From incandescent light bulbs to fluorescent tubes, from metal-halide lamps to LEDs, human beings have strived for more than a millennium to improve the efficiency of light generation from energy sources, including electricity. Three decades after the invention of organic light-emitting diodes, more commonly known as OLEDs, the latest trend in lighting and display is set by this emerging technology.
Although OLEDs offer higher efficiency and image quality compared to LCD displays, conventional OLED materials are constrained by a theoretical internal quantum efficiency (IQE) limit of 25%. Governed by quantum mechanics, 75% of the spin states formed during OLED operation are dark states, which are normally not allowed to emit light. Methods that could overcome this fundamental limit have been the key driving force for major technological breakthroughs. However, these approaches were either too costly or could not offer ideal performances under high-brightness conditions.
After finishing his first PhD in photovoltaic (PV) solar cells, Dawei Di became captivated by the reverse process of PV: instead of converting light to electricity, solar cells can also be used as LEDs to convert electricity back into light. He decided to do a second PhD at Cambridge University’s Cavendish Laboratory to study the physics of organic optoelectronics, including OLEDs. Di proposed new strategies that could realize very high LED efficiencies at low costs, marking important milestones in the research and development of next-generation light sources. His research has led to efficiency records for solution-processed OLEDs and perovskite LEDs.
In the field of OLEDs, Di co-discovered and demonstrated a high-efficiency light emission mechanism associated with spin-state degeneracy in carbene-metal-amide (CMA) molecules. Through molecular rotation, dark states can reach energetic equivalence with bright states. This process can efficiently convert dark states to bright states, leading to IQEs of nearly 100% and external quantum efficiencies (EQEs) of up to 27.5%, a record for solution-processed LEDs.
In the field of perovskite LEDs, his work identified the primary reasons that limit light emission efficiency, and successfully minimized the energy losses caused by non-radiative processes. He and his colleagues reported EQEs of over 20% and IQEs of close to 100% for the first time for perovskite LEDs, demonstrating their potential as an alternative technology to OLED and quantum-dot displays.
His contributions represent significant milestones in the development of next-generation high-performance, low-cost, environmentally-friendly light sources. His research results have the potential to be widely used in display, lighting, and communications applications.