Organic Electronics 11 (2010) 1974–1990
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Environmentally sustainable organic field effect transistors
Mihai Irimia-Vladu a,f,⇑, Pavel A. Troshin b, Melanie Reisinger a, Guenther Schwabegger c,
Mujeeb Ullah c, Reinhard Schwoediauer a, Alexander Mumyatov b, Marius Bodea d,
Jeffrey W. Fergus e, Vladimir F. Razumov b, Helmut Sitter c, Siegfried Bauer a,
Niyazi Serdar Sariciftci f
a
Department of Soft Matter Physics, Johannes Kepler University, A-4040 Linz, Austria
Institute of Problems of Chemical Physics of Russian Academy of Sciences, Semenov Prospect 1, 142432, Chernogolovka, Moscow Region, Russia
Institute of Semiconductor and Solid State Physics, Johannes Kepler University, A-4040 Linz, Austria
d
Institute of Applied Physics, Johannes Kepler University, A-4040 Linz, Austria
e
Materials Research and Education Center, Auburn University, Auburn, AL 36849, USA
f
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, A-4040 Linz, Austria
b
c
a r t i c l e
i n f o
Article history:
Received 16 July 2010
Received in revised form 3 September 2010
Accepted 4 September 2010
Available online 26 September 2010
Keywords:
Environmentally sustainable electronics
Organic field effect transistors
Natural materials
Biodegradable electronics
Biocompatible electronics
Edible electronics
a b s t r a c t
Environmentally sustainable systems for the design, production, and handling of electronic
devices should be developed to solve the dramatic increase in electronic waste. Sustainability in plastic electronics may be the production of electronic devices from natural materials, or materials found in common commodity products accepted by society. Thereby
biodegradable, biocompatible, bioresorbable, or even metabolizable electronics may
become reality. Transistors with an operational voltage as low as 6 V, a source drain current of up to 0.5 lA and an on–off ratio up to four orders of magnitude, with saturated field
effect mobilities in the range of 1.5 10 4 to 2 10 2 cm2/V s, have been fabricated with
such materials. Our work comprises steps towards environmentally safe devices in lowcost, large volume, disposable or throwaway electronic applications, such as in food packaging, plastic bags, and disposable dishware. In addition, there is significant potential to
use such electronic items in biomedical implants. As such, organic materials offer a unique
opportunity to guide electronics industry towards an environmentally safe direction.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Two major concerns in the world nowadays are plastic
consumption and waste. Due to the economic growth
and the increased demand in developing countries, plastics
consumption is projected to increase by a factor of two to
three during the current decade [1]. A consequence of this
incessant demand of plastics in the world is the accumulation of non-biodegradable solid waste and plastic litter
⇑ Corresponding author at: Department of Soft Matter Physics and Linz
Institute for Organic Sollar Cells, Johannes Kepler University, Altenberger
Strasse Nr. 69, 4040 Linz, Austria. Tel.: +43 732 2468 9293; fax: +43 732
2468 9273.
E-mail address: [email protected] (M. Irimia-Vladu).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.09.007
with negative effects on our environment. As an example,
the amount of municipal solid waste per person per year
averages 440 kg/yr for China, 550 kg/yr for the European Union and 790 kg/yr for the United States, with
roughly half of the waste being electronic products and
plastics [1,2]. Taking into account the expected increase
of plastic electronics in low-cost, large volume, disposable
or throwaway applications, plastic waste problems may
become even more dramatic. More effort will be necessary
in order to minimize the negative impact of the increasing
production, consumption and disposal of both polymer
materials and electronic circuits [3].
There have been so far several initial reports addressing
the use of biodegradable substrates in organic electronics,
such as poly (L-lactide-co-glycolide) [4], paper [5–9],
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
leather [10] or even silk [11,12]. Here we present an initial
study of using natural, nature-inspired and common commodity materials which are widely accepted by society for
sustainable electronics. The rationale for choosing materials from sugar family, small molecular nucleobases, betacarotene, indigo, food colors (indanthrenes) and cosmetic
colors (perylene diimide) on substrates such as glucose,
hard gelatine and biodegradable polymers is to show
how large the materials base may become for organic electronics, when low cost and sustainability is a concern. We
limit our work to the electrical characterization of these
materials and to the demonstration of field-effect transistors, a first step towards building more complex electronic
integrated circuits. We consider unusual substrates, such
as Ecoflex, hard gelatine capsules and caramelized glucose,
to enhance the substrate materials base for organic electronics. High-performance, biocompatible and biodegradable organic field-effect transistors operating at low
voltages are demonstrated by the evaporation of ultrathin
layers of natural nucleobase dielectrics (adenine and guanine) on inorganic oxide dielectrics, a viable alternative to
the passivation of such oxide dielectrics with self-assembled monolayers [13,14]. Semiconductors chosen include
natural compounds like beta-carotene and indigo, as well
as perylene diimide (a lipstick colorant), indanthrene yellow G and indanthrene brilliant orange RF, colorants used
for example in textile and food industry. Having identified
natural and nature-inspired p- and n-type semiconductors,
opens ways to fabricate integrated circuits, based on
inverters, ring oscillators, logical elements, etc. We hope
that this initial study initiates an intense search for new
materials and material combinations to finally end up with
sustainable plastic electronic products.
2. Experimental
2.1. Preparation and/or purification of materials
Dielectric and semiconductor compounds employed
(adenine, guanine, cytosine, thymine, caffeine, indigo,
indanthrene yellow G) were purchased from Sigma–
Aldrich and additionally purified by two vacuum sublimation cycles performed in a closed quartz tube. Indanthrene
brilliant orange RF was purchased from Shanghai Jucheng
Chemical Co., and purified by three vacuum sublimation
cycles; its chemical composition was proven by mass spectrometry. Poly (vinyl alcohol) (MowiolÒ40-88, electronic
grade) was purchased from Kuraray Specialities Europe
GmbH and used as received. D-(+)-glucose, lactose, and
beta-carotene were purchased from Sigma–Aldrich and
used without further purification. Ecoflex foil was purchased from BASF and hard gelatine capsules from a local
pharmacy in Linz, Austria.
2.1.1. Perylene diimide: synthesis and analysis
Perylene diimide (EH-PDI) was synthesized as shown in
Scheme 1. Perylene-3,4,9,10-tetracarboxylic acid dianhydride (5 g, 12.8 mmol) was mixed with 60 ml of freshly distilled quinoline, 15 g (116 mmol) of 2-ethylhexylamine
and ca. 100 mg of Zn(OAc)2H2O.
1975
The obtained mixture was heated at reflux for 2 h,
cooled down to room temperature and poured into
600 ml of 10% aqueous hydrochloric acid. The precipitate
formed was removed by filtration, extracted with methanol and dried in air. The resulting crude sample of perylene
diimide was purified by column chromatography. Elution
with a CH2Cl2–methanol mixture (95.5:0.5 v/v) produced
a pure compound EH-PDI. The final purification was performed by sublimation at 450–500 °C under a reduced
pressure of 10 2 mbar. The yield of EH-PDI was in the
range of 45–55%.
The NMR and FTIR spectra are summarized below:
1
H NMR (400 MHz, CDCl3,) d = 8.83 (d, 4H, H-Ar), 8.75
(d, 4H, H-Ar), 4.31 (m, 4H, NCH2), 2.14 (m, 2H, CH), 1.58
(m, 8H, CH2), 1.50 (m, 8H, CH2), 1.13 (t, 6H, CH3), 1.06 (t,
6H, CH3), ppm. Chemical analysis: C40H42N2O4.
Calculated: C, 78.15; H, 6.89; N, 4.56. Found: C, 77.95;
H, 6.97; N, 4.61.
