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Exciplex-sensitized Triplet-triplet Annihilation in Heterojunction Organic Thin-film Bo-Yen Lin, Connor J. Easley, Chia-Hsun Chen, Po-Chen Tseng, Ming-Zer Lee, Pin-Hao Sher, Juen-Kai Wang, Tien-Lung Chiu, Chi-Feng Lin, Christopher J. Bardeen, and Jiun-Haw Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16397 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

Exciplex-sensitized Triplet-triplet Annihilation in Heterojunction Organic Thin-film Bo-Yen Lin, 1 Connor J. Easley, 2 Chia-Hsun Chen, 1 Po-Chen Tseng,1 Ming-Zer Lee,1 Pin-Hao Sher,3 Juen-Kai Wang, 3 Tien-Lung Chiu, 4 Chi-Feng Lin, 5 Christopher J Bardeen,* ,2 and Jiun-Haw Lee* ,1 1

Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Road Taipei, Taiwan, R.O.C., 10617 2

Department of Chemistry, University of California, Riverside, Riverside, CA 92506, United States

3

Center for Condensed Matter Science, National Taiwan University, and Institute of Atomic and

Molecular Science, Academia Sinica, 1, Sec. 4, Roosevelt Road Taipei, Taiwan, R.O.C, 10617 4

Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Road, Taoyuan, Taiwan, R.O.C, 32003

5

Department of Electro-Optical Engineering, National United University, 1, Lienda Road, Miaoli, Taiwan, R.O.C., 36003

KEYWORDS: Triplet-triplet annihilation, exciplex, organic light-emitting diode, energy transfer, upconversion

ACS Paragon Plus Environment

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ABSTRACT: A new concept for organic light-emitting diode (OLED) is presented, which is called exciplex-sensitized triplet-triplet annihilation (ESTTA). organic

heterojunction

interface

of

4,

4’,

The exciplex formed at the

4"-tris(N-3-methyphenyl-N-phenyl-amino)

triphenylamine and 9,10-bis(2’-naphthyl) anthracene (ADN) is used to sensitize the triplet-triplet annihilation (TTA) process on the ADN molecules. This results in a turn-on voltage (2.2 V) of the blue emission from the OLED below the bandgap (2.9 eV).

From the transient

electroluminescence measurement, blue emission totally came from TTA process without direct recombination on the ADN molecules.

Blue singlet exciton by TTA process is seriously

quenched by yellow singlet, which was revealed with transient photoluminescence measurement, and can be prevented by blocking the energy transfer path and improving the radiative recombination rate of blue emission. With the insertion of the “triplet diffusion and singlet blocking (TDSB)” layer and the incorporation of the dopant material, an ESTTA-OLED with external quantum efficiency of 5.1% was achieved, which consists of yellow and blue emission coming from exciplex and ESTTA process, respectively.


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1. Introduction Sensitized triplet-triplet annihilation (STTA) is a nonlinear process which upconverts exciton energy.1 It is a non-coherent process and effective even at low incident power, which potentially has many applications, such as (bio-)imaging, organic light-emitting diode (OLED), solar cell, and data storage.2-5 The STTA process involves two species: a wide-bandgap TTA (triplet-triplet annihilation) organic emitter and a narrow-bandgap sensitizer. The low bandgap sensitizer is excited and then the singlet excitons undergo intersystem crossing to the triplet state. Energy transfer of triplet excitons takes place from sensitizer to the emitter, followed by TTA for the upconversion process. Here, the input is two low-energy excitons to generate one high-energy exciton, and hence the theoretical limit of external quantum efficiency (EQE) for STTA is 50%.6 Requirements of the sensitizer include a small singlet-triplet splitting and long excited lifetime. Suitable candidates include phosphorescent dyes, thermally activated delayed fluorescent (TADF) materials, charge transfer states, and quantum dots.7-10 ~40% EQE can be achieved in solventbased STTA with high molecular translational and rotational mobilities that facilitate intermolecular energy transfer.11-12 For a solar cell, STTA makes it possible to harvest the subbandgap photons which improves the output electrical current and leads to higher efficiency, and the solid-state thin-film is preferred to the liquidus form. 13-14 The success of STTA schemes for solar energy raises the question of whether it is possible to use STTA in an electrically pumped OLED device. This scheme, outlined in Figure 1, would enable an OLED to emit high energy photons at driving voltages as low as half the bandgap of the TTA emitter.3 In Figure 1, the low bandgap sensitizer was an exciplex generated at the interface of a bi-layer structure, consisting of an electron donor and acceptor material, 4, 4’, 4"-


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tris(N-3-methyphenyl-N-phenyl-amino) triphenylamine and 9,10-bis(2’-naphthyl) anthracene (ADN), respectively. The acceptor layer, ADN, was also the TTA medium. Singlet and triplet energy states of m-MTDATA, ADN, and the exciplex are shown in Fig. 1.15-16 The STTA process was achieved by triplet energy transfer (TET) from the exciplex to ADN, and then TTA in the ADN layer. In this paper, we demonstrate an OLED based on exciplex-sensitized triplettriplet annihilation (ESTTA) that generates blue emission (approximately 432 nm, corresponding to approximately 2.9 eV) at a low voltage of 2.2 V. The high energy emission can be generated without direct carrier recombination on the TTA molecules. In our electroluminescence (EL) and photoluminescence (PL) measurements, although the ESTTA signal was observed, it was seriously quenched by the exciplex. Hence device modification was done to block the singlet exciton quenching route and improve the ESTTA efficiency.

