influence of molecular weight on the short-channel effect in polymer-based field-effect transistors

Influence of Molecular Weight on the Short-Channel Effect in

Polymer-Based Field-Effect Transistors

Ali Veysel Tunc,1 Bernhard Ecker,1 Zekeriya Dogruyol,2 Sabrina Juchter,1 Ahmet Lutfi Ugur,3

Ali Erdogmus,3 Sait Eren San,2 Jurgen Parisi,1 Elizabeth von Hauff1

1Energy and Semiconductor Laboratory, Institute of Physics, Carl von Ossietzky University of Oldenburg, 26111 Oldenburg,

Germany

2Department of Physics, Gebze Institute of Technology, Gebze 41400, Kocaeli, Turkey

3Department of Chemistry, Yildiz Technical University, Davutpasa Campus, 34210 Esenler, Istanbul, Turkey

Correspondence to: Elizabeth von Hauff (E-mail: [email protected])

Received 26 July 2011; revised 5 August 2011; accepted 8 August 2011; published online 8 September 2011

DOI: 10.1002/polb.22353

ABSTRACT: In this study, we demonstrate how the intrinsic prop-

erties of a polymer can influence the electrical characteristics of

organic field-effect transistors (OFETs). OFETs fabricated with

three batches of poly[2-methoxy,5-(30,70-dimethyl-octyloxy)]-

p-phenylene vinylene (MDMO-PPV) were investigated. The prop-

erties of the polymers were initially investigated using Fourier

transform infrared spectroscopy (FTIR), impedance spectroscopy

(IS), gel permeation chromotography (GPC), and cyclic voltam-

metry (CV), respectively. The structure and purity of the polymer

batches were found to be very comparable, but the molecular

weight (Mn and Mw) and polydispersity (PDI ¼ Mw/Mn), varied

between the samples and the HOMO and LUMO levels of the

polymers were found to depend on the molecular weight proper-

ties. OFETs were then fabricated with the polymers and electri-

cally characterized. It was observed that the channel current

and the field-effect mobility increase with increasing polymer

molecular weight. The output characteristics of the transistors, on

the other hand, were found to depend on the PDI of the polymer.

Saturation of the channel current occurs at higher source–drain

voltages and short-channel behavior was observed to start at

longer channel lengths for polymers with a higher PDI. This

behavior is observed to be thickness dependent, and the short-

channel behavior was more pronounced for thicker MDMO-PPV

films. These results are explained in terms of influences of chain

packing and ordering and high bulk currents on the FET output

and transistor parameters. VC 2011 Wiley Periodicals, Inc. J Polym

Sci Part B: Polym Phys 50: 117–124, 2012

KEYWORDS: charge transport; conjugated polymers; disorder;

molecular weight distribution; structure-property relations

INTRODUCTION Organic semiconductors offer the possibilityto fabricate low-cost, light-weight, flexible devices for a widerange of applications. The organic field-effect transistor (OFET)is the most significant element for the production of faster andsmaller electronics to create high-density integrated circuits.1

Transistor performance is proportional to lFE/L,2 where L is

the channel length of the transistor, and lFE is the field-effectmobility,2,3 so not only is the charge carrier mobility decisivefor device performance but also the channel length. The highestfield-effect mobilities reported in solution processed organicmaterials (lFE � 1.1 cm2 Vs–1)4 are considerably lower thanthose in poly-Si (l � 102 cm2 Vs–1);5 for this reason, the real-ization of organic field-effect transistors with shorter channellengths is of interest for preparing faster electronics.

Reducing the channel length in OFETs, however, can result inthe onset of the short-channel effect. Short-channel behavioris the deviation from FET current-voltage characteristics by

lack of saturation of the channel current, Ids, at high source-drain voltages, Vds, for a constant gate voltage, Vgs.

