Magnesium-silver cathodes for efficient charge injection into Organic Light Emitting Diodes deposited by LTVA method
Graphical Abstract
Introduction
One of the key challenges in the field of high-quality organic light-emitting diodes (OLED) is the improvement of the optical and electrical characteristics of the metallic cathodes. A thin metallic cathode ensures light emission in both directions, through the anode and the cathode in the so-called transparent electrode OLED. To avoid the increase of driving voltage in OLED applications, a low sheet resistance is required when the cathode used is formed as a thin metallic film. Highly efficient electron injection from silver (Ag) is driven by the addition of alkaline and alkaline earth metals. While alkaline metals are highly hygroscopic, the use of alkaline earth metals, such as Mg, is more convenient.
The applications of functional nanoparticles (NPs) to optoelectronic devices, such as metallic oxide NPs, are of high significance in OLEDs as they provide feasible solutions to the enduring challenges related to higher device emission efficiency [1]. Some metallic oxides, like ferrite compounds, can influence the charge transport in optoelectronic devices balancing the electron and hole transport across the active layer to increase the recombination processes and hence the radiative processes [2], [3], [4], [5], [6], [7], [8], [9]. The roles of NPs in OLEDs, including the improvement of carrier injection abilities, local surface plasmonic resonance, light scattering, and magnetic field effects are discussed together with their latest research progress.
For more complicated optoelectronic devices, metallic oxides like aluminum oxide phosphate (ALPO) can be successfully used as gate dielectrics [10], [11], [12], [13], [14], [15] or as blocking layers [16].
Starting with the early paper of Tang et al., the usage of Mg:Ag cathodes decreased the driving voltage from the 100 V used in anthracene-based organic electroluminescent (EL) devices to achieve a significant light output [17], [18], [19], [20], [21]. OLEDs based on an 8-hydroxyquinoline aluminum (Alq3) emissive layer have the top electrodes formed as an alloy or a mixture of magnesium and silver with an atomic ratio of 10:1. The Mg:Ag cathodes were co-evaporated from two separate sources [21]. The driving voltage was reduced down to 5–10 V with an EL between 10–3 and 1 mW cm–2. The low work function of magnesium is well suited for electron injection into organic materials. While pure Mg cathodes are susceptible to atmospheric oxidation and corrosion, the alloys of magnesium with silver were found to retard these degradation processes.
The optimized Mg:Ag cathode with 1:10 ratio (wt%) shows a sheet resistance value Rsh as low as 5.2 Ω/sq, an average transmittance of 49.7%, a reflectance of 41.4%, and absorbance of 8.9% over the visible spectral region [22]. The work function varies between 4.0 eV in 10:1 Mg:Ag samples to 4.7 eV in 1:10 samples for a 20 nm thin layer. A proper Mg:Ag ratio will lead to a lower operating voltage and hence to low power consumption of the OLED.
Several Mg:Ag combinations were studied with the aim to increase the charge injection into OLED devices [23]. For a reasonable explanation of this effect the Ag:Mg phase diagram should be discussed first [24]. In Mg:Ag alloys containing from 0% to 8% of Mg, the main phase is Ag3Mg, while in alloys with Mg content between 8% and 48% the dominant phase is AgMg. With over 48% of Mg, the stable phase is AgMg3 for which electroluminescent devices achieve maximum current and power efficiencies. This fact suggests that electron injection is more efficient when Mg concentration is over 48%.
These two metals and their alloys were used extensively as semi-transparent cathodes for top-emitting OLEDs due to their high conductivity and relatively low work function, which amounts to 4.3 eV for Ag [25] and 3.7 eV for Mg [26]. The most common 10:1 ratio Mg:Ag was extensively studied because it induces low electron injection barriers [27], [28], [29], [30], but other mixing ratio Mg:Ag alloys were also explored [31], [32].
In this context, the co-deposition of Mg:Ag required novel technologies for optimal Mg:Ag concentrations, which improved the cathode’s electron injection property as well as its transparency. The most common method is the thermal evaporation technique, i.e. the co-evaporation of the two metals. This method maintains the already deposited organic layers and the ITO at temperatures close to room temperature and avoids the thermal degradation of the organic layers and, especially, of the emissive layer. Thermal analysis of Alq3 shows a stable chemical structure up to 350 °C [33] while the modern emissive layers based on Ir3+ ions are stable up to 300 °C [34] or even 350 °C [35], [36].
