Organic light-emitting diodes

DACT-II, which consists only of C, H and N, was synthesized as described in Supplementary Note 2 and Supplementary Fig. 2. DACT-II has favourable thermal properties, with melting and decomposition temperatures of 266 °C and 484 °C, respectively. The sublimation temperature under vacuum was 308 °C. The glass transition temperature (T g ) of bulk amorphous DACT-II (∼6 mg) measured by conventional differential scanning calorimetry (DSC) at heating rates of 1–100 K min−1 was 151–159 °C. We also attempted DSC measurements for DACT-II in vacuum-deposited amorphous thin films with a thickness of 100 nm (∼20 ng). The ultrasensitive measurements were successful using very fast heating rates of 500–3,000 K s−1, giving T g of 192–197 °C. T g of the thin films was higher than that of bulk samples, although it would be decreased by extrapolating to a lower heating rate. The higher T g probably reflects the higher thermal stability of vacuum-deposited thin films than bulk samples25 (Supplementary Note 3). DACT-II was then used as an emitting material for OLEDs (DACT-II devices). Supplementary Fig. 3 shows the materials for the devices; N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (NPD) or 4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC) was used for the hole-transport layer (HTL), ([1,1′-biphenyl]-4-yloxy)bis((2-methylquinolin-8-yl)oxy)aluminium (BAlq) or 3,3′′,5,5′′-tetra(pyridin-3-yl)-1,1′:3′,1′′-terphenyl (BmPyPhB) for the electron-transport layer (ETL), lithium quinolin-8-olate (Liq) for the electron-injection layer (EIL) and 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) for the host material of the emitter layer (EML). The device structure was indium tin oxide (ITO)/HTL (100 nm)/EML (40 nm)/ETL (30 nm)/Liq/Al (Fig. 1d and Supplementary Fig. 4). CBP doped with x wt% DACT-II was used for the EML. The devices are termed DACT-II- x devices, where x corresponds to the respective wt% of DACT-II. Hereafter, the DACT-II- x devices, containing TAPC and BAlq as HTL and ETL, respectively, are discussed unless otherwise stated. The energy-level diagram of the devices is shown in Fig. 1d. A neat DACT-II layer was also used for the EML in the DACT-II-100 device. The results and band diagrams for devices containing NPD or BmPyPhB are included in Table 1, Supplementary Tables 1 and 2, and Supplementary Figs 4 and 5. Typical fluorescent and phosphorescent devices were also fabricated for reference using mer tris(8-hydroxyquinoline) aluminum(III) (Alq 3 ) and fac tris(2-phenylpyridine) iridium(III) (Ir(ppy) 3 ) as emitters, respectively. The structures of the Alq 3 device and Ir(ppy) 3 device are shown in Supplementary Fig. 4.

Table 1 EQE values for selected DACT-II-x devices. Full size table

Figure 2a shows EL spectra of DACT-II devices at a current density of 1 mA cm−2. The OLEDs, containing DACT-II concentrations of 1 to 23 wt%, all emit green EL. The emission maxima (λ max ) range from 516 to 537 nm, with a small red shift observed with increasing DACT-II concentration. λ max of PL spectra has a stronger concentration dependence, as shown in Fig. 2b (see Supplementary Note 4). EL spectra with a logarithmic EL intensity scale are shown in Supplementary Fig. 6. The DACT-II-1 device contains emission from TAPC and/or CBP (Supplementary Figure 6 also shows the PL spectra of the respective materials). However, no obvious radiative emission from any other layer is detected for any of the other devices. This indicates that excitons are confined within the emission layer, despite the absence of blocking layers. In addition, no emission originating from CBP is observed, except for the DACT-II-1 device. This indicates that excitons are well-confined on DACT-II molecules, by direct charge injection and/or complete energy transfer from the CBP host. In Fig. 2c, EQE is shown as a function of luminance. The EQE for representative DACT-II- x devices are given in Table 1 (Data for all OLEDs without and with out-coupling sheets are listed in Supplementary Tables 1 and 2, respectively). The maximum EQE of 29.6% is obtained at 5 cd m−2 (5 μA cm−2) for the DACT-II-9 device (filled black circles in Fig. 2c). EQEs of a typical fluorescent Alq 3 device and phosphorescent Ir(ppy) 3 device are also shown in Fig. 2c, as black dot-dashed and solid lines, respectively. Maximum EQEs of the Alq 3 and Ir(ppy) 3 devices are 1.3% and 13.0%, respectively. Maximum EQEs of all fabricated DACT-II devices exceed those of the Alq 3 and Ir(ppy) 3 devices, except for the DACT-II-1 device. EQE values of the DACT-II-9 device are extremely large, even at the luminance levels of practical applications; 22.8% at 500 cd m−2 (for display applications) and 16.2% at 3,000 cd m−2 (for lighting applications) (Table 1). Although EQE decreases with increasing luminance, the efficiency roll-off is overcome by increasing the concentration of DACT-II in the EML. Figure 2c and Table 1 show that EQE values of 26.9% and 21.8% are obtained at 500 cd m−2 and 3,000 cd m−2, respectively, for the DACT-II-19 device.