6–11 Short-channel effects are well known in inorganic materials,9 forexample, in Si-based devices, in which the short-channeleffect is observed when changes to the channel length arecomparable to the total channel length, leading to a break-down of the gradual channel approximation. This effect isless understood for organic-based devices. The short-channeleffect has been already observed in soluble organic materi-als,5–8 and vacuum deposited small molecules.10 Many stud-ies have been done on short-channel OFETs to optimizeshort-channel devices,7,12–14 however, the mechanismsbehind this effect remain unclear for organic-based FETs.Most reports on the channel length dependence in OFETshave focused on the semiconductor film, the dielectric thick-ness, and contact effects. It was recently demonstrated thatthe short-channel effect in OFETs cannot be explained due todevice dimensions alone.15 It is reasonable to assume that

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efficient device performance is related not only to transistorstructure and geometry but also depends on the intrinsicproperties of the organic semiconductor, such as molecularweight, purity, side chain arrangement, and also molecularordering in the film.

In this study, we investigate how the molecular weight andpolydispersity of [2-methoxy,5-(30,70-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) influence OFET perform-ance, in particular, the onset of the short-channel effect. It isknown that the molecular weight of a conjugated polymereffects the interchain ordering and has a remarkable effecton the electrical characteristics of the semiconductor filmand the resulting device properties. Optical, electrical, andcrystalline properties of the semiconductor film are sensitiveto molecular weight and the solvent used for processing.4,16

It has been reported that the hole mobility in poly(p-phenyl-enevinylene) (PPV) materials depends strongly on the molec-ular weight17 and recently, mobilities of �10�2 cm2 Vs–1

have been achieved.18 The distribution of molecular weightswas found to influence the electrical characteristics of poly-mer-based light-emitting diodes due to the correlationbetween the energetic disorder in the polymer film andthe polydispersity.19,20 Recently, a study investigated theeffect of blending low and high molecular weight batches ofpoly(triaryl-amine) on the field-effect mobility and saturationcurrent.21 However, the combination of OFET channel length,polymer molecular weight, and polydispersity, on the electri-cal properties of polymer-based OFETs has not been wellstudied. Here, we demonstrate that the intrinsic propertiesof the polymer influence the electrical characteristics of theOFET. In particular, the molecular weight influences thetransport properties of the polymer, and the polydispersityaffects the quality of the output characteristics and the onsetof the short-channel behavior.

RESULTS

Analysis of the Intrinsic Properties of the MDMO-PPVBatchesThree batches of MDMO-PPV were investigated in these stud-ies which will be denoted as polymer A, B, and C. First, theintrinsic properties of the polymer samples were investi-gated to determine differences in structure, contamination,molecular weight, and material energetics.

Fourier transform infrared spectroscopy (FTIR) was per-formed on the samples and the results are shown in Figure 1.FTIR allows the determination of the structure and contami-nation of a material. The bands in the FTIR spectra could beassigned to characteristic features of PPV materials. Theband near 964 cm�1 was attributed to CAH out-of-planebending of the trans configuration of vinylene group, theband near 3,057 cm�1 was assigned to the trans-vinyleneCAH stretching mode, and the bands near 853 and 1,503cm�1 were assigned to p-phenylene CAH out-of-plane bend-ing and CAC ring stretching, respectively.22 Additionally, allsamples exhibited IR absorption consistent with PPV, withstrong bands at 557, 853, 964, 1,036, 1,462, 1,503, 2,924,2,951, and 3,057 cm�1, respectively.23 From the spectra, no

differences in structure between the MDMO-PPV samplescould be determined.

To investigate differences in the purities of the samples, whichlead to differences in the charge carrier densities, capacitance–voltage measurements on metal–insulator–semiconductor (MIS)diode structures were performed. The details of the samplepreparation are described in the Experimental section. Thecharge carrier density, NA, was extracted from the slope ofthe transition from the accumulation regime to the depletionregime of the MIS-diode observed in capacitance–voltage meas-urements using the standard Mott-Schottky relation.10

NA ¼ 2 � V � C2

e0 � es � q � A2(1)

where V is the applied voltage, C is the capacitance of thepolymer layer, e0 is the permittivity of free space, er is thedielectric constant of the polymer, q is the elementarycharge, and A is the active area of the MIS-diode.

