Organic light-emitting diodes (OLEDs) whose emission comes from a donor-acceptor exciplex — an intermolecular charge-transfer (CT) excited state — can convert normally wasted triplet excitons into singlet emission by thermally activated reverse intersystem crossing (RISC). Goushi, Yoshida, Sato and Adachi (Nature Photonics 6, 253-258, 2012) demonstrated this principle in m-MTDATA:3TPYMB devices with 5.4% external quantum efficiency and 86.5% RISC yield, and left a linked program of open questions: raising ηext\eta_{\mathrm{ext}} far above 5.4% through device optimization and outcoupling; identifying donor-acceptor pairs with energy-level alignment giving small ΔEST\Delta E_{\mathrm{ST}} and high photoluminescence quantum yield; establishing the triplet-confinement criterion (donor and acceptor neat-triplet levels above the exciplex singlet) across broader chemical families; and clarifying the origin, distribution, and device impact of the energetic inhomogeneity that makes exciplex photoluminescence kinetics deviate from simple two-state models, especially at low temperature. Later literature (2012-2026) closed the headline efficiency and materials targets — exciplex OLEDs now reach nearly 20% external quantum efficiency and design rules for confinement are articulated — but the microscopic origin and quantitative distribution of the inhomogeneity that broadens effective ΔEST\Delta E_{\mathrm{ST}} and governs the temperature dependence of RISC in solid-state exciplexes remain unsettled.

Background

In a conventional fluorescent OLED, electrical injection forms excitons with spin statistics fixing only about 25% as radiative singlets, so the internal quantum efficiency is capped near 25%; phosphorescent emitters harvest triplets but rely on rare, chemically less stable heavy-metal complexes. A donor (D) molecule with an electron-rich, shallow highest occupied molecular orbital (HOMO) and an acceptor (A) molecule with a deep lowest unoccupied molecular orbital (LUMO) can instead recombine charges into an exciplex, an intermolecular charge-transfer (CT) excited state whose singlet and triplet levels are nearly degenerate. The singlet-triplet energy gap ΔEST=E(S1)−E(T1)\Delta E_{\mathrm{ST}} = E(S_1) - E(T_1) of such a CT state is small because the electron and hole reside on different molecules, so the exchange splitting is strongly suppressed; the triplet can then be thermally re-populated back into the radiative singlet by reverse intersystem crossing (RISC), the spin-forbidden upward T1→S1T_1 \rightarrow S_1 transition. This cycle produces thermally activated delayed fluorescence (TADF), a delayed emissive component on microsecond time scales.

Goushi, Yoshida, Sato and Adachi (Nature Photonics 6, 253-258, 2012; DOI 10.1038/nphoton.2012.31) applied this to OLEDs using m-MTDATA (a triphenylamine-based hole-transport donor) paired with t-Bu-PBD and with 3TPYMB (a triarylborane acceptor). From the temperature dependence of the prompt and delayed photoluminescence components they extracted an Arrhenius activation energy ΔEST≈50\Delta E_{\mathrm{ST}} \approx 50 meV for m-MTDATA:t-Bu-PBD, a RISC quantum yield ΦRISC=0.865\Phi_{\mathrm{RISC}} = 0.865, and an external quantum efficiency (EQE, ηext\eta_{\mathrm{ext}}, the fraction of injected charges emitted as light out of the device) of 5.4% for the m-MTDATA:3TPYMB device — far above the conventional fluorescent limit once the modest photoluminescence quantum yield (PLQY, the fraction of absorbed photons re-emitted) of 26% and outcoupling are accounted for. A crucial design condition they identified is triplet confinement: the triplet energies of the neat donor and acceptor must lie above the exciplex singlet energy, so that the exciplex triplet is not drained by energy transfer to lower-lying molecular triplet states on the host materials. The same group simultaneously demonstrated (Uoyama, Goushi, Shizu, Nomura and Adachi, Nature 492, 234-238, 2012) that an intramolecular CT emitter (4CzIPN) with the same small-ΔEST\Delta E_{\mathrm{ST}} logic reaches 19.3% EQE, establishing TADF generally as a route to the 100% internal quantum efficiency regime.

Two physical complications define what remains unresolved. First, in a disordered solid blend the CT-state energy — and hence the local ΔEST\Delta E_{\mathrm{ST}} and RISC rate — varies from site to site: time-resolved measurements show non-exponential decay kinetics reflecting a distribution of exciplex lifetimes, and nanoscale imaging of CT-state motion in m-MTDATA:3TPYMB (Deotare et al., Nature Materials 14, 1130-1134, 2015) shows the bound electron-hole pair diffusing down an energetic disorder landscape. Second, the electronic structure of a solid-state exciplex is a many-molecule problem: quantum-chemical analysis (Moon, Huh, Kim and Kim, Chemistry of Materials 30, 5648-5654, 2018) shows that below roughly 2.8 eV the exciplex singlet and triplet are both CT in character and nearly degenerate, whereas above it the triplet pins to local-excited (LE) triplet levels of the constituent molecules, so whether RISC is energetically easy depends sensitively on where the CT state sits relative to the LE triplet manifold and on how the disorder of the blend broadens both.

