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42258-AC10
An Infrared Probe of Charge Injection in Polymer Field Effect Transistors

Dimitri Basov, University of California (San Diego)

This grant supports a program aimed at spectroscopic study of the charge injection and electronic transport in organic materials using IR spectroscopy. The PI has systematically investigated charge injection and charge dynamics in Poly(3-hexylthiophene) (P3HT) based FET devices and rubrene based transistors. These studies have allowed us to address and resolve many long-standing issues in organic materials.

1: Summary of published work

Experimental results obtained with the PRF support since the inception of the award have been published in 5 articles. This work has received broad media coverage: Photonics Spectra magazine (March 2006), Science@Berkeley Lab (April 2006), and ALS science highlights (July 2006) of Berkeley National Lab.

2: Research accomplishments from prior support

The proposed research program was focused on four areas that are among the central issues in the field of organic electronics:

1) Length scales of the charge injection process in organic FET;

2) Merits and pitfalls of high dielectric constant insulators in organic FET structures;

3) Low-energy electronic excitations and charge dynamics in P3HT;

4) The insulator-metal transition at high injection levels in P3HT FETs.

Systematic spectroscopic experiments carried out by the PI have lead to several significant advances in the understanding of charge injection and electronic transport in organic materials and organic FET devices1, 2, 3. Key accomplishments within the focused areas #1)- #4) and beyond will be outlined below.

Focus area 1) and 2): charge injection landscape and the role of gate insulators in organic FETs: The PI has successfully adapted the bottom-contact geometry for infrared/optical experiments with the polymer-based FET devices. The novelty of the device is in the large area “grid-electrode” structure displayed in Fig. 1A. We have succeeded for the first time in employing IR spectroscopy to directly probe the electronic excitations associated with the injected carriers in functional organic FET devices based on P3HT: IR active vibrational (IRAV) modes and polaron band. By spatially monitoring these spectroscopic fingerprints of the injected charges using IR microspectroscopy, one can study the charge injection landscape and obtain valuable information of the charge accumulation layer. IR microspectroscopy uncovers the critical role of the gate insulator in defining the charge injection length scales. In particular, we find that FETs employing high dielectric constant insulator TiO2 reveal finite charge injection length scale due to the limitations of the insulator, whereas FETs employing SiO2 gate insulator show no change of carrier density at least up to 1.6 mm away from the contacts limited only by the physical dimensions of our devices. This novel result can be applied in the search of optimized high-dielectric organic FET and the design of chemical or biological sensors.

Focus area 3): electronic excitations in P3HT FETs: The PI has carried out a comprehensive theoretical and experimental study of charge injection in P3HT FETs to explore the low-energy electronic excitations in P3HT. In collaboration with Professor Di Ventra's group, we calculate the optical absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. These theoretical scenarios are compared with data from IR spectroscopy on P3HT thin film FETs. IR measurements in conjunction with theoretical predictions suggest that charge-induced electronic states in P3HT FETs are bipolarons. This study has clarified the nature of the low-energy electronic excitations in P3HT based FET devices, which provides new insights into the novel magneto-optical functionalities of organic FET devices.

Focus area 4): insulator- metal transition in P3HT FETs: The PI has implemented a novel and general approach to evaluate the carrier density in the accumulation layer of organic FETs from an analysis of IR spectroscopic data. We find that the carrier density in P3HT reaches 1013 cm-2 at the highest gate voltage in our devices. In collaboration with Professor Di Ventra's group, we analyzed models for three scenarios under which charge-induced transitions between a bipolaron lattice and a polaron lattice or the merging between the localized states and the extended states can take place in doped P3HT. We predicted the critical doping thresholds above which metallic states can be expected in charge-injected P3HT. IR microspectroscopy in conjunction with detailed theoretical analysis shows that P3HT based FET devices operate in proximity of the insulator to metal transition, a fact which gives us high hopes to achieve the coveted metallic behavior in functional organic FET devices.

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