Lower limits for non-radiative recombination loss in organic donor/acceptor complexes

Understanding the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems is important for applications ranging from organic photovoltaics (OPV) to lasers and LEDs. We explore the role of charge-transfer (CT) energetics, lifetimes, and photovoltaic properties in the limit of very slow non-radiative rates by using a model donor/acceptor system with photoluminescence dominated by thermally activated delayed fluorescence (TADF). This blend exhibits an extremely high photoluminescence quantum efficiency (PLQY = similar to 22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only similar to 0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, k(nr), is on the order of 10(5) s(-1), approximately 4-5 orders of magnitude slower than typical OPV blends, thereby confirming that high radiative efficiency and low non-radiative voltage losses are achievable by reducing k(nr). Furthermore, despite the high radiative efficiency and already comparatively slow k(nr), we find that k(nr) is nevertheless much faster than predicted by Marcus-Levich-Jortner two-state theory and we conclude that CT-local exciton (LE) hybridization is present. Our findings highlight that it is crucial to evaluate how radiative and non-radiative rates of the LE states individually influence the PLQY of charge-transfer states, rather than solely focusing on the PLQY of the LE. This conclusion will guide material selection in achieving low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.

Automated summary

This study investigates the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems, highlighting the importance of reducing non-radiative rates to achieve high radiative efficiency and low voltage losses. The research demonstrates that in a model donor/acceptor system, high photoluminescence quantum efficiency and low non-radiative voltage losses can be achieved by reducing the non-radiative decay rate. Additionally, the presence of CT-local exciton (LE) hybridization is confirmed, suggesting the need to evaluate how radiative and non-radiative rates individually influence the PLQY of charge-transfer states.