Quantum efficiency of ambipolar light-emitting polymerfield-effect transistors

Jana Zaumseil,1 Christopher R. McNeill,1 Matt Bird,1 Darryl L. Smith,2 P. Paul Ruden,3

Matthew Roberts,4 Mary J. McKiernan,5 Richard H. Friend,1 and Henning Sirringhaus1,a�

1Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Ave.,Cambridge CB3 0HE, United Kingdom2Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA3University of Minnesota, Minneapolis, Minnesota 55455, USA4Sumation Co. Ltd., 2020 Cambourne Business Park, Cambourne, Cambridgeshire CB23 6DW,United Kingdom5Cambridge Display Technology Ltd., Building 2020, Cambourne Business Park,Cambridgeshire CB3 6DW, United Kingdom

�Received 15 October 2007; accepted 8 January 2008; published online 28 March 2008�

The emission characteristics and external quantum efficiencies of ambipolar polymer light-emittingfield-effect transistors are investigated as a function of applied voltage, current density, and ratio ofhole to electron mobility. Green-emitting poly�9,9-di-n-octylfluorene-alt-benzothiadiazole� �F8BT�with balanced electron and hole mobilities and red-emitting poly��9,9-dioctylfluorene�-2,7-diyl-alt-�4,7-bis�3-hexylthien-5-yl�-2,1,3-benzothiadiazole�-2� ,2�-diyl� �F8TBT� with stronglyunbalanced hole and electron mobilities as semiconducting and emissive polymers are compared.The current-voltage and light output characteristics of the two types of light-emitting transistorswere found to be fundamentally alike independent of mobility ratio. Device modeling allowing fora single �Langevin-type� charge recombination mechanism was able to reproduce the devicecharacteristics for both cases but could not replicate the experimentally observed dependence ofexternal quantum efficiency on current density. The increase of quantum efficiency with currentdensity up to a saturation value could be indicative of a trap-assisted nonradiative decay mechanismat the semiconductor-dielectric interface. Optical output modeling confirmed that the maximumexternal quantum efficiency of F8BT light-emitting transistors of 0.8% is consistent with completerecombination of all charges and a singlet exciton fraction of 25%. © 2008 American Institute ofPhysics. �DOI: 10.1063/1.2894723�

I. INTRODUCTION

Organic light-emitting field-effect transistors �LFETs�are a new type of bifunctional organic electronic devicescombining the switching behavior of transistors with theemissive behavior of light-emitting diodes �LEDs�. Due totheir planar structure allowing spatial resolution of the re-combination and emission zone and properties such as highercharge carrier mobilities and current densities compared toLEDs, they have received increased attention in recentyears.1 A number of LFETs have been demonstrated using awide range of materials from small molecules to conjugatedpolymers and blends thereof. Some of the demonstratedLFETs operate solely as unipolar transistors, being only ca-pable of conducting either holes or electrons.2–4,4,5 Injectionand recombination of opposite charges occurs only in thedirect vicinity of the drain electrodes and is often inefficient.

Recently, however, a number of ambipolar LFETs werereported.6–11 For suitable biasing conditions, these ambipolarFETs can accumulate both positive and negative charge car-riers in spatially separate regions of the device. They allowobservation of a narrow recombination and emission zonewhere the hole and electron accumulation layers meet. Thiszone can be moved and positioned from the source electrode

through the channel to the drain electrode and vice versa bychanging the applied gate �Vg� or source-drain voltage�Vds�.

7–9,11 Theoretical models supported by optical reso-lution of the emission zone showed that this ambipolar re-gime can be thought of as a saturated electron channel and asaturated hole channel in series within the overall transistorchannel. In the simplest case, an infinite recombination rateand, thus, an infinitesimal width of the recombination zoneare assumed such that the hole �Ih� and electron current �Ie�equal the source-drain current Ids= Ih= Ie.

10,12–14 Even for afinite recombination rate, for example, based on Langevin-type recombination and, hence, a certain interpenetration ofholes and electrons,15,16 this assumption holds true, if thechannel length is sufficiently long compared to the width ofthe recombination zone. As long as light is emitted from wellwithin the channel, the external quantum efficiency �EQE� ofambipolar light-emitting transistors is expected to be con-stant and should only depend on the singlet-triplet ratio, ra-diative yield of formed excitons, and light outcoupling effi-ciency irrespective of, for example, the ratio of hole toelectron mobility or voltage conditions.

In this paper, we test this assumption and investigate theemission characteristics and EQEs of ambipolar LFETs inbottom contact/top gate geometry, illustrated in Fig. 1�a�,based on two different conjugated polymers. One of them,a�Electronic mail: hs220@cam.ac.uk.

JOURNAL OF APPLIED PHYSICS 103, 064517 �2008�

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poly�9,9-di-n-octylfluorene-alt-benzothiadiazole� �F8BT��Fig. 1�b�, left�, is a green-emitting, semiconducting polymer�emission wavelength of �550 nm, photoluminescence �PL�efficiency of �54%�,17 which we previously demonstrated toexhibit efficient ambipolar transport and light emission inFETs with balanced hole and electron mobilities on the orderof �7–8��10−4 cm2 V−1 s−1.9 The other conjugated polymeris poly��9,9-dioctylfluorene�-2,7-diyl-alt-�4,7-bis�3-hexyl-thien-5-yl�-2,1,3-benzothiadiazole�-2� ,2�-diyl� �F8TBT��Fig. 1�b�, right�, which emits in the red �emission wave-length of �670 nm, PL efficiency of �34%�. F8TBT can beused both as the hole or as the electron transporting layer inphotovoltaic cells and also shows ambipolar transport andlight emission in bottom contact/top gate FETs.18 The holemobility in F8TBT transistors, however, is found to be anorder of magnitude higher than the electron mobility. In thispaper, we compare the emission characteristics and EQEs forthese two types of polymer LFETs and demonstrate theirfundamental similarity both experimentally and using ananalytical device model. We will show that the quantum ef-ficiency �QE� of these ambipolar LFETs increases with cur-rent density before saturating at values consistent with com-plete charge recombination and about 25% singlet excitons,pointing toward a trap-assisted mechanism for nonradiativerecombination in these devices.

