Near-infrared phosphorescent OLEDs exhibiting over 10% external quantum efficiency and extremely long lifetime using resonant energy transfer with a phosphorescent assist dopant

NIR emission is highly sought after for various applications, including vital, food, and analytical sensors. ^1–4) While halogen lamps and inorganic LEDs are used in those practical applications, the development of NIR organic LEDs (NIR OLEDs) ⁵⁾ lags far behind, primarily due to efficiency and lifetime challenges.

NIR OLEDs face issues with low external quantum efficiency (EQE), typically below several percent, and few examples surpassing 10% at wavelengths exceeding 750 nm. This is partially explained by the phenomenon in which the photoluminescence quantum yield (PLQY) decreases as the wavelength increases because of the influence of vibrational levels in narrow-gap organic molecules. ^5,6) In addition, material- and device-specific issues, such as aggregation-caused quenching (ACQ) ⁷⁾ and imperfect energy transfer, further hinder their performance.

Materials that have been reported for NIR OLEDs include phosphorescent metal complexes, ^8–15) NIR fluorescent dyes, ¹⁶⁾ and thermally-activated delayed-fluorescence materials. ¹⁷⁾ Because NIR-emitting materials have extended π-electron systems, the challenge lies in preventing aggregation that decreases PLQY. ⁷⁾ Currently, phosphorescent platinum complexes with planar ligands, such as porphyrins, are regarded as the most promising NIR-emitting materials. ^9–14) However, planar porphyrin compounds are susceptible to ACQ, and the emission is quenched in dense films.

Baldo first reported a phosphorescent platinum(II) octaethylporphyrin (PtOEP) doped device in a host, achieving a maximum EQE (EQE_max) of 4% with the electroluminescent emission peak (λ_max EL) of 650 nm. ⁹⁾ Borek and Sun reported EQE_max of 6.3% and 8.5% respectively, ^11,12) using platinum(II) tetraphenyltetrabenzoporphyrin [Pt(tpbp)] with λ_max EL at approximately 770 nm. Graham reported EQE_max of 9.2% using platinum(II) tetra(3,5-di-tert-butylphenyl)tetrabenzoporphyrin (Pt-Ar4TBP) with λ_max EL of 773 nm. ¹⁴⁾ These Pt complexes were dispersed in a host at a ratio of approximately 4–6 wt% to prevent concentration quenching. ^9–14) It was also reported that 3 mol% (4.6 wt%) doping ratio was too low because the emission of the host material was observed, and 10 mol% (15 wt%) was too high because the EQE decreased due to ACQ. ¹⁰⁾

Another challenge is a design of an energy-transfer system suitable for NIR-OLEDs. Generally, OLEDs comprise a host-guest system and the energy-transfer efficiency correlates with the ratio of the spectral overlap between the emission of the host and the absorption of the guest. ¹⁸⁾ However, because NIR-emitting materials have considerably longer absorption wavelengths than commonly used OLED materials, the energy transfer efficiency may be low. In many reports, Alq₃ is used as a host ^9–12,14) and a platinum porphyrin complex as a guest; however, the spectral overlap is limited since Alq₃ shows λ_max PL = 532 nm, while Pt(tpbp) shows λ_max Abs = 429 nm (Soret band) and 612 nm (Q band).

To address these problems, we employed a three-component system to maximize the spectral overlap between host and guest. We previously demonstrated an exciplex-host system consisting of a donor, an acceptor and a guest emitter in deep-red OLEDs, effectively achieving high efficiency by tuning the emission of the exciplex to match the absorption of the guest. ^19–21)

Using this three-component concept, we have introduced a phosphorescent assist dopant in this study to mediate energy transfer from the host to the guest NIR emitter. We carefully selected an adequate assist dopant for optimal spectral overlap between the emission of the assist dopant and the absorption of the NIR emitter, ensuring material stability. In this system, a triplet-triplet energy transfer is expected from the assistant dopant to the NIR emitter, realizing the maximum potential of the NIR material with a minimum concentration by resonant energy transfer between the two phosphorescent materials. ²²⁾

