HKU Engineering Team Reveals How Subtle Crystal 'Twists' Control Light In Perovskites, Paving Way For Improved LEDs, Solar Cells And Quantum Technologies

Research led by Professor Philip C. Y. Chow and his PhD student Ms. Yumeng Song from the Department of Mechanical Engineering under the Faculty of Engineering of the University of Hong Kong (HKU), has revealed how minute structural modifications in advanced perovskite materials critically influence their light-emission properties. This study, conducted in collaboration with research teams from The Hong Kong Polytechnic University and the Southern University of Science and Technology, provides valuable insights and practical guidance for designing brighter, more efficient, and versatile materials. The findings could accelerate the development of next-generation optoelectronic and quantum devices.

Two-dimensional hybrid perovskites are a new class of materials capable of converting light into electricity and produce light from electricity. Being lightweight, low-cost and solution-processable – similar to ink-based printing – they are particularly suited for large-area, flexible applications. Their tunable colour and brightness make them highly promising for solar cells, light-emitting diodes (LEDs), lasers and emerging quantum technologies.

In this study, the team systematically compared closely related bilayer (n = 2) perovskite crystals that differ in how their atomic layers are linked. Using a combination of advanced measurements and theoretical modelling, they demonstrated that crystals in the Dion–Jacobson (DJ) family exhibit slightly greater “twist” in their atomic building blocks than those in the Ruddlesden–Popper (RP) family. This extra twist facilitates the formation of tiny defects, specifically iodine vacancies. These vacancies act like potholes for moving charges, causing energy dissipation and reducing the material’s light emission efficiency.

In simple terms, the change in the material's structure also breaks its normal symmetry, leading to a special quantum effect called Rashba band splitting. This effect enables the material to emit circularly polarised light, a kind of light that twists like a corkscrew. This type of light is highly valuable for cutting-edge technologies like spintronics, secure optical communication, and certain quantum devices. Among the materials examined, a DJ-phase crystal based on the organic molecule 4-(aminomethyl) piperidinium (4AMP) showed the strongest structural distortion and the most pronounced circularly polarized photoluminescence. Although its room-temperature brightness is currently limited by defects, its unique optical behaviour marks it as a promising platform for future technologies once those defects are controlled.

Professor Chow commented, “By connecting crystal structure directly to both efficiency and advanced optical functionality, the study provides a practical roadmap for engineering perovskites with specific performance targets. In LEDs and display technologies, these insights can help minimize energy losses and boost brightness. In solar cells, reducing defect formation can improve stability and power conversion efficiency—two key hurdles to commercialization. And for information technologies that rely on the control of light’s polarization, the work points to material designs that can generate circularly polarised light on demand.”