# Synthesis and Electrical Properties of a New Bipolar Material Using Spacer Moiety
## 1 Introduction  
Since the introduction of organic light-emitting diode (OLED) technology in 1987 by Dr. [PERSON], researchers have focused on improving the efficiency and lifetime of OLED displays while reducing manufacturing costs [1]. OLEDs offer many advantages in terms of their light weight, thin panel construction, wide viewing angle, high color saturation, and fast response time. These properties have led to their use in numerous products, including smartphones, televisions, and wearable devices. In an attempt to achieve optimal full-color display, considerable effort has been expended regarding the identification of red-, green-, and blue-emitting materials with excellent performance [2]. In particular, there is an increasing need for deep-blue materials that satisfy the Commission Internationale de l'Eclairage (CIE) coordinates (0.14, 0.08) for deep blue, which was established by the National Television System Committee for OLED televisions and applications [3, 4]. Thus, there is active research regarding high-efficiency deep-blue dopants with bipolar characteristics. In particular, conjugated materials with such bipolar characteristics are crucial for enhancing charge transport and the efficiency of charge transfer in organic semiconductors. Emitting materials with bipolar characteristics are commonly composed of donor-acceptor structures. In these bipolar structures, it is difficult to realize deep blue because intramolecular charge transfer between donors and acceptors effectively reduces the band gap. However, deep blue can be achieved using the following three strategies (i-iii): (i) enhancement of steric hinderance between donors and acceptors; (ii) maintenance of an appropriate distance between the two moieties using a donor-spacer-acceptor type; and/or (iii) reduction of electron donating and accepting capacities [5, 6, 7, 8]. Without these strategies, it is difficult to achieve deep-blue emission with bipolar materials. However, bipolarcharacteristics are necessary in that general polycyclic aromatic hydrocarbon materials have more rapid hole mobility than electron mobility, hindering charge balance maintenance in OLED devices [9; 10]. Considering that charge injection and transport in OLED devices are the main factors that affect electroluminescence (EL) performance, bipolar characteristics that facilitate charge injection and movement are required. Additionally, bipolar materials maintain excellent charge balance at high luminance by efficiently generating excitons while exhibiting low roll-off [11]. [PERSON]'s group developed a donor-spacer-acceptor-type dopant composed of carbazole and 1H-phenanthro[9;10-d]imidazole, then separated the carbazole and imidazole components by introducing biphenyl as a spacer; non-doped devices with this as-described dopant showed an external quantum efficiency (EQE) of 6.0% and CIE coordinates of (0.16, 0.08) [12]. [PERSON] et al. reported bipolar material fabrication using mCP as the backbone while substituting 2,4-diphenylpyrimidine; a photoluminescence maximum (PLmax) value of 426 nm was observed in the film state for application as a green host, indicating that bipolar characteristics were utilized with excellent charge balance [13]. [PERSON]'s group synthesized two materials consisting of a symmetrical D-\(\pi\)-A-\(\pi\)-D structure (CzS1, CzS2); the OLED device using these two materials showed EQEs of 4.2% and 2.7% and CIE coordinates of (0.157, 0.055) and (0.157, 0.44), respectively [14]. In the present study, a novel deep-blue bipolar emitter of the donor-spacer-acceptor type (D-\(\pi\)-A), 9-(4-(4,6-diphenyl-1),3,5-triazin-2-ylyl)-2,5-dimethylphenyl)-9H-carbazole or DTPCZ, was designed and successfully synthesized. Carbazole (hereinafter, Cz) was used as the donor; Cz is the most commonly used donor moiety because it is cheap and easy to synthesize, and it exhibits excellent hole transport properties.