Purely organic electroluminescent material realizing 100% conversion from electricity to light. — TEI-tagged view

Hironori KajipersName, Hajime SuzukipersName, Tatsuya FukushimapersName, Katsuyuki ShizupersName, Katsuaki SuzukipersName, Shosei KubopersName, Takeshi KominopersName, Hajime OiwapersName, Furitsu SuzukipersName, Atsushi WakamiyapersName, Yasujiro MuratapersName, Chihaya AdachipersName (2015date). “Purely organic electroluminescent material realizing 100% conversion from electricity to light..” Nature Communicationstitle·j.

Object of study — the purely organic TADFterm·dom emitter DACT-IIterm and OLEDsterm·mth built from it

Research question — whether an emitter of abundant materials can reach full conversion of charge to light

Contribution — a TADFterm·dom material with ~100% PLQYterm·key and ~100% triplet up-conversion, giving OLEDsterm·mth of 29.6% EQEterm·dom

Full text — tagged

Purely organic electroluminescent material realizing 100% conversion from electricity to light.

Abstract Efficient organic light-emitting diodesterm·mth have been developed using emitters containing rare metals, such as platinum and iridium complexes. However, there is an urgent need to develop emitters composed of more abundant materials. Here we show a thermally activated delayed fluorescenceterm·dom material for organic light-emitting diodesterm·mth, which realizes both approximately 100% photoluminescence quantum yield and approximately 100% up-conversion of the triplet to singlet excited state. The material contains electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties. The typical trade-off between effective emission and triplet-to-singlet up-conversion is overcome by fine-tuning the highest occupied molecular orbitalterm·dom and lowest unoccupied molecular orbitalterm·dom distributions. The nearly zero singlet–triplet energy gap, smaller than the thermal energy at room temperature, results in an organic light-emitting diodeterm·mth with external quantum efficiency of 29.6%. An external quantum efficiency of 41.5% is obtained when using an out-couplingterm·key sheet. The external quantum efficiency is 30.7% even at a high luminance of 3,000 cd m −2 .

Electroluminescence (EL) was initially studied for fundamental interest 1 , 2 , 3 . Tang and VanSlyke 4 subsequently applied EL in organic light-emitting diodesterm·mth (OLEDsterm·mth), obtaining an external EL quantum efficiencyterm·dom (EQEterm·dom) of 1% for a double-layer fluorescent OLEDterm·mth. OLEDsterm·mth have since been studied extensively, with particular focus on increasing their efficiency in display and lighting applications. EQEterm·dom ( η EQEterm·dom ) is the product of the internal quantum efficiencyterm·dom (IQEterm·dom) and light out-couplingterm·key factor ( η out ), η EQEterm·dom =IQEterm·dom × η out . η out is typically 20–30% (refs 5 , 6 , 7 , 8 ). IQEterm·dom is obtained from IQEterm·dom= β × γ × Φ PL , where β is the exciton generation factor resulting in photons, γ is the carrier balance ratio of holes and electrons, and Φ PL is the photoluminescence (PL) quantum yield (PLQYterm·key). Singlet and triplet excitonsterm·key are theoretically generated in a 1:3 ratio by electronic excitation 3 . Triplet excitonsterm·key typically dissipate as heat rather than being converted into photons. Because only singlet excitons generate photons, the resulting β for fluorescent OLEDsterm·mth is limited to 25%. Even under conditions where γ and Φ PL are 100%, the theoretical maximum η EQEterm·dom of the resulting OLEDterm·mth is only 5–7.5%.

Increasing β and η out are therefore important challenges to maximize EQEterm·dom. β can be increased directly using triplet excitonsterm·key. The heavy-atom effect can convert triplet excitonsterm·key into phosphorescenceterm·dom. Intersystem crossingterm·dom (ISCterm·dom) involves transition from the singlet to triplet state, and can theoretically result in 100% of excitons being extracted as photons. The EQEterm·dom of the first device based on such a concept in 1998 was 4% (ref. 9 ), and subsequently reached 19% in 2001 (ref. 10 ). EQEsterm·dom exceeding 20% are now frequently reported for optimized devices incorporating well-designed materials 11 . However, phosphorescent materials for OLEDsterm·mth are currently restricted to Pt and Ir complexes, which have limited availability and are expensive.