Problem Statement

The 2012 demonstration left open, as one linked program, the question of whether exciplex-based reverse-intersystem-crossing OLEDs can become a generally applicable, high-efficiency, and quantitatively understood emitter technology. Specifically: (i) how far can the external quantum efficiency be pushed beyond the demonstrated 5.4% by systematic device optimization — layer thicknesses, carrier balance, electrode and interlayer choices, and light outcoupling — for exciplex emissive layers; (ii) which donor and acceptor molecules, with a shallow donor HOMO and a deep acceptor LUMO giving exciplexes with small ΔEST\Delta E_{\mathrm{ST}} and with intrinsically high photoluminescence quantum yields, can be identified or synthesized, and what design rules govern that choice across chemical families; (iii) under what conditions does the triplet-confinement criterion — that the neat donor and acceptor triplet levels lie above the exciplex singlet so the triplet exciplex is not quenched — hold for other donor-acceptor combinations; (iv) what is the microscopic origin, the distribution, and the device-level impact of the energetic inhomogeneity of exciplex charge-transfer states, which makes the photoluminescence decay deviate from a simple two-component (double-exponential) kinetic model at low temperature and indicates a broadened distribution of ΔEST\Delta E_{\mathrm{ST}} depending on the local molecular environment, and how does this inhomogeneity control the temperature dependence of the RISC rate; and (v) by what simultaneous optimization of RISC quantum yield, intrinsic photoluminescence quantum yield, carrier balance, and outcoupling can robust, high-efficiency, scalable exciplex devices be achieved. Items (i)-(iii) and (v) share the practical objective of an efficient, general exciplex OLED technology and are scientifically tied to (iv), the mechanistic photophysics that explains when and why the exciplex triplet-to-singlet conversion works. The unresolved question is the microscopic origin, the functional form and width, and the device-level impact of the site-dependent ΔEST\Delta E_{\mathrm{ST}} distribution in solid-state exciplexes, and the role of this inhomogeneity in setting the temperature dependence of the RISC rate, consistent with the observed deviation of the photoluminescence decay from a simple two-component (double-exponential) kinetic model.