IR spectrum (KBr pellet): m = 745, 809, 1178, 1247, 1308,
1346, 1380, 1403, 1441, 1577, 1594, 1615, 1650, 1695,
2859, 2873, 2930, 2958 cm 1.
2.2. Device fabrication
2.2.1. Glucose/caffeine-beta-carotene OFET on glass
Precursor solution of D-(+)-glucose (0.9 g/ml) and
caffeine (0.02 g/ml) was prepared in deionized water
(18 MX cm). The mixture was stirred for 60 min at
60 °C. Beta-carotene solutions (10 mg/ml in chloroform)
were prepared under inert atmosphere in a glove bow.
Thin films of glucose containing also caffeine were spun
at 2000 rpm on top of a 100 nm thick patterned aluminium
gate on a 1.5 1.5 cm glass substrate. The spun samples
were dried overnight in a vacuum oven at 55 °C. Betacarotene thin film was spin coated at 2000 rpm for 60 s
under inert atmosphere. A 100 nm thick gold layer was
evaporated through a shadow mask to pattern the top
source and drain electrodes. The channel dimensions
(length and width) was L = 100 lm, and W = 1 mm. The
measured capacitance per unit area (by impedance spectroscopy) of a 3.1 lm thick dielectric film of glucose with
caffeine was C0d = 1.9 nF/cm2. The measured unit capacitance of a 2.6 lm thick film cast from glucose solution
(0.9 g/ml in deionized water, with no caffeine addition)
was 2.15 nF/cm2.
2.2.2. PVA/Indigo OFET on glass
Precursor solution of PVA (0.08 g/ml) was prepared in
deionized water (18 MX cm) and the mixture was stirred
for 24 h at 90 °C. Thin films of PVA were spun at 2000 rpm
on top of a 100 nm thick patterned aluminium gate on a
1.5 1.5 cm glass substrate. 100 nm thick indigo layer
was vacuum processed in an Edwards evaporator. A
100 nm thick aluminium layer was evaporated through a
shadow mask to pattern the top source and drain electrodes. The evaporation rate of indigo was 0.1 nm/s. The
channel dimensions (length and width) was L = 100 lm,
and W = 1 mm. The measured capacitance per unit area
(by impedance spectroscopy) of a 2 lm thick dielectric film
of PVA was C0d = 3.1 nF/cm2.
1976
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
O
O
O
O
O
O
NH2
O
O
Zn(OAc)2.H2O
N
N
Quinoline, reflux, 2h
O
O
EH-PDI
Scheme 1. Fabrication route of the perylene diimide semiconductor.
2.2.3. Glucose/C60 and lactose/C60 OFET on glass
Precursor solutions of D-(+)-glucose (0.9 g/ml in deionized water) and lactose (0.3 g/ml in DMSO) were spun at
2000 rpm on top of a 100 nm thick patterned aluminium
gate electrodes on 1.5 1.5 cm glass substrates. Thick film
of fullerene (100 nm), C60, was evaporated in an Edwards
vacuum evaporator. Thick aluminium (100 nm) was evaporated through a shadow mask to pattern the top source and
drain electrodes. The evaporation rate of fullerene was
0.1 nm/s. The channel dimensions (length and width) was
L = 100 lm, and W = 1 mm. The measured capacitances
per unit area (by impedance spectroscopy) for a 2.6 lm
thick dielectric film of glucose, C0d, was 2.15 nF/cm2 and
for a 0.85 lm thick dielectric film of lactose was 6.8 nF/cm2.
2.2.4. Guanine–C60 OFET on glass
A 1 mm wide-100 nm thick aluminium was evaporated
onto 1.5 1.5 cm2 glass slides to pattern the gate electrode. Thick guanine (425 nm) and C60 (100 nm) formed
the organic dielectric and semiconductor layers, respectively, whereas 100 nm aluminium was used to pattern
the source and drain electrodes. The evaporation rates of
guanine and fullerene were 0.75 nm/s and 0.1 nm/s,
respectively. The channel dimensions for the source and
drain electrodes were: L = 100 lm and W = 1 mm. The
measured unit capacitance of the dielectric was C0d =
9.25 nF/cm2.
2.2.5. Cytosine–C60 OFET on glass
A 1 mm wide-100 nm thick aluminium was evaporated
onto 1.5 1.5 cm2 glass slides to pattern the gate electrode.
Thick cytosine (300 nm) and C60 (100 nm) were vacuum
deposited to form the organic dielectric and semiconductor
layers, respectively, whereas 100 nm aluminium was used
to pattern the source and drain electrodes. The evaporation
rates of cytosine and fullerene were 2 nm/s and 0.1 nm/s,
respectively. The channel dimensions for the source and
drain electrodes were: L = 100 lm and W = 1 mm. The
measured unit capacitance of the dielectric was C0d =
13.8 nF/cm2.
2.2.6. Aluminium oxide–adenine–C60/pentacene OFET on glass
A 1 mm wide-100 nm thick aluminium gate was evaporated onto 1.5 1.5 cm2 glass slides and subsequently
anodized by immersing in citric acid solution and passing
a step voltage (up to a maximum of 40 V) at a constant current of 0.06 mA. Adenine (i.e., 10 nm thick for the sample
with C60 and 150 nm thick for sample with pentacene)
was vacuum deposited on top of AlOx to form the
combined inorganic–organic dielectric. A 100 nm thick
C60 and 100 nm pentacene, respectively, formed the organic semiconductors for the n- and p-type of OFETs. A 100 nm
thick aluminium formed the source and drain electrodes
for the sample with fullerene semiconductor; a 100 nm
thick gold formed the source and drain electrodes for the
sample with pentacene semiconductor. The evaporation
rate of adenine was 3 nm/s, whereas pentacene and C60
and were evaporated at 0.1 nm/s, respectively. The channel
dimensions for the source and drain electrodes were:
L = 100 lm and W = 1 mm. The measured unit capacitances
of the dielectrics were C0d = 99 nF/cm2 for the dielectric
built for the n-type sample, and 19.55 for the dielectric
of the p-type OFET.
2.2.7. Aluminium oxide–thymine–C60 OFET on glass
A 1 mm wide-100 nm thick aluminium gate was evaporated onto 1.5 1.5 cm2 glass slides and subsequently
anodized using the same method as described above. Thymine (i.e., 75 nm thick) was vacuum deposited on top of
AlOx to form the combined inorganic–organic dielectric. A
100 nm thick C60 formed the organic semiconductor. A
100 nm thick aluminium formed the source and drain electrodes. The evaporation rate of thymine and C60 was 0.1 nm/
s. The channel dimensions for the source and drain
electrodes were: L = 100 lm and W = 1 mm. The measured
unit capacitances of the combined dielectric was C0d =
23.6 nF/cm2.
2.2.8. Aluminium oxide–guanine/adenine–indanthrene yellow
G OFET on glass
A 1 mm wide-100 nm thick aluminium gate was evaporated onto 1.5 1.5 cm2 glass slides and subsequently
anodized by immersing in citric acid solution and passing
a step voltage (up to a maximum of 40 V) at a constant current of 0.06 mA. A transmission electron microscopy (TEM)
picture of a cross-section of the anodized electrode revealed
a thickness of 55 nm of aluminium oxide. Two alternating
layers of guanine and adenine (i.e., 2.5 nm thick guanine
and 15 nm thick adenine) were vacuum deposited on top
of AlOx to form the combined inorganic–organic dielectric.
