Zirconium Tetrakis(8-hydroxyquinolinolate) and Lithium Schiff-Base Cluster Complex for Efficient Charge Injection and Tr
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.478
Zirconium Tetrakis(8-hydroxyquinolinolate) and Lithium Schiff-Base Cluster Complex for Efficient Charge Injection and Transfer in Green PHOLED processed by OVPD Gintautas Simkus1,2, Pascal Pfeiffer1, Simon Sanders1, Dominik Stümmler1, Peter K. Baumann3, Sivagnansundram Surendrakumar4, Muttulingam Kumaraverl4, Maxson Liu4, Seenivasagam Ravichandran4, Poopathy Kathirgamanathan4, Andrei Vescan1, Holger Kalisch1, Michael Heuken1,2 1 Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074 Aachen, Germany. 2
AIXTRON SE, Dornkaulstr. 2, 52134 Herzogenrath, Germany.
3
APEVA SE, Dornkaulstr. 2, 52134 Herzogenrath, Germany.
4
Organic Electronics, Wolfson Centre, Brunel University London, Uxbridge, UB8 3PH, UK.
ABSTRACT
Typical electron transport (2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)) and injection (Cs2CO3) materials are successfully replaced by zirconium tetrakis(8hydroxyquinolinolate) (Zrq4) and lithium 2-((o-tolylimino)methyl)-phenolate (EI-111) in simplified OLED (organic light-emitting diodes) processed by organic vapor phase deposition (OVPD). The performance of combining Zrq4 and EI-111 is analyzed in unipolar devices and compared to devices with configurations of Zrq4/Cs2CO3, TPBi/EI-111 and TPBi/Cs2CO3. Current density-voltage (J-V) measurements are performed and correlated to different carrier injection and transport properties. The investigated material combinations are implemented in the simplified OLED structures and compared to each other. To account for the high HOMO level of Zrq4, 5 nm of TPBi are added to confine holes and excitons in the emissive layer (EML) and to improve device performance. After tailoring the organic stack for OLED with Zrq4, a remarkable boost in device efficiency is observed. The luminous efficacy increased from 3.0 to 21.9 lm/W and the EQE from 2.1 to 11.0 % for a device with Zrq4/EI111. Furthermore, OLED having Zrq4/Cs2CO3 show an even greater enhancement to 26.3 lm/W and 11.7 %.
INTRODUCTION Over the past decade, tremendous advances in the area of OLED have been achieved, mainly through novel efficient charge-transporting materials [1,2] and improved device architectures [3]. However, the performance and cost of large-area
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OLED are critical issues at this time. Conventional vacuum thermal evaporation (VTE) tools have a relatively low material utilisation efficiency and require a high vacuum in the evaporation chamber [4]. Considering these drawbacks, OVPD technology providing fast, efficient and uniform deposition of organic semiconductors is a promising candidate for the production of large-area light sources and displays. Transferring a process from VTE to OVPD is not trivial though, beca
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