Doctoral Thesis: Strategies for High-Performance Solid-State Photon Upconversion

Ting-An Lin

Abstract:

Photon upconversion, a process that combines two low-energy photons into one higher energy photon, has promising applications such as photovoltaics, bio-imaging, and photo-chemistry. Among the techniques capable of achieving photon upconversion, manipulating the excited states of organic molecules is especially attractive for many applications thanks to its capability of being operated with low-intensity incoherent light sources. The performance in solid-state, however, is unsatisfactory for applications due to weak optical absorption, internal losses, and the fundamental limit from the upconverting process—triplet-triplet annihilation (TTA)—itself.

Here, we investigate strategies to tackle the limitations in solid-state photon upconversion. To address the external limit of weak optical absorption, an archetypical solid-state infrared-to-visible upconverter is embedded into an optical cavity, which results in 74-fold enhancement in absorption and two-orders-of-magnitude reduction in required excitation intensity down to subsolar flux. Next, we dive into the internal loss pathways—back transfer and material aggregation—and demonstrate that a bilayer structure with the absorbing layer diluted into a host material can simultaneously mitigate these losses. Finally, we explore the very interior of an upconverter—the potential to achieve efficient TTA beyond its fundamental limit by utilizing high-lying non-emissive excited states. The experimental results manifest our concept as a design rule for further developing limit-breaking TTA molecules. With the strategies to develop high-performance solid-state photon upconverters, we look forward to further advancement in modern technologies that benefit from photon upconversion.

Details

• Date: Monday, April 25
• Time: 10:00 am
• Location: 4-303

Additional Location Details:

Thesis Supervisor: Prof. Marc A. Baldo