A 100 nm thick indanthrene yellow G and 100 nm aluminium formed the semiconductor and source/drain electrodes,
respectively. The evaporation rates of adenine, guanine and
indanthrene yellow G were 3 nm/s, 0.75 nm/s and 0.1 nm/s,
respectively. The channel dimensions for the source and
drain electrodes were: L = 100 lm and W = 1 mm. The
measured unit capacitance of the dielectric was C0d =
81.6 nF/cm2.
2.2.9. Aluminium oxide–adenine/guanine–perylene diimide
OFET on glass
A 1 mm wide-100 nm thick aluminium gate was evaporated onto 1.5 1.5 cm2 glass slides and subsequently
anodized by immersing in citric acid solution and passing
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
a step voltage (up to a maximum of 40 V) at a constant current of 0.06 mA. Two alternating layers of guanine and adenine (i.e., 2.5 nm thick guanine and 15 nm thick adenine)
were vacuum deposited on top of aluminium oxide to form
the combo inorganic–organic dielectric. A 100 nm thick
perylene diimide and 100 nm aluminium patterned the
semiconductor and source and drain electrodes, respectively. The evaporation rates of adenine, guanine and perylene diimide were 3 nm/s, 0.75 nm/s and 0.1 nm/s,
respectively. The source and drain electrodes had the channel dimensions: L = 100 lm, W = 1 mm. The measured
capacitance per unit area of the combined dielectric was
C0d = 81.6 nF/cm2. The measured capacitance per unit area
of the plain AlOx was 140 nF/cm2.
2.2.10. Aluminium oxide–adenine/guanine–indanthrene
brilliant orange RF OFET on glass
A 1 mm wide-100 nm thick aluminium gate was evaporated onto 1.5 1.5 cm2 glass slides and subsequently
anodized by immersing in citric acid solution, using the
method described. Two alternating layers of guanine and
adenine (i.e., 2.5 nm thick guanine and 15 nm thick adenine) were vacuum deposited; followed by 100 nm thick
indanthrene brilliant orange RS and 100 nm gold source
and drain electrodes. The source and drain electrodes had
the channel dimensions: L = 35 lm, W = 7 mm. Gold,
rather than aluminium, was used for contacting the n-type
semiconductor. Adenine was evaporated at a rate of 3 nm/
s, guanine at 0.75 nm/s and indanthrene brilliant orange RF
at a rate of 0.1 nm/s. The measured capacitance per unit
area of the dielectric was C0d = 81.6 nF/cm2.
2.2.11. Adenine–perylene diimide OFET on Ecoflex
Aurin (0.1 g/ml in pure ethyl alcohol) was spun at
2000 rpm for 60 s onto 1.5 1.5 cm2 Ecoflex foils to act
as a smoothening layer. A 1 mm wide-100 nm thick aluminium gate was evaporated through a shadow mask; A
1.1 lm thick adenine and 100 nm thick perylene diimide
were evaporated in an Edwards high vacuum evaporation
system at a pressure of 10 6 torr to pattern the gate dielectric and semiconductor, respectively. A 100 nm thick
aluminium layer formed the top drain and source electrodes. The evaporation rate was 3 nm/s for adenine, and
0.1 nm/s for perylene diimide. The channel dimensions
were L = 100 lm, W = 1 mm. The measured capacitance
per unit area of the dielectric was C0d = 3.1 nF/cm2.
2.2.12. Adenine/guanine–indanthrene yellow G OFET on
caramelized glucose
As received powder of D-(+)-glucose was melted on top
of a hot plate at 225–250 °C and droplets were deposited
with a glass rod on aluminium foil wrapped around
1.5 1.5 cm glass slides. Wrapping aluminium around
the glass slides was necessary to prevent the solidified glucose from cracking during cooling to room temperature. A
thin layer of rosolic acid (50 nm) was vacuum evaporated
to serve a double role: (1) to act as a smoothening layer
and (2) to prevent the caramelized glucose substrate to
swollen during the gold gate electrode patterning. A
100 nm thick-1 mm wide gold gate electrode was evaporated in a metal evaporator. The gate dielectric was formed
1977
by four alternating layers of guanine and adenine (75 nm
thick each of the first three layers of guanine and adenine,
and 400 nm thick adenine-the fourth layer), and 100 nm
indanthrene yellow G was used as organic semiconductor.
Adenine was evaporated at a rate of 3 nm/s, whereas guanine and indanthrene yellow G at a rate of 0.1 nm/s. A
100 nm thick layer of gold patterned the source and drain
electrodes. The channel dimensions were: L = 100 lm and
W = 1 mm. The measured unit capacitance of the combined
dielectric was C0d = 5.6 nF/cm2.
2.2.13. Adenine/guanine–perylene diimide OFET on hard
gelatine capsules
Hard gelatine capsules were cut open and stuck with
the aid of a double-side scotch tape on 1.5 1.5 cm2 glass
substrates in order to force the gelatine substrate remain in
a flat-open position. Rosolic acid (0.1 g/ml in pure ethylalcohol) was spun at 2000 rpm for 60 s on top of capsule
substrates to act as a smoothening layer. A 1 mm wide100 nm thick gold gate was evaporated through a shadow
mask followed by four alternating layers of guanine and
adenine (75 nm thick the first and the third layer of guanine, 100 nm and 500 nm thick the second and fourth layer
of adenine) to form the gate electrode and gate dielectric,
respectively. A 100 nm thick perylene diimide was evaporated to pattern the semiconductor layer and a 100 nm
thick gold source and drain electrodes were subsequently
evaporated. The evaporation rate was 1 nm/s for guanine,
3 nm/s for adenine, and 0.1 nm/s for perylene diimide.
The source and drain electrodes had the channel dimensions: L = 100 lm, W = 1 mm. The measured capacitance
per unit area of the dielectric was C0d = 5.1 nF/cm2.
2.3. Device characterization
Steady state current–voltage measurements were performed with an Agilent E5273A instrument. Both transfer
and output characteristics were measured at a sweep rate
of 66 mV s 1, with 1 s for each of the hold, delay and step
delay times, respectively.
Dielectric characterization of the gate dielectrics was
performed with metal–insulator–metal capacitors using a
Novocontrol Alpha Analyzer. X-ray diffraction was performed using a Bruker AXS X-ray Diffractometer (Cu Ka)
X-ray Diffractometer. AFM investigation was performed
using a Digital Instruments Dimension 3100 microscope
working in tapping mode.
3. Results and discussion
3.1. Field effect transistors
Field effect transistors rely on an electric field (supplied
by the gate voltage, Vgs, applied between the grounded
source and the gate) to control the conductivity of a channel at the interface between the semiconductor and the
insulator, and hence the current between source and drain
contacts Ids. A measure of the quality of the dielectric is
given by the leakage current from the gate to the source
contacts through the insulator layer, Igs. Field effect
1978
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
transistor preparation requires substrates, smoothing layers when the substrate is rough, metal electrodes, gate
dielectrics and organic semiconductors. Fig. 1 illustrates
such a transistor with all the components employed in this
work presented in the schematic form. In the following, we
start discussing first substrates, then gate dielectrics and
Fig. 1. Natural materials or materials inspired by nature used for fabrication of environmentally sustainable organic field effect transistors. (a) Schematic of
bottom-gate, top-contact OFET employed in this work; (b) schematic of rosolic acid (aurin), used here as smoothener; (c) substrates investigated: Ecoflex
produced from potato and corn starch, hard gelatine capsule originating from collagen and caramelized glucose; (d) natural dielectrics materials in the
nucleobase and sugar families: adenine, guanine, cytosine, thymine glucose and lactose; (e) semiconductor materials: b-carotene and indigo are natural
p- and n-type organic semiconductors; indanthrene yellow G and indanthrene brilliant orange RF are semiconductors derived from natural anthraquinone
and perylene diimide, a cosmetic color. Glass, aluminium oxide, poly(vinyl alcohol), C60 and pentacene are examples of workhorse substrate, dielectric and
semiconductor materials widely employed in the field, used here for comparison. Transistors and integrated circuits produced from natural or nature
inspired materials may ultimately provide the basis for ‘‘sustainable green electronics”.
