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In solar cells and light-emitting diodes, maintaining the kinetics of excited molecular states against their annihilation is a race against time. These systems must find a careful balance between various processes that lead to energy losses and those that lead to the desired result.
A major loss mechanism, especially in the most efficient systems, is the so-called exciton-exciton annihilation, which leads to a reduction in solar efficiency and light output in LEDs. Controlling the extent of exciton-exciton annihilation is therefore an important lever affecting efficiency.
Researchers at the National Renewable Energy Laboratory (NREL) in collaboration with researchers at the University of Colorado Boulder sought to control exciton/exciton annihilation by coupling excitons with cavity polaritons, essentially photons trapped between two mirrors, to combat energy dissipation and potentially increase the efficiency of optoelectronic devices. As reported in a recent paper in the Journal of Physical Chemistry, They used transient absorption spectroscopy to demonstrate control of the loss mechanism by varying the distance between the two mirrors forming the cavity enclosing the 2D perovskite (PEA).2PbI4 (PEPI) layer. This perovskite material is a candidate for future LED applications.
“If we could control the exciton/exciton annihilation in the active materials used in an LED or a solar cell, we could reduce the energy losses and thus significantly increase their efficiency,” said NREL’s Jao van de Lagemaat, director of the Center for Chemistry and Nanoscience, who led the study.
As the energy exchange between light and matter systems exceeds their decay rates, strong coupling occurs between photonic and electronic states (i.e., excitons), creating polaritons, hybrid states of light and matter. The NREL researchers demonstrated ultrastrong coupling of the PEPI layer in a Fabry-Pérot microcavity consisting of two partially reflective mirrors. A PEPI layer more tightly coupled to the cavity produced a longer excited state lifetime and gave the researchers control over exciton-exciton annihilation—lowering the loss process by an order of magnitude.
The NREL researchers explained their observation by the quantum nature of the newly formed hybrid states. Polaritons switch back and forth extremely quickly between a more photonic and a more excitonic nature. Since photons do not annihilate each other when they meet, but excitons can, this ghostly “phase shift” between the two particle properties allows polaritons to pass through each other if they are more photonic at the precise moment of their interaction.
By adjusting the coupling strength, you can tune the relative time that polaritons spend as photons, and thus control the energy loss in these systems. “It was amazing how such a simple experiment of placing a material between two mirrors completely changes its dynamics,” said Rao Fei, a graduate student at the University of Colorado Boulder who created the cavities and performed ultrafast spectroscopy measurements.
“We have shown that strong coupling effects can be used to control the excited state dynamics of the PEPI system,” said van de Lagemaat. “The simplicity of the system suggests that this result can be transferred to other active materials in LEDs and solar cells and could potentially be integrated into these applications using simple manufacturing methods.”
Learn more about fundamental energy science at NREL and the U.S. Department of Energy Fundamentals of Energy Science program. Read “Controlling Exciton/Exciton Recombination in 2-D Perovskite Using Exciton–Polariton Coupling” in Journal of Physical Chemistry.
By Justin Daugherty, NREL.
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