Charge Injection in Solution‐Processed Organic Field‐Effect Transistors: Physics, Models and Characterization Methods

Abstract

A high-mobility organic semiconductor employed as the active material in a field-effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics. In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll-to-roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics. With the exception of electrolyte-gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution-processed and where the most common architectures are adopted, are ∼10 kΩ·cm for transistors with a field effect mobility in the 0.1-1 cm(2)/Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case-by-case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low-cost, large-area printed circuits.

Trends in molecular design strategies for ambient stable n-channel organic field effect transistors

“…34,136 In the next section, we will illustrate the continuing effort devoted to developing environmentally stable n-channel molecular materials based on electron withdrawing imide, cyano, carbonyl, and halogen substituted small molecule semiconductors.…”

Inkjet Printed Single‐Walled Carbon Nanotube Based Ambipolar and Unipolar Transistors for High‐Performance Complementary Logic Circuits

“…Concerning the electron current the same consideration for the fi rst regime is valid but a decreasing trend is present in the second regime. To explain this effect we have investigated the correlation between the number of printed layers, and the contact and channel resistances, for both holes and electrons, by using the differential method (DM) [ 41,42 ] and G-function method [ 43 ] (for details please see Supporting Information, Section 3: Contact Resistance Extraction). While an increasing trend in channel resistance for the electron is clearly visible, the contact resistance values are approximately constant within increasing printing passes.…”

High‐Mobility Naphthalene Diimide and Selenophene‐Vinylene‐Selenophene‐Based Conjugated Polymer: n‐Channel Organic Field‐Effect Transistors and Structure–Property Relationship

“…[1][2][3][4][5][6][7] Such progress coupled with the high compatibility of solution-processable organic semiconductors with plastic or metal foil substrates makes them ideal candidates for cost-effective, mechanically flexible electronic devices for various applications, such as printed radio frequency identification tags for item-level tagging, drivers for flexible displays, wearable electronics, distributed sensors, and integrated, nonvolatile memory devices. [8][9][10][11][12][13][14][15][16][17][18][19][20] [21][22][23][24][25] Such remarkable p-channel mobilities, obtained in devices with fairly promising shelf-life and operational stabilities, have been demonstrated with conjugated polymers based on new building units, such as diketopyrrolopyrrole, [24,26,27] isoindigo, [28,29] and indacenodithiophene.…”