The application of molybdenum oxide layers in electronic devices like solar cells and organic light emitting diodes (LEDs) has expanded considerably. *
Dielectric/metal/dielectric (DMD) transparent electrodes based on MoO3 have been applied in solar cells and organic light emitting diodes, by virtue of the favourable properties of MoO3 as hole-transport/hole-injection material.*
However, it was reported that poorly textured or amorphous MoO3 layers are extremely instable when exposed to ambient humidity and liquid water. *
In the article “Transparent electrodes based on molybdenum–titanium–oxide with increased water stability for use as hole-transport/hole-injection components” Selina Goetz, Rachmat Adhi Wibowo, Martin Bauch, Neha Bansal, Giovanni Ligorio, Emil List-Kratochvil, Christian Linke, Enrico Franzke , Jörg Winkler, Markus Valtiner and Theodoros Dimopoulos describe their study aimed to increase the water stability of sputtered, amorphous MoO3 layers without applying high temperatures, but by alloying with another refractory metal oxide to reduce hydrolysis and dissolution, while maintaining the essential electronic properties (i.e. wide band gap and high work function) of MoO3.*
In order to achieve this aim, the authors introduced titanium oxide to form a mixed molybdenum–titanium–oxide compound material. *
TiO2 is known to be stable in water and in a wide pH range of aqueous solutions. Additionally, TiO2 is also composed of octahedral TiO6 building blocks, analogous to the building blocks of MoO3, which gives both oxides a d-band dominated electronic band structure. *
In contrast to the previous reports, the article by S. Goetz et al. introduces for the first time compact, amorphous thin films, deposited by sputtering from a mixed molybdenum–titanium–oxide (MTO) compound target. The sub-oxidic composition of the sputter target allows fast deposition in direct current (DC) magnetron mode, achieving high sputtering rates that are relevant for industrial applications. *
Selina Goetz et al. demonstrate that the addition of Ti strongly increases the stability in water, while the desirable electronic properties of MoO3, specifically the high work function and wide bandgap, are maintained. *
The DMD electrodes, with Ag as metal layer, were fabricated on both rigid and flexible substrates, namely glass and polyethylene terephthalate (PET). *
The obtained electrodes have low sheet resistance around 5 Ω/sq and high average visible transmittance well above 0.7 (including the substrate). As a result of the MTO stability, processing with water-based solutions takes place without electrode degradation. *
To demonstrate the process compatibility for large-scale, industrial production, the DMDs were sputter-deposited by a roll-to-roll process on a 300 mm-wide PET foil, achieving similar electrode properties with the laboratory-scale samples. *
The surface characterization by atomic force microscopy was performed with a commercially available atomic force microscope using NANOSENSORS SuperSharpSilicon™ SSS-NCHR silicon high-resolution AFM probes for tapping mode with a typical AFM tip radius of 2 nm. *
Figure 6 from Selina Goetz et al. 2022 “Transparent electrodes based on molybdenum–titanium–oxide with increased water stability for use as hole-transport/hole-injection components”:
AFM images of a bare glass, b bare PET, c MTO40/Ag14/MTO10/30 on glass and d MTO40/Ag14/MTO10/30 on PET
Figure 6 displays the 1 × 1 µm2 AFM images of the DMD on glass and PET, as well as the corresponding bare substrate. The roughness of the electrodes is mostly affected by the underlying substrate. The extracted root mean square value (RMS) for the glass substrate (Fig. 6a) is 1.8 ± 0.1 nm and 11.5 ± 4.0 nm for the PET substrate (Fig. 6b). The RMS values of the DMD electrodes are even reduced, yielding RMS = 1.0 ± 0.1 nm for glass/MTO40/Ag14/MTO10/30 (Fig. 6c) and RMS = 4.8 ± 2.0 nm for PET/MTO40/Ag14/MTO10/30 (Fig. 6d).*
*Selina Goetz, Rachmat Adhi Wibowo, Martin Bauch, Neha Bansal, Giovanni Ligorio, Emil List-Kratochvil, Christian Linke, Enrico Franzke , Jörg Winkler, Markus Valtiner and Theodoros Dimopoulos
Transparent electrodes based on molybdenum–titanium–oxide with increased water stability for use as hole-transport/hole-injection components
Journal of Materials Science, Volume 57, pages 8752–8766, (2022)
DOI: https://doi.org/10.1007/s10853-022-07157-0
Open Access: The article “Transparent electrodes based on molybdenum–titanium–oxide with increased water stability for use as hole-transport/hole-injection components” by Selina Goetz, Rachmat Adhi Wibowo, Martin Bauch, Neha Bansal, Giovanni Ligorio, Emil List-Kratochvil, Christian Linke, Enrico Franzke , Jörg Winkler, Markus Valtiner and Theodoros Dimopoulos is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/.