Triplet excitonsterm·key can also potentially be exploited by reverse ISCterm·dom (RISCterm·dom), which involves transition from the lowest energy triplet state (T 1 ) to the lowest excited singlet state (S 1 ) 12 , 13 , 14 , 15 , 16 . Converting triplet excitonsterm·key to singlet excitons allows all excitons to be used to generate photons. This strategy has typically not been effective, because S 1 is at a much higher energy than T 1 . Spin inversion is also required for this transition. However, fabrications of efficient OLEDsterm·mth based on RISCterm·dom have been attempted in recent years 17 , 18 , 19 , 20 , 21 . Unlike normal fluorescenceterm·dom, the fluorescence resulting from triplet–singlet transition is delayed, so we term it thermally activated delayed fluorescenceterm·dom (TADFterm·dom).

TADFterm·dom occurs by two successive processes, shown in Fig. 1a as green arrows; RISCterm·dom from T 1 to S 1 , and subsequent radiative decay from S 1 to the ground state (S 0 ). RISCterm·dom is effective when the energy difference between S 1 and T 1 (Δ E ST ) is small. The theoretical background ( Supplementary Note 1 ) reveals that small Δ E ST is achieved by separating highest occupied molecular orbitalterm·dom (HOMOterm·dom) and lowest unoccupied molecular orbitalterm·dom (LUMOterm·dom) distributions. However, this HOMO–LUMO separationterm·dom decreases the oscillator strengthterm·dom ( f ) between S 1 and S 0 , which lowers PLQYterm·key 21 .

DACT-IIterm. ( a ) Schematic illustration of the TADFterm·dom process. ( b ) Structure of DACT-IIterm. ( c ) HOMOterm·dom and LUMOterm·dom distributions in DACT-IIterm calculated at the PBE0/6-31G(d) level of theory. ( d ) Device structure and energy band diagram of DACT-II- x OLEDsterm·mth . For the DACT-II-100 OLEDterm·mth , the EML consisted of DACT-IIterm without CBP. Energy band diagrams of the reference fluorescent and phosphorescent OLEDsterm·mth and other DACT-II- x OLEDsterm·mth are shown in Supplementary Fig. 4 .

Here we report an organic TADFterm·dom material, 9-[4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl]- N , N , N ′, N ′-tetraphenyl-9 H -carbazole-3,6-diamine, named DACT-IIterm ( Fig. 1b ). DACT-IIterm consists of electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties, chemically bonded at a torsion angle α ( Fig. 1b ). We will demonstrate that both nearly zero Δ E ST , which is smaller than the thermal energy at room temperature, and large f , resulting in a PLQYterm·key of 100%, are simultaneously achieved by precisely controlling the HOMOterm·dom and LUMOterm·dom distributions ( Fig. 1c and Supplementary Fig. 1 , the geometries were optimized at the PBE0/6-31G(d) level 22 , 23 using the Gaussian 09 software package 24 ), although the two factors are considered to be in a trade-off relationship. The small Δ E ST and large f of DACT-IIterm allow ‘all injected holes and electrons to be used in fluorescence', resulting in an IQEterm·dom of 100%. DACT-II-based OLEDsterm·mth exhibit a maximum EQEterm·dom of 29.6% in the absence of out-couplingterm·key treatment, indicating an IQEterm·dom of ∼100%. The only loss of quantum efficiency in the device is via light out-couplingterm·key. A maximum EQEterm·dom of 41.5% is obtained simply using an out-couplingterm·key sheet. Importantly, DACT-IIterm also allows the typical substantial decrease in EQEterm·dom with increasing luminance (efficiency roll-offterm·key) to be overcome. In DACT-IIterm, S 1 and T 1 excitons are rapidly converted to photons because of its large f and small Δ E ST . The rapid removalterm·dom of both S 1 and T 1 excitons in the optimized DACT-II-based OLEDsterm·mth suppresses triplet–triplet annihilationterm·dom (TTAterm·dom) and singlet–triplet annihilation, which are an origin of roll-offterm·key. Very high EQEterm·dom up to 30.7% is achieved at 3,000 cd m −2 .

