Organic light-emitting diodes (OLEDs) are used as energy-efficient light-emitting units e.g. for displays and lighting. Among the appealing features of organic materials for light-emitting devices are their relative ease of processing combined with the possibility of tuning the colour of the emitted light via chemical synthesis of molecular building blocks with the desired characteristics. Moreover, their mechanical properties can enable the realization of mechanically flexible devices. In addition, some types of organic small-molecules can self-assemble into crystalline nanofibers with additional features. At NanoSYD, we study both organic nanofibers and thin films used as the active layer in both OLEDs as well as transistor devices that function either as organic light-emitting transistors (OLETs) or organic phototransistors (OPTs). More specifically, the research topics are:
- Flexible/stretchable OLEDs
- Techniques for integration of organic nanofibers into devices
- In-situ growth
- Top-contact lamination
- Improvement of electrical and light-emission properties by functional interlayer materials
- Multicolor light-emission from nanofibers
- Organic/hybrid devices
Bendable OLED devices can be realized by fabricating the OLED layer stack on top of a flexible substrate – typically some type of plastic. The fabrication of a stretchable device is, however, more challenging since most of the device materials only tolerate relatively moderate levels of strain. At NanoSYD, we are working on the development of a new type of stretchable OLED based on a substrate with microscopic surface waves, which flatten out when the sample is stretched. This allows the OLED device to be installed on 3-dimensional surfaces and objects.
Techniques for the integration of organic nanofibers into devices
When organic films based on small molecules are used in devices, they are typically deposited onto the device substrate by physical vapor deposition. However, the crystalline nanofibers are formed only on specific surfaces that are typically unsuited for further processing, and special techniques for their integration into devices therefore needs to be developed. One technique relies on the initial growth of an array of nanofibers on a muscovite mica substrate and subsequent transfer onto a device substrate via roll-printing. The process is illustrated in the figure below, on which panel (a) shows the array of nanofibers prior to transfer, panel (b) shows the principle of the roll-printing method, and panel (c) shows the nanofiber array after transfer. Afterwards, electrical contact can be established by electrode material deposition through a nanostencil as shown in panel (d), while panel (e) and (f) show the contacted nanofibers observed with a fluorescence and white-light microscope, respectively.
An alternative integration technique uses a gold surface as the substrate for nanofiber self-assembly. On this surface, there is no epitaxial relation between the surface and the nanofibers and there is consequently no well-defined growth direction. However, if artificial pinning structures are realized on/in the gold surface, these can act as nucleation centers for the nanofiber growth and thereby steer the growth direction to enable, e.g. the growth between two closely spaced gold electrodes. Finally, we also study an alternative way of top contact deposition that do not rely on direct evaporation, but rather uses an elastomeric substrate on which a set of contacts is made via nanostenciling. Subsequently this substrate can be laminated onto the organic material to establish the electrical contact.
Improvement of electrical and light-emission properties by functional interlayer materials
The performance of OLETs is strongly affected by the transistor platform materials, which forms an interface to the organic semiconductor. For example, the source and drain electrode material forms a metal-semiconductor interface, across which the charge carriers must be injected. For some material combinations, an energy barrier is formed that will limit this injection and thereby the device performance. One way to remedy this is to include functional monolayers on the source and drain electrodes that will lower the injection barrier.
Also the gate dielectric forms an interface to the organic semiconductor and can therefore influence the device characteristics. It is known that some dielectric materials form charge traps at the interface and thereby limits the current flow. Therefore, alternative gate dielectrics such as certain types of polymers can help to improve the performance.
Multicolor light-emission from nanofibers
By stimulating an organic nanofiber-transistor with an AC voltage, localized light emission can be observed from the metal-organic nanofiber interfaces. This is the results of sequential injection of holes and electrons into the nanofiber and the subsequent electron-hole recombination leading to photon emission. The emission color is determined by the electronic band gap of the nanofiber material, and multicolor light- emission can therefore be achieved by integrating two different types of nanofibers with different fluorescence spectra onto the same transistor platform. The figure below shows emission from two different nanofibers as an overlay between the electrically stimulated light (grayscale) and a fluorescence microscope images from which the actual color can be seen.
Organic/hybrid phototransistor devices
While organic semiconductors have several advantages such as tunability combined with low price and fairly straight-forward processing on large areas, they typically exhibit only more moderate electrical transport properties. At NanoSYD, we work on the development of phototransistor devices based on either a pure organic semiconductor thin-film or a hybrid structure combining an organic semiconductor thin-film with graphene, which has excellent electrical transport properties. This allows the realization of sensitive photodetectors. More specifically, we investigate how the process parameters used during thin-film deposition are influencing the thin-film microstructure, and how this translates into improved device performance.
