Elsevier

Organic Electronics

Volume 7, Issue 6, December 2006, Pages 514-520
Organic Electronics

Effect of side chain length on molecular ordering and field-effect mobility in poly(3-alkylthiophene) transistors

https://doi.org/10.1016/j.orgel.2006.07.007Get rights and content

Abstract

The effect of alkyl side chain length on the molecular ordering and electrical properties of regioregular poly(3-alkylthiophene) (P3AT)-based field-effect transistors (FETs) was investigated using P3ATs with various alkyl side chain lengths (–(CH2)nCH3, n = 3, 5, and 7) as active materials. The inner structures and surface morphologies of the P3ATs thin films fabricated on an insulator substrate were characterized, and the electrical properties of FETs based on these films were correlated with the structure and alkyl chain length of the P3ATs. The FET based on poly(3-butylthiophene), which has the shortest alkyl side chains among the three P3ATs considered, showed the highest field-effect mobility, possibly because the film formed from molecules with short butyl side chains had a higher density of π-stacked ordered structures in the charge transport region.

Introduction

The high field-effect mobility, stability and solution processability of regioregular poly(3-alkylthiophene)s (P3ATs) has stimulated considerable interest in the utilization of these fascinating polymers as the active materials in organic field-effect transistors (OFETs) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. In particular, previous studies have shown that a preferential supramolecular two-dimensional ordering of the P3AT chains with high regioregularity gives rise to a high field-effect mobility [10] of up to 0.3 cm2 V−1 s−1, a value approaching that of single crystalline oligothiophenes.

Many factors affect the structures of P3AT films and the electric properties of FETs based on these films: molecular parameters such as regioregularity [3], molecular weight [16] and side chain length [17], [18], [19], [20], [21], [22], [23], [24], [25], and processing conditions such as solvent power [6], film thickness [26], doping level [27] and the method used to form the film [28]. For P3ATs, one of the key factors is the alkyl side chain length which greatly affects the solubility of these polymers in organic solvents. P3ATs with long alkyl side chains are highly soluble, which facilitates the fabrication of transistor devices by solution processing. However, for linear alkyl chains, the field-effect mobility is expected to decrease with increasing chain length due to the isolated nature of the alkyl substituents. Several groups have examined the effect of side chain length on the thin film structures and electrical properties of P3AT thin films in TFTs [20], [21], [22], [23], [24]. Despite this previous work, the role of the alkyl side chains is poorly understood and the correlation between the film structure and FET electrical properties has not been systematically studied. In the present study, we systematically characterized the inner structures and surface morphologies of thin films of P3ATs with three side chain lengths (alkyl = butyl [P3BT], hexyl [P3HT], octyl [P3OT]), and evaluated the electrical performances of FET devices based on these P3ATs. In addition, we determined the correlation between the thin film structure and electrical properties of the FETs according to the side chain length of the P3AT.

Section snippets

Experiments

The regioregular P3ATs used in this study, P3BT, P3HT, and P3OT, were obtained from the Rieke Metals Incorporation. The molecular weights of the alkylthiophenes had almost same values (Mn = 45–50 kg/mol). The coupling ratio of head–tail to head–head and tail–tail was estimated to be about 93% by NMR integration.

A highly doped Si substrate was used as the transistor substrate as well as the gate electrode. A thermally grown silicon dioxide (SiO2) layer of thickness 300 nm acted as the gate

Results and discussion

Fig. 1 shows AFM topographic and phase images of P3AT thin films on insulator substrates (Si/SiO2) [29], [30]. Before annealing, the spin-coated film shows featureless characteristics (Fig. 1(a)). However, after annealing by slow heating to the melting temperature above the order–disorder transition temperature of the P3AT backbone chain, followed by slow cooling, peculiar morphologies indicative of nanoribbon structures with the width of 20–40 nm are observed in the phase images (Fig. 1

Summary

The inner structure and surface morphologies of a P3BT, P3HT, and P3OT thin films on insulator substrates were characterized, and the electrical properties of FETs based on these polymers were correlated with the structure and alkyl side chain length of the P3ATs. Thermal annealing of the thin films above the order–disorder transition temperature of the P3AT allowed the P3AT molecules to rearrange into an ordered structure. X-ray reflectivity and GIXD studies of the P3BT, P3HT, and P3OT thin

Acknowledgements

This work was supported by the National Research Laboratory Program and ERC Program (R11-2003-006-03 005-0) of the MOST/KOSEF, a grant (F0 004 022) from the Information Display R&D Center under the 21st Century Frontier R&D Program and the Regional Technology Innovation Program (RTI04-01-04) of the MOCIE, and the BK21 Program of the Ministry of Education and Human Resources Development of Korea. The authors are grateful to the Pohang Accelerator Laboratory for access to synchrotron radiation at

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