Biocellulose Based Materials for Organic FieldEffect Transistors

Luı́s Alcácer,Jorge MorgadoQuirina Ferreira

Instituto de TelecomunicaçõesInstituto Superior Técnico

Av. Rovisco Pais,P-1049-001 Lisboa, Portugal

Édison PecoraroInstituto de Telecomunicações

Campus de SantiagoP-3810-193 Aveiro, Portugal

Carlos Pascoal NetoArmando J.D. Silvestre

Carmen S.R. FreireEliane Trovatti

Susana C.M. FernandesCICECO, University of Aveiro

Campus de SantiagoP-3810-193 Aveiro, Portugal

Abstract—Biocellulose and biocellulose composites are beinginvestigated as substrates and as gate dielectrics for organicfield effect transistors. Spin coated films of regioregularpoly(3-hexylthiophene) (RR-P3HT) and thermally evaporatedpentacene were used as semiconducting materials. Preliminaryresults on device characteristics are reported.

Keywords: Organic field effect transistors, Biocellulose, Bacte-rial Cellulose

I. INTRODUCTION

Adequate substrate and dielectric materials for organicelectronic devices are still a research subject, specially forcost-effective processing of high volumes using technologiessuch as inkjet printing and roll-to-roll processing. At presentthe best compromise for substrates, considering properties,availability and cost-effectiveness, is achieved with either(poly(ethylene naphtalate) (PEN) or poly(ethylene terephta-late) (PET)[1]. More critical still are dielectric materials whichshould be good insulators and form good quality thin films.

Common paper has been explored as substrate, but it lacksadequate smoothness and hydrophobicity character to enableadherence of the materials that need to be deposited on top.In all reported cases it had to be covered with a polymer layerin order to become water resistant and with a smooth surfaceto enable deposition of the active layers [2], [3], [4], [5].

Bacterial cellulose or biocellulose (BC) is produced by aparticular strain of Acetobacter xylinum in a wet state, but afterprocessing it looks like butter paper. It is a pure substance,biodegradable, renewable and it has a continuous fibrousnanostructure with high dimensional stability and mechanicalstrength. It can be chemically modified, coated with othermaterials and incorporated in nanocomposites giving a varietyof products with interesting properties which open the groundsfor many potential applications. Its properties compare veywell with those of PET and PEN, when aiming at applicationsin organic electronics. Its dielectric constant is in the range ofk = 1.6−1.9 [6] and it can incorporate carbon nanotubes [7],gold [8] and many kinds of nanoparticles [9],[10], [11]. It isstable up to 260 oC [12], which is better than PEN and PET(220 oC and 150oC, respectively). It has higher glass transition

temperatures: Tg = 200 − 230 oC [12] for biocellulosewhereas, Tg = 120 oC and Tg = 78 oC, for PEN and PETrespectively [1]. BC nanofibers and BC nanocomposites haveYoung modulus at 20 oC of the order of 138 Gpa and 21 Gpa,respectivelly [13] whereas, for PEN and PET it is 5 Gpa and4 Gpa, respectively. Most relevant is its low coefficient ofthermal expansion, (0.1-6 ppm/oC including in composites)[13]. Those of PEN and PET are 18-20 ppm/oC and 20-25 ppm/oC, respectively. Its hydrophobic character and surfaceenergy can be controlled to enable the deposition of activelayers [14],[15]. When homogenized, it can be spin coated ontop of various materials, giving smooth surfaces [16], [17],[18]. Transparent BC-acrylic nanocomposites, covered withsputtered transparent conductive ITO, have been proposedfor OLED substrates [13], [19], and ionically conductivebiocellulose has been proposed for electronic paper [20].

Apart from its use in headphone membranes (Sony), biocel-lulose is best known for medical applications and in the foodindustry. The commercial production is restricted to companiesthat possess the bacteria overproducer strain, namely a fewcompanies in Brazil, farmers in Southeast Asia and someresearch laboratories in Europe and USA. Companies donot reveal production costs, but it is presently estimated at2.3 euro/kg to 23 euro/kg depending on the feeding source[21]. However production costs of BC will certainly go downas demand and production volumes increase and may reachcompetitive values in the future for applications such asorganic electronics, attending that BC has better propertiesand is environmentally friendly.

Biocellulose has been produced in Portugal in laboratoryscale at the University of Minho and lately at the Universityof Aveiro. The Macromolecular and Lignocellulosic MaterialsGroup (LMG) from the University of Aveiro supplied biocellu-lose for this work. In 2009 the group established an RD projectaiming at cost-effectiveness and industrial scale production ofbiocellulose.

