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Godumala, Mallesham,Choi, Suna,Kim, Hyung Jong,Lee, Chiho,Park, Sungnam,Moon, Ji Su,Kim, Si Woo,Kwon, Jang Hyuk,Cho, Min Ju,Choi, Dong Hoon The Royal Society of Chemistry 2018 Journal of Materials Chemistry C Vol.6 No.15
<P>Correction for ‘Novel dendritic large molecules as solution-processable thermally activated delayed fluorescent emitters for simple structured non-doped organic light emitting diodes’ by Mallesham Godumala <I>et al.</I>, <I>J. Mater. Chem. C</I>, 2018, 6, 1160-1170.</P>
Godumala, Mallesham,Choi, Suna,Kim, Seong Keun,Kim, Si Woo,Kwon, Jang Hyuk,Cho, Min Ju,Choi, Dong Hoon The Royal Society of Chemistry 2018 Journal of Materials Chemistry C Vol.6 No.37
<P>New versatile host materials with good solubility in common organic solvents are greatly desired to promote solution-processable thermally activated delayed fluorescence organic light-emitting diodes. This paper describes the design and synthesis of two new host materials, namely 3′-(9<I>H</I>-carbazol-9-yl)-[1,1′-biphenyl]-3,5-diylbis(diphenylphosphine oxide) (CDPO) and 3′,5′-di-(9<I>H</I>-carbazol-9-yl)-[1,1′-biphenyl]-3,5-diylbis (diphenylphosphine oxide) (mCPDPO), featuring hole-transporting carbazole and electron-transporting diphenylphosphine oxide (DPO) entities. The donor strength was varied to a constant n-type DPO core to tune the functional properties. The detailed studies proved that the resulting two new materials exhibit excellent solubility in common organic solvents, high triplet energy (@@>@@2.80 eV), high glass transition temperature (up to 133 °C), and good bipolar electronic nature. Consequently, both compounds were used as hosts in solution-processable blue and green thermally activated delayed fluorescence (TADF) OLEDs. In particular, the devices using mCPDPO as a host in the emissive layer showed outstanding performance with a maximum current efficiency, power efficiency, and external quantum efficiency of 35.3 cd A<SUP>−1</SUP>, 17.0 lm W<SUP>−1</SUP> and 15.4% in the blue-emitting diode, and 61.5 cd A<SUP>−1</SUP>, 29.7 lm W<SUP>−1</SUP> and 18.8% in the green-emitting diode, respectively. These results corroborated the potential of carbazole-phosphine oxide derivatives as host materials in solution-processable blue and green TADF OLEDs.</P>
Godumala, M.,Choi, S.,Cho, M.,Choi, D. Royal Society of Chemistry 2016 Journal of Materials Chemistry C Vol.4 No.48
<P>Thermally activated delayed fluorescence (TADF) materials have attracted much attention in the field of organic light-emitting diodes (OLEDs), with their state-of-the-art performance in terms of external quantum efficiencies (EQEs), turn-on voltages, and color coordinates. TADF materials exhibited EQEs above 25% due to harvesting both singlet and triplet excitons via reverse intersystem crossing (RISC). A small singlet-triplet energy gap (Delta E-ST) is essential for TADF materials to exhibit efficient upconversion from the lowest triplet excited state (T-1) to the lowest singlet excited state (S-1, T-1 -> S-1). Moreover, these materials are purely organic and thus not costly. Therefore, the TADF approach provides the best alternative to conventional fluorescent and phosphorescent OLEDs, regarding device efficiency and cost. On the other hand, blue light-emitting devices are facing several issues related to their stability and efficiency, making their development quite challenging for researchers. Herein, we review the recent advances in the use of blue TADF dopants and hosts in OLEDs.</P>
Godumala, Mallesham,Choi, Suna,Cho, Min Ju,Choi, Dong Hoon Royal Society of Chemistry 2019 Journal of Materials Chemistry C Vol.7 No.8
