Exciplex: An Intermolecular Charge-Transfer Approach for TADF

Abstract

Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. In addition to pure TADF emitters achieved by the subtle manipulations of intramolecular charge transfer processes with sophisticated molecular structures, a new class of efficient TADF-based OLEDs with emitting layer formed by blending electron donor and acceptor molecules that involve intermolecular charge transfer have also been fabricated. In contrast to pure TADF materials, the exciplex-based systems can realize small Δ E (0-0.05 eV) much more easily since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the prospective to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. Therefore, the challenging issue of achieving small Δ E in organic systems could be solved. In this article, we summarize and discuss the latest and most significant developments regarding these rapidly evolving functional materials, wherein the majority of the reported exciplex forming systems are categorized into two subgroups, viz. (a) exciplex as TADF emitters and (b) those as hosts for fluorescent, phosphorescent and TADF dopants according to their structural features and applications. The working mechanisms of the direct electroluminescence from the donor/acceptor interface and the exciplex-forming systems as cohost for the realization of high efficiency OLEDs are reviewed and discussed. This article delivers a summary of the current progresses and achievements of exciplex-based researches and points out the future challenges to trigger more research endeavors to this growing field.

Tricomponent Exciplex Emitter Realizing over 20% External Quantum Efficiency in Organic Light‐Emitting Diode with Multiple Reverse Intersystem Crossing Channels

“…Therefore, the evidently improved Φ PL of DBT-SADF:PO-T2T:CDBP is actually ascribed to the better upconversion of triplet excitons with three RISC channels and prove the feasibility of our strategy to improve the exciton utilization of exciplex emitters. Moreover, to the best of our knowledge, this is the first report with an EQE over 20% for the OLEDs based on exciplex emitters at room temperature (Figure 4; Table S2, Supporting Information), [1,2,[4][5][6][7][8][9][10][11][12][13][14][17][18][19][20][21]30,31,[46][47][48] thus our novel strategy of tricomponent exciplexes will provide a valuable approach for further development of exciplex emitters. As the mixing ratios of exciplexes would change their charge balance and affect the device performance, [13,14] the carrier transporting properties of four exciplexes were carefully investigated.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Formed between electron-donating (D) and electron-accepting (A) molecules, group reported an exciplex-based TADF-OLEDs by employing a bipolar constituting molecule, and achieved a maximum EQE of 15.4%. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Formed between electron-donating (D) and electron-accepting (A) molecules, group reported an exciplex-based TADF-OLEDs by employing a bipolar constituting molecule, and achieved a maximum EQE of 15.4%.…”

“…The lower-energy exciplex could be beneficial due to the energy transfer between two exciplexes. [1,2,[4][5][6][7][8][9][10][11][12][13][14][17][18][19][20][21]30,31,[46][47][48] The outstanding performance of the DBT-SADF:PO-T2T:CDBP-based OLEDs not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of our novel tricomponent exciplex strategy for further development of exciplex emitters. According to our novel strategy, two tricomponent exciplex emitters, 13AB ((1,3-bis(9,9dimethylacridin)) benzene):PO-T2T ((1,3,5-triazine-2,4,6-triyl) tris(benzene-3,1-diyl) tris(diphenylphosphine oxide)):CDBP (4,4′-bis(9-carbazolyl)-2,2′-dimethylbiphenyl) and DBT-SADF Energy transfer diagrams of the conventional bicomponent TADF exciplexes, TADF-assistant bicomponent TADF exciplexes, and novel tricomponent TADF exciplexes (blue: conventional constituent material, orange: single-molecule TADF emitter).…”

“…

all exciplexes possess intermolecular charge-transfer (CT) transition and locate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) on the D and A molecules, respectively. [2,7,14,17] Therefore, exciplexes were widely investigated as the TADF emitters or hosts of OLEDs in recent years. [1,13,14] According to our previous work, by further controlling the T 1 energy levels of two constituting molecules higher than that of exciplex, exciplexes can exhibit significant thermally activated delayed fluorescence (TADF) behaviors, [12][13][14] which will promote the utilization of electrogenerated triplet excitons in the OLEDs.

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“…With these completely separated frontier molecular orbitals, exciplexes naturally have extremely small energy gaps (ΔE ST s) between their lowest singlet and triplet (S 1 and T 1 ) energy levels. [1][2][3][5][6][7][8][9][10][11][12][13][14][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] As the exciplexes can realize the balanced charge transport and suppress the triplet-triplet annihilation by upconvert triplet excitons to singlet excitons, the OLEDs using exciplexes as the hosts have realized remarkable progress till now. Besides, as the constituting D and A materials are generally capable of hole-and electron-transporting properties, respectively, exciplexes would possess bipolar transporting properties, and even reach a charge-transporting balance by adjusting the mixing ratio between D and A, which would be helpful to further simplify the structures and lower the driving voltages for OLEDs.…”