Stretched Contact Printing of One-Dimensional Nanostructures for
Hybrid Inorganic
−Organic Field Effect Transistors
Gen-Wen Hsieh,*
,†,‡JinJin Wang,
†Ken Ogata,
†John Robertson,
†Stephan Hofmann,
†and William I. Milne*
,††Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, U.K.
‡Institute of Lighting and Energy Photonics, National Chiao Tung University, No. 301, Gaofa third Rd., Guiren Township, Tainan
County 71150, Taiwan, R.O.C
ABSTRACT: We demonstrate a stretched contact printing technique to assemble one-dimensional nanostructures with controlled density and orientation from either dry or wet sources. The random, chaotically arranged nanostructures can gradually transform to a highly aligned configuration. Our results show that up to 90% of the printed nanowires are aligned within ±15° of the primary stretching direction. This approach is easily applicable to a variety of nanowires and
nanotubes on different substrates, and we demonstrate various field effect transistors with nanowire and hybrid nanowire− polymer networks. The hybrid inorganic−organic transistors based on a parallel aligned nanowire network and a semiconducting polymer revealed a significant enhancement in transistor mobility, a 10-fold reduction in subthreshold slope (∼0.26 V decade−1), and superior air stability compared to a pristine polymer host.
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INTRODUCTIONThe drive toward “ubiquitous electronics” that has been motivated by the rise of mobile communications and enabled by high quality displays has led to a commercial need for a new large area, flexible, cost-effective, and sustainable electronic technology. Several new materials systems are competing such as large grained silicon films, metal oxides, one-dimensional (1-D) nanostructures, and organic semiconductors. Organic semiconductors have, notably, been promoted as the potentially superior technology as they are compatible with very low cost patterning technologies based on printing.1,2 However, they suffer from low mobility and poor air stability compared with inorganic alternatives.3−5 Hence, a number of groups are investigating the use of nanostructured composite materials in which an inorganic nanomaterial is incorporated with an organic semiconductor with the aim of retaining the patterning advantages of the organic host, while enhancing performance by a path through the inorganic material.6−12
To enable the advancement of these hybrid material systems, one of the most important challenges is to manipulate nanoscale structures into a parallel aligned network which, in the active channel of transistors, could help charge transport without disturbing the orientation of organic conjugated molecules/polymers. Recent reports on nanostructure align-ment have showed a broad range of proof-of-concept self-assembly approaches. Wet solution-based self-assembly methods via microfluidic alignment,13−16 electric/magnetic field direct-ing,17−22 Langmuir−Blodgett films,23−25 blown bubble films,26,27and anisotropic evaporation of cylindrical droplets28 have been studied. However, achieving spatially specific, highly ordered 1-D nanostructure arrays from solution still continues
to be a significant challenge. Relatively simple dry assembly methods by shear-sliding contact printing29−32 and roll printing33−35 were recently developed to directly transfer regular arrays of 1-D nanomaterials from donor (mainly for vertical aligned networks) to receiver substrates. A poly-dimethylsiloxane (PDMS)-meditated or adhesive tape-assisted soft contact printing36−41 was also conducted, which demonstrated the large area capability of transferring prealigned (formed by guided growth during nanostructure synthesis) horizontal arrays. While these approaches are successful for assembling single layer arrays of functional NW/NT devices, the development of a comprehensive assembly approach dealing with both wet-based and dry-based 1-D nanomaterials remains a challenge.
Here we demonstrate a stretched contact printing technique to assemble 1-D nanostructures with controlled orientation and density from either as-grown substrates or solution-based dispersions. Unlike recent work on strain-release assembly of nanowires which relied on repeated operation of material transfer to prestrained substrates and strain release,42 our process is based on a simple one-step stretching of an elastic substrate covered with high-aspect ratio nanostructures. During the stretching process, the randomly orientated nanostructures gradually transform to highly aligned films. Subsequently, these stretching-aligned 1-D nanostructures can be transferred to other rigid or flexible substrates, forming a single layer of well-oriented network. Our results show that up to 90% of the
Received: October 27, 2011
Revised: February 18, 2012
Published: February 21, 2012
printed NWs are aligned within±15° of the primary stretching direction. Moreover, the feasibility of incorporating these stretching-aligned networks with organic semiconductors for generating field effect transistors (FET) devices is demon-strated. Through addition of such ordered networks to organic semiconductors, the flow of electric current can be accelerated, resulting in an enhancement of transistor mobility and of device stability in ambient air conditions.