The values of NA were found to be 1.8 � 1017 cm�3, 3.0 �1017 cm�3, and 3.2 � 1017 cm�3 for polymers A, B, and Crespectively. These results are summarized in Table 1together with the molecular weight data for the polymers.The charge carrier densities are comparable between thepolymers, with the lowest concentration in polymer A andthe highest in polymer C.

The molecular weights of the samples were determinedusing gel permeation chromotography (GPC). The results forthe number average (Mn) and the weight average (Mw)molecular weights, and the polydispersity (PDI ¼ Mw/Mn)are also presented in Table 1. Both the molecular weight andthe PDI of a polymer are expected to influence the orderingof the polymer chains in the film and therefore the transportproperties. For the polymer batches investigated in thisstudy, A was found to have the lowest molecular weight(both Mn and Mw) and the lowest PDI. Polymer B has the

FIGURE 1 FTIR spectra (transmission vs. wave number) for the

three MDMO-PPV samples, A, B, and C. Graphs were shifted in

intensity for clearer presentation.

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highest molecular weight of the three polymers, and aslightly higher PDI than A. C has a molecular weight betweenthe two other polymers and the highest PDI of the samples,namely 16.8 compared to 3.5 and 4.2, respectively.

It has been shown that the HOMO energy level increasesas the number of conjugated units along the backboneincreases.22 Cyclic voltammetry was used to determine theHOMO and LUMO levels of the polymers (in film). In particu-lar, the HOMO level of the semiconductor is expected toinfluence the efficiency of charge injection and extractionfrom the source and drain contacts, respectively, for holesin the OFET channel. The HOMO/LUMO values for the poly-mers were found to be 4.97 eV/2.85 eV for polymer A,4.90 eV/2.84 eV for polymer B, and 4.93 eV/2.85 eV forpolymer C. The results are summarized in Table 1. Theenergy gaps of the polymers correspond to expected trend;polymer A, the polymer with the lowest molecular weight,has the largest gap (2.12 eV) while polymer B, the highestmolecular weight polymer, has the smallest gap (2.06 eV).The trend is supported by absorption data of the polymersmeasured in solution (not shown here) which revealedoptical HOMO/LUMO gaps of 2.16, 2.14, and 2.15 eV forpolymers A, B, and C, respectively.

Dependence of the FET Performance on MolecularWeight and PolydispersityTransistors were prepared using polymers A, B, and C. AllOFETs had comparable MDMO-PPV film thicknesses (80 nm).OFETs with channel lengths L ¼ 2.5, 5, 10, and 20 lm wereinvestigated. In Figures 2–4, the output characteristics of theOFETs are shown.

The FET output follows a similar trend for all three polymers.At longer channel lengths, (20 lm) all devices demonstrateideal output, with linear current–voltage characteristics at lowVds and saturation of the channel current at higher Vds. It isclearly seen that the magnitude of the channel currentdepends on the molecular weight of the polymer. For deviceswith L ¼ 20 lm (ideal output) Ids is the highest for devicesprepared with polymer B and the lowest for devices preparedwith polymer A. At shorter channel lengths, the FET behaviordeviates from the ideal case and the saturation is less pro-nounced or lost. Short-channel behavior can be seen at channellengths of 2.5 lm up to 10 lm, depending on the polymer. Theonset of saturation behavior is observed to depend on the PDIof the polymer. This is demonstrated quantitatively by examin-ing the normalized channel current to determine the onset ofthe short-channel behavior.

