Excitons in organic semiconductors: Unlocking quantum entanglement and dynamics

Excitons encountered in technologies such as solar cells and television are quasiparticles formed by electrons and positively charged “holes” that move together in semiconductors. The excitons created when electrons are excited to a higher energy state carry energy without carrying a net charge. Although behavior in traditional semiconductors is well understood, excitons differ in organic semiconductors.

Recent research led by condensed material physicist Ivan Biagio focuses on understanding the mechanisms behind exciton dynamics, quantum entanglement and the dissociation of organic molecular crystals.

This paper is published in the Journal Physical Review Letters.

In organic materials, excitons must first move through the material and generate by dissociating the usable current. Biaggio’s lab uses lasers to excite these particles and observe quantum-level interactions. Researchers track exciton behavior via short laser pulses and fluorescence, and analyze “quantum beats” to study complex processes such as singlet splitting, triplet transport and triplet fusion. Singlet fission divides the initial excitation (spin 0, called singlet) into two triplet excitons (spin 1 each).

The lab investigates the properties of quantum extension pairs of triplet excitons produced after photoexcitation. Biaggio and his team cultivate Loubrene crystals, an organic semiconductor with high carrier mobility and allowing singlet exciton fission, and use lasers to excite and detect specific excitencies . They exploit the process in which excitons absorb light at different wavelengths, thereby allowing two triplet excitons to emit photons when they meet each other.

“The detection of fluorescence decay and the high-frequency ripples caused by quantum extension are quantum mechanical ways to observe what is happening,” says Biagio of Joseph A. Waldschmidt chair in Physics. .

“What do these excitons do not in terms of dissociating currents and creating them, but in terms of wandering through the crystals, and at some point they meet again and re-emit light. It is indirect because it depends on detection.

Biaggio’s latest experiments investigate how quantum extension of triplet exciton pairs persists as the two excitations wander independently in the crystal. His experiments unearthed a process in which triplet-exchitton pairs of clocks can escape from synchronization, despite each clock continuing to click at the same frequency.

This research could support semiconductor development or quantum information science. The long-term goal is to better understand the behavior of basic excitons, and ultimately affect solar energy harvesting, or perhaps quantum computing applications.