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EXPERIMENTAL METHODSPreparation of As-Grown Nanowires. The n-type zinc oxide (ZnO) NWs with diameters of ∼50−80 nm were prepared by vapor phase deposition. ZnO powder mixed with graphite at a ratio of 1:1 was placed at the center of a quartz reaction tube heated to 950°C in a flow of Ar/O2(50/1 sccm)
at a total pressure of 3 mbar. NW growth was promoted downstream on a Si substrate covered by a 1 nm thick gold layer.10
Intrinsic Si NWs with diameters of∼10−60 nm and ∼5 μm in length were synthesized by chemical vapor deposition using hydrogen diluted silane (H2/SiH4 ratio of 170/30 sccm, total pressure 15 mbar) at 400 °C and a 2 nm thick gold layer as catalyst on a thermally oxidized Si (200 nm SiO2) wafer
support.43
A PDMS film (∼2−4 mm thick) was formed by casting its prepolymer (Sylgard 184, Dow Corning), mixed with curing agent at a ratio of 15:1 (w/w), degassing in a vacuum oven for 30 min over a plastic mold, and curing in the oven at 50°C for 4−6 h. The PDMS film was placed on top of the NW growth substrate, and after applying gentle manual pressure from the Figure 1.Stretched contact printing. (a) Schematic illustration of process flow for the stretched contact printing of as-grown or solution-based 1-D nanostructures. (b−e) Resulting SEM images and the corresponding alignment histograms of the ZnO NWs transferred from different stretching conditions (%): (a) S0: 0%; (b) S1: 40%; (c) S2: 80%; and (d) S3: 120%, where each arrow pair indicates the stretching direction. Scale bar: 10 and 2μm (inset).
top, the film was quickly peeled to transfer the NWs to the PDMS surface (yield: 50−60%).
Dispersions of Nanowires and Nanotubes. The N,N-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13, Aldrich) NWs were prepared by adding methanol to a vial containing the solutions of PTCDI in chloroform (1.5 mg mL−1) to induce the precipitation and self-assembly by π−π interactions of PTCDI.44The bottom layer of the vial contains PTCDI dissolved in chloroform, and the upper layer contains methanol. An initial swirl of the vial immediately induced the supramolecular self-assembly into 1-D nanostructures, and in several minutes, the complete formation of NWs was observed. The beltlike PTCDI NWs were typically 50−110 nm in width and tenths of micrometers in length.
Single-walled carbon nanotubes (CNTs) (CoMoCAT, South Nanotechnologies) were suspended in aqueous sodium dodecyl benzensulfonate (SDBS; 1 wt %) where the concentration was typically 10 mg mL−1with an average bundle length of 1.0−2.0 μm and a diameter of ∼10 nm. No further purification steps were carried out after the suspension. These nanostructures were transferred to the PDMS films by drop-casting and dried at room temperature overnight.
Stretched Contact Printing for Assembling Nanowires and Nanotubes. The controlled stretched contact printing was carried out using a home-built setup consisting of two foldback clips, each fixed with a movable carrier that can easily be shifted along the length of a scale-engraved rail. The PDMS films with a random distribution of NWs (by stamping or casting) were held with two foldback clips and stretched to different distances at a controlled speed. NWs were transferred to the surface of a rigid Si/SiO2 substrate (functionalized by poly(L-lysine) to increase the yield30) or a flexible sheet by
stamping and then peeling back the PDMS at a relatively slow speed, forming a single layer of oriented NWs on the receiving substrate.
Nanowire and Nanocomposite Transistors. For nano-wire transistors, ZnO NWs and Si NWs were transferred to Si substrates covered by 120 nm thick SiO2(Ci = 23 nF cm−2).