TABLE 1 Molecular Weight (Mn, Mw, and PDI), charge carrier densities (NA), and HOMO/LUMO values determined for the MDMO-

PPV polymers

Samples Mn (g/Mol) Mw (g/Mol) PDI (Mw/Mn) NA (cm�3) HOMO (eV) LUMO (eV) Eg(cyclic V) (eV) Eg

Abs (eV)

A 25,000 88,000 3.5 1.8 � 10�17 4.97 2.85 2.12 2.16

B 450,000 1,900,000 4.2 3.0 � 10�17 4.90 2.84 2.06 2.14

C 85,000 1,400,000 16.8 3.2 � 10�17 4.93 2.85 2.08 2.15

FIGURE 2 Output characteristics of OFETs prepared with polymer A at channel lengths of 20, 10, 5, and 2.5 lm for Vgs ¼ 0 to �60 V.



Figure 5 shows the normalized Ids versus Vds for Vgs ¼ 60 Vfor channel lengths of 20, 10, and 5 lm. Saturation corre-sponds to Ids ¼ 1. At L ¼ 2.5 lm, the output characteristicsdemonstrate no clear saturation behavior, so these resultswere excluded. For all channel lengths, devices preparedwith polymer A demonstrate saturation behavior at lowerVds than devices prepared with polymers B and C, whiledevices prepared with polymer C demonstrate saturationonset at the highest Vds. This is observed to correlate with

the PDI values of the polymer: A (3.5), B (4.2), and C (16.8).It is known that the doping level of the polymer layer influ-ences the Vds at which saturation occurs, and pinch off isshifted to lower voltages for lower doping concentrations.24

Polymer A has the lowest doping concentration of the threebatches which may result in the lower Vds for saturation inthe OFETs prepared with this polymer.

Figure 6 shows the channel current, normalized for thechannel length, (Ids � L at Vgs ¼ Vds ¼ 60 V) for transistors

FIGURE 3 Output characteristics of OFETs prepared with polymer B at channel lengths of 20, 10, 5, and 2.5 lm for Vgs ¼ 0 to

�60 V.

FIGURE 4 Output characteristics of OFETs prepared with polymer C at channel lengths of 20, 10, 5, and 2.5 lm for Vgs ¼ 0 to

�60 V.


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prepared with polymer A, B, and C. For ideal transistorbehavior, Ids is independent of the channel length and theonset of an L-dependence indicates a deviation from idealoutput. For OFETs prepared with polymers A and B the nor-malized current remains constant with the reduction inchannel length for FETs until a channel length between 5and 10 lm, indicating an onset of short-channel behavior atL < 10 lm. The slope is higher for the data from polymer Athan for data from polymer B, so it is likely that the devia-tion from ideal output occurs at longer channel lengths forpolymer A than for polymer B. For polymer C the normalizedIds varies with channel length for every L investigated, indi-cating that short-channel behavior begins at much longerchannel lengths. Short-channel behavior at L ¼ 20 lm can-not be excluded for devices prepared with polymer C. Thesedata demonstrate that the molecular weight and PDI mayboth play a role in determining the onset of short-channelbehavior.

It is expected that the transistor parameters are also dependenton the molecular weight and PDI of the polymers. The transfercharacteristics (Ids vs. Vgs for Vds ¼ 5 V at L ¼ 20 lm) areshown in Figure 7 for FETs prepared with polymers A, B, andC. From the transfer characteristics, the transistor parameterssuch as the field-effect mobility (lFE), the threshold voltage(VTH), and the ION/IOFF ratio can be extracted. The lFE wasdetermined according to25

lFE ¼ @Ids@Vgs

L

WCiVds

where Ci is the capacitance of the gate oxide. Additionally,values of lFE were extracted from the FET output in thesaturation regime for comparison, at Vds ¼ 60 V according to

lFE ¼ @ Ids;sat� �1=2@Vgs

" #22L

WCi

The threshold voltage, VTH, was determined from the inter-cept of the transfer characteristics with the voltage axis. TheION/IOFF ratio was determined from ION ¼ Ids at Vgs ¼ 60 Vand IOFF ¼ Ids at Vgs ¼ 0 V. The OFET parameters aresummarized in Table 2.

