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Highly efficient deep blue fluorescent material (SiPIM) based on phenanthro[9, 10-d]imidazole (PPI) and triphenylsilane is designed and synthesized. SiPIM presents a narrow deep blue emission, high quantum yield, high thermal stability and good morphological stability. A non-doped vacuum-deposited device using SiPIM as active layer achieves an extremely high external quantum efficiency of 6.29% with true deep blue CIE coordinates of (0.163, 0.040). The solution-processed device is also tried due to the good solubility of SiPIM, which displays a maximum ηext of 2.40% and CIE coordinates of (0.157, 0.041).
Deep blue color is usually defined as the blue electroluminescence (EL) emission with a Commission International de L'Eclairage (CIE) coordinate of y < 0.08 according to the National Television System Committee and/or y < 0.06 according to the European Broadcasting Union (EBU) standard . The development of deep blue emitters is especially important for organic light emitting diodes (OLEDs), because deep blue emission can efficiently reduce the power consumption in full-color displays , i.e. the lower the y coordinate, the less the power consumption. In addition, deep blue emitters can be used to generate light of all colors by energy transfer to emissive dopants . Tremendous efforts have been devoted to investigate different kinds of non-doped deep blue fluorescent emitters, such as anthracene, fluorene, dimesitylborane and phenanthro-imidazole. It has been found that most of the excellent deep blue emitters exhibit very high fluorescence quantum efficiency (Φf) in dilute solution, such as 9,10-diphenylanthracene . But they tend to crystallize in the solid state, failing to afford homogeneous films. And some non-doped blue OLEDs usually turn on at voltages as high as 6-7 V resulting in low power efficiency. Even if some emitters have high Φf in solid state, achieving pure deep blue chromaticity is still difficult. For instance, the long-wavelength emissions are often observed in fluorene-based devices, which results in poor color purity . Up to now, there have been only a few reports on deep blue OLEDs with high efficiency and satisfactory CIE chromaticity. For example, Lin et al. reported a new carbazole-π-dimesitylborane bipolar fluorophore with color coordinates of (0.15, 0.09) and the maximum external quantum efficiency (ηext) of 4.3% . In 2011, Tong et al. reported a triphenylamine substituted PPI derivative exhibiting excellent CIE coordinates of (0.15, 0.09) with ηext of 3.08% . Also, S. Y. Park published a deep blue emitter based on anthracene with the maximum ηext of 4.61% and CIE coordinates of (0.15, 0.09) in 2012 . Particularly, the recent report by Kim et al. showed that a non-doped device using a highly twisted asymmetric anthracene emitter displayed a maximum ηext of 4.62% with CIE coordinates of (0.154, 0.049) . One reason for the scarcity of reports is that the wide energy bandgap of deep blue material would confine the molecular size to render a limited π-conjugation length, which leads to the difficulty in choosing efficient deep blue emitting building block. In addition, fluorescence can be easily quenched in the solid state due to aggregation, which usually results in the decreased efficiency and emission out of the deep blue spectral region in OLEDs. More importantly, it is difficult to simultaneously inject electrons and holes into such wide energy bandgap emitters since the restriction in π-conjugation length would result in the reduction of carrier injection and transport properties and hamper the devices performance. Overall, high performance deep blue light emitting materials are still rare, and the design for the blue emitters with good thermal stability, high efficiency, and reliable color purity is quite challenging .
Recently, we have successfully demonstrated a novel deep blue emitting material (PPI) possessing a wide energy bandgap of 3.46 eV, deep blue light emission in film (393 nm), and narrow full width half maximum (FWHM) (40 nm) in OLEDs , which provide a basic structure for color tuning within deep blue spectral region. PPI also exhibits good carrier injection and transport properties. However, its low glass transition temperature (Tg) of 62 °C, moderate Φf of 40% in the solid state and poor film-forming ability is far from being adequate for application in deep blue OLEDs. To address this problem, it appears rational to improve the thermal stability by enlarging the molecular size, to increase the Φf without red-shifting its deep blue emission, and to achieve the amorphous thin film by improving flexibility, while the wide bandgap and excellent carrier injection and transport characteristics should be sustained simultaneously.