Source and drain contacts were defined by e-beam lithography and thermal evaporation (50 nm thick Cr for ZnO NW, 50 nm thick Ni for Si NW). The Si NW samples were dipped in a HF solution for 10 s to remove the native oxide prior to the metallization. Reliable contact formation was promoted by subsequent annealing in an Ar atmosphere at 425°C for a few minutes, leading to a Ni silicidation of the Si NWs.45
For hybrid inorganic−organic nanocomposite transistors, the Cr/Au contacts were defined by conventional UV photo-lithography on Si/SiO2substrates with stretching-assembled Si NWs. Prior to the octyltrichlorosilane self-assembled mono-layer (OTS-SAM) surface treatment the Si NW samples were dipped in a HF solution. An organic semiconductor poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]-thiophene) (PBTTT, Merck) was spin-coated (at 600 rpm for 5 s and then 1300 rpm for 50 s) in a 1,2-dichlorobenzene solution (7−8 mg mL−1) onto substrates covered with the Si NWs and Cr/Au electrodes. The spin-coating was carried out inside a N2
glovebox. Pristine PBTTT devices were also fabricated in the same manner without the stretched contact printing process. The devices were annealed at 180°C in N2for 15 min.
Characterization of Nanowire and Nanocomposite Transistors. The NW/hybrid FET devices were electrically characterized under ambient air, in dim light at room temperature with a Cascade probe station equipped with an
Agilent B1500A analyzer. From the measured transfer characteristics (IDS vs VGS), the transconductance (gm),
threshold voltage (VT), and effective field effect mobility (μFE) of the FETs were extracted using the following
expressions governing the operation in the linear and saturation regions: = μ − = ∂ ∂ = μ I CW L V V V g I V C W LV ( ) , i i DS,lin FE GS T DS m DS,lin GS FE DS (1) = μ − = ∂ ∂ = μ − I CW L V V g I V C W L V V 1 2 ( ) , ( ) i i DS,sat FE GS T2 m DS,sat GS FE GS T (2)
where IDSis the drain current, VGSthe gate-source voltage, VDS
the drain-source voltage, Cithe gate dielectric capacitance per
unit area, W the channel width, and L the channel length. The hybrid NW−polymer and pristine polymer FETs were further characterized by tapping mode atomic force microscopy (Explorer, Veeco).
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RESULTS AND DISCUSSIONStretched Contact Printing. Figure 1 shows the processing steps for creating oriented NW/NT arrays from a wafer source or solution and transferring them to another rigid or flexible substrate. The sources of nanostructures were either as-grown randomly oriented forests (e.g., ZnO NWs, Si NWs) or solution-based dispersions (e.g., CNTs, organic NWs). The first step comprised the stamping of as-grown NWs, or casting of nanostructure dispersions, onto PDMS films, which generated a layer of randomly distributed NWs or NTs. The PDMS film, with initial length (L), was then held with two foldback clips and gently stretched to different lengths. During this process, the randomly orientated nanostructures were gradually transformed to a partially ordered and further to a highly aligned film, according to the extent of deformation of the elongated PDMS film (defined as L + dL, where dL is the increase in length of the PDMS film due to stretching). These NWs/NTs were then transferred to the surface of a rigid Si substrate or a flexible film by stamping, providing a single layer of oriented nanostructures. The difference between the PDMS−nanostructure and substrate−nanostructure van der Waals interactions enables an efficient transfer process.36Thus, the extent of stretching that can induce the alignment effect simply depends on the length and density of the employed nanostructures and the elasticity of the PDMS stamp.
Figures 1b−e show SEM images of ZnO NW arrays transferred on Si/SiO2 substrates for increasing dL/L ratios. The histograms present the average alignment success for over 200 NWs, as determined from an area of 10μm × 10 μm for at least five arbitrary locations over the entire printed area (∼10 mm × 10 mm) on the receiver substrates. The networks printed from a less stretched PDMS film (dL/L = 40−80%) display modest alignment effects of ordering 25−48% of the NWs within±10° from the elongated axis of the elastic film (Figure 1c,d), while those from a nonstretched film (dL/L = 0%) show a random distribution (Figure 1b). For the networks transferred from the more highly stretched (dL/L = 120%) PDMS film, as shown in Figure 1e, the angle distribution
histogram shows that more than 78% of the NWs are aligned within±10° of the primary stretching direction, and over 90% of NWs are within ±15°. Table 1 summarizes the average
alignment results based on the different stretching conditions. Note that the NW areal density decreases from 50 NWs to 20 NWs per 10 μm × 10 μm depending upon the elongation. Hence, the orientation and density of the NWs in the stretched film will mainly follow the elongating direction.