<P>Thermally activated delayed fluorescence (TADF) materials have attracted significant attention owing to their state-of-the-art performance in the field of organic light-emitting diodes (OLEDs). TADF materials have the capability of harvesting both singlet and triplet excitons through reverse intersystem crossing, thus, the maximum internal quantum efficiency can be expected to be 100%. Thanks to TADF materials that are developed from pure organic entities so that they are inexpensive and external quantum efficiencies as high as 38% have been achieved to date. Despite the high performance of doped OLEDs, precise control of the host-guest ratio and phase separation at high driving voltages are the foremost challenges. In order to circumvent these demerits, the use of numerous small molecules, dendritic molecules, and polymers as non-doped emitters has been reported. Non-doped OLEDs consist of only a single molecule emitter in the emissive layer, thus simplifying the device fabrication process. Furthermore, non-doped OLEDs exhibit better device stability compared to doped devices, because the former do not exhibit phase separation or crystallinity and maintain a homogeneous surface film morphology during device operation. As a result, non-doped OLEDs are capable of overcoming the most pivotal issues of device complexity and stability, which makes them effective in rendering OLEDs commercially viable. Although maximum external quantum efficiencies of nearly 23.0% in blue, 22.0% in green, 22.0% in yellow, 17.0% in orange, and 10.0% in red emitting materials have been realized with non-doped OLEDs, still further progress is essential to reach the doped devices. This review systematically describes the molecular design, photophysical properties, and electroluminescence performance data of non-doped TADF emitters. We strongly believe that this review will be beneficial in terms of presenting a specific direction for the design of a variety of non-doped TADF emitters in the near future.</P>
Godumala, Mallesham,Choi, Suna,Park, Seo Yeon,Cho, Min Ju,Kim, Hyung Jong,Ahn, Dae Hyun,Moon, Ji Su,Kwon, Jang Hyuk,Choi, Dong Hoon American Chemical Society 2018 Chemistry of materials Vol.30 No.15
<P>New electron-acceptor cores are necessary for developing highly efficient bipolar hosts, particularly for blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, chromenopyrazole (CP) was used for the first time as an electron-acceptor core to design and synthesize two novel blue bipolar hosts, viz., 8-(9<I>H</I>-carbazol-9-yl)-3-methyl-1-phenylchromeno[4,3-<I>c</I>]pyrazol-4(1<I>H</I>)-one (<B>CzCP</B>) and 8-(9<I>H</I>-[3,9′-bicarbazol]-9-yl)-3-methyl-1-phenylchromeno[4,3-<I>c</I>]pyrazol-4(1<I>H</I>)-one (<B>2CzCP</B>). The influence of donor strength on the photophysical, electrochemical, and electroluminescent performances was systematically investigated. <B>CzCP</B> and <B>2CzCP</B> both maintain high triplet energy (∼3.0 eV), appropriate highest occupied and lowest unoccupied energy levels (HOMO/LUMO), and bipolar nature. Consequently, OLEDs containing <B>CzCP</B> as a host in the emissive layer exhibited state-of-the-art performance with external quantum efficiency of 27.9% and CIE color coordinates of (0.15, 0.21), thus achieving excellent performance among all reported blue host materials in TADF-OLEDs. This work highlights the importance of the CP unit in developing new host materials and paves the way for the realization of high-efficiency blue TADF-OLEDs.</P> [FIG OMISSION]</BR>
Godumala, Mallesham,Choi, Suna,Kim, Hyung Jong,Lee, Chiho,Park, Sungnam,Moon, Ji Su,Si Woo, Kim,Kwon, Jang Hyuk,Cho, Min Ju,Choi, Dong Hoon The Royal Society of Chemistry 2018 Journal of Materials Chemistry C Vol.6 No.5