1979
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
Fig. 2. AFM pictograms of (a) plain Ecoflex foil (rms 70 nm); (b) aurin-coated Ecoflex foil (rms 8 nm); (c) plain hard gelatine capsule (rms 30 nm); (d)
aurin-coated hard gelatine capsule (rms 9 nm).
organic semiconductors, together with examples of fieldeffect transistors built on either glass or biodegradable/
biocompatible substrates.
3.1.1. Substrates for sustainable electronics
Fabric, hot-pressed cotton-fiber paper, leather and silk
have been recently reported as examples of biodegradable
and biocompatible-sustainable substrates for organic field
effect transistors [4,5,10,11]. Other examples of such substrates are presented here; they include hard gelatine,
commercially available biodegradable polymers and even
caramelized sugar for edible devices.
Hard gelatine capsule is a fully biocompatible and biodegradable substrate employed extensively in the pharmaceutical field for oral drug delivery. Gelatine capsules are
made from pork skin and bones and may contain small
additions of plasticizers (e.g., glycerin and sorbitol), preservatives, colors, flavors (e.g., ethylvanilin and essential oil),
sugars, etc. Their widespread availability, the ease of production in various forms and the complete biodegradability make gelatine capsules an interesting substrate for
electronics. Such electronics may even be ingested in the
body in biomedical applications. Ecoflex is a commercially
Table 1
Dielectric performance of investigated materials.
Material
Dielectrics
Adenine
Cytosine
Guanine
Thymine
Glucose
Lactose
Caffeine
PVA
AlOx
Dielectric constant
(at 1 kHz)
Breakdown
field (MV/cm)
Loss tangent
(at 100 mHz)
3.85
4.65
4.35
2.4
6.35
6.55
4.1
6.1
9
1.5
3.4
3.5
0.9
1.5
4.5
2
2
3.5
4 10
5 10
7 10
1 10
5 10
2 10
9 10
4 10
4 10
3
3
3
2
2
2
2
2
3
available (BASF) biodegradable polymer based on potato
starch, corn and polylactic acid. Ecoflex degrades in compost in 6 months without leaving any residue [15]. Since
the use of such polymers for various low-end, biodegradable applications will most likely increase, it is interesting
to explore them also as a substrate for organic electronics.
Caramelized glucose is employed as another example of
substrates for the preparation of OFETs; it allows for the
1980
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
Fig. 3. AFM of nucleobase thin films on aluminium covered glass a) 1000 nm adenine (rms 14.5); (b) 425 nm guanine (rms 3); (c) 350 nm cytosine (rms
24); (d) 450 nm thymine (rms 65).
fabrication of bio-metabolizable (edible) electronics. All
substrates mentioned here have large surface roughnesses,
with root-mean-square (rms) values between 30 nm and
80 nm, necessitating the addition of a smoothing layer in
order to decrease their roughness to values that allow fabrication of electronic components on such materials.
3.1.2. Smoothening layer
Initial work on paper substrates employed a smoothening layer formed by a thin spin-coated polymer film (polydimethylsiloxane, PDMS) to reduce the inherent roughness
of the substrate [5,10]. While being biocompatible and
solution processible, the polymer film required thermal
cross-linking to render the structure amenable for the patterning of electrodes and further device preparation. Aurin
(rosolic acid), a compound with a simple synthetic chemistry (i.e., produced by the reaction of phenol with oxalic
acid in concentrated sulfuric acid bath) and which occurs
also naturally (being extracted from the rhizomes of a free
growing plant (Plantago asiatica L.)) [16,17] is used here as
a smoothing layer. In traditional Chinese medicine,
Plantago asiatica L. is known for its medicinal properties,
used as anti-inflammatory, antiseptic, diuretic, expecto-
rant, etc. [18]. In the medical and pharmacological fields,
aurin is currently explored as heme-oxygenase activity
regulator in aortic endothelial cells or as a generator of
androgen receptors in binding assays that mimic the
functions of natural hormones [17,19]. Being a naturally
occurring compound with a simple chemistry and easy
biodegradability, aurin represents an interesting candidate
for the smoothening layer of various rough substrates.
Fig. 2a and c shows the surface roughness of Ecoflex and
gelatine capsules, with rms roughness values of 70 nm
and 30 nm, respectively. As presented in Fig. 2b and d,
the substrate roughness, rms, is reduced to 8 nm for
Ecoflex and 9 nm for hard gelatine after spin-coating aurin,
solution processed in ethyl-alcohol.
3.1.3. Natural dielectrics for sustainable organic field effect
transistors
Nature provides an overwhelming number of materials
that are degradable, so looking for natural dielectrics
appears to be a promising route for sustainability in organic
electronics. We first describe dielectrics from the sugar
family, followed by dielectrics from the nucleobase family,
both groups comprising naturally occurring-biodegradable
1981
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
29.26
450
Adenine Powder
Intensity (a.u.)
300
24.5
150
31 33.34
39.44
13.9 16.8
44.08
49.8 51.84
58.44
0
Adenine Thin Film
29.42
150
100
24.52
50
0
10
20
30
40
50
60
70
2θ (deg.)
Fig. 4. X-ray diffraction profiles of precursor adenine (a) and evaporated adenine (b); the corresponding crystallographic planes are conforming to the
pattern in a standard card.
(a)
-7
(b) 0.15
Vds = + 11 V
10
-8
-9
Ids (µA)
Ids (A)
10
Igs
Ids
10
-10
10
-11
10
0V
1.5 V
3V
4.5 V
6V
7.5 V
9V
10.5 V
12 V
0.10
0.05
0.00
-12
10
-12 -9 -6 -3 0
3
6
0
9 12
2
Vgs (V)
(c)
(d) 0.5
Ids (µA)
Ids (A)
Igs
Ids
-9
8
10
12
0V
2.5 V
5V
7.5 V
10 V
12.5 V
15 V
17.5 V
20 V
20.5 V
25 V
0.4
-8
10
10
6
Vds (V)
Vds = + 22 V
-7
10
4
-10
10
0.3
0.2
0.1
-11
10
0.0
-25-20-15-10 -5 0 5 10 15 20 25
0
5
10
15
20
25
V (V)
ds
Vgs (V)
Fig. 5. Transfer and output characteristics of OFETs with (a and b) solution processed lactose gate and C60 semiconductor channel; capacitance per unit
area, C0d = 6.8 nF/cm2, field-effect mobility l = 0.055 cm2/V s, subthreshold swing S = 2 V/dec and normalized subthreshold swing Sn = 13.6 V nF/cm2 dec; (c
and d) solution processed glucose gate and C60 channel. Capacitance: C0d = 2.15 nF/cm2, field-effect mobility l = 0.085 cm2/V s, subthreshold swing
S = 6.2 V/dec and normalized subthreshold swing Sn = 13.3 V nF/cm2 dec.
compounds with
biochemistry.
a
long
history
in
chemistry
or
3.1.3.1. Dielectrics from the
sugar family (lactose, glucose). Glucose and lactose are two of many other simple
small molecules in the sugar family. They are industrially
produced on a very large scale, are non-toxic and have
excellent film forming properties, when solution processed
from water and/or DMSO. The measured dielectric properties of films of glucose and lactose (dielectric constant,
breakdown field and loss tangent) are displayed in Table 1.
