Controlled insulator-to-metal transformation in printable polymer composites with nanometal clusters

S. Sivaramakrishnan¹, P.J. Chia^1,2, Y. C. Yeo², L.L. Chua¹ and P.K.H. Ho¹

Nature Materials 6 (2007) 149

¹Department of Physics, NUS

²Department of Electrical and Computer Engineering, NUS

This paper describes the synthesis of highly water and alcohol soluble gold clusters that can be homogeneoulsy dispersed into poly(3,4-ethylenedioxythiophene) to give a material with annealed d.c. conductivity tunable between 10^-4 and 10⁵ S cm^-1. These composites can inject holes efficiently into all printed polymer organic transistors. The more resistive compositions show a field-induced transformation. These materials therefore find applications as resistor, memory, interconnect and electrode materials for organic electronics. We further show that details of the nature and extent of the metal-to-insulator transformation can be followed by accurate modeling of the optical spectra of the films.

Solvent Effects on Chain Orientation and Interchain pi -Interaction in Conjugated Polymer Thin Films: Direct Measurements of the Air and Substrate Interfaces by Near-Edge X-ray Absorption Spectroscopy

P.K.H. Ho¹, L.L. Chua^1,2, M. Dipankar¹, X. Gao¹, D. Qi¹, A.T.S. Wee¹, J. F. Chang² and R.H. Friend²

Advanced Materials 19 (2007) 215

¹Department of Physics, NUS

²Cavendish Laboratory, University of Cambridge

This paper describes the near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) on regioregular P3HT films deposited from different solvents and processing conditions. We demonstrate that an unprecedented breakdown of the "building block" picture occurs, and show from ab initio quantum chemical calculations that it arises from strong interchain pi-pi interaction due to the face-to-face stacking (as opposed to herringbone stacking) in this material. This interaction differs between the air and substrate interfaces, and varies also with deposition solvents and conditions. This therefore provides a method to directly measure and optimise the orientation and interchain packing in the frontier layers (2-3 nm) relevant to charge-injection and field-effect conduction in organic semiconductor thin films. Our conclusion for FETs is that the already high FET hole mobility of regioregular P3HT in "optimised devices" is still limited by disorder and so can be potentially higher. This project is a partnership between ONDL, Andrew Wee's group at the Surface Science Laboratory (NUS) and Richard Friend's group at University of Cambridge.

Electromigration of the conducting polymer in organic semiconductor devices and its stabilization by cross-linking

R.Q. Png^1,2, P.J. Chia^1,3, S. Sivaramakrishnan¹, L.Y. Wong¹, M. Zhou¹, L.L. Chua¹ and P.K.H. Ho¹

Applied Physics Letters 91 (2007) 013511

¹Department of Physics, NUS

²Department of Materials Science, NUS

³Department of Electrical and Computer Engineering, NUS

X-ray photoelectron spectroscopy (XPS) measurement of the ratio of poly(3,4-ethylenedioxythiophene) (PEDT) to polystyrenesulfonate (PSS) reveals accumulation of PEDT⁺ at the interface between the PEDT:PSSH hole-injection layer and the organic semiconductor during diode operation. This ionic drift of PEDT⁺ occurs even at low fields of 1 Vcm^-1, which will have an impact on the operational stability of the characteristics of organic light-emitting diodes. XPS and Raman spectroscopy indicate that dedoping of PEDT⁺ does not occur significantly in hole-only devices.

General Photo-Patterning of Polyelectrolyte Thin Films via Efficient Ionic Bis(Fluorinated Phenyl Azide) Photo-Crosslinkers and their Post-Deposition Modification

S.H. Khong, S. Sivaramakrishnan, R.Q. Png, L.Y. Wong, P.J. Chia, L.L. Chua and P.K.H. Ho

Advanced Functional Materials 17 (2007) 2490

Department of Physics, NUS

We have extended the well known bisfluorinated(phenyl azide) (bisFPA) methodology to develop an ionic bisFPA process suitable for photo-crosslinking a wide variety of polyelectrolyte thin films. This method is based on the photo-induced formation of singlet nitrenes from FPAs and their insertion into unactivated C-H or other bonds, which thus general and not dependent on the presence of specific chemical functional groups. We found that efficient photo-crosslinking can then be achieved for polyelectrolytes even when they have nucleophilic groups. These include poly(styrenesulfonic acid) and their salts, poly(acrylic acid) and their salts, poly(dimethyldiallylammonium salts), as well as the electrically-conducting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic acid) complex (PEDT:PSSH). We further demonstrate using this ionic bisFPA methodology by both photo-patterning and post-deposition chemical modifications of polyelectrolyte thin films. This opens broad new possibilities in membrane, sensor and actuator technologies, as well as for organic semiconductor plastic electronics (such as field-effect transistors) and polyelectrolyte-based devices.

Injection-induced De-doping in a Conducting Polymer during Device Operation: Asymmetry in the Hole Injection and Extraction Rates

P.J. Chia^1,3, L.L. Chua¹, S.Sivaramakrishnan¹, J.M. Zhuo^1,2, L.H. Zhao¹, W.S. Sim², Y.C. Yeo³, P.K.H. Ho¹

Advanced Materials 19 (2007) 4202

¹Department of Physics, NUS

²Department of Chemistry, NUS

³Department of Electrical and Computer Engineering, NUS

In the paper, a mechanism is described that can alter the doping level of conducting polymers during normal electrical injection, which arises from differential charge extraction/ injection into transport sites, and which is likely to be an intrinsic feature of the hopping transport itself. This and not Joule heating is the cause of the conductance decay in PEDT:PSS at strong electrical injection, in which the polyelectrolyte counter-ion also plays a key role. Neutral (non-acidic) PEDT:PSS is more stable in this regard.