Figure 1. Schematic diagram of exciton energies and transitions of the organic materials for the ESTTA thin-films. Charge injection populates the exciplex, which then sensitizes the ADN triple state. These triplet states can then undergo TTA to generate blue emission. 2. Experimental Section


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OLED fabrication and measurement: In our OLEDs, we used a patterned indium tin oxide substrate with a conventional bottom emission structure. Devices were fully fabricated using thermal evaporation, then encapsulated with ultraviolet (UV) glue and a cover glass in a glove box containing less than 1 ppm of O2 and H2O. Current density-voltage characteristics were obtained using a source meter (Keithley 2400). A spectroradiometer (Minolta CS-1000) was used to obtain the EL of the OLEDs. The external quantum efficiency (EQE) was obtained by integrating the EL signals from different viewing angles over the supplied current density. To measure the EL spectrum at a low driving voltage, a monochromator (HORIBA Jobin Yvon MicroHR) connected to a thermoelectric cooled charge-coupled device (TE-cooled CCD) (Andor DU920P BR-DD) was used. Turn-off dynamics of the transient EL (TrEL) measurement were employed to determine the singlet and triplet populations in the fabricated OLED.

This

experiment consisted of a waveform generator (Agilent 335011B) to provide electrical pulse signals, a photomultiplier (PMT) (Hamamatsu H6780-20) to collect time-dependent optical signals, and an oscilloscope to obtain the photocurrent from the PMT with 60 ns time resolution. The pulse width was determined to be 100 µs and the frequency was 1 KHz. To obtain the blue emission without the yellow exciplex, a short-pass filter (Thorlabs FESH0450) was used that only transmitted photons with a wavelength less than 450 nm. TrPL measurement of organic thin film: m-MTDATA/ADN thin films were fabricated on a glass substrate by thermal evaporation followed by encapsulation, similar to OLED fabrication. Time resolved measurements were performed with a 1 kHz Coherent Libra regeneratively amplified Ti:Sapphire laser system. The 800 nm fundamental pulse was directed into a frequency doubling beta barium borate (BBO) crystal to produce the 400 nm excitation beam. The sample was mounted in an optical cryostat and the emission was collected using front face detection with a


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420 nm long pass filter. The emission was detected using a Hamamatsu C4334 Streakscope with 15 ps time resolution and 2.5 nm wavelength resolution. Typical laser powers were 10-20 microwatts, and no dependence of the early (< 100 ns) signal on laser power was observed, indicating that exciton-exciton annihilation was negligible in this time window. 3. Results and Discussions Figure 2 (a) shows the device structure of ESTTA-OLED. Holes were injected from an indium tin oxide anode into m-MTDATA and were blocked at the ADN interface. Electrons from the aluminum cathode were transported through BPhen into ADN, and were blocked by the mMTDATA. This resulted in exciplex emission at the m-MTDATA/ADN interface.17-18 The organic thicknesses were: m-MTDATA/ADN/BPhen= 30/10/20 nm. Low thicknesses of the organic layers were maintained to reduce the driving voltage and facilitate observation of the blue emission. The BPhen layer functioned as the electron transport layer, followed by LiF (0.9 nm) and Al (100 nm). Figure 2(b) illustrates the singlet and triplet energies for each layer.


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Figure 2. (a) Energy levels of organic materials and electrodes, and (b) singlet and triplet energies in the OLED.

Figure 3 (a) presents the luminance-voltage curves for the ESTTA-OLEDs. The turn-on voltage (2.2 V) was determined by the difference between HOMO (highest occupied molecular orbital) value of m-MTDATA and LUMO (lowest unoccupied molecular orbital) value of ADN due to exciplex formation at m-MTDATA/ADN interface. Figure 3 (b) illustrates the EL spectra at low driving voltages (2.0-2.2 V). When the driving voltage was 2.1 V, no EL emission was observed. When the voltage was increased to 2.2 V, exciplex emission centered at approximately 543 nm was clearly observed. This result was reasonable because the photon energy was approximately 2.3 eV. Regarding whether blue emission existed at such a low voltage, Fig. 3 (c) shows that a blue emission band appeared in the EL spectra at low driving voltage. When the driving voltage was 2.2 V, a small but observable hump was observed at approximately 432 nm. This blue emission originated from the ADN layer; however, the driving voltage (2.2 V) was lower than that derived for the photon energy (approximately 2.9 eV, or 432 nm). In one case, it has been reported that the turn-on voltage (2.05 V) of an OLED can be lower than the voltage of the emitted photon energy (2.4 V), due to assisted carrier diffusion from the electrode into the organic layers.