AFM graphs of the spin-cast films are essentially featureless and display a root-mean-square (rms) roughness in
the range of 0.5–1 nm. In addition to their excellent film
1982
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
(a)
(b) 0.12
Vds = + 4.75 V
-7
10
Ids (μA)
Ids (A)
-8
10
Igs
Ids
-9
10
0V
0.5 V
1V
1.5 V
2V
2.5 V
3V
3.5 V
4V
4.5 V
5V
0.08
0.04
-10
10
0.00
0
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
1
2
(c) 10-7
(d) 100
Vds = + 3.75 V
-8
Ids (nA)
Ids (A)
Igs
Ids
-9
4
5
0V
0.4 V
0.8 V
1.2 V
1.6 V
2V
2.4 V
2.8 V
3.2 V
3.6 V
4V
80
10
10
3
Vds (V)
Vgs (V)
-10
10
60
40
20
0
-11
10
-20
-4
-2
0
2
4
Vgs (V)
0
1
2
3
4
Vds (V)
Fig. 6. Transfer and output characteristics of OFETs with (a and b) Guanine gate and C60 channel. Capacitance per area, C0d = 9.25 nF/cm2, field effect
mobility l = 0.12 cm2/V s, subthreshold swing S = 1.4 V/dec and normalized subthreshold swing Sn = 12.9 V nF/cm2 dec; (c and d) cytosine gate and C60
channel. Capacitance per area, C0d = 13.8 nF/cm2, field effect mobility l = 0.09 cm2/V s, subthreshold swing S = 1.2 V/dec and normalized subthreshold
swing Sn = 16.6 V nF/cm2 dec.
forming properties, the two small molecules are good insulators as shown by the low dielectric losses measured over
a wide frequency range (10 kHz to 10 mHz). In addition,
glucose and lactose have relatively high breakdown voltages of 1.5 MV/cm and 4.5 MV/cm, respectively.
3.1.3.2. Dielectrics from the
nucleobase family (adenine,
guanine, cytosine and thymine). DNA, recently considered
for photonics as well as other electronic applications
[20,21], has been employed also as gate dielectric in organic field effect transistors [22–24]. The main drawback of
the use of DNA as an organic dielectric is the occurrence
of hysteresis in the transfer characteristics of organic field
effect transistors [22–24], probably caused by the presence
of mobile ionic impurities in the natural DNA dielectric.
However, DNA is composed of alternating sequences of
four nucleobases (adenine, guanine, cytosine and thymine). Being small molecules, nucleobases are amenable
for scrupulous purification and can be vacuum processed
in thin films of thicknesses scaled down to 2.5 nm, as it will
be shown in this work. Their natural abundance, low cost
and low toxicity make these materials interesting candidates for organic electronics. Guanine and adenine, for
example, occur naturally in many biological systems.
Guanine can be extracted from bat droppings, as well as
fish skin and scales [25]. Currently, the cosmetic industry
relies on small additions of guanine into shampoo, facial
creams and nail enamels to render them an iridescent bluish-white tint. Adenine is also a natural product, freely pro-
duced in most of the fruits, whole grains, raw honey and
propolis, etc.
The dielectric properties of thin evaporated films of
adenine, guanine, cytosine and thymine presented in Table
1 show low losses and featureless dielectric functions over
a wide frequency window, ranging from 10 kHz to 10 mHz.
The breakdown voltage of the four investigated nucleobases ranges from 0.9 MV/cm to 3.5 MV/cm, which makes
these materials suitable for use as gate dielectric in field effect transistors.
AFM surface investigations of thin films of adenine,
guanine, cytosine and thymine are presented in Fig. 3a–d.
Vacuum processed nucleobase thin films show a tendency
for crystallization, with increasing surface roughness starting from guanine (rms 3 nm) to adenine (rms 14.5 nm),
cytosine (rms 24 nm) and thymine (rms 65 nm). The
investigation of the fifth nucleobase, uracil, proved difficult
because of its extreme tendency for crystallization. With
this respect, thin films of uracil displayed pin-holes,
whereas thicker films had a roughness with root-meansquare values greater than 100 nm.
An example of the X-ray diffraction investigation for
adenine in the forms of precursor powder and 1 lm thick
evaporated film on aluminium coated glass slide is presented in Fig. 4. The crystallographic planes are conforming
to the pattern available in the standard card (JSPDS no. 241654) of the instrument. As shown in Fig. 4, adenine thin
films display crystallinity and preferred orientation compared to precursor powder, as it is suggested by strong
1983
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
(a)
(b)
Vds = + 0.5 V
-6
10
-0.5 V
-0.4 V
-0.3 V
-0.2 V
-0.1 V
0V
0.1 V
0.2 V
0.3 V
0.4 V
0.5 V
1.2
Ids (μA)
Ids (A)
-7
10
Igs
Ids
-8
10
0.8
0.4
-9
10
0.0
-10
10
-1000
-500
0
500
0
100
-6
(d)
Vds = - 12.5 V
10
-7
Ids (A)
300
400
500
-8
10
0.4
0.0
Igs
Ids
10
Ids (μA)
(c)
200
Vds (mV)
Vgs (mV)
-9
10
-10
-0.4
-1 V
-2.4 V
-3.8 V
-5.2 V
-6.6 V
-8 V
-9.4 V
-10.8 V
-12.2 V
-13.6 V
-15 V
-0.8
-1.2
10
-1.6
-11
10
-15 -12
-9
-6
-3
0
-2.0
-16 -14 -12 -10 -8
3
(e)
-6
10
(f) 2.0
Vds = + 14.5 V
1.6
-7
-9
10
Ids (μA)
Ids (A)
-8
-4
-2
0
0V
1.5 V
3V
4.5 V
6V
7.5 V
9V
10.5 V
12 V
13.5 V
15 V
10
10
-6
Vds (V)
Vgs (V)
Igs
Ids
-10
10
1.2
0.8
0.4
-11
10
0.0
-12
10
-9 -6 -3
0
3
6
9 12 15
Vgs (V)
0
3
6
9
12
15
Vds (V)
Fig. 7. Transfer and output characteristics of OFETs with (a and b) an AlOx–adenine gate and hot-wall epitaxial deposited C60 channel. Capacitance per area,
C0d = 99 nF/cm2, field effect mobility l = 5.5 cm2/V s, subthreshold swing S = 0.25 V/dec and normalized subthreshold swing Sn = 24.7 V nF/cm2 dec; (c and
d) AlOx–adenine gate and pentacene channel. Capacitance per area, C0d = 19.6 nF/cm2, field effect mobility l = 0.35 cm2/V s, subthreshold swing S = 2.5 V/
dec and normalized subthreshold swing Sn = 49 V nF/cm2 dec; (e and f) AlOx–thymine gate and C60 channel. Capacitance per area, C0d = 23.6 nF/cm2, field
effect mobility l = 0.5 cm2/V s, subthreshold swing S = 2 V/dec and normalized subthreshold swing Sn = 47.2 V nF/cm2 dec.
domination of the peak centered at 2h = 29.42°. Nevertheless, more work is required to understand the mechanisms
of film growth in nucleobase materials.