AlcacerText BoxProc EUROCON and CONFTELE 2011, Lisbon, Portugal, Vol. na, pp. na - na, April, 2011

II. MATERIALS PREPARATIONA. Biocellulose

The microorganism used for BC production was isolatedin LMG laboratories and identified as Gluconacetobacterxylinus. The strain was isolated from Kombucha tea. TheBC membranes were cultivated in 500mL Erlenmeyer flaskscontaining 50mL of Hestrin-Schramm’s liquid medium witha 10% (v/v) of pre-inoculum. The flasks were transferred toa static incubator at 30 ◦C for 96h. The BC membrane iswithdrawn from the culture media and purified by washingwith a 0.5 M NaOH solution at 90oC for 30 min. Thisprocedure is repeated three times to eliminate attached cells. Itis followed by washing with distilled water to remove culturemedium residues until pH neutralization. The drying processconsists on pre-pressing the membranes to remove 90% ofwater content. Next, they are transferred to an aluminiumplates press at 120oC for 30min to remove the remaining water,and cooled to room temperature [22].

B. Biocellulose nanocomposites

For the preparation of the BC-acrylic [23] and BC-chitosannanocomposites [24], two commercially available acryliccopolymer emulsions were first diluted fourfold with water,and 1.5 % solutions were prepared, by dissolving the powderedchitosan samples in aqueous acetic acid (1 % v/v), respectively.Different amounts of BC were added to the acrylic emulsionsand the chitosan solutions, in order to obtain BC-acrylic andBC-chitosan nanocomposites with BC contents of 1, 2.5, 5 and10 % (dry weight of BC in relation to the dry weight of thepolymeric matrices) and 5, 10 and 20 %, respectively. Then,the biocellulose was dispersed in these emulsions or solutionsand homogenized using an Ultra-Turrax stirrer for 40 minutes(500 rpm) and degassed to remove entrapped air. Both BC-acrylic and BC-chitosan nanocomposites were then preparedby casting at 30 oC in a ventilated oven for 16 h, using aPetri dish coated with a fluorinated aluminum foil (10 cmdiameter) as the mould. Finally, the BC-acrylic sheets werefurther pressed at 4MPa and 95 oC and 75 oC (depending onthe acrylic copolymer), during 10 minutes.

III. BIOCELLULOSE AND COMPOSITES AS SUBSTRATESThe surface of bare dried and pressed biocellulose is very

irregular with depths reaching ca. 100 nm, the average rough-ness being of the order of 40 nm (Fig. 1). This value maybe inappropriate for a substrate of a field effect transistor,but such high values could be suppressed by coating thesurface with either a polymer such as polymethylmethacry-late (PMMA), a PMMA-TiO2 composite or a biocellulosecomposite. Surface roughnesses of the order of 0.31 nmwere obtained for PMMA-TiO2 (1 µm thickness) (Fig. 2),while values of order of 12.5 nm (640 nm thickness) wereobtained for a biocellulose-acrylic composite substrate, coatedwith a biocellulose-chitosan composite (Fig. 3). The lineprofile of the PMMA:TiO2 coated substrate indicates a verysmooth surface comparing to the one of the bare biocellulosesubstrate. Those values were measured by AFM imaging.

-138 nm

92 nm

1

3

3

0.1

0.0

-0.1

-0.2

0

Length (µm)

He

igth

(µ

m)

1 2 3

1 2

2

0

0 µm

Fig. 1. AFM image (topography) and profile of the surface of bare driedand pressed biocellulose (average roughness: 40 nm).

-1.85 nm

1.61 nm

1.0

1.0

0.1

0.0

-0.1

-0.2

0

Length (µm)

He

igth

(µ

m)

0.5 1

0.5

0.5

0.0

0.0 µm

1 nm

0

-110 0.5 nm

Fig. 2. AFM image (topography) and profile of the surface of a biocellulosesubstrate coated with a 1 µm PMMA:TiO2 (average roughness: 0.31 nm).Insert: same profile with expanded heigth (in nm).

IV. ORGANIC FIELD EFFECT TRANSISTORS USINGBIOCELLULOSE BASED MATERIALS

These biocellulose and biocellulose composites were usedas substrates for organic field effect transistors (OFETs) com-bined with solution processable dielectrics such as PMMAand BC derivatives. Spin coated films of regioregular poly(3-hexylthiophene) (RR-P3HT) and thermally evaporated pen-tacene were used as semiconducting materials.

A scheme and photograph of an OFET in a bottom gateconfiguration on a biocellulose substrate, as those reportedin this paper, are shown in Fig. 4. The Channel length is100 µm and the channel width 4 mm. The RR-P3HT OFETwas manufactured in a glove box under dry nitrogen, but thepentacene transistors were manufactured in air, at ambientconditions. A biocellulose-chitosan composite was used as thegate dielectric in all transistors reported in this paper. Outputand transfer characteristics were traced on a HP 4140B pAMeter/DC Voltage Source. Figs. 5 and 6 show the outputand transfer characteristics, respectively, of an organic fieldeffect transistor on a glass substrate with ITO gate and thebiocellulose-chitosan composite (490 nm thick) as dielectric.The RR-P3HT semiconductor was deposited by spin coatingfrom a chloroform solution inside a glove box and the sourceand drain contacts were made of thermally evaporated gold.Due to the high sensitivity of this polymer, which becomesintensively doped upon exposure to ambient conditions (airand humidity), OFETs were kept inside the glove box whilebeing characterised. As shown in Fig. 5, there is a modulationof the source-drain current by the gate voltage. This modula-tion is only observed under negative bias, which is consistent

-24 nm

39 nm

2

8

6

8

0.1

0.0

-0.1

-0.2

0

Length (µm)

He

igth

(µ

m)

2 4 6 8

4 62

4

0

0 µm

Fig. 3. AFM image (topography) and profile of the surface of a biocellulose-acrylic composite (ACBC) substrate coated with a 640 nm thick biocellulose-chitosan composite (QBC) (average roughness: 12.5 nm).