These results indicate that the polymeric properties influ-ence the quality of the FET output, in particular the onset ofsaturation behavior, the short-channel effect and the transis-tor parameters. Polymer B has the highest molecular weight,and the highest channel currents. This polymer results in thebest transistor performance, with deviations from idealFET performance starting at the shortest channel lengthsL < 10 lm. A comparison of the transistor parametersfor all polymers at L ¼ 20 lm reveals that polymer B resultsin the highest hole mobility values, determined in both thesaturation (lFE ¼ 2.7 � 10�4 cm2 Vs�1) and linear (lFE ¼4.8 � 10�5 cm2 Vs–1) regime, and the highest ION/IOFF ratio(6 � 104). Polymer A, on the other hand, has the lowest

FIGURE 5 Normalized source-drain currents for Vgs ¼ �60 V

for polymers A (closed squares), B (open circles), and C (stars).

Ids ¼ 1 corresponds to saturation.

FIGURE 6 Source-drain current normalized for the channel length

(Ids � L) at Vds ¼ Vgs ¼ �60 V for polymers A (closed squares), B

(open circles), and C (stars).

FIGURE 7 Transfer characteristics for polymers A (closed

squares), B (open circles), and C (stars) (L ¼ 20 lm and Vds ¼�5 V).



molecular weight, and results in the lowest channelcurrents and the lowest lFE values (saturation lFE ¼ 2.6 �10�6 cm2 Vs–1 and linear lFE ¼ 2.6 � 10�6 cm2 Vs–1). TheION/IOFF ratio is lower than in devices prepared with poly-mer B, but higher than in devices prepared with polymer C.Short-channel behavior also sets in at channel lengthsbetween L ¼ 5 – 10 lm. Polymer C results in channelcurrent and mobility values (saturation lFE ¼ 4.5 �10�6 cm2 Vs–1 and linear lFE ¼ 7.0 � 10�6 cm2 Vs–1) thatare slightly higher than Polymer A, but considerablylower than polymer B. The channel length dependenceof the normalized current indicates that short-channelbehavior occurs in FETs with L > 10 lm. Polymer C has amolecular weight between the other two polymers but aroughly fourfold higher PDI than polymers A and B.We interpret these results to show that a high PDI negativelyimpacts the quality of the transistor output. Additionally,the ION/IOFF ratio is the lowest for polymer C due to thecomparatively high OFF current. The values for VTH are com-parable between the polymers, slightly higher for polymer A;which is consistent with the low doping concentration as

well as the low currents observed in FETs prepared withthis polymer.

On the basis of these results, we propose that the molecularweight influences the transport properties in the channelwhile the PDI determines the quality of the output character-istics, specifically the onset of the short-channel effect inpolymer-based OFETs. Higher molecular weight is expectedto lead to better charge transport due to reduced polymerchain ends and increased packing density.16 This is observedwhen comparing the channel currents and mobility values inFETs prepared with the high molecular weight polymer Band the low molecular weight polymer A. A high polydisper-sity of the polymer, on the other hand, leads to a higherdegree of disorder in the channel which correlates to alonger channel length for the onset of the short-channeleffect, seen in devices prepared with polymer C. This can beunderstood by considering the polymer film as a network oftransport paths with varying resistances.26 The PDI, whichreflects the disorder in the system, influences the flow ofcurrent through the resistor network. In OFETs, it is knownthat the current is comprised on the one hand of charge

TABLE 2 Field Effect Mobility lFE (Linear and Saturation Values), Threshold Voltage VT, and

ION/IOFF Values for OFETs Made with Polymers A, B, and C

Samples lFE (linear) (cm2/V s) lFE (saturation) (cm2/V s) Vth (V) Ion/Ioff

A 2.6 � 10�6 5.6 � 10�6 �30 5 � 104

B 4.8 � 10�5 2.8 � 10�4 �25 6 � 104

C 4.5 � 10�6 7.0 � 10�6 �25 1 � 104

The lFE (linear), VT, and ION/IOFF values were extracted from the transfer characteristics (Vds ¼ �5 V). The

lFE (saturation) was determined at Vds ¼ 60 V.