II. DEVICE PREPARATION AND EXPERIMENTALMETHODS

The schematic device structure of bottom contact/topgate LFETs is shown in Fig. 1�a�. Low sodium glass �Corn-ing 1737F� served as the substrate on which interdigitatedgold electrodes �thickness of 15–30 nm� were photolitho-

graphically patterned by a standard two-layer liftoff process�LOR 5B/Shipley 1813, MicroChem Corp.�. Vapor treatmentwith �3-mercaptopropyl�trimethoxysilane �Aldrich� beforemetal evaporation improved adhesion of the gold to theglass. Spinning 8–10 mg ml−1 solutions of F8BT or F8TBTin anhydrous m-xylene gave films of 50–70 nm thickness.Annealing at 290 °C removed residual solvent and inducedpolycrystallinity in F8BT films,17 while F8TBT films re-mained amorphous. A 450–520 nm thick poly�methyl meth-acrylate� �PMMA� �Mn=350 kg mol−1, Polymer Source,Inc., dielectric constant �PMMA=3.6� gate dielectric was de-posited by spin coating from an anhydrous n-butyl acetatesolution �35–40 mg ml−1�. Shadowmask evaporation of15 nm of gold as a semitransparent gate electrode completedthe devices.

All processing steps after liftoff as well as device char-acterizations were carried out in a dry nitrogen gloveboxexcluding oxygen and moisture. Current-voltage characteris-tics of the fabricated transistors were measured using an Agi-lent semiconductor parameter analyzer HP4155C. The ca-pacitance Ci of the gate dielectric was determined for eachsample using a capacitor structure and a Hewlett–Packardimpedance analyzer HP4192A. Saturation field-effect mo-bilities were extracted from the unipolar parts of the transfercharacteristics applying Ids

sat= �WCi�sat /2L��Vg−VTh�2 �W,channel width; L, channel length; Ids

sat, source-drain current insaturation; VTh threshold voltage; and �sat, saturation mobil-ity� and assuming gate voltage independent mobilities. Thelight output of LFETs was recorded with a silicon photodi-ode �Hamamatsu S1133-01, maximum dark current of 5 pA�,which was much larger than the device and placed in closeproximity to the device surface at a reverse bias of 4 V. Forcalculations of EQEs, it was assumed that all photons emit-ted from the surface �that is, through the gold gate electrode�were collected by the photodiode, which gave a lower boundfor the total EQE..

PL spectra and efficiencies of annealed F8BT andF8TBT thin films were measured at room temperature in anitrogen-purged integrating sphere with excitation from anargon ion laser at 457 nm. PL efficiencies were calculated asdescribed by de Mello et al.19

III. RESULTS AND DISCUSSION

Typical transfer and output characteristics of an ambipo-lar F8TBT transistor are displayed in Fig. 2. The transfercharacteristics �Figs. 2�a� and 2�b�� exhibit the characteristicsource-drain voltage dependence expected for ambipolarFETs and are similar to those previously reported for ambi-polar F8BT transistors.9 The significant difference betweenthe hole and electron mobilities is clearly reflected by theasymmetry of the hole �at low gate voltages� and electron �athigh gate voltages� currents. The hole mobility �5�10−4 cm2 V−1 s−1� in saturation is about an order of mag-nitude larger than the electron mobility �3�10−5 cm2 V−1 s−1�. This large disparity of hole and electronmobilities compared to F8BT can be rationalized when as-suming that electrons are largely localized on the benzothia-diazole �BT� unit of the polymer chain, whereas holes are

FIG. 1. �a� Schematic device structure of a bottom contact/top gate transis-tor with gold electrodes, PMMA as the gate dielectric, and F8BT or F8TBTas the semiconducting layer on a glass substrate. �b� Chemical structures andphotoluminescence spectra of F8BT �left� and F8TBT �right� thin films afterannealing.

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expected to be delocalized along the backbone. Such a dif-ference between the electron and hole polaron wavefunctionshas been predicted by quantum chemical calculations onF8BT,20 and a similar behavior is expected for F8TBT.21 Thedistance between BT units in F8TBT is much larger than inF8BT due to the additional thiophene units in the polymerbackbone. This increased hopping distance is likely to beresponsible for lowering the electron mobility in F8TBT sig-nificantly, while in F8BT, the hopping distance for electronsis sufficiently small so that electron and hole mobilities re-main approximately balanced.

In our device configuration containing gold source-drainelectrodes, ambipolar F8TBT transistors exhibit high contactresistance indicated by suppressed drain currents at lowsource-drain voltages as seen in the output characteristics�Fig. 2�c� and 2�d��. The contact resistance for hole injection,however, is notably lower than that of F8BT transistors �seeRef. 9�. This is due to the smaller bandgap of F8TBT �Egap

=2.2 eV� �Ref. 18� compared to F8BT �Egap=2.6 eV� andthe improved alignment of the F8TBT highest occupied mo-lecular orbital �HOMO� level ��5.4 eV� to the work func-tion of the injecting gold electrodes ��Au�4.8 eV� and, thus,lower injection barrier compared to F8BT �HOMO level of�5.9 eV�. The injection barrier for electrons into F8TBT isonly slightly lower, due to the more similar lowest unoccu-pied MO levels of F8TBT �3.15 eV� and F8BT �3.3 eV�.