Substrate

Dielectric

Semicondutor Source Drain

Gate

a) b)

Fig. 4. Scheme and photograph of an organic field effect transistor on abiocellulose substrate. Channel length = 100 µm; channel width = 4 mm.

with this being a p-type (hole-type) semiconductor, being thedevice operated in accumulation regime. As shown in Fig. 6,the transistor is normally off. This result is similar to previousreports using either organic or inorganic dielectrics (such assilicon dioxide) [25]. The On/Off ratio between 0 and −15 V,from the transfer curve in Fig. 6, is ca. 100, which is rathersmall. Saturation field effect mobility, assuming a dielectricconstant of 1.7 for the biocellulose-chitosan dielectric, is5.7 × 10−2cm2/Vs, as estimated from the linear part of theI1/2sd vs. Vg plot, using eq. (1).

Isd = µC0W

2L(Vg − Vt)2 (1)

For comparison, Fig. 7 shows the transfer curve for an OFETbased on RR-P3HT with silicon dioxide, 625 nm thick, asdielectric, prepared and tested under similar conditions. Forthis case a lower mobility of 4.3× 10−4 cm2/Vs is obtained.It is worth noting that the hysteresis found in the transfer curveof the device with the biocellulose-chitosan composite (Fig.6)

Fig. 5. Output characteristics of an organic field effect transistor on glass/ITOwith a biocellulose-chitosan composite (490 nm thick) as dielectric and RR-P3HT as the semiconductor

Fig. 6. Transfer curve of the device with output shown in fig. 5

Fig. 7. Transfer curves for P3HT-based OFETs having silicon dioxide (625nm thick) as gate dielectric.

is the reverse of that found in the silicon dioxide (Fig. 7).While the first is likely due to ion displacements within thedielectric, the hysteresis in SiO2-based devices is attributed tostress. OFETs were also prepared using biocellulose as flexiblesubstrate and pentacene, as organic semiconductor, which isdeposited by vacuum sublimation. Figure 8 shows the outputcharacteristics of a pentacene field effect transistor fabricatedover biocellulose coated with a 1 µm PMMA:TiO2 film, assubstrate, and using the biocellulose-chitosan composite (490nm thick) as dielectric. As evidenced in Fig. 8, a typicalbehaviour of a p-type transistor is observed. However, no clearsaturation of the output current is observed, showing insteadan almost linear behaviour. The On/Off ratio is very small.When a biocellulose-acrylic composite was used as substrateand a biocellulose-chitosan composite (QBC) as dielectric(392 nm thick), a modulation of the source-drain current isalso observed, as shown in Fig. 9. These results obtainedwith pentacene as semiconductor, though encouraging andevidencing the potential of using BC as substrate, requirefurther studies in order to improve devices performance.

V. CONCLUSION

The preliminary results reported in this paper show thatbiocellulose and its composites and derivatives are potentiallyinteresting materials for organic electronics to be used both assubstrates and as gate dielectric insulators. In fact, chitosan-biocellulose nanocomposites containing between 5 and 20%

Fig. 8. Output characteristics of an organic field effect transistor ona biocellulose substrate coated with a 1 µm PMMA:TiO2 film and abiocellulose-chitosan composite (490 nm) as dielectric, with pentacene as thesemiconductor.

Fig. 9. Output characteristics of pentacene transistor on a biocellulose-acryliccomposite substrate and a biocellulose-chitosan composite (QBC) as dielectric(392 nm thick).

biocellulose were shown to be reasonable gate insulators in or-ganic field effect transistors. In addition, biocellulose-polymercomposites such as biocellulose-acrylic resin nanocompositeswere sucessfully used as substrates for such devices. Evenbare biocellulose looks a promising substrate, in spite ofits surface roughness (of order of 40 nm or more). Thefact that biocellulose and derivatives are hygroscopic has, ingeneral, been neglected in this preliminary study, but this factcould eventually be an advantage for hygroscopic insulatorfield-effect transistors (HIFET), which can be produced bysolution processing performed without requiring controlledatmosphere, as first reported by Sandberg et al [26]. Suchdevices have potential applications in sensors.

ACKNOWLEDGMENT

This work was partly supported by Fundação paraa Ciência e a Tecnologia (FCT) through contractsPTDC/CTM/102144/2008 and Fis/72831/2006.

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