Organic electronics: Materials, technology and circuit design developments enabling new applications

ARTICLE IN PRESS Materials Science in Semiconductor Processing 11 (2008) 199–204

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Organic electronics: Materials, technology and circuit design developments enabling new applications D.M. de Leeuw a, E. Cantatore b, a b

Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

a r t i c l e in fo

abstract

Available online 11 December 2008

Organic electronics is growing to become an important new field in the global electronics market. RFID labels, flexible displays, solar cells, OLED-based lighting and displays are only some of the innovative products enabled by technologies based on organic semiconductors. Starting from a short overview of organic thin-film transistors research, this paper will concentrate on the recent developments in organic circuit design and on how the state of the art in this field can be further advanced with contributions from materials and processing research. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Organic integrated circuits Organic thin-film transistors (OTFTs) Organic semiconductors

1. Introduction Thanks to the large research effort in the field of materials, processing and circuit design, electronics based on organic semiconductors has evolved from a curiosity for scientists to a technology that enables completely new products. Simple electronic RFID tags manufactured using printing, displays that can be rolled-up and stowed in a fraction of the surface they occupy when used, cheap and stable solar cells, organic light-emitting diodes for lighting and television sets are only some of the products based on organic semiconductors that are quickly appearing on the market. This tremendous output in terms of innovative products is enabled by an increasingly more mature science and technology of organic semiconductors. Organic electronic devices include organic thin film transistors (OTFTs), organic light emitting diodes (OLEDs), sensors, memories and more. In this paper we will review research developments in materials, technology and circuit design for electronics based on organic thin film transistors and we shall try to find out which upcoming research is

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E-mail address: [email protected] (E. Cantatore). 1369-8001/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.10.001

likely to contribute to a further advancement of the state of the art. In order to determine these future research challenges we will take the perspective of a circuit designer, analyzing present OTFTs circuits and suggesting improvements at the circuit, device, technology and material level. The paper is organized as follows. Section 2 gives a short overview of the state-of-the-art of OTFT research, Section 3 presents some important research challenges still open in the field and Section 4 summarizes our analysis in a kind of ‘‘research agenda’’ for material scientist, technologists and circuit engineers interested to work with OTFTs.

2. Organic TFT research: a concise overview The organic TFT research in the last 15 years has been mainly focused on the development of new p-type semiconductors, on improved chemistry and on better technology of the semiconductor materials to improve charge carrier mobility. As witnessed in many overviews of the field [1,2], polymer semiconductors mobility has improved more than four orders of magnitude, from values in the order of 105 cm2/V s before the 1990s to values just below unity nowadays [2,3]. Small-molecule organic semiconductors

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Table 1 Organic semiconductor mobility for some state-of-the-art examples taken from open literature. Type

Organic semiconductor

Ref.

p

Solution processed pentacene Evaporated pentacene Single-crystal pentacene

[32]

1.8

[33] [34]

5.5 40

Evaporated PTCDI-C4F7

[7]

n

Max. mobility (cm2/V s)

0.7

have dramatically improved in a similar way, from the typical mobility of 103 cm2/V s of the early 1990s to values surpassing 10 cm2/V s nowadays. Processing and deposition techniques still play a significant role in determining the mobility of the semiconductor films. Depending on the degree of internal ordering, on the presence of traps and on the properties of the semiconductor/insulator interface, the mobility of a given material can vary in a transistor by more than one order of magnitude. Some examples of state-of-the-art mobility performance for the popular small-molecule pentacene, processed in different ways, are given in Table 1. Stability, i.e. shelf and operation lifetime of devices based on organic semiconductors, has always been a matter of concern and an important research domain. Very encouraging results have been obtained with p-type materials. In [4], for instance, OTFTs are shown, which have been fabricated and measured after 15-month storage in ambient conditions, showing only a 20% mobility variation on the original 0.35 cm2/V s value, and a small (interestingly) increment in the on–off ratio. The same devices have been stressed at a constant VDS ¼ 40 V and sweeping continuously the gate voltage from 40 to 40 V for about 3000 times, showing little noticeable variation in the main device parameters. Traditionally n-type materials have been characterized by extremely poor performance stability. Resent research, though, shows interesting results also in this field. Weitz et al. [5], for instance, discuss a cyanated perylene carboxylic diimide derivative1 with an initial mobility of 0.12 cm2/V s. The mobility of this n-type material degrades by one order of magnitude after about 400 days of storage in ambient conditions. State-of-the-art mobility in n-type materials reaches values around 1 cm2/V s [6] and can be reasonably air-stable [7]. Organic transistors and other sort of organic semiconductor-based devices (solar cells, light-emitting diodes) can be manufactured with large-area compatible technologies, using processing temperatures just above ambient and processing methods that may attain largethroughput like roll-to-roll industrial printing. Also, due to the very limited thermal budget, organic electronics can be produced on basically any substrate, including cheap and flexible films. The economic interest in organic 1 N,N0 -bis(1H,1H-Perfluorobutyl)-1,7-dibromo-perylene-3,4:9,10-bis (dicarboximide).