In this paper, we describe a strategy to achieve this goal by introducing a triphenylsilane segment into PPI. Compared with PPI, the new wide bandgap compound SiPIM has following characteristics and advantages. Firstly, the non-conjugated triphenylsilane can maintain the wide bandgap of PPI to afford emission within deep blue spectral region . Secondly, triphenylsilane with rigid bulky groups can enlarge the molecular size, which would provide high thermal stability. And the tetrahedral geometry of triphenylsilane could increase the steric hindrance, suppress intermolecular interactions and enhance the efficiency in solid state. Thirdly, silicon-carbon bond is longer than carbon-carbon bond due to the bigger atom radius of silicon, which could effectively increase the flexibility of the molecular to help conserve better morphological stability. As a result, a non-doped device using the new deep blue emitter as active layer displays a maximum ηext of 6.29% with true deep blue CIE coordinates of (0.163, 0.040). To the best of our knowledge, this newly synthesized material shows the deepest blue among reported OLEDs with extremely high efficiency and satisfactory color purity.
SiPIM used for this study was readily synthesized by the one-pot reaction with high yield according to our reported procedure , and the molecular structure was shown in Scheme 1. The molecular structure was confirmed by 1H and 13C NMR, high-resolution MS, FTIR, elemental analysis, and corresponded well with its expected structure.
SiPIM exhibited higher thermal stabilities than PPI on account of the introduction of triphenylsilane segment. The decomposition temperature (5% weight loss) of 430 °C for SiPIM was observed, which was 113 °C higher than PPI (317 °C). The Tg of SiPIM was measured to be at 121 °C (Fig. 1). SiPIM showed nearly 2-folded higher Tg than that of PPI (62 °C) with a rigid structural feature. Upon further heating beyond Tg, an exothermal peak was observed at 209 °C, which was ascribed to the crystallization temperature (Tc). The melting temperature (Tm) was observed at 268 °C. Compared with PPI, the Tc and Tm of SiPIM were increased by 80 °C and 60 °C, respectively. The high Tg value implies that SiPIM could form morphologically stable films upon thermal evaporation, which is highly important for its application in devices. AFM characterization presented that the vacuum-deposited film of SiPIM displayed fairly homogenous and smooth surface morphology with the root mean square roughness of 0.47 nm. After annealing at 90 °C for 3 h, the morphology of film was almost kept unchanged. In contrast, the surface of PPI film showed noticeable crystallization area after annealing. As we all known, the crystallization could induce the molecular self-aggregation. These results indicated that introduction of triphenylsilane could disrupt intermolecular interactions and suppress aggregation effectively in solid state, which is very important to achieve high Φf and pure blue chromaticity. Meanwhile, introduction of triphenylsilane segment also afforded the material with solution-processability. Its solubility could attain as high as 30 mg mL−1 in toluene, suggesting the greatly improved flexibility of the material compared to that of PPI. The thermal stability, film morphology and solubility of SiPIM had been successfully modified for application in deep blue devices with the rational design of molecular structure.
SiPIM showed quite similar absorption and emission spectra to PPI in dilute THF solution and the solid state, indicating that the non-conjugated triphenylsilane segment in SiPIM did not affect the conjugation length. The maximum absorption peak at 262 nm was attributed to the isolated benzene ring connected with imidazole (Fig. 2). The absorption band around 340 nm of SiPIM might be originated from the π–π* transition of PPI unit. The bandgap of SiPIM was calculated to be 3.28 eV according to the absorption edge in the film state. The emission peaks of PPI and SiPIM were all located at 372 nm and 394 nm in dilute THF solution. It is noteworthy that they all exhibited narrow emission with FWHM of only 44 nm in the spectra, which is helpful for obtaining saturated color with a low y coordinate in OLEDs. Very high Φf of 81% for SiPIM was observed in thin film, indicating that the bulky triphenylsilane moiety can effectively block the quenching process of PPI. Such high Φf was rarely observed for wide bandgap organic deep blue emitters .
Figure 2. Normalized absorption and emission spectra of SiPIM in THF (concentration: 10−5 mol L−1) and in thin film.
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Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were assessed by cyclic voltammetry . SiPIM exhibited one quasi-reversible oxidation wave corresponding to HOMO level of −5.57 eV (Fig. S6). The LUMO level of SiPIM was calculated to be −2.29 eV. In comparison, the HOMO and LUMO levels of PPI were calculated to be −5.53 eV and −2.07 eV, respectively. The LUMO level of SiPIM was much lower than that of PPI, suggesting a beneficial effect for the electron injection in devices. This could be attributed to the partly overlapping between π-π* conjugation in PPI and the d orbital of silicon atom. This effect was often observed in silicon containing compounds and was revealed by our theoretical calculation results (Fig. S7) .
