Joint Conference ofGCIM 2024 & IUMRS-ICYRAM 2024
& MRS-K SPRING MEETING

Perovskite Nanocrystals for Efficient Light-Emitting Diodes

Chemically synthesized metal halide perovskite nanocrystals (NCs) have recently emerged as a new class of efficient light emitting materials which are particularly interesting for development of light-emitting diodes (LEDs). Decreasing NC size increases radiative recombination due to the quantum confinement effect, but also increases the Auger recombination rate which leads to carrier imbalance in the emitting layers of LEDs. We demonstrate how to overcome this trade-off by increasing the exciton effective mass without affecting the size, which is realized through the trace Cd2+ doping of formamidinium lead bromide perovskite NCs. We observe an 2.7 times increase in the exciton binding energy benefiting from a slight distortion of the [BX6]4− octahedra caused by doping, while the Auger recombination rate remains almost unchanged. As a result, bright color-saturated green emitting perovskite NCs with a photoluminescence quantum yield of 96% are obtained. Cd2+ doping also shifts up the energy levels of the NCs, relative to the Fermi level so that heavily n-doped emitters convert into only slightly n-doped ones; this boosts the charge injection efficiency of the corresponding devices. LEDs based on those NCs reach a high external quantum efficiency (EQE) of 29.4% corresponding to a current efficiency of 123 cd A-1 and show improved device lifetime, with a narrow bandwidth of 22 nm and CIE coordinates of (0.20, 0.76) for the electroluminescence peak centered at 534 nm, thus being fully compliant with the latest standard for wide color gamut displays.
In another approach, we develop high-quality red emitters based on polymer-perovskite donor-acceptor composites, where the excitation of perovskite NCs happens via Förster resonance energy transfer from the capping polymer. Using poly(3-hexylthiophene-2,5-diyl) to cap CsPbI3 perovskite NCs results in almost 100% photoluminescence quantum yield and leads to better carrier confinement. Moreover, this hydrophobic polymer segregates to the NC surface and stabilizes the undercoordinated surface sites, which results in a significantly enhanced stability against phase transition, humidity, and temperature. This allows us to fabricate efficient perovskite LEDs with EQEs of 26.6% and 23.8% for deep-red and color-saturated-red emission, respectively. As compared with the perovskite-only control devices, their operational stability is improved, and efficiency roll-off is suppressed.

Skin-Interfaced Wearable Biosensors

The rising research interest in personalized medicine promises to revolutionize traditional medical practices. This presents a tremendous opportunity for developing wearable devices toward predictive analytics and treatment. In this talk, I will introduce our efforts in developing wearable biosensors for non-invasive molecular analysis. Such wearables can autonomously access body fluids (e.g., human sweat) across the activities and continuously measure a broad spectrum of analytes including metabolites, nutrients, hormones, proteins, and drugs. To manufacture these high-performance nanomaterial-based wearable biosensors at a large scale and minimal cost, we employ techniques such as laser engraving, inkjet printing, and 3D printing. The clinical utility of our wearable systems is assessed through a series of human trials, focusing on precision nutrition, stress and mental health evaluation, chronic disease management, fertility management, and drug personalization. I will also delve into our ongoing research into energy harvesting from both the human body and the surrounding environment, with the ultimate aim of achieving battery-free, wireless wearable biosensing. The integration of these wearable technologies has the potential to unlock a wide spectrum of applications, ranging from personalized monitoring and diagnostics to innovative therapeutic solutions.