Transparent metallic electrodes and conductors for Organic Electroluminescent Diodes

Project Director: Dr. Silviu POLOSAN

The project describes the manufacture of transparent metal electrodes for electroluminescent diodes, which allow the improvement of charge transport through multilayer OLED structures and the increase of light emission due to the electroluminescence obtained on both sides. As a novelty, the application of a patent based on low-energy electron irradiation of metal surfaces facilitates the obtaining of transparent anodes with improved electrical conductivity by decreasing the roughness, using the nanozonal melting of metal films under the action of an electron beam. Regarding the transparent cathodes, the project proposes a new strategy of electrodes with low mechanical extraction work, using alloys between alkaline-earth metals and metallic silver. Ag-Mg compositional optimization for cathodes with uniform distribution and optimal Ag/Mg ratio represents the second novelty of this project. This fact implies obtaining thin films as good quality alloys using thermal co-evaporation and comparing them with those obtained by vacuum thermionic arc deposition.

1. Depositing thin silver films by the thermionic arc method and thermal evaporation in a vacuum.
2. Irradiation of thin silver films with low-energy electrons, modification of the irradiation parameters in accordance with the objectives of the project (increasing the electrical conductivity of thin films).
3. Comparison of the electrical conductivity of these films before and after irradiation with low energy electrons.
4. Improving cathodes by using Mg-Ag alloys for an optimal charge injection in OLED structures.
5. The construction of OLED structures with optimized metal electrodes and the determination of their operating parameters, as a functional OLED demonstrator.

Coordinator - National Institute of Materials Physics - Romania

Summary of stage 1.
a) The installation of thermal evaporation and deposition by thermionic arc has been re-designed so that the thickness of the deposited metal films is monitored with the help of a quartz balance. Also, the evaporators were brought closer with the help of spacers so that the deposition angle is as small as possible, as well as the nacelle-substrate distance, thus ensuring an efficient deposition of the metal films.
b) The thermionic emission process was adjusted by using two DC sources necessary to accelerate the emitted thermoelectrons to:
- Low voltages (0-400 V) and currents of the order of hundreds of microamperes for low-energy electron irradiation of metal surfaces. The method was tested on a tantalum metal plate, establishing the main electron irradiation parameters.
- High voltages (0-2 kV, 0-2 A) for the realization of the thermionic arc (TVA), i.e. the deposition of the films in its own plasma. The main deposition parameters (filament current, anodic voltage and current, breakdown voltage and deposition time) were established for silver films deposited on a glass substrate.
c) The vacuum thermal evaporation parameters were established for the silver and magnesium films, by depositing three silver samples of different thicknesses and a very thin magnesium sample.
d) A co-evaporation of magnesium-silver in a ratio of 1:10 was carried out at different evaporation rates, but the preliminary results require refining the method to obtain a ratio of 10:1 silver-magnesium.
e) A silver-magnesium co-evaporation was achieved by the modified thermionic arc method with the application of a 532 nm laser beam to homogenize the alloy on the glass substrate, but the ratio was 1:4, a fact that requires additional investigations regarding the preliminary silver mixture and magnesium as well as adjusting the laser power to obtain the desired ratio.

Summary of stage 2.
a) The silver films deposited on glass and Si/SiO2 in the previous step were subjected to low-energy electron irradiation (640 eV) in order to improve their electrical conductivity, at various electron fluences. Thus it was demonstrated that irradiation with a dose below 100 C/m2 preserves the conductive character avoiding the formation of oxides on the metal surface.
b) Morphological and structural analysis of the silver films obtained in the previous phase by means of X-ray diffraction measurements, scanning electron microscopy and atomic force microscopy. The results demonstrated the deposition of thin films between 22 nm and 53 nm that preserve the transparency of these electrodes. Amorphous films are mainly obtained, but at very small thicknesses a slight crystallization (below 3%) of the silver is observed. Film thickness measurement was performed by three techniques: diffuse optical reflectance, small angle X-ray scattering (Xrr) and atomic force microscopy (AFM). The results were correlated and the Xrr and AFM studies revealed an average roughness.
c) Electrical conductivity measurements showed an increase in resistivity with decreasing film thickness, so that their thickness presents an optimum between transparency and electrical conductivity. The optical transparency of the silver films is 50-60% in the visible range at thicknesses of 22 nm and 31 nm, and the wettability measurements showed a hydrophilic character of these films and therefore a good roughness, useful in charge injection processes.
d) Silver electrodes deposited on 31 nm glass were coated with p-type conducting polymers such as PEDOT:PSS and NPB with thicknesses of approx. 45 nm and then an 80 nm AlQ3 electroluminescent layer was deposited. The transparency of these sandwich structures is kept at 50-60%, and the optical absorption indicates the absorption bands of the AlQ3 film in the ultraviolet, the other films being non-absorbing in the visible range. The quantum emission efficiency of the electroluminescent film was calculated to be 39-40% with a luminance of 147 cd/m2.
e) In order to obtain the cathode type electrodes, it was tried to obtain Mg-Ag alloys through the co-evaporation technique, but the subsequent characterization of these films demonstrated the obtaining of non-uniform and implicitly electrically non-conductive structures due to the presence of lamellar deposited magnesium.
f) Significantly improved results for Mg-Ag alloys were obtained by laser assisted thermionic arc deposition. The films obtained showed a Mg3Ag type alloy and the wettability measurements showed a slightly hydrophobic character but with a much improved roughness compared to the films obtained by co-evaporation.
g) Electrical resistivity measurements demonstrated a much better conductivity than in the case of films obtained by co-evaporation which improves the charge injection in the light emitting diodes and leads to an increase in the luminance of these diodes (external quantum efficiency).
h) Capacitance-voltage measurements indicated a mechanical work of extraction lower than in the case of magnesium films but much better than in the case of silver films, thus favoring charge injection into the diodes. The theoretical calculation model of the mechanical extraction work in the Mg-Ag alloy indicated a value of 4.21 eV very close to the value obtained by the graphical method from the capacitance-voltage measurements of 4.25 eV.

1. Vladoiu, R.; Mandes, A.; Dinca, V.; Matei, E.; Polosan, S.
"Synthesis of Cobalt–Nickel Aluminate Spinels Using the Laser-Induced Thermionic Vacuum Arc Method and Thermal Annealing Processes"
Nanomaterials 2022, 12, 3895.
2. S. Polosan , C. C. Ciobotaru , I.C. Ciobotaru , M. Enculescu , D. Iosub, A. Mandes , R. Vladoiu, „Electron Irradiation of Titanium-Doped Chromium Nanostructured Thin Films for Higher Conductive Electrodes”, IEEE TRANSACTIONS ON NANOTECHNOLOGY, Vol.21, p. 823-829 (2022).
3. I. C. Ciobotaru , M. Enculescu , S. Polosan, I. Enculescu, C. C. Ciobotaru, „Organic Light-Emitting Diodes with Electrospun Electrodes for Double-Side Emissions”, Micromachines, 14(3), 543 (2023).
4. Development of a national patent with the title "Magnesium-silver alloys for cathodes used in electroluminescent diode technologies", authors S. Polosan, A. Nițescu, A. Mandeș, V. Dincă, R. Vladoiu,
No. OSIM A/00752

Dr. Silviu Polosan, PhD
Senior Researcher I
National Institute of Materials Physiscs
Functional nanostructures Laboratory
Tel: +40-(0)21-2418 268


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