Reducing Contact Resistance in p- and n-Channel Thin-Film Organic Field-Effect Transistors
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Philipps-Universität Marburg
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Abstract
Performance of organic field-effect transistors (OFETs) is generally defined by two processes: injection of charge carriers at the electrode-organic interface and transport of charge carriers across the conduction channel at the gate dielectric-organic interface. In recent years, progress in understanding the electronic structure of π-conjugated organic solids and advancements in methods of chemical synthesis have allowed to develop organic semiconductor (OSC), whose effective mobility in devices exceeds that of amorphous silicon. However, as the efficiency of charge transport in the conduction channel increases, the inefficiency of the charge-carrier injection starts to play an ever more important role in the performance of OFETs. Nowadays contact resistance – a property that summarizes the effects of an inefficient charge-carrier injection in a device – is one of the main factors limiting the performance of OFETs. Therefore, in order to further advance the field of organic electronics, it is crucial to be able to understand the physical origin of contact resistance and its interplay with other performance characteristics of a device.
While contact resistance has been a topic of extensive research in recent years, its effects on the device performance are not yet completely understood. In particular, this is influenced by the fact that methods used for characterisation of OFETs are often adopted directly from the field of inorganic semiconductors. However, the van der Waals nature of intermolecular bonds in organic solids results in a much higher rates of disorder in comparison to inorganic semiconductors. Therefore, even extremely pure organic single crystals only exhibit a band-like transport, while the theory of inorganic semiconductors mostly deals with ideal electronic band structures. Another contribution stems from the specifics of thin films of OSCs not being taken into account during the preparation and electrical characterisation of devices. This includes, in particular, exposure to ambient air, which is known to affect the current-voltage characteristics of both p- and n-channel OFETs. Furthermore, the measured current-voltage characteristics implicitly include a complex interplay of phenomena at metal-organic and dielectric-organic interfaces, making the development of a simple analytical model a rather challenging task. As a result, unravelling the effects of individual interfaces on the device performance is often not possible. Finally, the importance of investigating devices with well-defined interfaces to establish a proper physical understanding of the involved phenomena cannot be underestimated.
To address the aforementioned issues, it was decided to develop a full-high vacuum (HV) process chain, which encompasses preparation and electrical characterisation of OFETs. Devices with a bottom gate-bottom contact geometry are used, which ensures well-defined interfaces between the organic layer, the source-drain electrodes and the gate dielectric. The use of a full-vacuum process allows to exclude the effects of exposure to ambient air and thus isolate the influence of the interfaces on the device performance. This makes it possible to demonstrate an interplay between the contact resistance and the effective mobility in OFETs. Furthermore, it is shown that even a short exposure to ambient air drastically alters the current-voltage characteristics, influencing extracted performance characteristics. To allow a deeper insight into the charge-carrier injection and transport phenomena in OFETs, a variable-temperature transfer length method (TLM) analysis is developed and used in combination with the full-HV process chain. As a result, a connection between the height of the injection barrier and the activation energy of charge transport is uncovered in both ?- and ?-channel devices. This connection indicates that a low activation energy of charge transport is required to ensure a low contact resistance. Furthermore, the reproducibility of the TLM analysis is investigated, whereby the importance of precisely defining the actual channel length of devices is demonstrated.
The results of this dissertation advance the understanding of the contact resistance and its relation with charge transport in the conduction channel of OFETs. The developed full-HV process chain and analysis methods are flexible and have great potential for future studies, in particular in investigation of electrode functionalisation techniques to achieve true ohmic contacts in organic electronics.
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Created: 2025Issued: 2025-08-06Updated: 2025-08-06
Faculty
Fachbereich Physik
Publisher
Philipps-Universität Marburg
Language
eng
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DoctoralThesis
Keywords
contact resistanceAktivierungsenergie des LadungstransportsMolekularstrahlabscheidungactivation energy of charge transportinjection barrierFallenzuständetrap statesSchottky-BarriereInjektionsbarriereenvironmental stabilityOrganic field-effect transistorsOrganische FeldeffekttransistorenKontaktwiderstandmolecular beam depositionUmweltstabilitätSchottky barrier
DFG-subjects
KontaktwiderstandSchottky-KontaktFeldeffektOrganischer Transistor
DDC-Numbers
530
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Radiev, Yurii (0000-0001-6969-546X): Reducing Contact Resistance in p- and n-Channel Thin-Film Organic Field-Effect Transistors. : Philipps-Universität Marburg 2025-08-06. DOI: https://doi.org/10.17192/z2025.0492.