In the experiment, a three-component emitting layer consisting of a host, an assistant dopant, and a NIR emitter was constructed, and evaluated for its optical and device properties. Figure 1 illustrates the molecular structures of the three materials used in the emitting layer and the spectral overlap between the materials. The host material chosen was 2,4-diphenyl-6-bis (12-phenylindolo) [2, 3-a] carbazole-11-yl)−1, 3, 5-triazine (DIC-TRZ) ^23,24) known for its ambipolar carrier-transport properties and a small energy gap between the singlet and triplet excited states. Tris [1-phenylisoquinolinato-C²,N]iridium(III) (Ir(piq)₃), ^25–27) served as the assistant dopant. Ir(piq)₃ is known as a stable and efficient red-phosphorescent material used in practical applications. For the NIR emitter, Pt(tpbp) ^11,12) was chosen due to its reasonably high PLQY of 35% in deoxygenated toluene, a λ_max PL of 773 nm, ¹⁴⁾ and a stable molecular structure.

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Although the spectral overlap between the PL of DIC-TRZ (λ_max PL: 508 nm) and the absorption of Pt(tpbp) was small (Fig. S1), the PL spectrum of Ir(piq)₃ (λ_max PL: 630 nm) showed a large overlap with the Q-band absorption of Pt(tpbp), as shown in Fig. 1(b), indicating the possibility of efficient energy transfer from Ir(piq)₃ to Pt(tpbp). For confirmation, co-deposited films using the three materials were prepared with a composition of [DIC-TRZ: 10, 20, 30, and 40 wt% Ir(piq)₃:1 wt% Pt(tpbp)], and the PL spectra were measured at an excitation wavelength of 370 nm. Pt(tpbp) was preferentially emitted by the efficient energy transfer at all blend ratios (Fig. S2). The PLQY values of the co-deposited film of DIC-TRZ: Ir(piq)₃:1 wt% Pt(tpbp) were 24%–36% (Table S1).

OLED devices were fabricated with and without the phosphorescent-assisted dopant, varying the assistant dopant concentration from 0 to 99 wt%, where 99 wt% means the host material was Ir(piq)₃ only. The device structure was as follows; [ITO (130 nm)/polymer buffer ²⁸⁾ (20 nm)/NPD (20 nm)/DIC-TRZ: X wt% Ir(piq)₃: 1 wt% Pt(tpbp) (30 nm)/nBPhen (70 nm)/Liq (1 nm)/Al (80 nm)]. Figure 2 illustrates the molecular structures, energy diagram of the device, the EL spectra, and the device performance. Figure S3 illustrates the device structure. The detailed device performance is summarized in Table I.

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NIR OLEDs with λ_max EL of 769 nm were successfully fabricated at all the doping ratios, mainly deriving the EL peak from Pt(tpbp) (Fig. S4). It was found that the three-component devices exhibited lower driving voltages than the two-component devices as shown in J–V and R–V characteristics. Using DIC-TRZ is essential to decrease the driving voltage; however, the simultaneous use of Ir(piq)₃ and DIC-TRZ further reduces the driving voltage, because the iridium complex accepts electrons directly from the electron transport layer. ²⁷⁾ The electron injection barrier from nBPhen to Ir(piq)₃ seems to be low in terms of energy band schemes [Fig. 2(b)].

The three-component devices demonstrated a significant improvement in EQE [Fig. 2(f)]. We observed that 35 wt% was the optimal ratio of Ir(piq)₃ (Table I and Fig. S5). The EQE at 0.01 mA cm⁻² was 10.5%, which was approximately eight times higher compared to that of the device without using the assist dopant. This is one of the best-reported efficiencies obtained in this NIR wavelength region using platinum-porphyrin complexes. The presence of these three components is essential; however, the ratio of the assistant dopant is not critical. An adequate carrier balance was achieved at approximately 64 wt% DIC-TRZ and 35 wt% Ir(piq)₃ as a double-host scheme. ²⁹⁾ The radiance reached 10 W sr⁻¹ m⁻² at 6.7 V for all the three-component devices.

For further investigation, the Pt (tpbp) doping ratio in the emitting layer was varied. Figure 3 and Table II summarize the performance of the devices of [ITO (130 nm)/polymer buffer (20 nm)/NPD (20 nm)/DIC-TRZ: 35 wt% Ir(piq)₃: Y wt% Pt(tpbp) (30 nm)/nBPhen (70 nm)/Liq (1 nm)/Al (80 nm)], where the doping ratio of Pt(tpbp) (Y wt%) varied at 1, 3, and 5 wt%. The obtained EL spectra were nearly identical, with a tiny peak at approximately 630 nm (derived from Ir(piq)₃) found in the 1 wt% device, which was suppressed by increasing the doping ratio to 3 or 5 wt% [Fig. 3(a)]. There was no significant change in the J–V characteristics upon increasing the Pt (tpbp) doping ratio. The radiance of the 1 wt% Pt(tpbp) device was higher than those of the other two devices because the 1 wt% device showed the highest EQE among the three devices.