3.1.3.3. Natural dielectrics-based OFETs. Organic field effect
transistors with solution processed thin film gate dielectrics of glucose (in deionized water) and lactose (in DMSO)
and vacuum processed fullerene, C60, as semiconductor are
presented in Fig. 5a–d. The transfer and output characteristics display a minimal hysteresis, which being counterclockwise can be attributed to the finite presence of mobile
ionic impurities in the two dielectric films [26,27]. The
field effect mobility of the organic semiconductor, C60,
deposited on lactose and glucose dielectrics was in the
range of 5.5 10 2 to 8.5 10 2 cm2/V s, and the capacitances per area of the two dielectrics employed were
6.8 nF/cm2 for lactose and 2.15 nF/cm2 for glucose dielectric. The method used to calculate the field effect mobility
was reported in Ref. [28]. The measured subthreshold
swing values of the fullerene were 2 V/dec for the OFET
with a lactose dielectric and 6.2 V/dec for the glucose
dielectric device. As described in the review article (Ref.
[28]), the normalized subthreshold swing (the product of
the dielectric capacitance per area and the subthreshold
swing) is a more useful metric of comparison of semiconductor films deposited on different dielectrics or on
dielectrics of various thicknesses. The values of the
normalized subthreshold swing (Sn) for fullerene grown
on lactose and glucose dielectrics are 13.6 V nF/cm2 dec
and 13.3 V nF/cm2 dec, respectively. The saturated electron
mobility of C60 deposited on solution processed small molecules in the sugar family is somewhat lower compared to
1984
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
(a)
Ids
-9
10
Ids (nA)
Ids (A)
(b) 0
Vds = - 75 V
-8
10
Igs
-10
10
-11
10
-4
0V
- 10 V
- 20 V
-30 V
-40 V
-50 V
-60 V
-70 V
-80 V
-90 V
-100 V
-8
-12
-12
10
-100 -80
-60
-40
-20
0
-100 -80
Vgs (V)
Ids (A)
(d) 20
Vds = + 90 V
-8
10
10
-20
0
0V
20 V
40 V
60 V
80 V
100 V
16
Igs
Ids
-9
-40
Vds (V)
Ids (nA)
(c)
-60
-10
10
12
8
4
0
-11
10
-20
-4
0
20
40
60
80
100
Vgs (V)
0
20
40
60
80
100
Vds (V)
Fig. 8. (a and b) Transfer and output characteristics of organic field effect transistors with glucose and caffeine gate and solution processed beta-carotene
(from chloroform) channel. Capacitance per unit area, C0d = 1.9 nF/cm2, field effect mobility l = 4 10 4 cm2/V s, subthreshold swing S = 11 V/dec and
normalized subthreshold swing Sn = 21 V nF/cm2 dec; (c and d) transfer and output characteristics of organic field effect transistors with poly(vinyl alcohol)
(PVA) gate and vacuum processed indigo organic semiconductor channel. Capacitance per unit area C0d = 3.1 nF/cm2, field effect mobility
l = 1.5 10 4 cm2/V s, subthreshold swing S = 34 V/dec and normalized subthreshold swing Sn = 105.4 V nF/cm2 dec.
values reported elsewhere for fullerene semiconductors
[29,30]. This lower electron mobility may be explained
by the finite presence of moisture in the two dielectric
films.
Vacuum processed organic dielectrics on the other hand
allow fabrication of OFETs operating at low voltages
[31,32]. Organic field effect transistors with vacuum processed guanine and cytosine gate and C60 channel are
displayed in Fig. 6a–d, for transistors operating at voltages
as low as 4–5 V. The measured saturated field effect mobility of C60 vacuum processed on cytosine and guanine
dielectrics was 0.09 cm2/V s and 0.12 cm2/V s, respectively; the dielectric capacitance per area was 9.25 nF/
cm2 for guanine and 13.8 nF/cm2 for cytosine. The measured subthreshold swing and normalized subthreshold
swing values for the structure built on guanine were
S = 1.4 V/dec and Sn = 12.9 V nF/cm2 dec, respectively; the
respective values for the fullerene film grown on cytosine
were 1.2 V/dec and 16.6 V nF/cm2 dec.
Higher operating currents as well as higher saturated
field effect mobilities are possible when organic and inorganic dielectrics (i.e., anodized aluminium) are combined
in hybrid structures [33]. Transistors with such hybrid gate
structures are shown in Fig. 7a–f where thin layers of
adenine (a–d) and thymine (e and f) were evaporated on
aluminium oxide electrochemically grown in citric acid
to produce high performance gate dielectrics. The calculated field effect mobility of transistors with hot-wall
epitaxially deposited C60 and the hybrid gate (capacitance
per area, C0d = 99 nF/cm2) listed in Fig. 7a and b was
5.5 cm2/V s, with a calculated subthreshold swing of
0.25 V/dec and a normalized subthreshold swing of
24.7 V nF/cm2 dec. The operating voltage of the respective
device was as low as 500 mV. It is important to note that
all the vacuum processed nucleobase dielectrics are also
amenable to work in combination with p-type semiconductors (e.g., pentacene); an example of such an OFET is
shown in Fig. 7c and d. In the latter case, 150 nm adenine
was vacuum processed on 55 nm aluminium oxide (AlOx
with a capacitance per unit area, C0d = 140 nF/cm2). The
field effect mobility in the saturation regime of pentacene
was 0.35 cm2/V s, subthreshold swing S = 2.5 V/dec and
normalized subthreshold swing Sn = 49 V nF/cm2 dec, for
a capacitance per unit area of the combined dielectric
C0d = 19.6 nF/cm2. Despite its relatively low dielectric constant (i.e., 2.4 as shown in Table 1) and its high tendency of
crystallization that renders films of high roughness
(Fig. 3d), thymine passivates aluminium oxide very well.
In the transistor shown in Fig. 7e and f, a 75 nm thick
thymine film was vacuum processed on top of anodized
aluminium, and vacuum processed C60 was used for the
organic semiconductor. The respective OFET was hysteresis free in both transfer and output characteristics, while
displaying a C60 mobility of 0.5 cm2/V s, a subthreshold
swing of 2 V/dec and a normalized subthreshold swing of
47.2 V nF/cm2 dec, for a capacitance per unit area of
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
1985
Fig. 9. AFM of natural semiconductor thin films (a) solution processed beta-carotene in chloroform on glucose + caffeine dielectric. Average roughness,
rms = 19 nm, measured in the shallow region of small grains; (b) vacuum processed indigo on poly(vinyl alcohol) dielectric. Average roughness,
rms = 17 nm. Measurements were recorded in the channel of OFETs devices displayed in Fig. 8.
23.6 nF/cm2 of the hybrid gate dielectric. Surprisingly, the
extreme roughness of vacuum processed small molecule
dielectrics described here (adenine, cytosine and thymine),
as well as in our previous work (melamine), Ref. [32]), was
not a deterrent in generating hysteresis-free devices. The
only exception was guanine. Interestingly, guanine formed
smoother films when compared with adenine, cytosine,
thymine or even melamine. We presume that the intimate
binding that occurs at the interface between the dielectric
and the semiconductor layers generates the occurrence of
hysteris-free behavior in various OFETs. Further investigations at the molecular level are required to elucidate the
mechanism of hysteresis formation in transistors with vacuum processed small molecules dielectrics and semiconductors of high purity.
3.1.4. Semiconductors for sustainable electronics
Encouraged by the performance of our transistors based
on vacuum processible sustainable dielectrics and traditional workhorse semiconductors we went on identifying
organic semiconductors, which may be viewed sustainable.
A short, and by no means comprehensive list of such semiconductors, includes natural (indigo, beta-carotene), nature-inspired (anthraquinone vat dyes), and common
commodity materials (perylene diimide). All these semiconductors have in common large scale production, ease
of synthesis and low price, coupled with low toxicity, biodegradability and wide social acceptance [34].
3.1.4.1. Natural semiconductors for
organic field effect
transistors. Beta-carotene is a material with an old history.
