate a first-of-its-kind, Mn-based white OLED device and a green OLED device with record-high efficiency. Courtesy of Dongguk University.

From this process, the researchers obtained an MnBz complex that demonstrated bright phosphorescence under ultraviolet (UV) light with high quantum yield and a narrow emission spectrum. MnBz exhibited a green emission peak at 517 nm, with excellent stability and a photoluminescence quantum yield of 53%.

The team then used the single crystals of MnBz to fabricate a Mn(II) complex-based, RGB, warm-white OLED, using the Mn(II) complex as the green component. The warm-white OLED demonstrated a color rendering index of 78 with a correlated color temperature of 3740 Kelvin. The researchers also used MnBz to design a green phosphorescent OLED device that demonstrated excellent performance. The light emitters displayed a record-breaking quantum efficiency of 11.4% and current efficiency of 56.84 cd A^-1, outperforming the currently available Mn(II) complex-based OLEDs.

The exceptional brightness of the MnBz-based devices, in response to low turn-on voltages, could inspire the future development of cheaper OLEDs for a variety of emission applications and commercial displays and provide a path to energy-efficient, OLED-based consumer electronics and lighting systems.

The research was published in the Chemical Engineering Journal (www.doi.org/10.1016/j.cej.2023.145936).