FIGURE 8 FET output and normalized channel current (Ids � L) for polymers A, B, and C with film thicknesses of 240 nm.


POLYMER SCIENCE


carrier flow restricted within the first few nanometers abovethe semiconductor-dielectric interface27 (channel current)which is modulated by Vgs and on the other hand of a cur-rent within the bulk of the semiconductor film (bulk current)which is modulated by Vds.

7,27,28 For ideal OFET output char-acteristics, the channel current dominates. In the case of adecreased bulk resistance of increased charge injection intothe bulk, the bulk current can begin to dominate, and a devi-ation from ideal OFET output characteristics is observed.29

We interpret the results here as a demonstration that a highPDI, that is, a high degree of disorder in the channel of thepolymer OFET, leads to an increase in the relative contribu-tion of the bulk current compared to the channel current,leading to a deviation from the ideal output characteristicsin the OFET. This effect is demonstrated in the next section,in which FETs are prepared with thicker MDMO-PPV layersresulting in a high bulk current, and more pronouncedshort-channel behavior.

Influence of Polymer Film Thickness on theShort-Channel EffectThe influence of polymer molecular weight was observedto be more obvious in OFETs prepared with MDMO-PPVfilm thicknesses around 240 nm. In Figure 8(a–c), the outputcharacteristics of FETs with channel lengths L ¼ 10 lmare shown for polymers A, B, and C; respectively. Theshort-channel behavior is apparent in all of the devices tovarying degrees, most strongly in the FET prepared withpolymer C.

In Figure 8(c), the normalized channel current is plottedagainst the channel length. It can be seen that Ids � L varieswith the channel length for all FETs prepared with polymersA, B, and C over all channel lengths investigated. Theincrease in the film thickness is expected to influence theamount of charges being injected from the contacts into thebulk of the semiconductor film, influencing the quality of theoutput characteristics.

In addition, differences in the HOMO values of the polymercan lead to differences in the injection efficiency in the de-vice. As Au is expected to have a work function of �5.1–5.2eV30,31 polymer C should exhibit the lowest injection barrierfor holes from the Au-source contact, which can also resultin more charges being injected into the bulk of the semicon-ductor, leading to an onset of short-channel behavior at lon-ger channel lengths.

The analysis of the polymers with FTIR and in MIS-diodestructures demonstrated comparable intrinsic properties,such as structure and doping concentrations. On the basis ofthese results, we conclude that it is the PDI which has amajor influence on the electrical properties of the OFET andgoverns the onset of short-channel behavior in OFETs. Theseresults are significant when considering material design anddevice production for polymer-based electronics.

CONCLUSIONS

In this study, we investigated three batches of MDMO-PPV,which varied in molecular weight (Mn and Mw) and in PDI

(Mw/Mn). We demonstrate that the output characteristicsand parameters of MDMO-PPV-based OFETs depend stronglyon the molecular weight and PDI of the polymer. High molec-ular weight results in higher channel currents and field-effect mobilities. A high PDI, on the other hand, correlates toa decrease in the quality of the FET output characteristics. Itis observed that saturation occurs at higher Vds and short-channel behavior at longer channel lengths for when the PDIof the polymer is increased, and the ION/IOFF ratio is lowerthan in OFETs prepared with polymers with a lower PDI.These results are explained in terms of chain packing, whichis more efficient in high molecular weight and low PDI poly-mers. Increasing the MDMO-PPV film thickness leads tomore pronounced short-channel effect, which is attributed toan increase in the bulk current, leading to a deviation in theideal OFET output.

