Organic vapor phase deposition (OVPD) technology, providing fast, particle-free, uniform deposition on large substrates, is a potential candidate for the production of large-area displays. Nevertheless, classical vacuum thermal evaporation (VTE) materials for electron injection layers (EIL) such as LiF or Cs2CO3 cannot be used in pure OVPD processes because of their high sublimation temperatures. This limits the choices for combinations of efficient EIL and transport layers (ETL) in OLED fabrication by OVPD. Kathirgamanathan et al.  have reported on a novel ETL material – zirconium tetrakis(8-hydroxyquinolinolate) (Zrq4) – combined with lithium Schiff-base cluster complexes as EIL, which show comparable performance to LiF and 8-hydroxyquinolinolato lithium (Liq). Due to the oligomeric nature (clusters) of lithium complexes, such molecules can be evaporated at a relatively low temperature of 200 – 400 oC.
In this work, we examine lithium 2-((o-tolylimino)methyl)phenolate (EI-111-2Me) and Zrq4, as EIL and ETL, respectively, in green PHOLED processed by an AIXTRON Gen1 OVPD tool. For comparison, reference devices having TPBi as ETL and Cs2CO3 as EIL (deposited by VTE) are fabricated. All devices are prepared on pre-structured ITO-on-glass substrates with a 2 nm layer of MoOx on the ITO surface to enhance hole injection. The OLED stack consists of a graded Ir(ppy)3/CBP organic layer serving as combined transport and emission layer. After depositing ETL and EIL, an opaque Al (120 nm) cathode is formed by VTE. To investigate the impact of EI-111-2Me and Cs2CO3 on electron injection, unipolar devices consisting of 150 nm undoped Zrq4 in combination with the respective EIL are produced. Electron-only devices of Zrq4 show a significant improvement in electron injection with reduced turn-on voltages from 11.1 V down to 4.4 V or to 4.3 V after introducing EI-111-2Me (1 nm) and Cs2CO3 (2 nm) as EIL, respectively.
Reference ETL and EIL composed of TPBi and Cs2CO3 are successfully replaced by Zrq4 and EI-111-2Me in the PHOLED stack. However, the HOMO level of Zrq4 is not sufficient for an effective confinement of holes in the emissive layer, therefore an additional 5 nm hole-blocking layer of undoped TPBi is added. After implementation of this hole-blocking layer, the luminous efficacy strongly increases from 7.7 to 21.9 lm/W and the EQE from 2.1 to 11.0 % in the investigated PHOLED containing Zrq4 and EI-111-2Me. Further enhancement of emission properties to 26.3 lm/W and 11.7 % is observed for OLED with Zrq4 and Cs2CO3 (all values at 1000 cd/m2). Obtained values are comparable to those of the reference PHOLED with TPBi and Cs2CO3 (24.4 lm/W and 11.2 %), whereas combining TPBi and EI-111-2Me yielded inferior results.
1 Kathirgamanathan P. et al., J. Mater. Chem., 2012, 22, 6104-6116
The research leading to these results has received funding from the European Union‘s Horizon 2020 Research and Innovation Programme under grant agreement No 674990 (EXCILIGHT).