Both F8BT and F8TBT ambipolar FETs show lightemission from within the channel region, as shown in Figs.3�a� and 3�b�. For both transistors, the position of the emis-sion zone can be adjusted from close to the source electrodethrough the entire channel to the drain electrode by changingthe applied gate or source-drain voltage as previously re-

ported for F8BT �Ref. 9� and other ambipolar LFETs.7,8,11

Figure 3�c� shows the position of the emission zone andsource-drain current �Ids� of a F8BT LFET during a transferscan with respect to the applied gate voltage and at constantsource-drain voltage. The symmetric form of the sigmoidalposition curve centered around the current minimum ob-served in the transfer characteristics is expected for a devicewith balanced hole and electron mobilities.10 Light-emittingcarbon nanotube FETs show an equivalent dependence of theemission spot position on gate voltage.22 The shift of thecenter point of the curve away from the ideal condition ofVg=Vds /2 is due to the somewhat different threshold volt-ages for holes �−29 V� and electrons �25 V� and the reducedeffective Vds due to contact resistance. For the F8TBT LFET,on the other hand, a very asymmetric position-versus-gate-voltage curve is observed reflecting the disparity of mobili-ties �Fig. 3�d��. Since electrons exhibit a lower mobility thanholes in F8TBT, a larger voltage drop across the electronaccumulation layer is needed to obtain an electron currentequivalent to the hole current. For equivalent voltage condi-tions the electron channel must be shorter than in the case ofbalanced mobilities. Hence, for a wide range of gate volt-ages, the emission zone is located closer to the electron in-jecting electrode, and only for a very narrow voltage range, itremains closer to the hole injecting electrode �Fig. 3�d��.Note that in both cases, there is a wide range of gate voltagesfor which the emission zone is located away from the inject-ing electrodes and, thus, complete recombination of holesand electrons is expected. This notion is further supported bythe narrow shape of both emission zones. In the case ofF8BT LFETs, the width at half maximum is about 4 �m. Theintensity profile along the recombination zone shows a

FIG. 2. Current-voltage characteristics of an ambipolarF8TBT transistor with L=20 �m, W /L=500, and Ci

=7.1 nF cm−2. Transfer characteristics at different �a�negative and �b� positive source-drain voltages. Outputcharacteristics for gate voltages �c� from 0 to −80 Vand �d� from 20 to 100 V in steps of 10 V. The holeand electron mobilities at saturation for this device are5�10−4 and 3�10−5 cm2 V−1 s−1, respectively.

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grainy texture reflecting the polycrystalline morphology ofthe F8BT film. This polycrystalline texture also leads tosome broadening of the emission zone. In contrast, the amor-phous F8TBT films shows a much narrower emission zonewith a width at half maximum of about 2 �m with no visibleasymmetry. The correlation of emission images and micro-structure of the polymer films suggests that the linewidthobserved by optical microscopy reflects the true width of therecombination zone and is not determined by limited reso-lution of the microscope setup.

Figure 4 shows the current-voltage and correspondinglight output characteristics of F8BT and F8TBT LFETs to-gether with their EQEs as a function of gate and source-drainvoltages. The EQE of the F8BT LFET reaches a plateau ataround 0.55% centered around the source-drain current mini-mum for Vds=−100 V. However, at Vds=−80 V, the maxi-mum efficiency is lower than that by almost 20%. The EQE,thus, seems to increase with increasing Vds. Furthermore, forVds=−80 and −90 V, a dip in efficiency coincident with thecurrent minimum is evident where the emission zone shouldbe in the middle of the channel, as shown in Fig. 3�c�.

A similar behavior of the external efficiency has beendescribed recently by Smits et al. for a bottom-gate near-infrared light-emitting ambipolar FET based on a solution-processable small-bandgap squarylium dye.10 This suggeststhat this dependence is a common feature among ambipolarLFETs irrespective of their geometry or organic semiconduc-tor. A similar apparent increase of efficiency with source-drain voltage can be found for F8TBT light-emitting transis-tors �Fig. 4�f��. F8TBT transistors show generally lower QEscompared to F8BT LFETs due to the lower PL efficiency ofF8TBT �34%� and the maximum EQE is reached at 0.4%. Incontrast to the F8BT case, however, this maximum is notcentered around the current minimum but shifted towardlower Vds and higher Ids. There is also no clear plateau of

efficiency but rather a peak that becomes broader as Vds in-creases. From Fig. 3�d�, one would expect the emission zoneof the F8TBT LFET to be located within the channel, awayfrom the electrodes, for a gate voltage range from about50 to 80 V for Vds=100 V. Thus, complete recombination ofholes and electrons should take place resulting in a constantQE. However, clearly in Fig. 4�f�, the efficiency is alreadysignificantly lowered at about 65 V even before the draincurrent reaches its minimum and, thus, before the emissionzone has reached the drain electrode. There is, however, akink in the efficiency curve at the gate voltage with the low-est drain current and the emission efficiency decreases lesssteeply from there. Notably, as the source-drain voltage in-creases, the shape of the efficiency curve becomes broaderand seems to approach a plateau. While the maximum QE ateach Vds remains unchanged, the efficiency at higher gatevoltages increases with respect to the maximum value lead-ing to a more plateaulike curve shape.