The non-doped vacuum-deposited device was fabricated with a typical sandwiched structure of ITO/MoO3 (10 nm)/TAPC (40 nm)/SiPIM (20 nm)/TmPyPb (40 nm)/LiF (0.5 nm)/Al (100 nm) to investigate the potential application of the material in deep blue OLEDs. The device exhibited deep blue EL spectra similar to the photoluminescent spectra of the thin films. The maximum peak of EL emission was at 420 nm with very narrow spectra distribution. The FWHM was only 60 nm, which effectively guaranteed saturated deep blue color. The CIE coordinates of (0.163, 0.040) was obtained, in which the y coordinate was much smaller than 0.06 and matched well with the requirement of EBU standard blue CIE coordinates. Significantly, at 100 cd m−2 the CIE coordinates were still kept at (0.179, 0.056), indicating excellent color stability for the non-doped deep blue OLEDs. Such high color purity has not yet been reported as far as we know. OLEDs containing wide bandgap emitters generally led to difficulty in carriers injection and caused the high turn-on voltage of devices. Here, this device showed a relatively low turn-on voltage of 4.2 V, and low operation voltage (100 cd m−2 at 7 V).
The vacuum-deposited device utilizing SiPIM as emissive layer showed an exceedingly enhanced maximum ηext of 6.29% with the current efficiency (ηc) of 1.94 cd A−1 (Table 1). This high EL efficiency is consistent with its high Φf in solid state, appropriate flexibility and high Tg. It is also suggested that the present device structure could effectively confine the carriers and excitons in the emissive layer. Moreover, it is worthy to note that the device showed very low roll-off of the luminance efficiency. The results further demonstrated that the strategy of molecular design is feasible. To our knowledge, however, there has been no report to date, a material exhibiting the color purity with the y value below 0.05 and high ηext exceeding 6% at the same time.
Table 1. Performance of the vacuum-deposited and the solution-processed devices for SiPIM
|SiPIM[V]||420||4.2||4.73, 6.29||1950||1.94||(0.163, 0.040), (0.179, 0.056)|
|SiPIM[S]||416||5.5||2.36, 2.40||630||0.80||(0.157, 0.041), (0.157, 0.042)|
Because of the good solubility of SiPIM, we also tried to fabricate the device by solution-processed method with the structure of ITO/PEDOT:PSS/SiPIM (60 nm)/TmPyPb(5 nm)/TPBi(40 nm)/LiF(1.2 nm)/Al(150 nm). The device also exhibited high performance with a maximum ηext of 2.40%, ηc of 0.80 cd A−1, and CIE coordinates of (0.157, 0.041). The EL emission spectra were similar to that of the vacuum-deposited device. In particular, the EL spectra and CIE coordinates were kept unchanged over the entire applied voltage from 6 V to 11 V. To our knowledge, the performance of this deep blue device based on SiPIM is one of the best performances of the solution-processed small-molecule devices reported so far. Due to the low visibility of deep blue and near-ultraviolet emission, and our limited testing equipments (PR650), emission less than 380 nm can't be detected (Fig. 3). This means that the real efficiency would be 20% higher than the result shown here.
Figure 3. (a) EL emission of vacuum-deposited and solution-processed devices at 6 V. Insert: CIE coordinates. (b) The external quantum efficiency and current efficiency of vacuum-deposited and solution-processed devices for SiPIM.
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In summary, a new deep blue emitter, SiPIM, using phenanthro[9, 10-d]imidazole and triphenylsilane as the basic building blocks, was obtained. The non-doped vacuum-deposited device based on SiPIM achieves extremely high ηext of 6.29% (1.94 cd A−1) with true deep blue CIE coordinates of (0.163, 0.040). Its solution-processed device also provides excellent color purity and high EL efficiency. It can be distinctly ascribed to the introduction of triphenyl-substituted silane, which entitles SiPIM with promising physical properties, including high quantum yield of deep blue emission, appropriate flexibility, and high thermal and morphological stabilities. In addition, this intriguing material firstly demonstrates the deep blue emitter which can be processed by both solution-processed and vacuum-deposited methods. All of these results gives us a new foreground for the design of deep blue material and inspire its application in the future.