TY - JOUR
T1 - Lower limits for non-radiative recombination loss in organic donor/acceptor complexes
AU - Liu, Yun
AU - Zheng, Zilong
AU - Coropceanu, Veaceslav
AU - Brédas, Jean Luc
AU - Ginger, David S.
N1 - Funding Information:
The work at the University of Washington (YL and DSG) was supported by the Office of Naval Research through grant numbers N00014-17-1-2201 and N00014-20-1-2191. The theoretical work was supported by the Department of Energy through grant number DEEE0008205 (at the Georgia Institute of Technology), the Office of Naval Research in the framework of Award No. N00014-20-1-2110 (at the University of Arizona), as well as by the University of Arizona. Ellipsometry and time-resolved PL measurements were conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington, which is supported in part by the National Science Foundation (grant ECC-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, and the National Institutes of Health. We thank Dr Jian Wang, Dr Mark Ziffer, and Karisse Yamamoto for valuable discussions.
Publisher Copyright:
© 2021 The Royal Society of Chemistry.
PY - 2022/1
Y1 - 2022/1
N2 - 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 = ∼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 ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, knr, is on the order of 105 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 knr. Furthermore, despite the high radiative efficiency and already comparatively slow knr, we find that knr 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.
AB - 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 = ∼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 ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, knr, is on the order of 105 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 knr. Furthermore, despite the high radiative efficiency and already comparatively slow knr, we find that knr 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.
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U2 - 10.1039/d1mh00529d
DO - 10.1039/d1mh00529d
M3 - Article
C2 - 34842253
AN - SCOPUS:85122879661
VL - 9
SP - 325
EP - 333
JO - Materials Horizons
JF - Materials Horizons
SN - 2051-6347
IS - 1
ER -