It is highly unlikely that a hole or electron can passthrough several micrometers of an accumulation layer of op-posite charge without recombining, and the narrowness ofthe emission zone supports the assertion that all charges arerecombining in a LFET with sufficient channel length. Thus,as long as the emission is located within the channel, holeand electron currents must be balanced irrespective of thesource-drain voltage or mobility ratio. Increased gate leakageas a possible source of additional emission can be excluded.The gate-source current in these LFETS is always two tothree orders of magnitude lower than the source-drain cur-rent, and there is no correlation between gate leakage andemission. The origin of the changing EQE must, therefore,be due to a change of either outcoupling efficiency or radia-tive yield. An effect of source-drain voltage on the outcou-pling efficiency is unlikely as the lateral position of the emis-sion zone is always far away from the source-drain

FIG. 3. �Color online� Optical images of light emissionfrom �a� F8BT and �b� F8TBT light-emitting FETs withinterdigitated source-drain electrodes �dark areas� withL=20 �m. The emission zone of the F8BT LFET re-flects the polycrystallinity of the polymer film, whilethe emission from the amorphous F8TBT is featureless.Position of emission zone with respect to source�grounded� electrode and source-drain current vs gatevoltage during a transfer scan of an ambipolar �c� F8BTand �d� F8TBT transistor with L=20 �m.

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electrodes and remains localized at the gate dielectric inter-face excluding interference effects.23 A dependence of theradiative yield on applied field and, thus, Vds could be pos-sible at very high fields; however, no corresponding correla-tion of efficiency with the channel length and, thus, fieldcould be found. The pronounced dip of efficiency for theF8BT LFETs at lower Vds and the lowered efficiency at lowerdrain currents in the case of F8TBT LFETs suggest that theobserved changes of EQE might be caused by different cur-rent densities rather than source-drain voltage or lateral field.

Before looking more closely at a possible charge densitydependence of the QE, we will compare the experimentaldevice characteristics to a recently developed analyticalmodel for organic ambipolar LFETs.16,14,24 We calculated thesalient device characteristics using the gradual channel ap-proximation based approach described in Ref. 16 and in-cluded the effects of nonvanishing contact resistances. Thecontact resistance values were chosen to be Re=3�106 � cm and Rh=105 � cm for both F8BT and F8TBTdevices, reflecting the expected larger contact resistance forelectron injection compared to hole injection from the goldcontacts. Carrier density dependent and, hence, gate voltagedependent, mobilities for electrons and holes were modeledas in Refs. 16 and 14, with threshold �VT,e and VT,h� andslope �Ve and Vh� parameters given by VT,e=15 V, VT,h=−20 V and Ve=100 V, Vh=200 V for F8BT, and by VT,e

=20 V, VT,h=−20 V and Ve=100 V, Vh=100 V for F8TBT.Saturation mobilities corresponded to the experimental satu-ration mobilities �Fig. 4�, as did the other independently de-termined device parameters, i.e., gate-channel capacitancesand channel lengths and widths. With the “source” grounded,the source is the electron injecting contact �large contact re-sistance� and the drain injects holes for positive drain andgate biases, while negative drain and gate biases imply thatthe source is the hole injecting contact �smaller contact re-sistance� and the drain injects electrons.

First, we explore the dependence of the recombinationposition as a function of gate voltage for fixed Vds in com-parison with the experimental data in Figs. 3�c� and 3�d�.Figures 5�a� and 5�b� show results for F8BT and F8TBTdevices with Vds=100 and −120 V, respectively. Evidently,the model reproduces well the qualitative differences in theshapes of these curves that are directly attributable to thesignificantly different electron and hole mobility ratios of thetwo materials as described above.

Next we examine the drain current, light output, and QEresults for the two different devices. For both F8BT andF8TBT devices, the calculated transfer curves shown in Figs.6�a� and 6�d� are in satisfactory agreement with the measureddata �Figs. 4�a� and 4�d��. Qualitative differences, i.e., the

FIG. 4. �a� Source-drain current, �b� light output, and �c� external quantum efficiency vs gate voltage �forward and reverse� of a light-emitting F8BT �green�transistor with L=20 �m, W /L=500, Ci=5.9 nF cm−2, �e=5�10−4 cm2 V−1 s−1, �h=6�10−4 cm2 V−1 s−1. �d� Source-drain current, �e� light output, and �f�external quantum efficiency vs gate voltage �forward and reverse� of a light-emitting F8TBT �red� transistor with L=20 �m, W /L=1000, Ci=6.8 nF cm−2,�e=3�10−5 cm2 V−1 s−1, �h=6�10−4 cm2 V−1 s−1.

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strong asymmetry of the F8TBT transfer curves comparedwith those of the F8BT device, can be traced back to the verydifferent electron/hole mobility ratios.

