Can A Double-Doped Device Modification of A Standard Bilayer OLED Improve the Photo- And/or Electro-luminescence Efficiency? A Case Study of Architecture Design in Fluorescent Devices with A Potential Roadmap for High-Efficiency Phosphorescent Devices

ABSTRACT

This paper provides the sixth manifestation of a new tradition by which the editors of Comments on Inorganic Chemistry wish to lead by example, whereby we start publishing original research content that, nonetheless, preserves the Journal’s identity as a niche for critical discussion of contemporary literature in inorganic chemistry. (For the previous manifestations, see: Comments Inorg. Chem. 2018, 38, 1–35; 2019, 39, 1–26; 2019, 39, 188–215; 2020, 40, 1–24; 2020, 40, 277–303.) Coordination compounds are responsible for multiple quantum leaps in the performance of organic light-emitting diodes (OLEDs). The first breakthrough was via the green-fluorescent main-group complex tris-(8-hydroxyquinoline)aluminum (Alq₃) which acts as both light-emitting and electron-transporting material in combination with triarylamine as a hole-transporter. To optimize the performance of such standard bilayer devices, herein we provide a double-doped structure into the emissive region consisting of 20 nm N,N’-diphenyl-N,N’-bis(1,1ʹ-biphenyl)-4,4ʹ-diamine (NPB) and 10 nm Alq₃ utilized as buffer layers for facilitating charge injection from the electrodes, and a broad emissive region stacked by two doped layers with a 5% Alq₃ doped in a 50-nm thick NPB layer – as well as a 5% NPB doped in a 40-nm-thick Alq₃ layer from the anode side to the cathode side. The double-doped device achieves a decreased turn-on voltage of 2.44 V and maximum brightness of 17,300 cd/m² as well as enhanced electroluminescence efficiency and moderately reduced efficiency roll-off over single-doped and standard bilayer devices. We have also found ~50% improvement of the photoluminescence quantum yield, with some subtle color shift upon doping 10% of NPB or Alq₃ into the other vs. neat Alq₃ (~0.3 vs. ~0.2 ${\varphi }_{PL}$) which nonetheless led only to ~20% improvement in EQE (~1.0% vs. ~0.8%), suggesting additional device optimization is warranted. Furthermore, two typical fluorescent OLEDs architectures – a graded or uniformly mixed device – have been exploited, which together with the double-doped approach would be feasible to boost EL efficiencies in both fluorescent and phosphorescent OLEDs with neat bilayer structures. The approach is not suitable for the more common doped phosphorescent devices, the optimization of which has been reviewed earlier by Nazeeruddin and coworkers in this Journal (Comments Inorg. Chem. 2017, 37, 117–145); in combination with this article, we hope that the reader will have an educational experience on OLED design and optimization from an inorganic chemistry perspective vis-à-vis a materials science perspective that dominates the OLED literature.

GRAPHICAL ABSTRACT

KEYWORDS:

• tris-(8-hydroxyquinoline)aluminum (Alq₃)
• double-doping
• electroluminescence
• photoluminescence
• fluorescent OLEDs

Acknowledgments

M.A.O. thanks the Robert A. Welch Foundation (Grant B-1542) and the National Science Foundation (Grant CHE-1413641) for supporting this research to his research group at the University of North Texas. A.-M.M.R. and M.A.O. also acknowledge support of their OLED-related collaborative efforts at Yarmouk University by the Scientific Research Support Fund (SRSF) of the Ministry of Higher Education and Scientific Research in Jordan, including the acquisition of the Trovato vacuum deposition system for organic device fabrication and supporting Master’s thesis completion work of two Yarmouk University graduate student authors herein (M.K. and D.A.) under their co-supervision. A.-M.M.R. and M.A.O. also thank the Abdul Hameed Shoman Foundation (Grant 1/2019) and Yarmouk University/Deanship of Scientific Research and Graduate Studies for additional funding of infrastructure and research activities in Jordan. C.M.B. acknowledges support of her contribution by the National Science Foundation’s Research Experience for Undergraduates (REU) program (Grant CHE-1461027). C.A.M.’s work is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1746050. This work was performed in part at the Materials Research Facility at the University of North Texas, a shared research facility for multi-dimensional fabrication and characterization, with the assistance of its cleanroom facility manager, Dr. Jianchao Li. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Disclosure statement

No potential conflict of interest was reported by the author(s).