Figure 2: EL and PL of DACT-II. (a) EL spectra for the DACT-II- x devices. A photograph of EL emission for the DACT-II-9 device is also shown. (b) PL spectra of DACT-II in CBP films and a neat DACT-II film. Photographs of PL emission are also shown. (c) EQE versus luminance for the DACT-II- x devices. EQE values of the Alq 3 device and Ir(ppy) 3 device are also shown. Full size image

Although the devices are relatively thick (Fig. 1d), the DACT-II devices exhibit low power consumption, especially at high DACT-II concentrations. The turn-on voltage (voltage at 1 cd m−2) decreased with increasing DACT-II concentration and was 2.6 V for the DACT-II-19 device. This suggests that with increasing DACT-II concentration, charges tend to be injected directly into DACT-II molecules rather than via the CBP host, as supposed from Fig. 1d.

Photoluminescence quantum yield

We investigate the origin of the excellent EQE of DACT-II by first discussing the PLQY (Supplementary Note 5). The PLQY of DACT-II in toluene is 46.0%, which decreases to 26.6% on bubbling O 2 through the solution. Bubbling with Ar excludes O 2 and results in a PLQY of 63.7%. This suggests that some excitons, transferred to T 1 via ISC, are quenched by dissolved O 2 (refs 17, 18). In the Ar-purged system, excitons in T 1 are not quenched and revert to S 1 through RISC, resulting in a higher PLQY. Phosphorescence can be discounted as demonstrated below. The PLQY of the neat film is only 43.6±4%, but PLQY values of 101.1±4% are obtained for the 9 wt% DACT-II-doped CBP thin films, when DACT-II is directly excited. The PLQYs are 100.3±4% for the 9 wt% doped films even when CBP is excited, indicating that all the excitons transfer from CBP to DACT-II without any loss (ultraviolet–visible spectra of DACT-II and CBP films are given in Supplementary Fig. 7).

Exciton generation factor

Another important factor influencing EQE is β, that is, the singlet/triplet branching ratio. In PL experiments, excitation produces excitons in S 1 . Figure 3a shows the transient PL decay of DACT-II in toluene. When O 2 is removed by Ar bubbling, two components with prompt and delayed PL decays are observed. This differs from the result for the as-prepared solution containing O 2 , in which only the prompt decay component is observed. The results indicate that the prompt decay arises from fluorescence directly from the S 1 -to-S 0 transition. The delayed component arises from phosphorescence from T 1 via ISC, or delayed fluorescence from S 1 through ISC and subsequent RISC, because it is quenched by the O 2 triplet state in the presence of O 2 (refs 17, 18). Continuous transitions between S 1 and T 1 are theoretically possible, but also result in either phosphorescence or delayed fluorescence.

Figure 3: TADF characteristics of DACT-II. (a) Transient PL decay characteristics of DACT-II in toluene at room temperature. (b) Doping concentration dependence of transient PL decay of DACT-II doped into CBP thin films at room temperature. (c) Arrhenius plot of the RISC rate constant (k RISC ). ΔE ST of 9.0 meV was obtained from least-squares fitting (solid line). (d) Temperature dependence of Boltzmann factor for DACT-II (ΔE ST =9.0 meV) and 4CzIPN (ΔE ST =82.6 meV). Full size image

Prompt and delayed PL decay components are also observed for thin films. Figure 3b shows the doping concentration dependence of the transient PL decays for DACT-II-doped CBP thin films (The temperature dependence is shown in Supplementary Fig. 8). Both prompt and delayed components are observed for all films, and the delayed component increases with increasing DACT-II concentration. Phosphorescence and delayed fluorescence are easily distinguished from the temperature dependence of the transient PL decay. Delayed fluorescence exhibits an increase in delayed emission with increasing temperature. Conversely, inverse temperature dependence is found in the case of phosphorescence. The intensity of the delayed component is absent at 10 K but gradually increases with increasing temperature (Supplementary Fig. 8), indicating that the delayed emission is thermally activated. Thus, the delayed component originates from TADF.

TTA is another possible origin of delayed fluorescence, which can also improve OLED efficiency26,27,28,29. However, β obtained by TTA is theoretically limited to 62.5%. The IQE, and therefore β, in this study reached 100% as clearly shown below, confirming that the delayed fluorescence originates from RISC.

The radiative rate constants for the prompt (k p ) and delayed (k d ) components, rate constants for ISC (k ISC ) and RISC (k RISC ), and total (Φ PL ) and fraction of delayed (Φ PL d) emission quantum yields are shown in Table 2 (see Supplementary Note 6 for the analysis). ΔE ST determined from the Arrhenius plot of k RISC is 9.0 meV (Fig. 3c). The currently reported TADF emitting material with the highest efficiency is the carbazolyldicyanobenzene derivative 4CzIPN19, whose experimental ΔE ST is 82.6 meV. ΔE ST of DACT-II is markedly smaller, reflecting the temperature dependence of k RISC .