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In previous reports, general doping ratio of Pt complexes in Alq₃ was approximately 4–6 wt%, and EQE decreased at lower doping ratios. ^9–14) However, in our experiment, higher efficiency was obtained even with a doping ratio as low as 1 wt% (0.63 mol%), which is a typical doping ratio for a device having resonant energy transfer systems not only for fluorescent materials but also for bright phosphorescent materials. ²²⁾

For comparison, two-component devices were fabricated using DIC-TRZ as the host and Pt(tpbp) as the guest, varying the Pt(tpbp) ratio from 1 to 3–5 wt%. Higher EQEs were obtained at higher doping ratios (Fig. S6); however, the EQE never exceeded those obtained for the three-component devices. This is possibly attributed to the concentration quenching of the platinum complex and carrier imbalance in the two-component system.

Considering a PLQY of 24% and a light outcoupling efficiency of 30%, a maximum EQE of 10.5% in the three-component system is higher than the theoretical value of 7.2% most likely due to the direct carrier trapping to Ir(piq)₃. The platinum complex appears to be well dispersed at 1 wt% ratio with a reduced ACQ, and efficient energy transfer occurs because of the good spectral overlap with the assistant dopant.

The phosphorescence lifetimes were measured using a fluorescence lifetime spectrometer (Fig. S7). The PL lifetime of Pt(tpbp) in the three-component film was 24.1 μs, which was slightly shorter than that in the two-component film showing 31.0 μs at 365 nm excitation. It is assumed that a change in the refractive index or the circumstances surrounding Pt(tpbp) with Ir(piq)₃ might affect the PL lifetime.

The most notable result is the enhanced operational lifetime of the NIR OLEDs. The three-component device exhibited an extremely long lifetime of over 4800 h under accelerated conditions of 100 mA cm⁻² [Fig. 3(d)]. The 1 wt% Pt(tpbp) device showed a 10% luminance decay lifetime from an initial luminance (T₉₀) of 3000 h and a 11% decay lifetime (T₈₉) of > 4800 h. Both the 3 wt% and 5 wt% Pt (tpbp) devices showed a 5% luminance decay lifetime (T₉₅) of > 4800 h. The results corresponded to a lifetime of more than 200 000 h at a current density of 10 mA cm⁻² when the acceleration factors were calculated using a previously reported method. ¹¹⁾

The previously reported lifetime of NIR OLED is T₉₀ >1000 h by extrapolation at 40 mA cm⁻² for a device doped with 6 wt% Pt(tpbp) in Alq₃. ¹¹⁾ Our devices showed longer lifetimes at even higher current densities (Fig. S8). It is assumed that these ultralong lifetimes were achieved by changing the host material from electrochemically unstable Alq₃ ³⁰⁾ to more stable DIC-TRZ, ²³⁾ introducing molecularly rigid and stable Ir(piq)₃ ²⁶⁾ and Pt(tpbp), and by improving the carrier balance and energy transfer efficiency using the assist dopant. The 3 wt% and 5 wt% devices showed even longer lifetimes than the 1 wt% device possibly because the added Pt(tpbp) contributed in preventing excess electron injection into the electrochemically unstable NPD because porphyrin complexes have a hole-transporting nature. ³¹⁾

In conclusion, we introduced the red-phosphorescent iridium complex as an assist dopant in NIR-OLED devices to maximize the potential of the NIR emitter of the phosphorescent platinum complex. The maximum EQE of 10.5% and an extremely long lifetime of over 4800 h at 100 mA cm⁻² were obtained in the three-component emitting-layer devices by the efficient energy transfer between the two phosphorescent materials, well-tuned carrier balance, and by the employment of stable molecules of DIC-TRZ, Ir(piq)₃ and Pt(tpbp). This three-component system is proved to be effective for fabricating highly-efficient and long-lived NIR OLEDs.

Acknowledgments