Results Organic light-emitting diodesterm·mth DACT-IIterm, which consists only of C, H and N, was synthesized as described in Supplementary Note 2 and Supplementary Fig. 2 . DACT-IIterm 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-IIterm (∼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-IIterm 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 samples 25 ( Supplementary Note 3 ). DACT-IIterm was then used as an emitting material for OLEDsterm·mth ( DACT-IIterm 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(9 H -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-IIterm was used for the EML. The devices are termed DACT-II- x devices , where x corresponds to the respective wt% of DACT-IIterm. 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-IIterm 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 .

EQEterm·dom values for selected DACT-II- x devices.

Figure 2a shows EL spectra of DACT-IIterm devices at a current density of 1 mA cm −2 . The OLEDsterm·mth, containing DACT-IIterm 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-IIterm 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-IIterm molecules, by direct charge injection and/or complete energy transfer from the CBP host. In Fig. 2c , EQEterm·dom is shown as a function of luminance. The EQEterm·dom for representative DACT-II- x devices are given in Table 1 (Data for all OLEDsterm·mth without and with out-couplingterm·key sheets are listed in Supplementary Tables 1 and 2 , respectively). The maximum EQEterm·dom 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 ). EQEsterm·dom 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 EQEsterm·dom of the Alq 3 and Ir(ppy) 3 devices are 1.3% and 13.0%, respectively. Maximum EQEsterm·dom of all fabricated DACT-IIterm devices exceed those of the Alq 3 and Ir(ppy) 3 devices , except for the DACT-II-1 device . EQEterm·dom 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 EQEterm·dom decreases with increasing luminance, the efficiency roll-offterm·key is overcome by increasing the concentration of DACT-IIterm in the EML. Figure 2c and Table 1 show that EQEterm·dom 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 .

EL and PL of DACT-IIterm. ( 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-IIterm in CBP films and a neat DACT-IIterm film. Photographs of PL emission are also shown. ( c ) EQEterm·dom versus luminance for the DACT-II- x devices . EQEterm·dom values of the Alq 3 device and Ir(ppy) 3 device are also shown.

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

Photoluminescence quantum yield We investigate the origin of the excellent EQEterm·dom of DACT-IIterm by first discussing the PLQYterm·key ( Supplementary Note 5 ). The PLQYterm·key of DACT-IIterm 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 PLQYterm·key of 63.7%. This suggests that some excitons, transferred to T 1 via ISCterm·dom, 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 RISCterm·dom, resulting in a higher PLQYterm·key. Phosphorescenceterm·dom can be discounted as demonstrated below. The PLQYterm·key of the neat film is only 43.6±4%, but PLQYterm·key values of 101.1±4% are obtained for the 9 wt% DACT-II-doped CBP thin films, when DACT-IIterm is directly excited. The PLQYsterm·key 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-IIterm without any loss (ultraviolet–visible spectra of DACT-IIterm and CBP films are given in Supplementary Fig. 7 ).

Exciton generation factor Another important factor influencing EQEterm·dom is β , that is, the singlet/triplet branching ratio. In PL experimentsterm·mth, excitation produces excitons in S 1 . Figure 3a shows the transient PL decay of DACT-IIterm 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 phosphorescenceterm·dom from T 1 via ISCterm·dom, or delayed fluorescenceterm·dom from S 1 through ISCterm·dom and subsequent RISCterm·dom, 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 phosphorescenceterm·dom or delayed fluorescenceterm·dom.

TADFterm·dom characteristics of DACT-IIterm. ( a ) Transient PL decay characteristics of DACT-IIterm in toluene at room temperature. ( b ) Doping concentration dependence of transient PL decay of DACT-IIterm doped into CBP thin films at room temperature. ( c ) Arrhenius plot of the RISCterm·dom rate constant ( k RISCterm·dom ). Δ E ST of 9.0 meV was obtained from least-squares fitting (solid line). ( d ) Temperature dependence of Boltzmann factor for DACT-IIterm (Δ E ST =9.0 meV) and 4CzIPN (Δ E ST =82.6 meV).

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-IIterm concentration. Phosphorescenceterm·dom and delayed fluorescenceterm·dom are easily distinguished from the temperature dependence of the transient PL decay. Delayed fluorescenceterm·dom exhibits an increase in delayed emission with increasing temperature. Conversely, inverse temperature dependence is found in the case of phosphorescenceterm·dom. 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 TADFterm·dom.