NanoSYD members working within the research area
Jakob Kjelstrup-Hansen, Associate Professor
Jes Linnet, PhD student
Anders Løchte Jørgensen, PhD student
Martin Søndergaard Andersen, Master student
Tobias Schiby, Bachelor student
Andreas Christian Holm, Bachelor student
Jenn Tian Vuong, Master student
The Stretchable OLED display, Innovation Fund Denmark, Industrial PhD project in collaboration with Polyteknik A/S, 2020-2023.
Methodologies for Hyperspectral Thermal Imaging, Innovation Fund Denmark, Industrial PhD project in collaboration with Newtec Engineering A/S, 2018-2021
Work function difference of naphthyl end-capped oligothiophene in different crystal alignments studied by Kelvin probe force microscopy, M. N. Larsen, M. S. Peters, R. Lemos-Silva, D. A. da Silva Filho, B. Jørgensen, O. Albrektsen and J. Kjelstrup-Hansen, Organic Electronics 89 (2021) 106060 https://doi.org/10.1016/j.orgel.2020.106060
Surface-Controlled Crystal Alignment of Naphthyl End-Capped Oligothiophene on Graphene: Thin-Film Growth Studied by In Situ X-ray Diffraction, M. K. Huss-Hansen, M. Hodas, N. Mrkyvkova, J. Hagara, P. Siffalovic, B. B. E. Jensen, A. Osadnik, A. Lützen, E. Majková, F. Schreiber, L. Tavares, J. Kjelstrup-Hansen and M. Knaapila, Langmuir, 36/8 (2020) 1898 https://doi.org/10.1021/acs.langmuir.9b03467
Structural evaluation of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene in organic field-effect transistors with n-octadecyltrichlorosilane coated SiO2 gate dielectric, A. Lauritsen, M. Torkkeli, O. Bikondoa, J. Linnet, L. Tavares, J. Kjelstrup-Hansen and M. Knaapila, Langmuir 34/23 (2018) 6727 https://doi.org/10.1021/acs.langmuir.8b00972
Transparent and conductive electrodes by large-scale nano-structuring of noble metal thin-films, J. Linnet, A. R. Walther, C. Wolff, O. Albrektsen, N. A. Mortensen, and J. Kjelstrup-Hansen, Optical Materials Express, 8/7 (2018) 1733 https://doi.org/10.1364/OME.8.001733
Laser-induced charge separation in organic nanofibers: A joint experimental and theoretical investigation, L. Tavares, Y. Liu, D. Behn, J. Siebels, T. Kipp, A. Mews, and J. Kjelstrup-Hansen, Organic Electronics, 53 (2018) 20 https://doi.org/10.1016/j.orgel.2017.10.032
Enhanced photoresponsivity in organic field effect transistors by silver nanoparticles, Jes Linnet, Anders Runge Walther, Ole Albrektsen, Luciana Tavares, René Lynge Eriksen, Per Baunegaard With Jensen, Andreas Osadnik, Søren Hassing, Arne Lützen, and Jakob Kjelstrup-Hansen, Organic Electronics, 46 (2017) 270 https://doi.org/10.1016/j.orgel.2017.04.019
Multicolor nanofiber based organic light-emitting transistors. Per Baunegaard With Jensen, Jakob Kjelstrup-Hansen, and Horst-Günter Rubahn. Organic Electronics 14/12 (2013) 3324 http://dx.doi.org/10.1016/j.orgel.2013.10.001
Localized and guided electroluminescence from roll printed organic nanofibres. L Tavares, J Kjelstrup-Hansen and H-G Rubahn. Nanotechnology, Vol. 23 (2012) 425203 DOI: 10.1088/0957-4484/23/42/425203
Efficient Roll-On Transfer Technique for Well-Aligned Organic Nanofibers. Luciana Tavares, Jakob Kjelstrup-Hansen, and Horst-Günter Rubahn. small 7(17)(2011)2460-2463, Wiley-VCH DOI: 10.1002/smll.201100660
AC-biased organic light-emitting field-effect transistors from naphthyl end-capped oligothiophenes. X Liu, I. Wallmann, H. Boudinov, J. Kjelstrup-Hansen, M. Schiek, A. Lützen, and H.-G. Rubahn. Organic Electronics, 11(2010)1096-1102 DOI: 10.1016/j.orgel.2010.03.015