Researchers from Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) have developed an innovative method to activate thermally activated delayed fluorescence (TADF) molecules by trapping them with light in an “optical cavity.” The finding could have implications for Organic Light-Emitting Diodes (OLEDs) and improve our understanding of how light behaves in a cavity.
To understand the research, it's essential to know about TADF molecules. In regular fluorescence, a compound absorbs energy when light is shined on it and then glows for a while by releasing that energy in the form of light of a different wavelength, before fading away. Thermally Activated Delayed Fluorescence (TADF) is a special way where some materials don’t immediately glow when the light is shined. Instead, it gets "stuck" in a state where it can’t easily release light (this is called the triplet state). Normally, it would stay here for a long time and fade slowly. If there’s enough heat around, however, the molecule can use that energy to "jump" back to a state where it can glow, a state called singlet state. This jump is called reverse intersystem crossing (RISC). Once it reaches this new state, it finally releases light, but with a slight delay, hence the name delayed fluorescence.
Traditionally, TADF molecules are fabricated using complex molecular synthesis to tune the delay precisely according to requirements and glow at the right time. In their new study, the researchers have proposed instead to use light trapped in a cavity to tune the delay of a TADF molecule.
In their study, researchers used 4CzFCN molecules, which are TADF molecules that emit blue light. The molecules are embedded in special structures known as optical cavities. Imagine an optical cavity like a house of mirrors for light, trapping it in a space where it bounces back and forth. In this specific study, the team used a type of cavity known as a Fabry–Pérot cavity, made from a thin film of material placed between two silver mirrors.
Inside this cavity, light-matter coupling occurs, a process where the light trapped in the cavity and the TADF molecules influence each other. When the light-matter interaction in the cavity is strong enough, it leads to the formation of hybrid energy states in the molecule. These hybrid states create a barrier-free pathway for the molecule's transition from a triplet state to a singlet state, where it can finally glow.
The researchers found they could manipulate these pathways by changing the angle at which light enters or exits the cavity. This angle adjustment alters the interaction between light and molecules, allowing scientists to tune the emission properties of TADF molecules. The study reported reduced fluorescence lifetime by 10 microseconds and narrower emission widths, this means the molecules emit light more quickly and efficiently.
TADF molecules play an important role in making OLED screens brighter and more efficient. By improving how TADF molecules emit light using optical cavities, scientists can enhance the efficiency of OLEDs without relying on complex and expensive molecular synthesis. The insights from this study could also lead to advancements in organic lasers, offering new avenues in fields that require precise and efficient light sources. The approach demonstrates significant potential for enhancing the performance of optoelectronic devices, especially by enabling tailored emission properties through controlled light-matter interactions.
The findings also provide new insights into photon–exciton coupling dynamics, which have the potential to improve light emission efficiencies in organic materials. Moreover, the study shines light on the fundamental nature of light and how it interacts with matter when confined to a cavity.