electronics is generated by the fact that organic devices may be produced with tremendous throughput, at very low cost per unit area, on large surfaces, and on any substrate. Early ground-breaking research on inkjet printing used to manufacture OTFTs [8] has been followed by integration of different devices fabricated on separate sheets by inkjet printing, screen printing and shadowmask evaporation, as shown in [9,10]. Important application-related work on truly high-throughput printing techniques has been done [11], resulting recently in the first products (simple RFID labels) based on printed OTFT technology [12]. Several other groups are also working in the field of large-throughput OTFT and organic integrated circuit (IC) fabrication, like reported in [13], exploring as well alternative techniques like transfer printing using stamps [14–16]. In the field of circuit design, the very first encouraging results reporting functional ICs [17] have been followed by much research work to improve the yield, robustness and complexity of digital circuits based on OTFTs. State-of-theart digital designs contain routinely more than 1000 transistors [18,19]. 3. Future organic TFT research challenges: a circuit designer’s point of view In this section, starting from a more detailed analysis of the state of the art of circuit design with organic TFTs, we will point out some challenges for further high-impact research in the field. The section is organized in three subparts dealing with different kinds of circuits and their specific demands: digital, RF and analogue. 3.1. Challenges arising in digital circuits 3.1.1. Noise margin Much work [19] has been devoted in recent years to designing manufacturable digital electronics using p-type-only organic TFTs. One of the major issues there is to design digital gates with enough noise margin and low-enough noise margin variability. Only under these conditions, in fact, it is possible to build digital circuits with a complexity of hundreds or more logic gates that would function properly in a real working environment [20]. The noise margin, defined for an inverter as the side of the largest square that can be inscribed between the input–output characteristics (Vout vs. Vin) and the mirrored characteristic that gives Vin as a function of Vout (drawn with a stippled line in Fig. 2a), turns out to be strongly dependent on the threshold voltage Vth of the transistors used to build the inverter. Considering the schematic of an inverter with zero-Vgs load, given in Fig. 1, the noise margin of the inverter manufactured using both for the load and for the driver transistors OTFTs with slightly positive threshold is about 1 V (Fig. 2a). Using a hypothetic technology where transistors with two different threshold voltages would be available (one with slightly positive and the other with negative Vth), the noise margin could be strongly improved. In Fig. 2b, for

ARTICLE IN PRESS D.M. de Leeuw, E. Cantatore / Materials Science in Semiconductor Processing 11 (2008) 199–204

example, the driver transistor has received in simulation a threshold voltage of 8 V and the load transistor retains the small positive threshold typical of the technology employed, resulting in a noise margin of about 5 V. As it has been shown using the noise margin as an example, threshold voltage control is crucial to allow better performance in circuit based on p-type organic transistors. In spite of some research attempts to control the Fermi level using monolayers of dipolar molecules and the successful use of doping in organic light-emitting diodes [21–23], no proven and reliable technique is still available (to the author’s knowledge) to control the

threshold voltage in organic TFTs. Alternative approaches to threshold control have also been explored in order to improve noise margin in OTFT circuits. Logic gates made using ambipolar transistors, for instance, guarantee a good noise margin as the pull-up and pull-down device can be biased, respectively, in the p-type region and n-type region, to emulate the way complementary logic works [25]. Ambipolar transistors, however, never fully switch off when one of the stable output voltages of the gate (corresponding to the logic high or logic low) is reached, and thus do not allow to cut the leakage path between the power supplies when the logic gate does not switch. Truly complementary OTFTs technologies, i.e. technology platforms where both n- and p-type transistors are available, have also been studied [29,30]. The availability of complementary transistors improves dramatically the noise margin and may result in better power consumption too, as complementary gates only need power when the output switches from one stable output voltage to the other. Complementary OTFT technologies deserve, thus, the full attention of scientists active in the field.