It was first reported by the German chemist Hernan
Wachenroder, who in 1831 extracted red crystals from carrot roots, that he ultimately called ‘‘carotene” [35]. Until
now, applications of beta-carotene and related products
have been limited to the medical/biomedical field, as
anti-aging and heart-disease prevention drug [36]. However, the optical, non-linear optical, fluorescence and even
semiconducting properties of beta-carotene have been recently investigated by various groups [37–41].
Indigo is a naturally occurring compound, which has
historically been extracted from plants in the Indigofera
genus. Nowadays the synthetic production of indigo (initiated by Adolf Baeyer in 1882) has made possible high scale
production of blue cotton yarn cloths with the main application of the compound remaining in the blue jeans industry [42]. Although the electronic and energetic levels of
indigo were recently investigated, no report of using indigo
as organic semiconductor in field effect devices appeared
so far [43,44].
Organic field effect transistors with solution processed
p-type beta-carotene and vacuum processed n-type indigo
as organic semiconductors are presented in Fig. 8a–d. The
organic dielectrics are glucose with small additions of caffeine in a and b and poly(vinyl alcohol) in c and d. The AFM
investigations of the two dielectrics (glucose + caffeine and
PVA) revealed featureless surfaces with rms 1.5 nm and
1 nm, respectively. The calculated field effect mobilities
of the two natural semiconductors were 4 10 4 cm2/V s
for beta-carotene and 1.5 10 4 cm2/V s for indigo for
capacitances per area C0d = 1.9 nF/cm2 for glucose and caffeine dielectric and C0d = 3.1 nF/cm2 for PVA dielectric. The
calculated subthreshold swing and normalized subthreshold swing values of the two natural semiconductors were
11 V/dec and 21 V nF/cm2 dec for beta-carotene; 34 V/dec
and 105.4 V nF/cm2 dec for indigo, respectively. Higher
performance may be achieved by optimizing transistor design and fabrication, corroborated with scrupulous purification of the chemicals.
AFM measurements of beta-carotene and indigo, performed in the channel of the measured devices shown in
Fig. 8, are depicted in Fig. 9. The top surface of beta-carotene
revealed small grains with a diameter in the range of
60–150 nm and an average size of 105 nm. However, in all
the locations investigated (like for example the region
shown in Fig. 9a), large grains were also visible, with diameters scaling up to 900 nm. The average roughness, rms, of
1986
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
(a)
(b) 0.20
Vds = + 5.75 V
-7
10
Ids (μA)
Ids (A)
-8
10
Igs
Ids
-9
10
0V
0.6 V
1.2 V
1.8 V
2.4 V
3V
3.6 V
4.2 V
4.8 V
5.4 V
6V
0.16
0.12
0.08
0.04
-10
10
0.00
-11
10
-4
-2
0
2
4
0
6
1
2
Vgs (V)
(c)
(d) 0.12
Vds = + 8.5 V
-7
10
0.09
Ids (μA)
Ids (A)
Igs
Ids
-9
4
5
6
0V
1V
2V
3V
4V
5V
6V
7V
8V
9V
-8
10
3
Vds (V)
10
0.06
0.03
-10
10
0.00
-11
10
-6
-3
0
3
6
0
9
3
Vgs (V)
-6
(e) 10
10
Igs
Ids
-8
10
9
6
9
9V
8.1 V
7.2 V
6.3 V
5.4 V
4.5 V
3.6 V
2.7 V
1.8 V
0.9 V
0V
0.4
Ids (μA)
Ids (A)
(f) 0.5
Vds = + 8.75 V
-7
6
Vds (V)
-9
10
0.3
0.2
0.1
-10
10
0.0
-11
10
-6
-3
0
3
6
9
Vgs (V)
0
3
Vds (V)
Fig. 10. (a and b) Transfer and output characteristics of organic field effect transistors with AlOx–guanine–adenine gate and perylene diimide channel.
Capacitance per unit area C0d = 81.6 nF/cm2, field effect mobility l = 0.016 cm2/V s, subthreshold swing S = 1.5 V/dec and normalized subthreshold swing
Sn = 122.4 V nF/cm2 dec; (c and d) OFET with AlOx–guanine–adenine gate dielectric and indanthrene yellow G channel. Capacitance per unit area
C0d = 81.6 nF/cm2, field effect mobility l = 0.01 cm2/V s, subthreshold swing S = 2.4 V/dec and normalized subthreshold swing Sn = 196 V nF/cm2 dec; (e and
f) OFET with AlOx–guanine–adenine gate and indanthrene brilliant orange RF channel. Capacitance per unit area C0d = 81.6 nF/cm2, field effect mobility
l = 1.9 10 3 cm2/V s, subthreshold swing S = 1.9 V/dec and normalized subthreshold swing Sn = 155 V nF/cm2 dec.
beta-carotene measured in the shallow regions of small
grains was 19 nm. Indigo on the other hand displayed a
more uniform surface in terms of grain size and distribution. Most of the indigo grains were elongated in shape
and had a diameter in the range of 150–550 nm, with an
average of 250 nm.
3.1.4.2. Nature-inspired semiconductors for organic field
effect transistors. Indanthrene yellow G and indanthrene
brilliant orange RF (derivatives of natural occurring anthraquinone-a well known laxative drug [45]) are both used
widely in textile industry as vat dyes for fabrics coloring
as well as in electronics industry as color filter in image
forming applications [46,47]. However, their low toxicity
[48], biodegradability [49,50] and ability to metabolize
[51] has led to these compounds being proposed for coloring sausage skin in food industry [52,53].
Although having a synthetic route that can be distantly
considered as starting from natural naphthalene, perylene
diimide is not a truly nature-inspired compound [54]. Nevertheless, we want to list it here as environmentally sustainable material. The chemical inertness of perylene
diimides is a prerequisite for their very low acute toxicity
that opens up numerous applications in cosmetic industry
as red pigments for hair colorants, nail enamels and lipsticks. Nowadays many perylene dyes are produced on an
industrial scale and commercially available under trade
names such as Red Dye 190 or LumogenÒF [55]. Perylene
1987
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
(a) 10-7
(b)
Vds = + 12 V
0V
1.5 V
3V
4.5 V
6V
7.5 V
9V
10.5 V
12 V
13.5 V
15 V
0.12
-8
Igs
Ids
-9
10
Ids (μA)
Ids (A)
10
0.08
0.04
-10
10
0.00
-11
10
-15 -10
-5
0
5
10
15
0
5
(c) 10-6
(d) 0.75
Vds = + 47.5 V
Ids (μA)
Ids (A)
15
0V
5V
10 V
15 V
20 V
25 V
30 V
35 V
40 V
45 V
50 V
0.50
-7
10
10
Vds (V)
Vgs (V)
Igs
Ids
-8
10
0.25
0.00
-9
10
-50
-25
0
25
0
50
10
Vgs (V)
(e) 10-7
(f) 0.12
-8
Ids (μA)
Ids (A)
-9
Igs
Ids
-10
40
50
0V
2V
4V
6V
8V
10 V
12 V
14 V
16 V
18 V
20 V
10
10
30
Vds (V)
Vds = + 19 V
10
20
0.08
0.04
-11
10
0.00
-12
10
-10
-5
0
5
10
15
20
Vgs (V)
0
5
10
15
20
Vds (V)
Fig. 11. (a and b) Transfer and output characteristics of an OFET on a biodegradable Ecoflex substrate; the roughness of the Ecoflex foil is reduced with a
smoothing layer of aurin. Adenine forms the dielectric and perylene diimide is the semiconductor; C0d = 3.1 nF/cm2, l = 0.01 cm2/V s, S = 3.6 V/dec and
Sn = 11.1 V nF/cm2 dec; (c and d) Transfer and output characteristics of biocompatible and biodegradable OFETs on caramelized glucose substrates; guanine
and adenine form the gate dielectric and indanthrene yellow G is the organic semiconductor. C0d = 5.6 nF/cm2, l = 8 10 3 cm2/V s; (e and f) Transfer and
output characteristics of an edible OFET on a hard gelatine capsule substrate; adenine and guanine form the gate dielectric and perylene diimide is the
organic semiconductor; C0d = 5.1 nF/cm2, l = 0.02 cm2/V s, S = 3.1 V/dec and Sn = 15.8 V nF/cm2 dec. Electrodes are aluminium (a and b) and gold (c and f).
diimides are also known as good n-type organic semiconductors widely used in organic solar cells [56].