Moreover, the separation between the parallel aligned NWs can further be controlled by a secondary contact printing stretched in an orthogonal direction. As shown in Figure 2,
another elastic PDMS film was applied onto a substrate with a single layer of highly oriented NWs, whereby the alignment direction of the NWs is perpendicular to the initial elongating direction of the PDMS. The action of this secondary printing will be to simply expand the separation space of these aligned NWs. For example, an initial NW network separation of∼2−3 μm was increased to ∼4−6 μm by the stretching process (dL′/ L′ = ∼100%, where, similarly, dL′ stands for the increased length of the PDMS film in the orthogonal direction). This separation-controllable feature can be applied to the fabrication of large area regular nanodevice arrays to make a much more efficient use of the 1-D nanostructures. However, it should be noted that nonfully aligned NWs might become more disordered due to this orthogonal elongation procedure.
Other 1-D Nanostructures by Stretched Contact Printing. To highlight the generality of our approach, we transferred a variety of 1-D nanostructures from either as-grown substrates or solution dispersions. Figure 3a shows that high density, ordered, uniform single layered Si NW networks were achieved by using the stretched contact printing method on as-grown randomly oriented Si NWs. Figure 3b,c shows good alignment also for solution-based single-walled CNTs and organic conjugated PTCDI NWs, both of which were drop-cast on PDMS films prior to the stretched contact printing process. Other work using dispersed and solution-synthesized NWs is in progress. We highlight that our approach is also applicable to
form various types of multilayer arrangements (e.g., crossbar, triangle) of 1-D nanostructures (data not shown here).
Taking advantage of the conformal contact offered by the soft PDMS films, we also used the stretched contact printing approach to transfer aligned NW or NT films onto both nonplanar and flexible substrates. In Figure 3d, for example, a single layer of ZnO NWs was transferred to the surface of a glass vial (by rolling a vial on a stretched PDMS film including assembled NWs). The as-transferred NWs are highly aligned and have uniform separation across the entire substrate.
The data clearly show that our stretched contact printing approach has several important advantages, including low-temperature processing and applicability to a wide range of substrates and 1-D nanomaterials. Importantly, for the latter our approach is applicable both to solution-based dispersions and as-grown samples supported on substrates.
Nanowire Transistor Arrays. In order to demonstrate the potential application of this stretched contact printing approach for electronic devices, we fabricated single- and multi-NW FET arrays based on the highly aligned NWs. We characterized FETs based on individual ZnO NWs (Figure 4a), which show typical n-channel transistor behavior. Figure 4b shows transfer and output characteristics for a typical FET based on ∼5−7 aligned Si NWs. The device exhibits ambipolar behavior, with comparable ON currents for holes and electrons, showing that the accumulation of both holes and electrons occurs for negative and positive gate voltages, respectively. The hysteresis effect for those NWs is well-known14,46,47and is thought to be due to charge traps at the surface of the nanostructures, especially when the measurement is carried out in air without passivation. Nevertheless, these results illustrate the viability of making nanodevices via the stretched contact printing technique. More work on large area NW transistor arrays and their application on logic devices will be reported at a later date. Table 1. Average Alignment Results Based on Different
Stretching Conditions avg. NW alignment (%), within sample type stretch extent (dL/L, %) NW coverage (per 10× 10 μm2) ±5° ±10° ±15° S0 0 50 10 13 18 S1 40 42 17 25 32 S2 80 26 33 48 62 S3 120 21 65 78 90
Figure 2. Secondary contact printing stretched in an orthogonal direction for separation control. Left: NW arrays before stretching; Right: NW arrays after stretching. Scale bar: 10μm.
Figure 3. Dark field optical images of different aligned 1-D nanostructures by stretched contact printing. (a) Si NW arrays transferred from an as-grown substrate. Scale bar: 50 and 10 μm (inset). (b) Single-walled CNT arrays transferred from a randomly dispersed solution. Scale bar: 50μm. (c) Organic PTCDI NW arrays transferred from a randomly dispersed solution. Scale bar: 50μm. (d) Optical images of parallel aligned ZnO NWs on the surfaces of three glass cylinders. Scale bar: 10μm.
