Organic light-emitting diode (OLED) technology has shown great potential for applications in next-generation displays and lighting. Binary emissive layers (EMLs) have natural advantages in low film-fabrication cost, thus making them the mainstream solution in current OLED production lines. However, it has been difficult for previously reported OLEDs with binary EMLs to simultaneously exhibit the desired comprehensive performance to date, e.g., high colour purity, high efficiency, low efficiency roll-off, and long operational lifetime.
To address this bottleneck, our recent work published in Nature Communications proposed a promising molecular design strategy, i.e., hybridization of organoboron-nitrogen and carbonyl fragments for constructing a multiple resonance (MR) framework; thus, we designed a proof-of-concept thermally activated delayed fluorescence (TADF) molecule consisting of only Period-2 elements, i.e., h-BNCO-1.
Figure 1. Chemical structures and theoretical calculation results of h-BNCO-1, BNCZ, and BNBNB. a, Chemical structures of h-BNCO-1, BNCZ and BNBNB. b, Optimized geometric structures of their ground (S0) states via the ωB97XD functional with the nonempirically tuned ω value. c, Calculated excited-state energies and spin-orbit couplings via the high-level STEOM-DLPNO-CCSD method. The estimated kRISC values are also shown. d, Difference density plots of the S1, T1, and T2 excited states calculated via the high-level STEOM-DLPNO-CCSD method.
Theoretical calculations (Figure 1) indicated that compared with the control compound without carbonyl BNCZ, h-BNCO-1 shows a much more distorted geometry. Due to the electron-withdrawing effect of the carbonyl groups and the electronic delocalization of the carbonyl groups, the S1- and T1-state energies both decrease, with a reduced ΔES1T1 of 0.12 eV. Moreover, ΔET2T1 decreases to 0.18 eV, which is important for accelerating the RISC process. Although the SOC(S1-T1) is negligible, the SOC(S1-T2) is very large, ca. 1.26 cm−1, which could contribute greatly to the RISC process. This difference mainly results from their excited-state characteristics. In contrast to the T1 state, which shows a ππ* excitation character similar to that of S1, the ππ* excitation character of the T2 state on the BN main body shows an apparent difference. Moreover, the T2 state on the carbonyls shows only nπ* excitation, which is different from the ππ* excitation character of the S1 state on the groups. Eventually, such a combination of small ΔES1T1, small ΔET2T1, and large SOC(S1-T2) gives rise to a fast Boltzmann-averaged kRISC value of ~ 1.25×105 s−1.
Moreover, we also replaced the two carbonyls of h-BNCO-1 with electron-withdrawing phenyl-boron groups and thus designed a proof-of-concept BN-MR molecule (named BNBNB) without carbonyl groups. The results of our theoretical calculations on BNBNB clearly indicated that the introduction of phenyl-boron groups also leads to a smaller ΔES1T1 (0.08 eV) and ΔET2T1 (0.18 eV), similar to those of h-BNCO-1. However, the SOC value (0.69 cm−1) between the S1 and T2 states is smaller than that in h-BNCO-1 due to the absence of nπ* excitation in the T2 state. Eventually, the Boltzmann-averaged kRISC value (~ 1.70×103 s−1) in BNBNB is two orders of magnitude lower than that in h-BNCO-1. In short, our calculation results demonstrated that the proposed h-BNCO strategy not only reduces ΔES1T1 and ΔET2T1 but also increases the SOC, eventually substantially accelerating the RISC process.
Our photophysical measurements of h-BNCO-1 agreed well with our estimations. The ΔES1T1 value of h-BNCO-1 was computed to be 0.03 eV. The kRISC of h-BNCO-1 reaches an impressive value of 1.79 × 105 s−1, which is much faster than that of BNCZ under the same conditions (1.24 × 104 s−1).
Figure 2. Optimized EL performances of BNCZ- and h-BNCO-1-based OLEDs. a, Device structures with ionization potential and electron affinity (in eV) for each material and the relevant chemical structures of the materials used in the EMLs. b, Normalized EL spectra. c, EQE−luminance characteristics and efficiency roll-offs of the OLEDs, in which the black dashed line marks the luminance of 1000 cd·m−2, and the green and blue marks represent the device roll-off results at 1000 cd·m−2 of the devices based on h-BNCO-1 and BNCZ, respectively.
To evaluate the electroluminescent potential of h-BNCO-1, we fabricated an OLED with a binary EML of 1 wt% h-BNCO-1 doped in the bipolar TADF material DMIC-TRZ as the host matrix (Figure 2). The h-BNCO-1-based device shows a green EL peak at 528 nm and an FWHM of 39 nm (173 meV), corresponding to high-quality green CIE coordinates of (0.24, 0.71). Moreover, the device based on h-BNCO-1 exhibited a maximum EQE of 40.1%, which is among the highest EQEs of TADF OLEDs. Importantly, at an initial brightness of 1000 cd m−2, the h-BNCO-1-based device still maintains a decent EQE of 34.6%, corresponding to a relative roll-off of 14%. These device results further demonstrate the importance of h-BNCO in accelerating the RISC process and relieving exciton quenching at high current densities.
Figure 3. Operational stability of the h-BNCO-1-based OLED. a, The relative luminance versus operating time characteristics at an initial luminance of 1000 cd m−2. b, Summary of the operational lifetime (LT95 at an initial brightness of 1000 cd m−2)-wavelength peak-CIE-y coordinate of the reported MR-OLEDs based on binary-EMLs.
Traditionally, the severe roll-off efficiency and poor operational stability of TADF-based devices could be mainly ascribed to triplet accumulation, which originates from the slow RISC process from the triplet state to the singlet excited state. To further validate the benefits of the accelerated RISC process, the operational EL stability based on h-BNCO-1 was subsequently evaluated by changing with more stable functional materials to fabricate binary-EML OLEDs (Figure 3). Benefiting from the high kRISC (>1×105 s−1) of h-BNCO-1 and the device structure, a lifetime reaching 95% of the initial luminance (LT95, starting from 1000 cd m−2) was measured at approximately 140 hours. To our knowledge, this operational lifetime is one of the best performances reported for MR-OLEDs based on a binary EML. Additionally, our device stability is comparable to that of its ternary-EML analogous via extra assistance by TADF molecules or phosphors. Therefore, the impressive performance and stability obtained via a simple binary-EML structure composed of only pure organics indicates the potential for future low-cost commercialization.
In terms of OLED material chemistry, our work provided a promising chemical paradigm of h-BNCO for designing more MR-TADF molecules; in terms of OLED devices, the impressive performance and stability obtained via a simple binary-EML structure with h-BNCO-1 demonstrated the potential for low-cost commercialization based on h-BNCO-type MR-TADF molecules.
This work titled “Efficient, narrow-band, and stable electroluminescence from organoboron-nitrogen-carbonyl emitter” was published in the Nature Communications.