In order to reproduce the light output and QE character-istics, we extended the model by evaluating the total recom-bination rate and QE. This was done by integrating the spa-tial recombination profile between the source and draincontacts, suppressing emission from regions very close to thecontacts �about 1 /50th of the channel length� to account forthe low probability of radiative recombination in close prox-imity of the metal contacts. The integrated recombinationprobability, which is assumed to be proportional to the lightoutput, is plotted in Figs. 6�b� and 6�e�. The model repro-duces well the overall �twin peak� shape of the experimentalcurves �Figs. 4�b� and 4�e��. In the case of the F8TBT device,the high-voltage peak is largely suppressed because the cur-rent in that voltage range is electron dominated and, due tothe low electron mobility, significantly smaller than the cur-rent in the hole-dominated low-voltage range. Finally, Figs.6�c� and 6�f� show the calculated QEs as a function of thegate voltage. The calculated QEs, in agreement with themeasured data, display less structure than the light outputversus gate voltage curves. In particular, the strong asymme-try of the light output results is not found in the QEs of theF8TBT device because the effect of the lower electron mo-bility is largely cancelled out in the QE. The present model islimited to a single �Langevin� recombination mechanism,which is interpreted as a radiative process and leads to aconstant efficiency �that is, a flat efficiency plateau� for both

FIG. 5. Calculated source-drain current and position of recombination zonevs Vg at constant Vds for ambipolar transistors with �a� F8BT ��h /�e=1.2,Vds=100 V� and �b� F8TBT ��h /�e=20, Vds=−120 V� as in Fig. 3.

FIG. 6. �a� Calculated source-drain current, �b� light output, and �c� quantum efficiency vs gate voltage of a light-emitting F8BT �green� transistor withbalanced hole and electron mobilities ��h /�e=1.2�. �d� Calculated source-drain current, �e� light output, and �f� quantum efficiency vs gate voltage of alight-emitting F8TBT �red� transistor with unbalanced hole and electron mobilities ��h /�e=20�.

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F8BT and F8TBT in contrast to the experimental efficiencycurves. Competing mechanisms, possibly with different car-rier density dependencies,25 may reduce the internal QE. Theslopes of the calculated total recombination rate and QEcurves are steeper than those of the experimental results forlight output and EQE. We attribute this difference primarilyto the width of the calculated recombination profile��0.5 �m�, which is significantly narrower than the experi-mentally observed width of the light emission zone here�2–4 �m� and in other LFETs.26 The width of the calculatedrecombination profile is underestimated in the present model,primarily because the use of the gradual channel approxima-tion throughout the recombination region neglects the carrierspreading away from the semiconductor/insulator interfaceand the associated lowering of the longitudinal electric fieldin that region.

Evidently, device models that include only a single �ra-diative� recombination mechanism cannot reproduce the in-crease of QE with Vds and the dip of efficiency at the plateaudemonstrated in Figs. 4�c� and 4�f�, which again suggeststhat other charge density dependent recombination processescould play a role. In order to deconvolute the influences ofVds and source-drain current on QE in LFETs, we performedconstant current measurements on both types of polymertransistors. For these measurements, the source-drain currentwas kept constant while sweeping the gate voltage, adjustingthe source-drain voltage accordingly and recording the emit-ted light. Figure 7 shows the corresponding Vds ��a� and �c��and light output ��b� and �d�� versus Vg characteristics of aF8BT and F8TBT LFET.

During a gate voltage sweep with constant source-drain

current, the emission zone is again moved through the entirechannel; for a certain gate voltage range, the emission zonemust be located within the channel away from either elec-trode and complete charge recombination should take place.For both polymer transistors, this regime is indicated by anearly constant maximum light output and, hence, QE �pla-teau region in Figs. 7�b� and 7�d��. For values of Vg at theedges of the plateau �corresponding to lower and higher Vg�,the efficiency is reduced as the emission zone is located nextto the source or drain electrode resulting in incomplete re-combination and/or luminescence quenching by energytransfer. At the plateau, the light output is nearly constant fora range of Vg and Vds, as shown in Figs. 7�b� and 7�d�, unlikein the transfer curves of Figs. 4�c� and 4�f�. For example,while in Fig. 4�c�, a change of Vds from −80 to −100 Vcauses an increase of QE of about 20%, in Fig. 7�b�, the lightoutput and, hence, the QE for a constant source-drain currentof 500 nA remains unchanged for source-drain voltages be-tween 80 and 100 V. This suggests again that Vds is notdirectly responsible for the increasing QE in Figs. 4�c� and4�f� but rather indirectly through increasing the current den-sity.

From these constant current measurements, one can ex-tract the light output and QE for a range of source-draincurrents encompassing more than two orders of magnitudefrom 20 nA cm−1 up to 8 �A cm−1, as shown in Fig. 8�a� forF8BT and from 20 nA cm−1 up to 1.5 �A cm−1 in Fig. 8�b�for F8TBT. In Fig. 8, the source-drain current was normal-ized against channel width in order to be able to comparedevices with different W /L ratios and give a measure of cur-rent density. The QE of both types of LFETs initially in-

FIG. 7. Constant current measurements of LFETs inFig. 4: �a� Source-drain voltage and �b� light output vsgate voltage for various fixed negative source-drain cur-rents of a F8BT-LFET. �c� Source-drain voltage and �d�light output vs gate voltage for various fixed positivesource-drain currents of a F8TBT-LFET. The voltagerange for which a plateau of the light output is observedcorresponds to the emission zone moving away fromthe source electrode through the channel before reach-ing the opposite drain electrode.

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creases strongly with increasing source-drain current andthen starts to saturate approaching a constant efficiency atsource-drain currents exceeding 1.5 �A cm−1.