3.1.4.3. OFETs with sustainable semiconductors on glass
substrates. After having shown a large number of potential
low cost organic semiconductors, we now proceed by presenting field effect transistors of these materials on glass
substrates. Fig. 10a–f display transistors formed with
indanthrene yellow G and indanthrene brilliant orange
RF, as well as perylene diimide. Thin layers of these organic
semiconductors have been evaporated on hybrid inorganic–organic gate dielectrics (55 nm aluminium oxide
and thin alternating layers of guanine (2.5 nm) and adenine (15 nm), with a total capacitance per area of
81.6 nF/cm2. The reason for using different organic dielec-
tric layers was the intention to produce dense and pinhole
free layers at lower thickness than using one organic
dielectric layer alone. The saturated field effect mobilities
of perylene diimide and indanthrene yellow G were
0.015 cm2/V s, indanthrene brilliant orange RF had a
mobility of 2 10 3 cm2/V s. The mobilities of these
materials of low cost and large scale availability are on
par with mobilities reported recently for synthetic
poly(p-phenylene vinylene) or for ambipolar polyselenophene organic semiconductors [57,58]. The calculated
subthreshold swing values for the investigated natureinspired semiconductors fell in a close range: 1.5 V/dec
for perylene diimide, 2.4 V/dec for indanthrene yellow G
and 1.9 V/dec for indanthrene brilliant orange RF. The OFET
devices in Fig. 10 have been fabricated using identical
1988
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
Fig. 12. AFM of various nature-inspired semiconductors employed in this work. Measurements were performed in the channel of the OFET devices shown
here; (a) perylene diimide displayed in Fig. 10a and b; average surface roughness, rms 17.5 nm (b) indanthrene yellow G displayed in Fig. 10c and d;
average surface roughness, rms 10.5 nm; (c) indanthrene brilliant orange RF displayed in Fig. 10e and f; average surface roughness, rms 34 nm; (d)
indanthrene yellow G displayed in Fig. 11c and d; average surface roughness, rms 22 nm.
dielectric material (a hybrid inorganic–organic layer, having a capacitance per area C0d = 81.6 nF/cm2). The much
lower values of the subthreshold swing recorded for the
nature-inspired semiconductors displayed in Fig. 10, compared to the respective values of the natural semiconductors (beta-carotene and indigo) shown in Fig. 8, are not a
surprise, since the subthreshold swing is highly dependent
on both the semiconductor mobility and the dielectric
capacitance per unit area [28]. It also shows the need for
more detailed investigations on naturally occurring organic semiconductors.
3.1.4.4. OFETs with
sustainable semiconductors on
biodegradable and biocompatible substrates. Having been
able to demonstrate transistors with sustainable gate
dielectrics and semiconductors, we have tried to fabricate
such devices also on natural caramelized glucose, biodegradable Ecoflex and biocompatible hard gelatine capsule
substrates. OFET transfer and output characteristics for
such devices are presented in Fig. 11a–f where small molecule-vacuum processed nucleobases (adenine in Fig. 11a
and b or alternating layers of adenine and guanine in
Fig. 11c–f) have been used for gate dielectrics; perylene
diimide and indanthrene yellow G have been used for organic semiconductors. The on–off ratio of transistors fabricated with these sustainable materials ranged from 102 to
105, whereas the mobility of organic semiconductor was
8 10 3 for indanthrene yellow G and 1 10 2 to
2 10 2 cm2/V s for perylene diimide. The calculated subthreshold swing values of the perylene diimide semiconductor employed in Fig. 11a, b e, and f were 3.6 V/dec
and 3.1 V/dec, respectively; the normalized subthreshold
swing values were 11.1 V nF/cm2 and 15.8 V nF/cm2. The
recorded values of normalized subthreshold swing show
a good correlation of its dependence on mobility and
dielectric capacitance (i.e., 3.1 nF/cm2 for adenine dielectric in Fig. 11a and b and 5.1 nF/cm2 for guanine and
adenine in Fig. 11e and f).
AFM investigations of the nature-inspired semiconductors employed here (perylene diimide, indanthrene yellow
G and indanthrene brilliant orange RF), performed in the
channel of the measured devices presented in Figs. 10a–f
and 11c and d are displayed in Fig. 12. The grain size of
uniformly distributed perylene diimide (Fig. 12a) is in the
range of 80–160 nm, with an average size of 125 nm.
Indanthrene yellow G and indanthrene brilliant orange
M. Irimia-Vladu et al. / Organic Electronics 11 (2010) 1974–1990
RF form also grains with a uniform distribution (Figs. 12b
and c, respectively). The indanthrene yellow G grain size
spans the range from 60 nm to 130 nm, with an average
size of 90 nm, whereas the indanthrene brilliant orange
RF grains are somewhat larger, ranging from 100 nm to
300 nm, with an average grain size of 200 nm. Fig. 12d)
shows the surface of indanthrene yellow G deposited on
alternating layers of guanine and adenine on caramelized
glucose substrate, the OFET device displayed in Fig. 11c
and d). The surface of the latter material shows a uniform
grain distribution, with grain diameters from 70 nm to
190 nm, and an average grain size of 125 nm.
It is interesting to observe that the grain sizes of the two
semiconductor materials displaying on par mobilities in
Fig. 10 (perylene diimide and indanthrene yellow G) were
also comparable, whereas materials that rendered films of
larger grain sizes (indanthrene brilliant orange RF, betacarotene and indigo) recorded significantly lower mobilities. Moreover the subthreshold swing of various OFET
devices was in the same range for devices showing similar
semiconductor mobilities and having dielectrics of comparable capacitances per unit area.
4. Conclusions
In this work, we have initiated a search on low cost
materials that may find applications in sustainable biodegradable and biocompatible organic field effect transistors.
We have shown that different gate dielectrics from sugars
and nucleobases can be used in field effect transistors based
on workhorse p and n-type organic semiconductors such as
C60 and pentacene. Large volume prepared dyes used in textiles, cosmetics or food industry have shown surprisingly
large mobilities in field effect devices on glass and biodegradable/biocompatible substrates. Transistors on degradable substrates, such as Ecoflex, gelatine and caramelized
sugars, could be operated with voltages around 15–20 V
and source drain currents of 0.15 lA. We hope to have initiated with this work an intense search for new materials
for organic electronics, to make the vision of sustainability
in plastics electronics coming closer to reality.
Acknowledgements
The work was financially funded by the Austrian Science Foundation ‘‘FWF” within the National Research Network NFN on Organic Devices (P20772-N20, S09712-N08,
S09706-N08 and S9711-N08) as well as by the Russian
Foundation for Basic Research (grant 10-03-00443), by
the Russian Ministry for Science and Education (contract
02.740.11.0749) and by the Russian President Foundation
(grant MR-4305.2009.3). We thank Philipp Stadler,
Michael Ramsay, Roland Resel and Christian Teichert for
valuable discussions and suggestions.
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