Hybrid Inorganic−Organic Nanocomposite Thin Film Transistors. Stretching-aligned 1-D nanostructures can be used to produce hybrid inorganic−organic semiconducting composites with improved performance. Here we demonstrate hybrid inorganic−organic FETs with typical channel lengths (L) of 20μm and widths (W) of 10 000 μm. The channel layer consisted of a stretching-aligned Si NW network (aligned parallel to the channel direction and with an average area density of ∼20 NWs per 10 × 10 μm2 with ∼1−1.5 μm separation) mixed with a solution-processable organic semi-conductor PBTTT. The interdigitated (Cr/Au = 10 nm/50 nm thick) source and drain contact patterns were defined by photolithography and thermal evaporation onto Si/SiO2 substrates covered with transferred Si NWs. Prior to deposition of PBTTT, the samples were dipped in HF and subsequently treated with OTS in a N2 environment. After PBTTT spin-coating and device annealing (both carried out inside a N2
glovebox), the electrical performance of these hybrid Si-PBTTT FETs was characterized in ambient air with dim light and compared to measurements on pristine organic PBTTT FETs. Note that no field effect was observed from these stretching-aligned NW networks, indicating that the source-drain electrodes were not directly connected by Si NWs.
Figure 5a,b shows the characteristics of the hybrid Si-PBTTT and PBTTT devices. The values of IDS in the output
characteristics of the Si-PBTTT FET are much higher than those of the pristine PBTTT device. The contact resistance effect observed at low VDS is of similar order for both hybrid
and organic devices and can be reduced by using surface modification for the electrodes.48 The saturation field effect mobility of the pristine PBTTT extracted from the IDS−VGS
curves is 0.031 cm2V−1s−1, and the ON/OFF current ratio is 4
× 104 (note: the values are inferior to previously reported
results,49probably because the employed PBTTT material has been in the laboratory for more than three years). For the hybrid Si-PBTTT FET, its ON/OFF current ratio is 1× 107
and the carrier mobility is increased to 0.101 cm2 V−1 s−1, representing a∼3 times enhancement over the pristine PBTTT device. Moreover, a much sharper subthreshold slope of 0.258 V decade−1is achieved for the hybrid device, which represents a 10-fold reduction with respect to that of the pristine device. This improvement in field effect mobility and subthreshold slope upon the addition of aligned Si NWs is probably due to the superior electrical properties of the NWs, where they serve to increase the overall speed of charge carrier transport in the active channel.
Tapping mode AFM images of the pristine PBTTT and hybrid Si-PBTTT films after annealing are shown in Figures 5c and 5d, respectively. The topography and the size and structure of the crystalline domains are similar for both films (see insets Figures 5c,d). The aligned Si NWs are clearly revealed in the phase image (Figure 5d). Our data indicates that stretching-aligned Si NWs are embedded in the polymer film without interrupting the formation of polycrystalline PBTTT domains. Hence, these Si NWs may serve as a speedy pathway to transport a certain amount of charge carriers in the active channel. We believe this to be the key mechanism contributing to the increase in field effect mobility with reduced subthreshold slope in the hybrid device.
The possible charge transport routes via the hybrid Si-PBTTT film are illustrated in Figure 6. The first route is via PBTTT polycrystalline grains (black path), and more significantly, the second route is via a combinational path of PBTTT-Si NW-PBTTT (red path), which is likely to be the most desired route to enhance the charge carrier transport. However, to gain access to the red path, it is very important to Figure 4.Transfer and output characteristics of (a) a single-nanowire ZnO transistor with Cr contact and channel length of 2.0μm and (b) a multinanowire Si transistor (5−7 NWs) with NiSi contacts with reduced channel length of ∼0.5−1.0 μm.
remove the native silicon oxide shell surrounding each Si NW during the hybrid FET fabrication, as shown in Figure 6b. Otherwise, the oxide shell (∼1−2 nm thick) will inhibit charge transport (Figure 6c).