EXPERIMENTAL

The MDMO-PPV used in this study is commercially available.All processing of the organic thin films was performed in aglove box under nitrogen atmosphere. MDMO-PPV was dis-solved in dichlorobenzene and spin coated onto the sub-strates. The samples were then dried and annealed at110 �C for 10 min to evaporate residual solution. To achievethe same layer thicknesses for all samples, different concen-trations of MDMO-PPV in dichlorobenzene were used[20, 10, and 11 mg mL–1 for thick films (240 nm) and 10, 5,and 7 mg mL–1 for thin films (80 nm), respectively] andthe spin coating speed was varied between 1,200 and800 rpm. The film thicknesses were measured using a Dektakprofilometer.

FTIR spectra (KBr pellets) were recorded on a PerkinElmerSpectrum One Spectrometer. Thermogravimetric analysis(TGA) is a thermoanalytical method, in which the weight var-iation of a sample heated at a constant rate is measured con-tinuously. From the temperature derivative of these spectra,differential thermogravimetric analysis (DTG), it is possibleto obtain peak temperatures associated with a maximumrate of weight loss. Thermogravimetric analysis (TGA) wasdone using a TA SDT Q600. Nitrogen gas was set to run at20 mL min–1 to provide a controlled combustion environ-ment. TGA was carried out in the temperature range of 30–1,500 �C.

To determine the HOMO/LUMO levels of the polymers, cyclicvoltammetry was performed on MDMO-PPV films using aCHI660C Electrochemical Workstation with a scan rate of0.05Vs�1. The experiments were performed in an air-tightcell using acetonitrile (99.9% from Acros Organics) as thesolvent and TBAPF6 as the electrolyte salt. The workingelectrode was Cr/Au (5/150 nm) on glass, the counterelectrode was platinum and the reference electrode was anon-aqueous Ag/Agþ electrode.

Thin-film polymer field-effect transistors were fabricated onhighly n-doped silicon substrates with a thermally grown siliconoxide (SiO2) insulating layer (thickness 230 6 10 nm) with20 transistors per substrate. The substrate served as the gate



electrode. Source and drain contacts are interdigitated structures(10 nm ITO, 60 nm Au) with channel lengths L ¼ 2.5, 5, 10,and 20 mm and channel width W ¼ 1 cm. The substrates werepurchased from Fraunhofer IPMS (Dresden). Substrates werecleaned in acetone and isopropyl alcohol in an ultrasonic bathfor 10 min and then etched with O2 plasma for 2 min beforeapplying a silinization treatment to the SiO2 surface. Octadecyl-trichlorosilane (OTS) was applied to the SiO2 surface by immer-sion in a solution of 10 mM OTS in toluene for 20 minmaintained at 60 �C, and the surface was highly hydrophobicwith a water contact angle between 94� and 96�.

MIS-diodes were prepared on the same silicon substrates asused for the OFETs but without contact structures. The samecleaning recipe and procedure to deposit the organic layerswas used for these substrates as described above for theOFETs. The MDMO-PPV layers were 80 nm thick. The diodestructures were finalized by thermal evaporation of 20 nmof MoO3 and 100 nm Ag through a shadow mask, resultingin MIS-diodes with an active area of 16 mm2.

The electrical characterization of the OFETs and MIS-diodeswas carried out in a cryostat at 10�6 mbar in the dark.Computer-controlled source measure units (Keithley 236)were used for OFET characterization. A Solartron 1260 com-bined with a dielectric interface (Solartron 1296) was usedto record capacitance–voltage curves used for the extractionof charge carrier densities.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Dr. Susanne Heun fromMerck for helpful discussions and insights and the financialsupport from the DFG SPP1355 ‘‘Elementary processes inorganic photovoltaics’’ and the EWE-Nachwuchsgruppe‘‘Dunnschichtphotovoltaik’’ funded by the EWE AG Oldenburg.

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influence of molecular weight on the short-channel effect in polymer-based field-effect transistors

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