A Heterogeneous Kinetics Model for Triplet Exciton Transfer in Solid-State Upconversion

Area of Science:

• Materials Science
• Nanotechnology
• Quantum Chemistry

Background:

• Semiconductor nanocrystal (NC)-based photon upconversion devices achieve high internal quantum efficiency.
• Current devices utilize a single monolayer of active NCs, limiting external quantum efficiency due to low photon absorption.

Purpose of the Study:

• To investigate the reasons behind the limited performance increase in thicker NC layers for photon upconversion devices.
• To unravel the mystery of why increasing NC layers does not proportionally increase upconverted light output.

Main Methods:

• Kinetic modeling and transient photoluminescence spectroscopy were employed.
• A stochastic transfer matrix model was developed, drawing rates from a probabilistic distribution to represent NC polydispersity.
• A reaction network with realistic connectivity was constructed to fit complex photoluminescence traces.

Main Results:

• The study explains the thickness-dependent performance of upconversion devices.
• Reduced efficiencies in thicker layers are attributed to low excitonic diffusivity within NC layers.
• Increased back transfer of singlets from the organic annihilator rubrene further limits efficiency.

Conclusions:

• The energetic disorder and polydispersity of NCs govern upconversion kinetics.
• Low excitonic diffusivity and significant back transfer are key limitations in multilayered NC devices.
• Future device designs should aim to overcome these identified limitations for improved performance.