Table 2 Temperature dependence of various rate constants and PLQYs for CBP films doped with 6 wt% DACT-II. Full size table

ΔE ST of DACT-II is smaller than the thermal energy at 300 K (25.9 meV). Figure 3d compares the temperature dependence of the Boltzmann factors, exp(–ΔE ST /k B T), for DACT-II and 4CzIPN. The Boltzmann factor of DACT-II at 300 K is 0.71, much larger than that of 4CzIPN, 0.041. This indicates that RISC occurs more readily in DACT-II than 4CzIPN. The temperature dependence of the Boltzmann factors suggests effective up-conversion from T 1 to S 1 for DACT-II even at low temperature. This is supported by the total PLQY remaining at ∼100% at lower temperatures, as shown in Table 2. T 1 excitons are more easily up-converted to S 1 at higher temperature. This is an advantage of DACT-II TADF systems over other TADF and phosphorescent systems, the latter two systems cannot exhibit high PLQY across a wide temperature range.

The PL spectra of the prompt and delayed PL decay components are shown in Supplementary Fig. 9. These spectra are indistinguishable within experimental error, irrespective of temperature. This supports the fact that the delayed component originates from TADF, not phosphorescence. In phosphorescent OLEDs, the energy corresponding to ΔE ST is lost during ISC. Although this does not affect the quantum yield, such energy loss is avoided (or there is even an energy gain) during RISC in TADF systems by exploiting the thermal energy at room temperature. This results in lower power consumption.

Time-dependent density functional theory30,31 (TD-DFT) calculations indicate that the torsion angle (α) between the donor and acceptor moieties affects the spatial overlap of the HOMO and LUMO (Supplementary Fig. 1, α=0° for planar configuration and 90° for perpendicular configuration). The value of f decreases with increasing α, so precise control of f by adjusting α is important in designing highly luminescent TADF compounds. The value of f for DACT-II (0.2400) is higher than that for 4CzIPN (0.0708). S 0 , T 1 and S 1 have energy minima at α of 48°, 85° and 89°, respectively. Subtracting the energy minimum of T 1 (at α=85°) from that of S 1 (at α=89°) gives a calculated ΔE ST of 5.2 meV (using TD-DFT with the PBE0/6-31G(d) basis set22,23). This is smaller than the ΔE ST of 11.3 meV calculated for 4CzIPN at the same level of theory. We realized both ∼100% up-conversion from T 1 to S 1 and ∼100% radiative emission, that is to say, 100% conversion from electricity to light, through the fine-tuning of ΔE ST and f, by precisely controlling the overlap of the HOMO and LUMO distributions in DACT-II. In real OLEDs, the energy levels of S 1 and T 1 likely have distributions, and ΔE ST may fluctuate. This is due to the distributions of conformations and environments of individual DACT-II molecules because they are in an amorphous state.

Orientation of transition dipole moment

The orientation of transition dipole moments of emitter molecules greatly affects out-coupling efficiency32,33,34,35. Here the orientation of DACT-II was quantified by two methods, variable angle spectroscopic ellipsometry (VASE)36,37,38 and angular-dependent PL measurements32,33,39,40 (Supplementary Note 7). Figure 4a shows the results of VASE measurements. The ordinary and extraordinary optical constants differ in the spectra, indicating that the orientation of DACT-II molecules is not random. Quantitative analysis indicates that DACT-II tends to be parallel to the substrate with an order parameter S of −0.32±0.01. The result is confirmed by the angular-dependent PL measurements (Fig. 4b), which gave S=−0.29±0.05, agreeing well with the above VASE result.

Figure 4: Orientation of transition dipole moment and out-coupling. (a) Results of VASE measurements; extinction coefficient k (black) and refractive index n (red). The solid and dashed lines represent spectra for the ordinary and extraordinary optical constants, respectively. (b) Results of angular-dependent PL experiments. (c) Optical simulations for the DACT-II-9 device. The numbers in parentheses for respective modes are obtained by the integration with respect to emission wavelengths with weighting of the PL intensity. (d) Dependence of the out-coupling mode on the thicknesses of hole and electron-transport layers. Full size image

Out-coupling and carrier balance ratio

We performed electrical and optical simulations41,42,43 for the DACT-II-9 device (Supplementary Note 8). Electrical simulations revealed that the injected electrons and holes effectively recombine in the EML near the interface with the HTL. Figure 4c shows the results of optical simulations for the DACT-II-9 device. IQE=100% and S=−0.29 were used for the simulation. The contributions of respective optical modes are given as a function of emission wavelengths. EQE can be calculated by integrating the fraction of out-coupled mode with respect to emission wavelengths with weighting of the PL intensity in Fig. 2b. Figure 4d shows the dependence of EQE on HTL and ETL thicknesses (see Supplementary Fig. 10 for all modes). The maximum EQE of 29.0±0.9% was obtained for HTL and ETL thicknesses of 100 nm and 30 nm, respectively (for S=−0.29±0.05). The simulation results agree well with the experimental maximum EQE of 29.6%. The agreement clearly confirms the IQE≈100%; that is, β≈γ≈Φ PL ≈100% for the conditions of maximum EQE.