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Solid-State Electronics 52 (2008) 53–59 www.elsevier.com/locate/sse
MIS polymeric structures and OTFTs using PMMA on P3HT layers M. Estrada b
a,*
, I. Mejia a, A. Cerdeira a, B. In˜iguez
b
a Departamento de Ingenierı´a Ele´ctrica, CINVESTAV, Av. IPN No. 2508, Apto. Postal 14-740, 07300 DF, Mexico Departament dE´nginyeria Electronica Ele´ctrica i Automatica, Universitat Rovira i Virgili, Avda. Paisos Catalans 26, 43007 Tarragona, Spain
Received 25 November 2006; received in revised form 3 July 2007; accepted 11 July 2007 Available online 23 August 2007
The review of this paper was arranged by Prof. Y. Arakawa
Abstract In this paper we present a detailed characterization of metal–isolator–semiconductor MIS structures and organic thin film transistors (OTFTs) using poly(methyl methacrylate) (PMMA) as gate dielectric on top of a semiconductor poly(3-hexylthiophene) (P3HT) layer. The PMMA layer was spin coated from a 6% dilution of PMMA in anisole. The P3HT layer was spin coated from a 0.66 wt% dilution of P3HT in chloroform. OTFTs with upper gate were fabricated using photolithographic processes. The current density across the dielectric is below 1 · 106 A/cm2. The interface states density is below 1 · 1011 cm2, while the flat band voltage shift is less than 0.5 V for bias stress in the range of ±20 V. Accumulation occurs for gate voltage below 10 V, allowing OTFTs to work in the voltage range below 30 V, with threshold voltage around 2.5 V. Mobility was 2.5 · 103 cm2/V s, which is among highest values reported for P3HT OTFTs working in this voltage range. 2007 Elsevier Ltd. All rights reserved. Keywords: Polymeric MIS structures; Polymeric TFTs; OTFTs; PMMA and P3HT properties
1. Introduction Polymeric materials are widely studied looking for better characteristics in organic thin film transistors (OTFTs) applications. Several polymeric materials have been studied as semiconductor layers for OTFTs. Among them, poly (3-alkylthiophene)s (P3AT), which are derivatives of the polythiophene (PT), have been widely studied because of their higher mobility with respect to other materials [1,2]. One of these derivatives, poly(3-hexylthiophene) (P3HT), has provided high mobility values, for example, 0.045 pffiffiffiffiffiffiffi cm2/V s [3], calculated from the slope of the curve I DS versus VGS in the drain and gate voltage range near 100 V. The device had W = 250 lm, L = 12 lm and 300 nm of SiO2 as a gate dielectric.
*
Corresponding author. E-mail address:
[email protected] (M. Estrada).
0038-1101/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2007.07.007
Several polymers have also been studied to use as gate dielectric [4–6], see Table 1, although most of them have been used with pentacene and other small molecule organic layers. OTFTs with semiconductor polymers have been fabricated mostly with SiO2 as dielectric, due to the lack of compatibility between the diluents of the semiconductor and dielectric polymer. However, what is really a main goal is to fabricate all polymeric OTFTs, where both semiconductor and gate dielectric layers, as well as the electrodes are polymers. Regarding this objective, we have found only one report showing an OTFT fabricated using a poly (methyl methacrylate) (PMMA)-based dielectric with P3HT as semiconductor and poly(ethylenedioxythiophene)–poly(styrene sulfonic acid) (PEDOT/PSS) as metal contact [7]. However, authors do not specify the characteristics of the PMMA-based polymer, nor present any characterization of the MIS structure formed by the two polymers used as dielectric and semiconductor. They only present the output characteristic and some properties of the OTFTs they fabricated.
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M. Estrada et al. / Solid-State Electronics 52 (2008) 53–59
Table 1 Properties of dielectric polymers used as gate dielectric Dielectric polymer Cyanoethyl pullulan (CEP) PM = 489,000, 9 wt% diluted in N,N-dimethyl formamide Poly Methyl Methacrylate (PMMA) PM = 950,000, 3 wt% diluted in chlorobenzene PMMA PM 967,000 5.6–11.2 wt% diluted in anisole Cross-linked (PVP) poly-4-vinylphenol and poly(melaminecoformaldehyde) diluted in propylene glycol monomethyl ether acetate (PGMEA) Poly-4-vinylphenol-co-2-hydroxyethylmethacrylate (PVP copolymer) diluted in n-methylpyrrolidinone (NMP)
Fcrit (V/cm)
ki
6
J (A/cm2)
Reference
6
[4]
ki = 16 (at 1 MHz). After thermal annealing (TA) at 90–150 C, ki = 20 (at 150 C, ki = 2.8 ki = 3, 6
