Henning Sirringhaus

Two-dimensional charge transport in self-organized, high-mobility conjugated polymers

Self-organization in many solution-processed, semiconducting conjugated polymers results in complex microstructures, in which ordered microcrystalline domains are embedded in an amorphous matrix. This has important consequences for electrical properties of these materials: charge transport is usually limited by the most difficult hopping processes and is therefore dominated by the disordered matrix, resulting in low charge-carrier mobilities (⩽10-5 cm2 V-1 s-1). Here we use thin-film, field-effect transistor structures to probe the transport properties of the ordered microcrystalline domains in the conjugated polymer poly(3-hexylthiophene), P3HT. Self-organization in P3HT results in a lamella structure with two-dimensional conjugated sheets formed by interchain stacking. We find that, depending on processing conditions, the lamellae can adopt two different orientations—parallel and normal to the substrate—the mobilities of which differ by more than a factor of 100, and can reach values as high as 0.1 cm2 V-1 s-1 (refs 3, 4). Optical spectroscopy of the field-induced charge, combined with the mobility anisotropy, reveals the two-dimensional interchain character of the polaronic charge carriers, which exhibit lower relaxation energies than the corresponding radical cations on isolated one-dimensional chains. The possibility of achieving high mobilities via two-dimensional transport in self-organized conjugated lamellae is important for applications of polymer transistors in logic circuits and active-matrix displays.

General observation of n-type field-effect behaviour in organic semiconductors

Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity or low-bandgap organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric—such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6)—can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10-3 to 10-2 cm2 V-1 s-1 have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylenevinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor–dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO2 dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed.

25th Anniversary Article: Organic Field‐Effect Transistors: The Path Beyond Amorphous Silicon

Over the past 25 years, organic field-effect transistors (OFETs) have witnessed impressive improvements in materials performance by 3–4 orders of magnitude, and many of the key materials discoveries have been published in Advanced Materials. This includes some of the most recent demonstrations of organic field-effect transistors with performance that clearly exceeds that of benchmark amorphous silicon-based devices. In this article, state-of-the-art in OFETs are reviewed in light of requirements for demanding future applications, in particular active-matrix addressing for flexible organic light-emitting diode (OLED) displays. An overview is provided over both small molecule and conjugated polymer materials for which field-effect mobilities exceeding > 1 cm2 V–1 s–1 have been reported. Current understanding is also reviewed of their charge transport physics that allows reaching such unexpectedly high mobilities in these weakly van der Waals bonded and structurally comparatively disordered materials with a view towards understanding the potential for further improvement in performance in the future.

Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices

We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene-diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm(2) V(-1) s(-1), which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC(71)BM gave a power conversion efficiency of 5.4%.

Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid-crystalline phase

A method is demonstrated by which liquid-crystalline self-organization in rigid-rod nematic conjugated polymers can be used to control the microstructure of the active semiconducting layer in solution-processed polymer thin-film transistors (TFTs). Enhanced charge carrier mobilities of 0.01–0.02 cm2/V s and good operating stability have been achieved in polyfluorene copolymer TFTs by preparing the polymer in a nematic glassy state and by aligning the polymer chains parallel to the transport direction with the help of an alignment layer. Mobility anisotropies of 5–8 for current flow parallel and perpendicular to the alignment direction have been observed that are of the same order of magnitude as optical dichroic ratios.

Low-temperature, high-performance solution-processed metal oxide thin-film transistors formed by a ‘sol–gel on chip’ process

At present there is no ‘ideal’ thin-film transistor technology for demanding display applications, such as organic light-emitting diode displays, that allows combining the low-temperature, solution-processability offered by organic semiconductors with the high level of performance achievable with microcrystalline silicon1. N-type amorphous mixed metal oxide semiconductors, such as ternary oxides Mx1My2Oz, where M1 and M2 are metals such as In, Ga, Sn, or Zn, have recently gained momentum because of their high carrier mobility and stability2, 3 and good optical transparency, but they are mostly deposited by sputtering. So far no route is available for forming high-performance mixed oxide materials from solution at low process temperatures <250 °C. Ionic mixed metal oxides should in principle be ideal candidates for solution-processable materials because the conduction band states derived from metal s-orbitals are relatively insensitive to the presence of structural disorder and high charge carrier mobilities are achievable in amorphous structures2. Here we report the formation of amorphous metal oxide semiconducting thin-films using a ‘sol–gel on chip’ hydrolysis approach from soluble metal alkoxide precursors, which affords unprecedented high field-effect mobilities of 10 cm2 V−1 s−1, reproducible and stable turn-on voltages Von≈0 V and high operational stability at maximum process temperatures as low as 230 °C.

High-performance ambipolar diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymer field-effect transistors with balanced hole and electron mobilities.

Ambipolar OFETs with balanced hole and electron field-effect mobilities both exceeding 1 cm(2) V(-1) s(-1) are achieved based on a single-solution-processed conjugated polymer, DPPT-TT, upon careful optimization of the device architecture, charge injection, and polymer processing. Such high-performance OFETs are promising for applications in ambipolar devices and integrated circuits, as well as model systems for fundamental studies.

Downscaling of self-aligned, all-printed polymer thin-film transistors

Printing is an emerging approach for low-cost, large-area manufacturing of electronic circuits, but it has the disadvantages of poor resolution, large overlap capacitances, and film thickness limitations, resulting in slow circuit speeds and high operating voltages. Here, we demonstrate a self-aligned printing approach that allows downscaling of printed organic thin-film transistors to channel lengths of 100-400 nm. The use of a crosslinkable polymer gate dielectric with 30-50 nm thickness ensures that basic scaling requirements are fulfilled and that operating voltages are below 5 V. The device architecture minimizes contact resistance effects, enabling clean scaling of transistor current with channel length. A self-aligned gate configuration minimizes parasitic overlap capacitance to values as low as 0.2-0.6 pF mm(-1), and allows transition frequencies of fT = 1.6 MHz to be reached. Our self-aligned process provides a way to improve the performance of printed organic transistor circuits by downscaling, while remaining compatible with the requirements of large-area, flexible electronics manufacturing.

Close look at charge carrier injection in polymer field-effect transistors

Parasitic contact resistance effects are becoming a major issue in organic transistors in that they can severely limit or even dominate their overall transistor performance. We present a systematic study of the contact resistance in bottom-contact polymer field-effect transistors made from poly(3-hexylthiophene) (P3HT) as well as poly-9,9′dioctyl-fluorene-co-bithiophene (F8T2). A microscopic approach based on noncontact scanning-probe potentiometry was used to directly separate the transport properties of the transistor channel and the electrode/polymer contacts, giving very accurate experimental access to both the source and drain contact resistance. The influence of the relevant parameters (temperature, electrode work function, ionization potential of the polymer, charge carrier mobility) on the source/drain contact resistance is investigated. We find that for “good” source/drain contacts that give rise to relatively small overall contact resistances (⩽50 kΩ cm), e.g., P3HT with chromium–gold electrodes...

Noncontact potentiometry of polymer field-effect transistors

We report on high-resolution potentiometry of operating organic thin-film field-effect transistors by means of scanning Kelvin probe force microscopy. It is demonstrated that the measured potential reflects the electrostatic potential of the accumulation layer at the semiconductor/insulator interface. We present data revealing gate bias and lateral electric field dependence of the field-effect mobility in poly(hexylthiophene) at temperatures from 50 to 300 K.