[paper] Flexible Megahertz Organic Transistors

Jakob Leise^1,4, Jakob Pruefer^1,4, Ghader Darbandy¹, Aristeidis Nikolaou^1,4, Michele Giorgio², Mario Caironi², Ute Zschieschang³, Hagen Klauk³, Alexander Kloes¹, Benjamin Iñiguez⁴

and James W. Borchert⁵

Flexible megahertz organic transistors and the critical role of the device geometry on their dynamic performance

Journal of Applied Physics 130, 125501 (2021); 

DOI: 10.1063/5.0062146

  
1NanoP, TH Mittelhessen University of Applied Sciences, Gießen 35390, Germany
2Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Milano 20133, Italy
3Max Planck Institute for Solid State Research, Stuttgart 70569, Germany
4DEEA, Uniersitat Rovira i Virgili, Tarragona 43007, Spain
5Georg August University of Goettingen, Goettingen 37077, Germany
  

Abstract: The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance.

Fig: Schematic cross-section of a top-contact (TC) organic TFT. Here, the semiconductor layer separates the source and drain contacts from the gate dielectric and thus from the gate-field-induced charge-carrier channel; hence, these transistors are also referred to as staggered TFTs. The overlap regions are assumed as a series connection of two capacitances. However, when the organic semiconductor (OSC) is operated in accumulation, the accumulation charges change the behavior of the series connection. The charge density at the source end of the channel is assumed to be found in the entire gate-to-source overlap region. 

Acknowledgments: The authors thankfully acknowledge funding for this project from the German Federal Ministry of Education and Research (“SOMOFLEX,” No. 13FH015IX6) and EU H2020 RISE (“DOMINO,” No. 645760), and the German Research Foundation (DFG) under Grant Nos. KL 1042/9-2, KL 2223/6-1, and KL 2223/6-2 (SPP FFlexCom). The authors would like

[paper] Organic Semiconductor Devices

D. Oussalah^1,2, R. Clerc², J. Baylet¹, R. Paquet¹, C. Sésé¹, C. Laugier¹, B. Racine¹

and J. Vaillant¹

On the minimum thickness of doped electron/hole transport layers 

in organic semiconductor devices 

Journal of Applied Physics 130, 125502 (2021);

DOI: 10.1063/5.0060429

  
1Université Grenoble Alpes, CEA, Leti, Grenoble 38000, France
2Université de Lyon, UJM-Saint-Etienne, CNRS, IOGS, Lab. Hubert Curien, UMR5516 St-Etienne, France

  

Abstract: Doped hole (respectively electron) transport layers [HTLs (respectively ETLs)] are commonly used in evaporated organic devices to achieve high work function hole contact (respectively low work function electron contact) in organic LEDs to inject large current, in solar cells to increase the open circuit voltage, and in photodetectors to minimize the dark current. However, optimization of the HTL thickness results from a delicate trade-off. Indeed, on the one hand, to minimize the impact of HTLs on light propagation and series resistance effects, it is commonly admitted that HTLs must be kept as thin as possible. In this work, a model, validated by drift and diffusion simulations, has shown that, depending of the doping level, a minimum thickness between 10 and 20 nm was needed to prevent the transport layer work function from degradation due to field effects. Experiments have been performed on template p-only devices featuring a single HTL of various thicknesses and doping, confirming the validity of the model. Finally, simulations have been performed on a p-i-n device featuring both HTL and ETL. These results constitute precious indications for the design of efficient evaporated organic LEDs, solar cells, or photodetectors.

Fig: Image of a top view of the 200 mm silicon wafer processed to realize TiN/STTB:F4TCNQ/ZnPc:C60/Ag devices.

[paper] Model for Organic Thin-Film Transistor

Physically Based Compact Mobility Model for Organic Thin-Film Transistor

T. K. Maiti, L. Chen, H. Zenitani, H. Miyamoto, M. Miura-Mattausch and H. J. Mattausch

in IEEE Transactions on Electron Devices, vol. 63, no. 5, pp. 2057-2065, May 2016.

doi: 10.1109/TED.2016.2540653

Abstract: A physically based compact mobility model for organic thin-film transistors (OTFTs) with an analysis of bias-dependent Fermi-energy (EF) movement in the bandgap (Eg) is presented. Mobility in the localized and extended energy states predicts the drain-current behavior in the weak and strong accumulation operations of OTFTs, respectively. A hopping mobility model as a function of the surface potential is developed to describe the carrier transport through localized energy states located inside Eg. The Poole-Frenkel parallel-field-effect mobility and vertical-field-effect mobility are considered to interpret the bandlike carrier transport in the extended energy states. The parallel field effect on mobility is more pronounced for shorter channel length OTFTs and is considered by developing a channel-length-dependent mobility model. The vertical field effect on mobility is included to account for the effect of mobility on carrier transport at high gate-voltage-induced fields. We also compared the model results with 2-D device simulations and measurements to verify the developed mobility model [read more...]

Compact DC Modeling of Organic Field-Effect Transistors: Review and Perspectives

In spite of impressive improvements achieved for organic field-effect transistors (OFETs), there is still a lack of theoretical understanding of their behaviors. Furthermore, it is challenging to develop a universal model that would cover a huge variety of materials and device structures available for state-of-the-art OFETs. Nonetheless, currently there is a strong need for specific OFET compact models when device-to-system integration is an important issue. We briefly describe the most fundamental characters of organic semiconductors and OFETs, which set the bottom line dictating the requirement of an original model different from that of conventional inorganic devices. Along with an introduction to the principles of compact modeling for circuit simulation, a comparative analysis of the reported models is presented with an emphasis on their primary assumptions and applicability aspects. Critical points for advancing OFET compact models are discussed in consideration of the recent understanding of device physics.

[1] Kim, C.-H.; Bonnassieux, Y.; Horowitz, G., "Compact DC Modeling of Organic Field-Effect Transistors: Review and Perspectives," Electron Devices, IEEE Transactions on , vol.61, no.2, pp.278,287, Feb. 2014
doi: 10.1109/TED.2013.2281054
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