The maximum EQE of F8BT LFET at high source-draincurrents is about �0.8�0.1�%. In order to estimate the lightoutcoupling efficiency for our LFETs, we carried out opticalmodeling of the multilayer structure of glass/F8BT/PMMA/Au with light collected above the gold electrode asfollows. The power loss spectrum P�k� ,� ,� ,d� of a singledipole source with fixed free space wavelength �, orientation�, and position d within a multilayer structure is calculatedusing a scattering matrix approach to solving Maxwell’sequations.27 Couplings to surface emission modes, substrateguided modes, surface plasmon-polariton modes, and lossysurface waves28 are all included in this treatment. Intrinsicnonradiative losses of the material are estimated from the PLefficiency of thin films and included in the calculation asdescribed by Lu and Sturm29 The outcoupling efficiency isobtained by calculating the ratio of the power lost to thesurface emission modes to the total loss rate. This calculationis weighted over a realistic distribution of dipole positions,orientations, and emission wavelengths. The distribution ofdipole positions �or exciton formation zone profile� is takenas an exponentially decaying function peaking at the F8BT-PMMA interface with decay width of 15 nm. The averagedipole orientation �x ,y ,z�= �0.44,0.44,0.11� is estimatedfrom the ratio of in-plane to out of plane extinction coeffi-cients obtained from spectroscopic ellipsometry �kz /kx

=0.25�.30,31 The emission wavelength distribution is esti-mated from the PL spectrum of a F8BT film on quartz.

Figure 9�a� shows experimental and simulated transmis-sion spectra of the multilayer optical structure for differentthicknesses of the gold gate electrode. The model is found toreproduce the experimental transmission spectrum well andis, therefore, expected to allow accurate calculation of out-coupling efficiencies and expected EQEs depending on thethicknesses of F8BT, PMMA, and Au, as shown in Figs.9�b�–9�d�. The EQE is obtained by multiplying the outcou-pling efficiency by the singlet yield, estimated here to be25%. We find that an experimental EQE of 0.8% is wellwithin the range of the expected values. The thicknesses ofF8BT and PMMA affect this value only slightly. Both theoutcoupling efficiency and the absolute transmittance areshown to be the most sensitive to the gold thickness, asmight be expected. The best fit is obtained for 15 nm gold,suggesting an EQE of �0.75%. We note that due to thecomplete recombination of charges in ambipolar LFETs, theycould be used to determine the singlet-triplet ratio of organicelectroluminescent materials, which has been a matter of de-bate for some time.32 Our results here are consistent with atleast 25% singlets being formed in F8BT. More detailed ef-ficiency measurements and optical modeling will be neededto give a more reliable estimate. The optical modeling showsthat the experimentally measured EQE of 0.8% is evidenceof very efficient LFET operation. What might at first appearsto be a low EQE value is, in fact, entirely due to the chosendetection geometry because only relatively little light is out-coupled through the gate electrode.

Based on the observed dependence of QE on Ids in Fig.

FIG. 8. Light output �squares� and external quantum efficiency �triangles� vschannel-width-normalized source-drain current of �a� F8BT �W=1 cm� and�b� F8TBT �W=2 cm� transistors. Data points were extracted from plateaupart of light output vs Vg of constant current measurements, as shown in Fig.7.

FIG. 9. �a� Experimental �open circles� and modeled �lines� transmissionspectra of a multilayer stack of glass/F8BT �60 nm�/PMMA �420 nm�/goldwith different modeled thicknesses of gold �13 nm, dashed line; 15 nm,solid line; and 17 nm, dash-dot line�. Calculated external quantum efficien-cies based on the multilayer stack in �a� for different thicknesses of �b�F8BT, �c� PMMA, and �d� gold assuming 55% PL efficiency, 25% singletexciton fraction, and an emitting dipole orientation of kz /kx=0.25.

064517-8 Zaumseil et al. J. Appl. Phys. 103, 064517 �2008�

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8, we can now easily explain the observed variations of ef-ficiency shown in Fig. 4. The source-drain current for whichthe emission zone is located within the transistor channelincreases with Vds, explaining the concomitant increase inEQE. As experimentally observed at very high Vds, the effi-ciency increases less with Vds than at lower values because itstarts to saturate at higher currents. Likewise, when thesource-drain current reaches its minimum during a transferscan �for example, −230 nA for Vds=−80 V in Fig. 4�a��, theefficiency goes down as well, causing the dip in the effi-ciency plateau �Fig. 4�c��. When the source-drain voltage isincreased to −100 V and, thus, the minimum source-draincurrent increases to about −1 �A, this dip becomes less pro-nounced because EQE already approaches saturation.

In this way, the asymmetric efficiency plots for theF8TBT transistors become comprehensible as well. Becausethe electron mobility is so much lower than the hole mobil-ity, the source-drain currents at high Vg in Fig. 4�d� are muchlower than at lower Vg, which leads to a lower light outputbut also lower QE at this point despite total charge recombi-nation. The maximum QE of 0.4% for the F8TBT LFET inFig. 4�f� is achieved for source-drain currents of about8–10 �A �that is, 4–5 �A cm−1� and is almost independentof source-drain voltage. As the source-drain current increaseswith increasing source-drain voltage in Fig. 4�f�, the QE alsostarts to approach this maximum value in the high Vg regionleading to a more plateaulike efficiency curve shape.

This behavior is reminiscent of the increasing QE ofpolymer LEDs at low forward bias and, thus, currentdensities.33 This increase of efficiency in single layer poly-mer LEDs, however, was explained with nonradiative recom-bination losses when the emission is predominantly locatednext to the metal cathode at low biases.34 This effect can beruled out for ambipolar light-emitting transistors where emis-sion takes place far away from the source-drain electrodes.The distance of 400–500 nm to the gate electrode given bythe thickness of the gate dielectric excludes emissionquenching by energy transfer to the metal. However, lightemission in a LFET is anticipated to predominantly takeplace close to the interface between the gate dielectric andthe emitting polymer. Nonradiative recombination processesat this interface could, thus, play a large role.