Gnd

Vin

201

Driver Transistor M1 Vout

Load

3.1.2. Speed in digital circuits Let us consider now the dynamic performance of digital circuits, again taking as example the simple zeroVgs load inverter of Fig. 1. A principle diagram showing the time response of this inverter to a variation of the incoming signal Vin from a logic low to a logic high value and back is shown in Fig. 3. When the input voltage goes high to reach a logic high value, the driver transistor is switched off and the net pull-down current is approximately equal to the current flowing for Vgs ¼ 0 in the wide load transistor. This current is small and heavily depends on the threshold voltage of the load transistor, as shown in some measured transfer characteristics of TFTs having different threshold voltages (Fig. 4). The time needed to

Cload

Transistor M2

-Vdd Fig. 1. Circuit schematic of a zero-Vgs load inverter, employing only ptype OTFTs. The load capacitance is schematically indicated in the drawing: in practice, this capacitance will be caused by the input capacitance of the logic gates connected to the inverter output.

5

5

0

0 -5

-10

Vout [V]

Vout [V]

-5

-10 -15

-15

-20

-20

-25

-25 -20

-15

-10

-5

Vin [V]

0

5

-30 -20

-15

-10

-5

0

5

Vin [V]

Fig. 2. (a) Measured input–output characteristics of a zero-Vgs load inverter (open symbols) and corresponding noise margin. (b) Input–output characteristics of a zero-Vgs load inverter obtained in simulation making the threshold voltage of the driver transistor negative (Vth ¼ 8 V) and corresponding noise margin.

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more than in the mobility of the semiconductor used (which has a stronger influence on the pull-up). In conclusion, for optimal design of zero-Vgs load inverters, achieving large noise margin and balanced pull-up and pull-down speeds, the threshold voltage of the driver transistor should be negative and the threshold voltage of the load transistor should be somewhat positive, while the mobility of the semiconductor is less important [24]. Speed may also be improved using complementary technologies, as in complementary logic style the pull-up or pull-down actions are always performed by a fully on device, exploiting the maximum current-driving capability for a given gate (and thus load) capacitance.

Vin V

Vout

t Fig. 3. Schematic time response of a zero-Vgs load inverter to an input pulse.

Id

1.00E-05

Vth1 Vth2

1.00E-07

1.00E-09

1.00E-11

1.00E-13 -30

-20

-10

0 Vgs

10

20

30

Fig. 4. Transfer characteristics of two transistors with identical layout and different, positive threshold voltages. The threshold Vth2 is more positive than Vth1; thus the transistor Vth2 conduces, at Vgs ¼ 0, a much higher current than transistor Vth1.

pull down the output node capacitance is about inversely proportional to the current flowing at Vgs ¼ 0 in the load transistor, and thus again strongly depends on the threshold voltage. When the input voltage goes down, on the other end, the driver transistor switches heavily on and is capable of pulling up the capacitance connected to the output node towards the high voltage supply (Gnd) in a short time. The pull-up time depends on the output capacitance and on the pull-up current, and thus will be inversely proportional to the mobility and proportional to the square of the transistor length (assuming all transistors share the same length and that the capacitive load is only due to transistors). If one looks at the total transit time of an input pulse though a zero-Vgs inverter, the pull-down time being much slower than the pull-up time, the bottleneck in achieving better speed is, for this popular kind of inverters, in the threshold voltage of the load transistor (which influences the pull-down action)

3.2. Challenges arising in RF circuits One of the most interesting applications of organic and printed electronics is item-level radiofrequency identification (RFID) of goods, which uses electronics labels, called RFID tags. In this field low-cost, flexible electronic labels made using printed electronics have the potential to enable a huge market. In RFID systems the reader sends electromagnetic waves that are captured by the antenna on the label: using this energy a rectifier provides the DC power needed to read an identification code stored in the label memory and the code is sent back to the reader using the same radio link used to send power [19]. The frequencies used for the power and communication link are usually 13.56MHz or 860–960 MHz (the so-called UHF band) according to the most used industrial standards. A non-linear circuit element is needed to rectify the e. m. wave captured by the antenna and transform it into DC current, which can be used to power the electronics on board of the RFID tag. State-of-the-art organic electronics does not provide efficient rectifying p–n junctions, so rectifiers are normally implemented using Schottky junctions or diode-connected OTFTs. The speed of these rectifiers, i.e. the maximum incoming frequency at which these rectifiers will work, depends on the time needed for the charge carriers to traverse the device when the incoming signal has the right polarity. Although the specific mechanisms behind the conduction are rather complicated and depend on the specific kind of device (diode or diode-connected transistor), the maximum working frequency depends on the semiconductor mobility and on the thickness of the semiconductor layer to cross. For this reason Schottky diodes [26] are inherently faster than diode-connected transistors [27]. Thus, in this application field, semiconductor mobility is of high importance and, at the state of the art, it is already possible to rectify UHF radio waves using organic rectifiers [28]. 3.3. Challenges in analogue circuits Analogue circuits are very important as they are needed to interface our physical world (via sensors, actuators, radio communication channels, etc.) to the processing performed using digital electronics. Organic analogue circuits would be needed, for instance, to