References

  1. Kenichi Goushi, Kou Yoshida, Keigo Sato, Chihaya Adachi, "Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion", Nature Photonics 6, 253-258 (2012), DOI 10.1038/nphoton.2012.31, https://doi.org/10.1038/nphoton.2012.31
  2. Hiroki Uoyama, Kenichi Goushi, Katsuyuki Shizu, Hiroko Nomura, Chihaya Adachi, "Highly efficient organic light-emitting diodes from delayed fluorescence", Nature 492, 234-238 (2012), DOI 10.1038/nature11687, https://doi.org/10.1038/nature11687
  3. Tianyou Zhang, Bei Chu, Wenlian Li, Zisheng Su, Qi Ming Peng, Bo Zhao, Yongshi Luo, Fangming Jin, Xingwu Yan, Yuan Gao, Hairuo Wu, Feng Zhang, Di Fan, Junbo Wang, "Efficient Triplet Application in Exciplex Delayed-Fluorescence OLEDs Using a Reverse Intersystem Crossing Mechanism Based on a ΔE_S–T of around Zero", ACS Applied Materials & Interfaces 6, 11907-11914 (2014), DOI 10.1021/am501164s, https://doi.org/10.1021/am501164s
  4. David Graves, Vygintas Jankus, Fernando B. Dias, Andrew Monkman, "Photophysical Investigation of the Thermally Activated Delayed Emission from Films of m-MTDATA:PBD Exciplex", Advanced Functional Materials 24, 2343-2351 (2014, published online December 2013), DOI 10.1002/adfm.201303389, https://doi.org/10.1002/adfm.201303389
  5. Vygintas Jankus, Przemyslaw Data, David Graves, Callum McGuinness, Jose Santos, Martin R. Bryce, Fernando B. Dias, Andrew P. Monkman, "Highly Efficient TADF OLEDs: How the Emitter-Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency", Advanced Functional Materials 24, 6178-6186 (2014), DOI 10.1002/adfm.201400948, https://doi.org/10.1002/adfm.201400948
  6. P. B. Deotare, W. Chang, E. Hontz, D. N. Congreve, L. Shi, P. D. Reusswig, B. Modtland, M. E. Bahlke, C. K. Lee, A. P. Willard, V. Bulović, T. Van Voorhis, M. A. Baldo, "Nanoscale transport of charge-transfer states in organic donor-acceptor blends", Nature Materials 14, 1130-1134 (2015), DOI 10.1038/nmat4424, https://doi.org/10.1038/nmat4424
  7. Yanlian Lei, Qiaoming Zhang, Lixiang Chen, Yongzhou Ling, Ping Chen, Qunliang Song, Zuhong Xiong, "Ultralarge Magneto-Electroluminescence in Exciplex-Based Devices Driven by Field-Induced Reverse Intersystem Crossing", Advanced Optical Materials 4, 694-699 (2016), DOI 10.1002/adom.201600015, https://doi.org/10.1002/adom.201600015
  8. Hin-Wai Mo, Youichi Tsuchiya, Yan Geng, Takehiro Sagawa, Chika Kikuchi, Hajime Nakanotani, Fuyuki Ito, Chihaya Adachi, "Color Tuning of Avobenzone Boron Difluoride as an Emitter to Achieve Full-Color Emission", Advanced Functional Materials 26, 6703-6710 (2016), DOI 10.1002/adfm.201601257, https://doi.org/10.1002/adfm.201601257
  9. Chang-Ki Moon, Jin-Suk Huh, Jae-Min Kim, Jang-Joo Kim, "Electronic Structure and Emission Process of Excited Charge Transfer States in Solids", Chemistry of Materials 30, 5648-5654 (2018), DOI 10.1021/acs.chemmater.8b02011, https://doi.org/10.1021/acs.chemmater.8b02011
  10. Xi Zhao, Xiantong Tang, Ruiheng Pan, Jing Xu, Fenlan Qu, Zuhong Xiong, "Direct observation of reverse intersystem crossing from fully confined triplet exciplexes using magneto-electroluminescence", Journal of Materials Chemistry C 7, 10841-10850 (2019), DOI 10.1039/c9tc02896j, https://doi.org/10.1039/c9tc02896j
  11. Tien-Lin Wu, Ssu-Yu Liao, Pei-Yun Huang, Zih-Siang Hong, Man-Ping Huang, Chih-Chun Lin, Mu-Jeng Cheng, Chien-Hong Cheng, "Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair", ACS Applied Materials & Interfaces 11, 19294-19300 (2019), DOI 10.1021/acsami.9b04365, https://doi.org/10.1021/acsami.9b04365
  12. Marian Chapran, Piotr Pander, Marharyta Vasylieva, Gabriela Wiosna-Salyga, Jacek Ulanski, Fernando B. Dias, Przemyslaw Data, "Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex", ACS Applied Materials & Interfaces 11, 13460-13471 (2019), DOI 10.1021/acsami.8b18284, https://doi.org/10.1021/acsami.8b18284
  13. Marco Colella, Andrew Danos, Andrew P. Monkman, "Identifying the Factors That Lead to PLQY Enhancement in Diluted TADF Exciplexes Based on Carbazole Donors", The Journal of Physical Chemistry C 123, 17318-17324 (2019), DOI 10.1021/acs.jpcc.9b03538, https://doi.org/10.1021/acs.jpcc.9b03538
  14. Hwang-Beom Kim, Jang-Joo Kim, "Recent progress on exciplex-emitting OLEDs", Journal of Information Display 20, 105-121 (2019), DOI 10.1080/15980316.2019.1650838, https://doi.org/10.1080/15980316.2019.1650838
  15. Xi Zhao, Xiantong Tang, Hongqiang Zhu, Caihong Ma, Ying Wang, Shengnan Ye, Linyao Tu, Zuhong Xiong, "Room-Temperature Observation for Reverse Intersystem Crossing in Exciplex-Based OLEDs with Balanced Charge Injection", ACS Applied Electronic Materials 3, 3034-3043 (2021), DOI 10.1021/acsaelm.1c00259, https://doi.org/10.1021/acsaelm.1c00259
  16. Tomas Serevičius, Rokas Skaisgiris, Sigitas Tumkevičius, Jelena Dodonova-Vaitkūnienė, Saulius Juršėnas, "High Reverse Intersystem Crossing Rate Diminishes the Impact of Conformational Disorder Phenomenon in Solid-State TADF", Advanced Optical Materials 12, 2401819 (2024), DOI 10.1002/adom.202401819, https://doi.org/10.1002/adom.202401819
  17. Huitian Du, Yaming Hao, Xijian Zhang, Shuyuan Zhang, Qiang Liu, Zhiyong Pang, "Recent progress in high-performance thermally activated delayed fluorescence exciplexes based on multiple reverse intersystem crossing channels", Organic Electronics 136, 107159 (2025), DOI 10.1016/j.orgel.2024.107159, https://doi.org/10.1016/j.orgel.2024.107159