Emitting layer thickness dependence of color stability in phosphorescent organic light-emitting devices
Introduction
Organic light-emitting device (OLED) has heralded a new chapter in chemistry and physics since a stack of organic thin films was demonstrated to be an efficient light-emitting structure at the end of the twentieth century [1]. During the early period of development, monochromatic OLED was one of the most hotly researched topics. Through the evolution of fluorescent to phosphorescent materials for enhancing the radiation efficiency [2] and the introduction of specific thin films for engineering carrier injection and transport [3], [4], [5], researchers have aggressively pushed to boost the efficiency of monochromatic OLED to its ultimate physical limits. Nowadays, the thriving market for consumer electronics (e.g. monitors) and power-saving lighting devices have further triggered a strong demand for white OLED (WOLED) due to its ultra-thin and high-quality surface emission [6], [7], [8], [9], [10], [11].
In addition to higher device efficiency for WOLED, higher color stability across a wider luminance range is also an important consideration for commercial products. Since the electroluminescence (EL) of a WOLED is in general determined by at least two different emitting materials, high color stability can theoretically be accomplished as long as the exciton competition among the emitting sites is delicately managed [7], [11], [12], [13], [14], [15], [16]. For instance, a dichromatic or trichromatic OLED with a stack of different emitting layers (EML) generally suffers from lower color stability due to the variations in exciton distribution at different driving voltages [17], [18], [19]. In this case, a spacer inserted in between the different EMLs can greatly alleviate color variation by stabilizing the exciton distribution [7], [9], [14], [20]. Instead of using spacers, adopting phosphorescent sensitization as the main emission mechanism or fabricating independent emitting units in a pixel open up another possibility for stabilizing the color emission [14], [21]. In this paper, we investigated the dependence of the color stability of green–blue OLED on the thickness of the EML and measured the carrier mobilities of N,N′-dicarbazolyl-3, 5-benzene (mCP) and 4,7-diphenyl-1,10-phenanthroline (BPhen). On account of the stronger electric field dependence of the carrier mobility of mCP than that of BPhen, and the larger voltage drop across the blue-EML (B-EML) than across the green-EML (G-EML) within the devices, the higher sensitivity of the EL spectrum to the applied driving voltage in the device with a thinner EML was attributed to the sharply increasing electric field. For an optimal device, the CIE coordinates were slightly shifted from (0.256, 0.465) to (0.259, 0.467) with increased luminance from 48.7 to 12,700 cd/m2. Based on the backbone of this green–blue OLED, a highly efficient WOLED with maximum efficiencies of 26.4 cd/A and 19.8 lm/W was fabricated. It also exhibited high color stability, that is, the CIE coordinates shifted from (0.310, 0.441) to (0.318, 0.446) in the practical luminance range from 1050 to 9120 cd/m2.
Section snippets
Experiments
Indium-tin-oxide (ITO) glass with sheet resistance of 30 Ohm/sqr and thickness of 100 nm was used as the substrate. The active region size of a test pixel was 2 × 2 mm2 and the fabrication process consisted of pre-cleaning of the ITO, thermal deposition of the materials, and finally encapsulation with a glass cap [22]. The pre-cleaning process of the ITO-coated glass adhered to a standard solvent cleaning procedure, followed by an oxygen plasma treatment. The thermal deposition was performed in a
Electrical properties of dichromatic (B + G) OLEDs with different B-EML thicknesses
In a blue–green dichromatic OLED, excitons would naturally be formed in or diffuse toward the G-EML due to its lower exciton energy. This EML structure often exhibited higher green emission at lower carrier injection conditions due to the abovementioned nature of the excitons, but lower green emission at higher carrier injection conditions due to the saturation of the emitting sites in the G-EML or the shift of the carrier recombination zone toward the B-EML [14]. These carrier injection
Conclusion
Due to the relatively larger voltage drop across a thinner B-EML, stronger electric field dependence of the electron mobility of mCP, and energy barrier lowering at the B-EML/ETL and spacer/B-EML interfaces, it was suggested that the recombination zone extended into the G-EML for devices with a thinner B-EML as the applied voltage was increased. The extension of the recombination zone in conjunction with more significant TTA in the thinner B-EML resulted in a rather low color stability.
Acknowledgements
This work was supported by the National Science Council of Taiwan, under Grant No. NSC 98-2221-E-002-038-MY3. We also acknowledge the financial support by Chung-Shan Institute of Science and Technology of the Republic of China.
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2017, Organic ElectronicsCitation Excerpt :We prepare the DMAC-DPS devices with either mCP or NPB/mCP hole transport layer i.e. the devices with the structure of ITO/MoO3 (5 nm)/HTL (40 nm)/DMAC-DPS (30 nm)/DPEPO (10 nm)/TmPyPB (40 nm)/CsF (1 nm)/Al (150 nm), in which HTL is NPB (30 nm)/mCP (10 nm) (device A) or mCP (40 nm) (device B)and the properties of the devices are shown in Fig. 1. The current density of device A at a given voltage is smaller compared to that of device B, which can be attributed to the presence of a 0.7 eV energy barrier for hole injection from NPB into mCP layer (Scheme 1) despite that the hole mobility of NPB (8.8 × 10−4 cm2V−1s−1) [28] is larger than that of mCP (1.2 × 10−4 cm2V−1s−1) [29]. Device A and B show similar EQEs.