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interface and digitalize the signals coming from organic sensors using an integrated circuit approach. The field of analogue circuit design is still in its infancy. Analogue circuit design needs mature characterization and modeling of the transistors as well as deeper insight by the designer in device physics. Analogue design is thus more challenging and usually follows the development of transistor-level logic circuits design. Some studies on basic analogue building blocks like amplifiers and differential amplifiers using p-type-only OTFTs have been published [31], but quite some work needs to be done here. The efforts of modeling and design experts will probably concentrate on this field in the near future. The availability of complementary transistors would be very positive also for analogue design as it can be exploited to improve performance and compactness in this kind of circuits. 4. Conclusion: a possible research agenda In the field of p-type-only OTFT technologies, the availability of transistor with different (namely positive and negative) thresholds is of paramount importance. This would allow building reliable digital gates characterized by high noise margin faster digital circuits and would also provide design freedom to improve analogue circuit performance. In general research on the use of doping in OTFT is of fundamental importance. Mobility improvements are fine, but play a less-crucial role here. In spite of the progress made, more work to enhance shelf and especially operational OTFT stability is very welcome too. The role of circuit design to improve operational lifetime decreasing threshold sensitivity and introducing systemlevel countermeasures should also be investigated. In the emerging field of complementary OTFTs we can also list some interesting research challenges, mainly

 the mobility (and the thresholds) of the p- and n-type transistors needs to be balanced,

 more work should be done on operational and shelf stability,

 device modeling and circuit design with complementary OTFTs should receive attention, especially in the new area of analogue circuits. As a concluding remark, it is fundamental to further strengthen the efforts aimed at using high-throughput processing to manufacture organic electronics products. Many startups and research institutions are actively working in this field, which is a key factor for the sustained commercial success of organic transistor technology. References [1] Klauk H. Organic circuits on flexible substrates. In: IEDM conference proceedings, 2005. p. 446–9. [2] Zaumseil J, Sirringhaus H. Electron and ambipolar transport in organic field-effect transistors. Chem Rev 2007;107:1296–323. [3] McCulloch I, Heeney M, Bailey C, Genevicius K, Macdonald I, Shkunov M, et al. Liquid-crystalline semiconducting polymers with high charge-carrier mobility. Nat Mater 2006;5:328–33.