Hsieh et al. investigated the influence of nonradiativerecombination processes on the QE of light-emitting carbonnanotube transistors and found that a trap-assisted Shockley–Read–Hall-type nonradiative recombination mechanismwould lead to an increase of efficiency with current density.25

A similar process could be responsible for the increasing QEin polymer LFETs. If a charge is trapped at the interface or ata defect in the bulk of the emitting polymer, recombinationwith an opposite charge is likely to be nonradiative assumingthat a charged trap is also a luminescence quenching site. Asthe source-drain current increases the number of defects atthe interface, or in the bulk, thus, the fraction of nonradiativerecombination events become less significant compared tothe number of radiative recombination events between mo-bile charge carriers resulting in increased QE. The exact na-ture of such traps, however, remains unclear. The relativelylarge threshold voltages for both hole and electron transport

in F8BT and F8TBT as well as the hysteresis of the current-voltage characteristics in Figs. 2 and 4 indicate that there is asignificant number of charge traps present at the interface.Whether those charge traps or other chemical or structuraldefects of the polymer chains at the interface are responsiblefor non-radiative recombinations remains to be investigated,for example, by means of temperature-dependent QE mea-surements. The fact that there is always a perfect balancebetween holes and electrons and a known position of therecombination zone within the devices will be helpful tostudy the nature of this nonradiative recombination process.

In order to compare the observed QEs of F8BT andF8TBT LFETs and their dependence on the source-drain cur-rent to those of other LFETs and polymer LEDs, one needsto determine the current density within the emission zone.Calculating the actual current density in a light-emitting tran-sistor is not straightforward. It is not known how much holesand electrons remain confined to the interface within the re-combination zone compared to their respective accumulationregions �within about 1–2 nm of the interface35� or howmuch they spread out away from the interface and, if so, howthis depends on voltage conditions. Nevertheless, it is helpfulto estimate a lower and an upper limit for the current densityin light-emitting transistors taking into account the currentflow through an area that is defined by the channel width andthe height of the emission zone. For a normalized source-drain current of 1 �A cm−1, the corresponding upper limit is10 A cm−2 �assuming all charges are confined to within 1 nmat the active interface�, while the lower limit of current den-sity is calculated to be 0.2–0.14 A cm−2 using the thicknessof the polymer film �50–70 nm�. However, this calculationdoes not yet take into account the finite width of the emissionzone in LFETs �2–4 �m�, which indicates that charge re-combination is less spatially confined in a LFET than in apolymer LED,15,16 and that the exciton concentration withinthe recombination zone of the LFET is not as high as indi-cated by the calculated current densities. Therefore, onecould expect the QE to be noticeably affected by a fixednumber of quenching sites as described above and, thus, toincrease with increasing source-drain current. Higher currentdensities at which such a quenching mechanism ceases to bepredominant are possible in LFETs with shorter channellengths and thinner gate dielectrics. The upper limits of QEin polymer LFETs, peak brightness, and possible quenchingmechanisms at high current densities are currently under in-vestigation.

IV. CONCLUSIONS

We have demonstrated experimentally and using an ana-lytical device model that ambipolar LFETs based on twoconjugated polymers with either balanced �F8BT� or stronglyunbalanced �F8TBT� hole and electron mobilities show fun-damentally the same emission characteristics. The emissionzone of both transistor types can be moved through the chan-nel by changing the applied gate voltage, thus, adjusting thepoint at which recombination of charge carriers takes place.Balanced hole and electron mobilities are not necessary forachieving a well-controlled ambipolar regime in organic

064517-9 Zaumseil et al. J. Appl. Phys. 103, 064517 �2008�

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FETs. The ratio of hole channel length to electron channellength for certain voltage conditions simply shifts accordingto the mobility ratio. For very unbalanced mobilities, thismeans that the voltage range must be extended significantlyin order to observe the emission zone moving through thetransistor channel and not being solely localized at the drainelectrodes. For both F8BT and F8TBT LFETs, the EQE wasfound to depend on current density. At low current densities,the EQE increases strongly and then approaches a saturationEQE, the value of which is consistent with complete chargerecombination and a singlet exciton fraction of about 25%.This dependence of efficiency on current density explains theobserved emission characteristics of organic LFETs, such asan apparent dependence of efficiency on applied source-drainvoltage or the dip in QE at the drain current minimum of thetransfer characteristics. The mechanism for increasing EQEwith current density is believed to be related to nonradiativerecombinations, such as recombinations involving trappedcharge carriers at the interface between the gate dielectricand the emissive polymer. A detailed understanding of themicroscopic mechanism will require temperature-dependentefficiency measurements and theoretical modeling of theLFET device characteristics including nonemissive recombi-nation processes.

ACKNOWLEDGMENTS

The authors thank Cambridge Display Technologies Ltd.for providing F8BT and F8TBT. J.Z. thanks the Gates Cam-bridge Trust for financial support. The work at Los AlamosNational Laboratory was supported by DOE Office of BasicEnergy Sciences Work Proposal No. 08SCPE973.

1M. Muccini, Nat. Mater. 5, 605 �2006�.2A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, and H. von Seg-gern, Phys. Rev. Lett. 91, 157406 �2003�.