<P>Solution-processable thermally activated delayed fluorescence (TADF)-assisted materials have been identified as promising materials for future applications as organic light-emitting diodes (OLEDs) owing to their numerous advantageous such as easy fabrication, large area applications, low cost, and state-of-the-art performance. Herein, two new carbazole-dendronized TADF emitters, namely, TB2CZ-ACTRZ and TB14CZ-ACTRZ, were designed and synthesized. Two different-sized carbazole dendron wedges were utilized as the encapsulating groups for the TADF core <I>via</I> methylene groups. The influence of the encapsulated dendrons on the thermal, optical, electrochemical, and OLED device performances of both dendritic molecules was studied in detail. The photophysical studies of TB2CZ-ACTRZ and TB14CZ-ACTRZ disclosed their extremely small singlet-triplet energy gaps (Δ<I>E</I>ST) of 79 and 134 meV, respectively. Consequently, the solution-processed non-doped OLEDs without any hole injection/transport layers featuring TB2CZ-ACTRZ and TB14CZ-ACTRZ as the TADF emitters demonstrated the maximum external quantum efficiencies (EQEs) of 9.5 and 8.1%, respectively, while the device fabricated with their simple emissive core ACTRZ had an EQE of only 1.2%. These results clearly demonstrated that the development of multifunctional TADF dendritic emitters is an extremely worthwhile objective for the realization of highly efficient solution-processable non-doped OLEDs with simple device architectures.</P>
Optimized structure of silane-core containing host materials for highly efficient blue TADF OLEDs
Choi, Suna,Godumala, Mallesham,Lee, Ji Hyung,Kim, Gyeong Heon,Moon, Ji Su,Kim, Jun Yun,Yoon, Dae-Wi,Yang, Joong Hwan,Kim, Jinook,Cho, Min Ju,Kwon, Jang Hyuk,Choi, Dong Hoon Royal Society of Chemistry 2017 Journal of Materials Chemistry C Vol.5 No.26
Jeong, Cheol Hun,Godumala, Mallesham,Yoon, Jiwon,Choi, Suna,Kim, Yong Woo,Choi, Dae Hyuk,Cho, Min Ju,Choi, Dong Hoon American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.19
<P>A new side-chain polymer (<B>X-TPACz</B>) bearing hole-transporting pendant groups accompanying a thermally crosslinkable entity was synthesized using <I>N</I>-([1,1′-biphenyl]-4-yl)-<I>N</I>-(4-(9-(4-vinylbenzyl)-9<I>H</I>-carbazol-3-yl)phenyl)bicyclo[4.2.0]octa-1(6),2,4-trien-3-amine (<B>6</B>) via addition polymerization. The <B>X-TPACz</B> could be spontaneously crosslinked without using any further reagents and showed a good film-forming property upon low-temperature thermal treatment. The thermal curing temperature for the <B>X-TPACz</B> film was optimized to be 180 °C based on a differential scanning calorimetry thermogram. Moreover, the thermal degradation temperature of <B>X-TPACz</B> measured to be over 467 °C using thermogravimetric analysis demonstrated that it shows excellent thermal stability. In particular, <B>X-TPACz</B> exhibits the highest occupied molecular orbital (HOMO) energy level to be −5.26 eV, which is beneficial for facile hole injection and transportation. Consequently, the thermally activated delayed fluorescence organic light-emitting diodes fabricated using <B>X-TPACz</B> as the hole-transporting material showed state-of-the-art performances with a low turn-on voltage (<I>V</I><SUB>on</SUB>) of only 2.7 V and a high external quantum efficiency (EQE) of 19.18% with a high current efficiency (CE) of 66.88 cd/A and a high power efficiency (PE) of 60.03 lm/W, which are highly superior to those of the familiar poly(9-vinylcarbazole) (PVK)-based devices (<I>V</I><SUB>on</SUB> = 3.9 V, EQE of 17.42%, with CE of 58.33 cd/A and PE of 33.32 lm/W). The extremely low turn-on voltage and high EQE were found to be due to the higher-lying highest occupied molecular orbital energy level (<I>E</I><SUB>HOMO</SUB> = −5.23 eV) and better hole-transporting property of <B>X-TPACz</B> than those of PVK.</P> [FIG OMISSION]</BR>