TTAterm·dom is another possible origin of delayed fluorescenceterm·dom, which can also improve OLEDterm·mth efficiency 26 , 27 , 28 , 29 . However, β obtained by TTAterm·dom is theoretically limited to 62.5%. The IQEterm·dom, and therefore β , in this study reached 100% as clearly shown below, confirming that the delayed fluorescenceterm·dom originates from RISCterm·dom.

The radiative rate constants for the prompt ( k p ) and delayed ( k d ) components, rate constants for ISCterm·dom ( k ISCterm·dom ) and RISCterm·dom ( k RISCterm·dom ), 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). Δ E ST determined from the Arrhenius plot of k RISCterm·dom is 9.0 meV ( Fig. 3c ). The currently reported TADFterm·dom emitting material with the highest efficiency is the carbazolyldicyanobenzene derivative 4CzIPN 19 , whose experimental Δ E ST is 82.6 meV. Δ E ST of DACT-IIterm is markedly smaller, reflecting the temperature dependence of k RISCterm·dom .

Temperature dependence of various rate constants and PLQYsterm·key for CBP films doped with 6 wt% DACT-IIterm.

Δ E ST of DACT-IIterm is smaller than the thermal energy at 300 K (25.9 meV). Figure 3d compares the temperature dependence of the Boltzmann factors, exp(–Δ E ST / k B T ), for DACT-IIterm and 4CzIPN. The Boltzmann factor of DACT-IIterm at 300 K is 0.71, much larger than that of 4CzIPN, 0.041. This indicates that RISCterm·dom occurs more readily in DACT-IIterm than 4CzIPN. The temperature dependence of the Boltzmann factors suggests effective up-conversion from T 1 to S 1 for DACT-IIterm even at low temperature. This is supported by the total PLQYterm·key 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-IIterm TADFterm·dom systems over other TADFterm·dom and phosphorescent systems, the latter two systems cannot exhibit high PLQYterm·key 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 TADFterm·dom, not phosphorescenceterm·dom. In phosphorescent OLEDsterm·mth, the energy corresponding to Δ E ST is lost during ISCterm·dom. Although this does not affect the quantum yield, such energy loss is avoided (or there is even an energy gain) during RISCterm·dom in TADFterm·dom systems by exploiting the thermal energy at room temperature. This results in lower power consumption.

Time-dependent density functional theory 30 , 31 (TD-DFT) calculations indicate that the torsion angle ( α ) between the donor and acceptor moieties affects the spatial overlap of the HOMOterm·dom and LUMOterm·dom ( 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 TADFterm·dom compounds. The value of f for DACT-IIterm (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 Δ E ST of 5.2 meV (using TD-DFT with the PBE0/6-31G(d) basis set 22 , 23 ). This is smaller than the Δ E 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 Δ E ST and f , by precisely controlling the overlap of the HOMOterm·dom and LUMOterm·dom distributions in DACT-IIterm. In real OLEDsterm·mth, the energy levels of S 1 and T 1 likely have distributions, and Δ E ST may fluctuate. This is due to the distributions of conformations and environments of individual DACT-IIterm 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-couplingterm·key efficiency 32 , 33 , 34 , 35 . Here the orientation of DACT-IIterm was quantified by two methods, variable angle spectroscopic ellipsometry (VASE) 36 , 37 , 38 and angular-dependent PL measurements 32 , 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-IIterm molecules is not random. Quantitative analysis indicates that DACT-IIterm 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.

Orientation of transition dipole moment and out-couplingterm·key. ( 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 experimentsterm·mth. ( 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-couplingterm·key mode on the thicknesses of hole and electron-transport layers.

Out-couplingterm·key and carrier balance ratio We performed electrical and optical simulations 41 , 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 . IQEterm·dom=100% and S =−0.29 were used for the simulation. The contributions of respective optical modes are given as a function of emission wavelengths. EQEterm·dom 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 EQEterm·dom on HTL and ETL thicknesses (see Supplementary Fig. 10 for all modes). The maximum EQEterm·dom 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 EQEterm·dom of 29.6%. The agreement clearly confirms the IQEterm·dom≈100%; that is, β ≈ γ ≈ Φ PL ≈100% for the conditions of maximum EQEterm·dom.