3C. Santato, R. Capelli, M. A. Loi, M. Murgia, F. Cicoira, V. A. L. Roy, P.Stallinga, R. Zamboni, C. Rost, S. E. Karg, and M. Muccini, Synth. Met.146, 329 �2004�.

4T. Oyamada, H. Uchiuzou, H. Sasabe, and C. Adachi, J. Soc. Inf. Disp. 13,869 �2005�.

5T. Kono, D. Kumaki, J. I. Nishida, T. Sakanoue, M. Kakita, H. Tada, S.Tokito, and Y. Yamashita, Chem. Mater. 19, 1218 �2007�.

6C. Rost, S. Karg, W. Riess, M. A. Loi, M. Murgia, and M. Muccini, Appl.Phys. Lett. 85, 1613 �2004�.

7J. Zaumseil, R. H. Friend, and H. Sirringhaus, Nat. Mater. 5, 69 �2006�.8J. S. Swensen, C. Soci, and A. J. Heeger, Appl. Phys. Lett. 87, 253511�2005�.

9J. Zaumseil, C. L. Donley, J. S. Kim, R. H. Friend, and H. Sirringhaus,Adv. Mater. �Weinheim, Ger.� 18, 2708 �2006�.

10E. C. P. Smits, S. Setayesh, T. D. Anthopoulos, M. Buechel, W. Nijssen, R.Coehoorn, P. W. M. Blom, B. de Boer, and D. M. de Leeuw, Adv. Mater.�Weinheim, Ger.� 19, 734 �2007�.

11K. Yamane, H. Yanagi, A. Sawamoto, and S. Hotta, Appl. Phys. Lett. 90,162108 �2007�.

12R. Schmechel, M. Ahles, and H. von Seggern, J. Appl. Phys. 98, 084511�2005�.

13E. C. P. Smits, T. D. Anthopoulos, S. Setayesh, E. vanVeenendaal, R.Coehoorn, P. W. M. Blom, B. de Boer, and D. M. de Leeuw, Phys. Rev. B73, 205316 �2006�.

14D. L. Smith and P. P. Ruden, Appl. Phys. Lett. 89, 233519 �2006�.15G. Paasch, T. Lindner, C. Rost-Bietsch, S. Karg, W. Riess, and S. Schei-

nert, J. Appl. Phys. 98, 084505 �2005�.16D. L. Smith and P. P. Ruden, J. Appl. Phys. 101, 084503 �2007�.17C. L. Donley, J. Zaumseil, J. W. Andreasen, M. M. Nielsen, H. Sir-

ringhaus, R. H. Friend, and J. S. Kim, J. Am. Chem. Soc. 127, 12890�2005�.

18C. R. McNeill, A. Abrusci, J. Zaumseil, R. Wilson, M. J. McKiernan, J. H.Burroughes, J. J. M. Halls, N. C. Greenham, and R. H. Friend, Appl. Phys.Lett. 90, 193506 �2007�.

19J. C. de Mello, H. F. Wittmann, and R. H. Friend, Adv. Mater. �Weinheim,Ger.� 9, 230 �1997�.

20J. Cornil, I. Gueli, A. Dkhissi, J. C. Sancho-Garcia, E. Hennebicq, J. P.Calbert, V. Lemaur, D. Beljonne, and J. L. Bredas, J. Chem. Phys. 118,6615 �2003�.

21K. G. Jespersen, W. J. D. Beenken, Y. Zaushitsyn, A. Yartsev, M. Ander-sson, T. Pullerits, and V. Sundstrom, J. Chem. Phys. 121, 12613 �2004�.

22J. Tersoff, M. Freitag, J. C. Tsang, and P. Avouris, Appl. Phys. Lett. 86,263108 �2005�.

23H. Becker, S. E. Burns, and R. H. Friend, Phys. Rev. B 56, 1893 �1997�.24P. P. Ruden and D. L. Smith, Organic Thin-Film Electronics—Materials,

Processes, and Applications, MRS Symposia Proceedings No. 1003E, ed-ited by A. C. Arias, J. D. MacKenzie, A. Salleo, and N. Tessler �MaterialsResearch Society, Warrendale, PA, 2007�, Paper no. 1003-004-08.

25C. T. Hsieh, D. S. Citrin, and P. P. Ruden, Appl. Phys. Lett. 90, 012118�2007�.

26J. S. Swensen, J. Yuen, D. Gargas, S. K. Buratto, and A. J. Heeger, J.Appl. Phys. 102, 013103 �2007�.

27D. M. Whittaker and I. S. Culshaw, Phys. Rev. B 60, 2610 �1999�.28W. L. Barnes, J. Mod. Opt. 45, 661 �1998�.29M. H. Lu and J. C. Sturm, J. Appl. Phys. 91, 595 �2002�.30C. M. Ramsdale and N. C. Greenham, Adv. Mater. �Weinheim, Ger.� 14,

212 �2002�.31J. M. Winfield, C. L. Donley, and J. S. Kim, J. Appl. Phys. 102, 063505

�2007�.32M. Segal, M. A. Baldo, R. J. Holmes, S. R. Forrest, and Z. G. Soos, Phys.

Rev. B 68, 075211 �2003�.33P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. Vissenberg, and J.

N. Huiberts, Synth. Met. 122, 95 �2001�.34D. E. Markov and P. W. M. Blom, Appl. Phys. Lett. 87, 233511 �2005�.35C. Tanase, E. J. Meijer, P. W. M. Blom, and D. M. de Leeuw, Org. Elec-

tron. 4, 33 �2003�.

064517-10 Zaumseil et al. J. Appl. Phys. 103, 064517 �2008�

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