Organic Electronics 11 (2010) 1974–1990
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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.
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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.
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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
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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
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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”.
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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
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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
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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
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(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)
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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
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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
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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
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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
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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
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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
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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
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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. References [1] E. Rudnik, Compostable Polymer Materials, first ed., Elsevier, Amsterdam, 2008. [2] R.J. Slack, J.R. Gronow, N. Voulvoulis, Hazardous components of household waste, Crit. Rev. Environ. Sci. Technol. 34 (2004) 419–445.
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