After more than 25 years of development, the Thermionic Vacuum Arc (TVA) technology has found its firm place among the different procedures for thin film deposition [37], not only for carbon [38], [39], [40], but also for other single elements or oxides [41], [42], [43]. The TVA system’s versatility, the creation of alloys with controllable percentages of components, the possibility to form sandwich-like multilayer structures, as well as nanocomposites with superior film properties, are just a few of the advantages when achieving the desired co-deposited materials [44], [45], [46], [47].
The TVA method can be used for deposition of Ti-doped GaN nano-honeycombs on polyethylene terephthalate (PET), since in the TVA deposition process the substrate temperature can be maintained rather low (40–50 °C) [48]. Also, the diamond-like carbon (DLC) layers on the same PET polymer can be deposited; here the substrate temperature can be maintained at 100–150 °C [49]. These substrate temperatures are much lower than those needed for the chemical stability of the organic layers in OLED structures, thus avoiding their thermal decomposition. Overall, the TVA method can be applied to the top or bottom deposition of the metallic cathodes for OLED structures and to obtain metallic alloys with excellent electrical conductivity and low work function.
In this paper, a new technique called Laser-induced TVA (LTVA) is developed to obtain low sheet resistance metallic layers. The LTVA method allows simultaneous deposition of Mg:Ag alloys that is very useful for the cathodes applied in OLEDs. The thin layers of the Mg:Ag alloys obtained by the LTVA technique were then characterized by standard structural methods. Furthermore, the electrical measurements proved the rather high electrical conductivity of these thin metallic alloys. The basic plasma parameters, the electron temperature Te and the electron density Ne, were measured during the deposition process by a heated probe.
Section snippets
The LTVA system
In this paper we report for the first time the thin films synthesized in the TVA magnesium/silver plasma induced by an adjustable power laser beam (Laser-induced Thermionic Vacuum Arc - LTVA). Fig. 1 shows the schematic overview of the upgraded experimental setup. Beside the typical system of the TVA description [50], in the LTVA configuration the laser beam is provided by a QUANTEL Q-Smart 850 Nd:YAG compact Q-switched laser with second harmonic module.
A diffusion pump backed by a rotary
Plasma diagnostics
From the recorded probe characteristic data we estimated the four basic plasma parameters: the floating potential Vfl, the plasma potential Vpl, the electron temperature Te and the electron density Ne. The electron temperature was calculated from the slope of the electron current part of the probe characteristic in semi-logarithmic scale in the electron retarding regime:
The electron density was determined from the electron current at the plasma potential :
Conclusions
We studied a preparation of Mg:Ag alloys as a material for stable metallic OLED cathodes by means of the optimized Laser-induced Thermionic Vacuum Arc method. The performed structural and compositional analysis estimated the atomic concentration of Mg in the Mg:Ag sample prepared by the LTVA technology to be around 75% and that for the Ag to be around 25%. This confirmed the formation of the AgMg3 layer by the LTVA technique, in comparison with the simple Mg metallic layer that was formed when
CRediT authorship contribution statement
Rodica Vladoiu: Conceptualization, Sample preparation methodology. Aura Mandes: Methodology, Software. Virginia Dinca: Sample preparation, Visualization. Pavel Kudrna: Plasma methodology, Investigation. Milan Tichy: Plasma diagnosis, Methodology, Data curation, Writing- Original draft preparation. Silviu Polosan: Writing - Original draft preparation, Writing- Reviewing and Editing, Methodology, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The partial financial support of the Czech Science Foundation, project No. 19-00579S, Charles University Grant Agency, Grant No. 1188218, and of EUROfusion is gratefully acknowledged. The authors acknowledge funding through POC-G project MAT2IT (contract 54/2016, SMIS code 105726, Intermediary Body-Romanian Ministry of Research and Innovation).
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2023, Inorganic Chemistry CommunicationsCitation Excerpt :Thermionic vacuum arc (TVA) is one type of PVD coating method. In TVA discharge, ion energies are very high compared to the other deposition technologies [20,21,22,23,24,25]. In general, graphene layers were employed to deposit by CVD technology at high temperatures (>800 °C).