Accelerated discovery of organic semiconductors
The Organic Materials in Electronics Consortium

Introduction

Organic semiconductors have been available for many years, but they have been limited by poor performance, and low atmospheric stability. More recently a small number of stable compounds, based on thiophene oligomers with attached fluoro-carbon groups, have been developed.

In this programme these stable compounds serve as leads for the parallel synthesis of libraries of related compounds. Each compound is to be characterised, screened and evaluated in device fabrications. Of particular interest to us are air-stable p- and n- type semiconductors.

It is important to realise that not only must we get the right compounds, but we must also process them correctly to give ordered materials that have acceptable properties! the determination of the proper processing regime for each compound must also be carried out in parallel...

Above right: Part of the crystal structure of a cyclopentadithiophene monomer, showing two closely packed stacks of molecules. These molecules have been designed so that the alkyl chain substituents are held in the same plane as the rings, to assist close packing, and improve carrier mobility. (You can adjust the viewing angle to investigate the stack structure by moving your mouse over the image).

Preparing a library of organic semiconductors

Strategies have been developed at OMEC that allow us to synthesise libraries of compounds (such as that shown above right), and select new lead compounds based on their properties. This work is now being extended to incorporate fluoro-substituted alkyl chains, of the types that have been shown to offer increased stability for the conjugated thiophene conductive unit.

The oligomers are purified by gradient sublimation, carried out in parallel, before being evaluated for applications.

Determination of physical properties

The semi-conducting properties of organic materials are highly sensitive to the bulk structure. In particular, the presence of a thermotropic liquid crystal phase can be very valuable in material processing.

Initial screening is therefore aimed at elucidating the basic thermo-mechanical behaviour of the new compounds. The basis of this evaluation is a Surface Acoustic Wave (SAW) device. This has been modified to allow measurements over a wide temperature range (from 50° to 350°C), and units are coupled into banks of ten, to allow parallel evaluations.

Above right: Melting endotherms for polyethylene oxide of low (Mn = 2,000) and moderate molecular weight (Mn = 20,000) determined using the SAW-DTA device.

Additional information can be obtained by the use of polarised optical microscopy, which can be conducted in parallel with the SAW measurements. Any potential ambiguities can be resolved using SAXS measurements.

Nanoscale measurements using scanning force microscopy and variants will provide topographical data. Additionally, recent advances in electrochemical AFM permit us to probe the electronic properties of each compound at the molecular level, so allowing us to evaluate the relative contribution to carrier mobility from the thiophene moieties, and their substituents.

Device fabrication

The new compounds to be tested are deposited on 10x10 grids of source/drain electrode pairs, with a range of channel lengths from 10 to 100 µm. Insulated gate electrodes are then lowered onto the layer of compound under test to complete the devices.