During the I−V measurements, we found that the output and transfer characteristics for hybrid Si (with oxide shell)-PBTTT
FETs were very similar to those of pristine PBTTT FETs, and no enhancement of, for instance, ON current, mobility, subthreshold slope was observed. Table 2 summarizes the key device parameters measured for pristine PBTTT, Si-PBTTT FETs, and Si (with oxide shell)-PBTTT devices. This suggests that the desired enhanced conductivity path of PBTTT-Si NW-PBTTT is prevented by the oxide shell. Hence, a removal of the oxide coating is essential to the processing of these hybrid devices.
Figure 7 shows device performance data measured periodi-cally over 600 h after fabrication. Our hybrid transistors were relatively stable in ambient conditions (exposed to air, moisture, and indoor light) with a reduction of 20% in mobility (from 0.101 to 0.080 cm2V−1s−1) and an ON/OFF
ratio remaining higher than 105, demonstrating superior electrical performance without any encapsulation. In contrast, the PBTTT-only devices showed a pronounced degradation with mobility decreased by ∼50%, and the ON/OFF ratio reduced to 103. Although the interactions and electronic
Figure 5.(a) Representative output characteristics of a hybrid Si-PBTTT FET and a pristine PBTTT organic FET. (b) Transfer characteristics of the same devices in the saturation regime at VDS=−50 V. The measurement was performed at room temperature under ambient air environment, after
N2annealing at 180°C for 15 min. The channel length and width of these devices are 20 and 10 000 μm, respectively. AFM phase images of (c) a
PBTTT film and (d) a hybrid Si-PBTTT film after annealing (insets: topography). All images (4μm × 4 μm) are randomly acquired on the active channel area of the composite devices. Scale bar: 1.0μm.
Figure 6.Schematic illustrations of possible charge transport routes via (a) crystalline PBTTT film (black-path), (b) crystalline PBTTT and semiconducting Si NWs (red path), where the NW oxide shell is removed by HF dipping, and (c) crystalline PBTTT and semi-conducting Si NWs with oxide shell (red path prevented). Note: the orange dot is the Au catalyst used for NW growth.
Table 2. Field Effect Mobility (μFE), ON/OFF Current Ratio, ON Current (ION), OFF Current (IOFF), Threshold Voltage (VT),
and Subthreshold Slope (S) of Pristine PBTTT, Si (HF Dipped)-PBTTT, and Si (with SiO2Shell)-PBTTT FET Devices atVDS
=−50 V
FET HF dip (10 s) μFE(cm2V−1s−1) ON/OFF current ratio I
ON(A) IOFF(A) VT(V) S (V decade−1)
PBTTT yes 0.031 4× 104 −2 × 10−4 −5 × 10−9 −13 2.551
Si-PBTTT (no SiO2shell) yes 0.101 1× 107 −1 × 10−3 −1 × 10−10 −11 0.258
properties at the interface between the polymer chains and the NWs are not fully understood yet, our results highlight that this heterogeneous interface plays an important role in the mobility degradation, i.e., improving resistance to oxygen doping.
For hybrid inorganic−organic FET applications, there is plenty of room for further enhancement via a variety of NWs with p-channel, n-channel, or ambipolar semiconductor proper-ties. Further experiments will examine the effect of different network densities and orientations of NWs on the hybrid FET characteristics. Ambipolar NWs would be of most interest for the realization of CMOS devices such as inverters or oscillators.
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CONCLUSIONSIn summary, stretched contact printing provides a simple, convenient, and low-cost method to fabricate highly oriented 1-D nanostructure networks over large areas. It is easily applicable to a variety of substrates and materials, including inorganic NWs, CNTs, and organic NWs and uniquely compatible to both solution-based dispersions and as-grown nanostructures supported on substrates. The various fabricated device structures, ranging from single- and multi-NW FETs to hybrid NW-polymer FETs, demonstrate the potency and feasibility of this technique for different electronic applications.
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AUTHOR INFORMATIONCorresponding Author
*E-mail [email protected], Tel +886 (0) 6303 2121 ext 57797, Fax +886 (0) 6303 2535 (G.-W.H.). E-mail wim@eng. cam.ac.uk, Tel +44 (0) 1223 748333, Fax +44 (0) 1223 748348 (W.I.M.).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTSWe thank Merck for the organic semiconductors. G.-W.H. acknowledges support from the W. Wing Yip & Brothers Bursary Award.
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