19

EL assisted by thermal energy is another mechanism for such low turn-on

voltage (1.9 V to generate photons at 523 nm, corresponding to 2.38 eV). 20 To provide direct evidence that the blue emission came from the ESTTA process, rather than from direct recombination on ADN molecules, the transient electroluminescence (TrEL) was analyzed. Measuring the turn-off delay of a conventional TTA-OLED typically reveals a fast decay (~ns) followed by a slow one (~µs), which are engendered by the singlet fluorescence and TTA, respectively.21 Figure 3 (d) shows the turn-off delay of the blue emission (< 450 nm) of the


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ESTTA-OLED, revealing no fast decay, which means that the blue emission came entirely from ESTTA, rather than a direct recombination on ADN molecules.3 The decay time decreased with an increase in the current density because of the exciton-polaron quenching.3 The following equation is typically used to relate the output and input power (Ioutput and Iinput) of the STTA process:

Ioutput = k Iinputn

(1)

where k is a constant, and n = 2 is the ideal case. In our case, assuming that the exciplex emission increases linearly (i.e., cd/A is constant) and that the slope of in Fig. S1 represents the value of (n - 1), n = 1.2, which is far lower than the ideal case. As shown in Fig. 1, a possible quenching path from the singlet of ADN to the exciplex existed, which reduced the blue emission efficiency. For this ESTTA-OLED device, the external quantum efficiency of blue emission was very low, approximately 0.1% at 100 mA/cm2. Notably, we used a three-layer structure rather than a mixed structure, because strong ADN singlet quenching resulted in the vanishing of blue light emission in a mixed m-MTDATA:ADN OLED (Fig. S2).


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Figure 3. (a) luminance versus voltage, (b) EL spectra at low voltages (2.0-2.2 V), (c) zoom-in of the spectra in the short wavelength region (400-550 nm), and (d) turn-off dynamics of TrEL measurement of the ESTTA-OLED.

In order to clarify the origin of the low yield of upconverted light, transient photoluminescence (TrPL) experiments were performed.

Optical pumping of the ADN is

different from electrical pumping of the exciplex state, and Fig. 4 (a) shows the possible energy transfer routes. After optical pumping, fluorescence from ADN molecule competes with singletsinglet energy transfer to the exciplex. Figure 4 (b) shows the TrPL measurement in 6 ns window for ADN and m-MTDATA/ADN thin films with different ADN thicknesses (30 and 100 nm). A biexponential function was used to fit the curves:


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ష೟

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ష೟

I(t) = ‫ܣ‬ଵ ݁ ഓభ + ‫ܣ‬ଶ ݁ ഓమ

(2)

where I(t) is the time-dependent PL intensity, A1 and A2 are prefactors for the fast and slow components of decay times, τ1 and τ2, respectively. Those values are shown in Table S1. The decay of a neat ADN film has a 65% component with a decay time τ1=0.69 ns, while the rest of the signal decays as a single exponential with decay time τ2 = 2.5 ns. The slow decay component can be assigned to molecular ADN, while the fast component likely originates from molecules that are quenched by solid-state packing effects. For the bi-layer with an ADN thickness of 30 nm, there is now a fast decay (τ1 = 0.22 ns) due to the energy transfer from ADN to the exciplex state. This initial decay comprises 96% of the TrPL signal and is followed by a slow decay with τ2 = 2.5 ns that reflects the contribution from unquenched ADN molecules. Note that the singlet and triplet state energies of m-MTDATA are 3.1 and 2.7 eV, respectively, which cannot quench the ADN emission. Strong quenching of the ADN by the m-MTDATA/ADN exciplex helps explain the low EQE in the ESTTA-OLED. When the ADN thickness is increased from 30 to 100 nm, the ADN TrPL decay approaches that of ADN by itself, indicating that at this thickness the majority of the singlet excitons avoid quenching at the m-MTDATA/ADN interface. On nanosecond timescales, the PL behavior reflects singlet energy transfer to the exciplex at the m-MTDATA/ADN interface.

On microsecond timescales, the TrPL signal contains

contributions from two species: the exciplex emission at 525 nm and the ADN singlet at 430 nm. Note that the central wavelengths of the spectra are slightly shifted from those seen in the EL


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devices due to different detection geometries and self-absorption effects. Fig. 4 (c) compares the fluorescence decays on longer timescales (0.7 µs) for m-MTDATA/ADN bilayers with different ADN thicknesses. For the pure ADN thin film, the PL decays monotonically within 200 ns. On the other hand, the bi-layers exhibit clear biexponential decays that depend on ADN thickness (Fig. 4 (c)). The exciplex emission at 525 nm dominates the 30 nm ADN decay, but the 100 nm decay contains a well-defined 430 nm peak that dominates the spectrum at longer timescales. This can be seen in Fig. 4 (d) and (e), which show the spectra at different timescales (< 1 ns, 0.40.9 µs, and 2-3 µs) for m-MTDATA/ADN = 30/30 and 30/100 nm, respectively. All the samples emitted blue emission at short timescales (

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