203

[4] Meng H, Sun F, Goldfinger MB, Jaycox GD, Li Z, Marshall WJ, et al. High-performance, stable organic thin-film field-effect transistors based on Bis-50 -alkylthiophen-20 -yl-2,6-anthracene semiconductors. J Am Chem Soc 2005;127:2406–7. [5] Weitz RT, Amsharov K, Zschieschang U, Barrena Villas E, Goswami DK, Burghard M, et al. Organic n-channel transistors based on corecyanated perylene carboxylic diimide derivatives. J Am Chem Soc 2008;130:4637–45. [6] Kumaki D, Ando S, Shimono S, Yamashita Y, Umeda T, Tokitoa S. Significant improvement of electron mobility in organic thin-film transistors based on thiazolothiazole derivative by employing selfassembled monolayer. Appl Phys Lett 2007;90:053506. [7] Hak Oh J, Liu S, Bao Z, Schmidt R, Wu¨rthnerb F. Air-stable n-channel organic thin-film transistors with high field-effect mobility based on N,N0 -bis(heptafluorobutyl)-3,4:9,10-perylene diimide. Appl Phys Lett 2007;91:212107. [8] Sirringhaus H, Kawase T, Friend RH, Shimoda T, Inbasekaran M, Wu W, et al. High-resolution inkjet printing of all-polymer transistor circuits. Science 2005;290:2123–6. [9] Sekitani T, Takamiya M, Noguchi Y, Nakano S, Kato Y, Sakurai T, et al. A large-area wireless power transmission sheet using printed organic transistors and plastic MEMS switches. Nat Mater 2007; 6:413–7. [10] Noguchi Y, Sekitani T, Someya T. Organic-transistor-based flexible pressure sensors using ink-jet-printed electrodes and gate dielectric layers. Appl Phys Lett 2006;89:253507. [11] Knobloch A, Manuelli A, Bernds A, Clemens W. Fully printed integrated circuits from solution processable polymers. J Appl Phys 2004;96(4):2286. [12] Clemens W, Mildner W, Bergbauer B. New high volume applications with printed RFID and more. MST-News 2007;3(5):10–2. [13] Huebler AC, Doetz F, Kempa H, Katz HE, Bartzsch M, Brandt N, et al. Ring oscillator fabricated completely by means of mass-printing technologies. Org Electron 2007;8:480–6. [14] Ofuji M, Lovinger AJ, Kloc C, Siegrist T, Maliakal AJ, Katz HE. Organic semiconductor designed for lamination transfer between polymer films. Chem Mater 2005;17:5748–53. [15] Hines DR, Ballarotto VW, Williams ED, Shao Y, Solin SA. Transfer printing methods for the fabrication of flexible organic electronics. J Appl Phys 2007;101:024503. [16] Lee HH, Brondjik JJ, Tassi NG, Mohapatra S, Grigas M, Jenkins P, et al. Appl Phys Lett 2007;90:233509. [17] Hart CM, de Leeuw DM, Matters M, Herwig PT, Mutsaerts CMJ, Drury CJ. Low-cost all-polymer integrated circuits. In: 24th European solid-state circuits conference proceedings 1998. p. 30–4. [18] van Lieshout P, van Veenendaal E, Schrijnemakers L, Gelinck G, Touwslager F, Huitema E. A flexible 240  320-pixel display with integrated row drivers manufactured in organic electronics. ISSCC digest of technical papers 2005. p. 578–618. [19] Cantatore E, Geuns TCT, Gelinck GH, van Veenendaal E, Gruijthuijsen AFA, Schrijnemakers L, et al. A 13.56-MHz RFID system based on organic transponders. IEEE J Solid State Circuits 2007;42:84. [20] Cantatore E, Hart CM, Digioia M, Gelinck GH, Geuns TCT, Huitema HEA, et al. In: ISSCC digest of technical papers, 2003. p. 382–501. [21] Appleyard SFJ, Day SR, Pickford RD, Willis MR. Organic electroluminescent devices: enhanced carrier injection using SAM derivatized ITO electrodes. J Mater Chem 2000;10:169–73. [22] Bharathan JM, Yang Y. Polymer/metal interfaces and the performance of polymer light-emitting diodes. J Appl Phys 1998;84(6): 3207–11. [23] Pfeiffer M, Leo K, Zhou X, Huang JS, Hofmann M, Werner A, et al. Doped organic semiconductors: physics and application in light emitting diodes. Org Electron 2003;4(2–3):89–103. [24] Cantatore E, Meijer EJ. Transistor operation and circuit performance in organic electronics. In: 29th European solid-state circuits conference proceedings 2003. p. 29–36. [25] Anthopoulos TD, Setayesh S, Smits E, Colle M, Cantatore E, de Boer B, et al. Air-stable complementary-like circuits based on organic ambipolar transistors. Adv Mater 2006;18:1900–4. [26] Steudel S, Myny K, Arkhipov V, Deibel C, De Vusser S, Genoe J, et al. 50 MHz rectifier based on an organic diode. Nat Mater 2005; 4(8):597–600. [27] Steudel S, De Vusser S, Myny K, Lenes M, Genoe J, Heremans P. Comparison of organic diode structures regarding high-frequency rectification behavior in radio-frequency identification tags. J Appl Phys 2006;99:114519.

ARTICLE IN PRESS 204

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[28] Steudel S, Genoe J, Heremans P. Organic rectifiers reaching UHF frequencies. MST-News 2007;3(5):41–2. [29] Klauk H, Halik M, Zschieschang U, Eder F, Rohde D, Schmid G, et al. Flexible organic complementary circuits. IEEE Trans Electron Dev 2005;52(4):618. [30] Klauk H, Zschieschang U, Pflaum J, Halik M. Ultralow-power organic complementary circuits. Nature 2007;445:745–8. [31] Gay N, Fischer W, Halik M, Klauk H, Zschieschang U, Schmid G. Analog signal processing with organic FETs. In: ISSCC digest of technical papers 2006. p. 1070–9.

[32] Park SK, Jackson TN, Anthony JE, Mourey DA. High mobility solution processed 6,13-bis(triisopropyl-silylethynyl)pentacene organic thin film transistors. Appl Phys Lett 2007;91: 063514. [33] Lee S, Koo B, Shin J, Lee E, Park H, Kim H. Effects of hydroxyl groups in polymeric dielectrics on organic transistor performance. Appl Phys Lett 2006;88:162109. [34] Jurchescu OD, Popinciuc Mihaita, van Wees BJ, Palstra TTM. Interface-controlled high-mobility organic transistors. Adv Mater 2007;19:688–92.