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Lower limits for non-radiative recombination loss in organic donor/acceptor complexes
Yun Liu1, Zilong Zheng2,3, Veaceslav Coropceanu2,4
1Department of Chemistry, University of Washington, Seattle, WA, 98195-2120, USA. dginger@uw.edu.
Materials Horizons
|November 29, 2021
Summary
Reducing non-radiative decay rates in organic systems is key for efficient organic solar cells and LEDs. This study shows that slowing these rates significantly lowers voltage losses and boosts performance.
Area of Science:
- Organic electronics
- Photophysics
- Materials science
Background:
- Charge transfer (CT) states are crucial for organic electronics like organic photovoltaics (OPV), lasers, and LEDs.
- Controlling radiative and non-radiative transition rates in these systems impacts device efficiency.
- Thermally activated delayed fluorescence (TADF) is a key mechanism for high photoluminescence quantum efficiency (PLQY).
Purpose of the Study:
- To investigate the role of CT energetics, lifetimes, and photovoltaic properties under very slow non-radiative rates.
- To explore a model donor/acceptor system exhibiting TADF and high PLQY.
- To understand the factors influencing non-radiative decay and their impact on organic solar cell performance.
Main Methods:
- Utilized a model donor/acceptor system with photoluminescence dominated by TADF.
- Measured photoluminescence quantum efficiency (PLQY) and photoluminescence (PL) lifetime.
- Evaluated photocurrent external quantum efficiency (EQE) and non-radiative voltage losses in solar cells.
Main Results:
- Achieved high PLQY (∼22%) and a long PL lifetime with a model TADF system.
- Observed significant free charge generation (EQE of 24%) and low non-radiative voltage losses (∼0.1 V).
- Determined non-radiative decay rates (k_nr) on the order of 10^5 s^-1, significantly slower than typical OPV blends.
- Found k_nr to be faster than predicted by Marcus-Levich-Jortner theory, indicating CT-local exciton (LE) hybridization.
Conclusions:
- Reducing non-radiative decay rates (k_nr) is critical for achieving high radiative efficiency and low non-radiative voltage losses in organic electronics.
- CT-LE hybridization plays a role and necessitates evaluating LE states' radiative and non-radiative rates individually for CT states.
- Findings guide material selection for low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.
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