Thus far, we have made no effort to improve the out-couplingterm·key efficiency. The current OLEDsterm·mth have a high η out = η EQEterm·dom /( γ × β × Φ PL )=29.6/(1.0 × 1.0 × 1.0)=29.6%. The IQEterm·dom is already ∼100%, so an increase in out-couplingterm·key efficiency is required to further increase EQEterm·dom. Several methods have been reported to achieve this 44 , 45 , 46 , 47 , 48 . We opted for a simple, economical attachment of an out-couplingterm·key sheet to the OLEDterm·mth. Using an out-couplingterm·key sheet ( Supplementary Fig. 11 and Supplementary Note 9 ) with a microlens array that also contained light-scattering particles yielded EQEterm·dom values of 41.5% at maximum (filled blue circles in Fig. 2c ), and 37.6% and 30.7% at 500 cd m −2 and 3,000 cd m −2 , respectively (filled green circles in Fig. 2c , see also Table 1 and Supplementary Table 2 ).

Discussion We obtained a TADFterm·dom material, DACT-IIterm. OLEDsterm·mth containing DACT-IIterm have ∼100% IQEterm·dom, that is, 100% conversion from electricity to light. DACT-IIterm has excellent thermal properties, and devices containing DACT-IIterm are expected to perform well across a wide temperature range. By further optimizing device structures and surrounding materials, as seen for phosphorescent 11 , 49 and TADF-based 40 OLEDsterm·mth, DACT-IIterm devices are expected to deliver much higher performance, such as higher EQEterm·dom at high luminance and lower power consumption. Improving the out-couplingterm·key efficiency will further increase the EQEterm·dom of DACT-II-based OLEDsterm·mth .

Methods Summary of DACT-IIterm synthesis DACT-IIterm was synthesized according to Supplementary Fig. 2 and Supplementary Note 2 with a 26% total yield. 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded using JEOL ECA 600 MHz (Japan) and Bruker Avance III 800 MHz (Germany) spectrometers. DACT-IIterm was purified by silica gel column chromatography using dichloromethane/hexane as an eluent, recrystallization and temperature-gradient sublimation.

Device fabrication and characterization OLEDsterm·mth with active areas of 4 mm 2 were fabricated by vacuum deposition at ∼10 −5 Pa on clean ITO-coated glass substrates with a deposition apparatus (SE-4260, ALS Technology, Japan). The deposition rates of HTL, EML, ETL, EIL, and Al were 0.1–0.2, 0.3–0.4, 0.1–0.2, 0.01, and 0.1–0.2 nm s −1 , respectively. After fabrication, devices were encapsulated with a glass cap using epoxy glue in a N 2 -filled glove box. Calcium oxide was incorporated into the encapsulated package as a desiccant. OLEDterm·mth characteristics were measured with a source meter (2400, Keithley, Japan) and an absolute EQEterm·dom measurement system (C9920-12, Hamamatsu Photonics, Japan). The integrating sphere for the measurements was calibrated before the experimentsterm·mth.

Additional information How to cite this article: KajipersName, H. et al . Purely organic electroluminescent material realizing 100% conversion from electricity to light. Nat. Commun . 6:8476 doi: 10.1038/ncomms9476 (2015).

Supplementary Material Supplementary Figures 1-11, Supplementary Tables 1-2, Supplementary Notes 1-9 and Supplementary References Acknowledgments This research was funded by the Japan Society for the Promotion of Science (JSPS) through the ‘Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)', initiated by the Council for Science and Technology Policy (CSTP). This research was also supported by a Grant-in-Aid for Scientific Research (A) and by the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant # 2015- 14). Out-couplingterm·key sheets were kindly provided by Yuji Hotta, Optmate Corporation. Computation time on a supercomputer system was provided by the Institute for Chemical Research, Kyoto University, Japan, and the Academic Center for Computing and Media Studies (ACCMS), Kyoto University, Japan. NMR measurements were supported by the Joint Usage/Research Center (JURC) at the Institute for Chemical Research, Kyoto University, Japan.

Footnotes References Associated Data Supplementary Materials Supplementary Figures 1-11, Supplementary Tables 1-2, Supplementary Notes 1-9 and Supplementary References