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Synthetic Metals 162 (2012) 1919– 1922

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

journa l h o me page: www.elsev ier .com/ locate /synmet

ffect of CsF buffer layer on charge-carrier mobility in organic light-emittingiodes based on a polyfluorene copolymers by admittance spectroscopy

ui Zhenga,b, Wenbo Huanga,∗, Wei Xua, Yong Caoa

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, People’s Republic of ChinaDepartment of Materials, Taizhou University, 605 East Road, Linhai, Taizhou 317000, Zhejiang, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 12 April 2012eceived in revised form 8 June 2012ccepted 8 August 2012vailable online 13 October 2012

eywords:

a b s t r a c t

Insert an ultrathin insulating layer at the organic/metal interface is a promising way to increase thedevice efficiency of organic light-emitting diodes (OLEDs). Here we have fabricated OLEDs, whichwere sandwich structure with Al/poly [9,9-dioctylfluorene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole](PFO-DBT15)/PEDOT/indium tin oxide (ITO) OLEDs, with and without ultrathin CsF buffer layer at theorganic/metal interface. Special attention was paid to polymer/electrode interface modification. Trans-port of carriers in copolymer with and without CsF buffer layer was investigated by means of admittance

sF buffer layerdmittance spectroscopyharge-carrier mobilityrganic light-emitting diodes

spectroscopy, respectively. We compare the charge-carrier mobilities of double-carrier (with Al/CsF cath-ode) and hole-only (with Al cathode) devices. CsF buffer layer is shown to significantly influence theelectron mobilities while the hole mobilities are left unchanged and thereby carrying out a better bal-ance of carrier in the device. The diffusion of CsF from electrode into copolymer is clearly observed byscanning electron microscopy and energy dispersive spectrometer, which resulting in enhance electric

e carr

injection and improve th

. Introduction

OLEDs are important in applications to full-color flat-panelisplays because of their low operating voltages and low poweronsumption. Their electrical characterization has an importantubject for a long time [1,2]. In order to improve the device effi-iency, it is necessary that modifying and optimizing the cathodeo establish efficient electron injection into the adjoining organicayer. An alternative approach is to insert an ultrathin insulat-ng layer [3–6], such as CsF [4,5], at the organic/metal interface.sF is widely used in OLEDs as cathode materials, which cannhance electron injection and improve luminescence efficiency.he mechanism of CsF for OLEDs was deeply discussed in manyapers [5], which suggest Al reacts with CsF and releases some

ow work-function metal atoms. According to the principle of ther-odynamics, this reaction may take place in certain situations.

s is a reactive metal with low work function, which may behe reason that it enhances the electron ejection. In this article,

e further study the effect of CsF buffer layer on charge-carrierobility in saturated red emitter based on polyfluorene copoly-ers by admittance spectroscopy. Admittance spectroscopy (AS)

∗ Corresponding author at: State Key Laboratory of Luminescent Materials andevices, South China University of Technology, 381 Wushan Road, Guangzhou10641, People’s Republic of China.

E-mail address: pswbh@scut.edu.cn (W. Huang).

379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.synthmet.2012.08.008

ier balance in a double carrier device.© 2012 Elsevier B.V. All rights reserved.

is a powerful tool that can be used to infer charge transport prop-erties of material [7–12]. In an admittance experiment, the chargerelaxation driven by a small harmonic voltage modulation vac isprobed. The amplitude and the phase difference of the ac currentiac are monitored as a function of frequency f, and the admit-tance Y is given by Y(ω) = iac(ω)/vac(ω) = G(ω) + iωC(ω), where Gis the conductance, C the capacitance, i the imaginary unit, andω = 2�f the angular frequency. By superimposing a forward dcbias Vdc to the harmonic voltage modulation, free carriers canbe injected into a sample having a diode structure. In case ofinjection, the frequency dependence of Y is determined by thetransit time �−1

tr effect of the injected carriers. In trap-free con-ditions, the capacitance makes a step around the frequency of�−1

tr , C tending at higher frequencies to the dielectric or geomet-rical capacitance of the sample, Cgeo, and decreasing to a lowervalue toward lower frequencies. However, this ideal behavior isnot observed if traps are present in the sample and the capaci-tance spectrum is found to be affected at low-frequencies by trapcontribution.

In the past decade AS has been applied to the investigation ofcharge-carrier mobility in organic materials [8–10], with the advan-tage of using films of just a few hundred nm instead of microns,as required by other common techniques such as time-of-flight or

dark injection. In this letter, AS technique is used to investigate themobility of carriers in an alternating polyfluorene copolymer calledPFO-DBT15 of hole-only and double-carrier devices (CsF/Al cathodereplacing Al cathode).

1 etals 162 (2012) 1919– 1922

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. Experimental details

The devices used in this study were prepared in the sandwichedtructures ITO/PEDOT (40 nm)/PFO-DBT15 (200 nm)/Al (100 nm)device A), ITO/PEDOT (40 nm)/PFO-DBT15 (200 nm)/CsF (1 nm)/Al100 nm) (device B), where ITO is indium tin oxide and PEDOTork function is 5.0–5.1 eV. The copolymer highest occupiedolecular orbital/lowest unoccupied molecular orbital levels, esti-ated by the onset of its oxidation/reduction potentials, are at5.64 and −3.58 eV, respectively, thus a barrier is expected at

TO/PEDOT/PFO-DHTBT15 for hole injection while Al should act as blocking contact for electrons under forward bias. The copolymerayer was spin-coated from a p-xylene solution to a thickness of00 nm. A thin layer of CsF (1 nm) and subsequently 100 nm of Alere vacuum-evaporated subsequently on the top of EL polymer

ayer under a vacuum of 1 × 10−4 Pa. The glass substrates were thenarried in a glove box without exposure to laboratory atmospherend sealed by a cap-glass with epoxy resin adhesives. The device is

hole-only device in the sense that only holes are injected from ITOlectrode at low dc bias voltage. The AS was recorded by impedancenalyzer (HP4192A) with a four-terminal pair configuration in arequency range from 10 Hz to 1 MHz under the condition of oscil-ating voltage of 50 mV during dc voltage scanning at 0.5, 1.0 and.5 V. The measurement area was 0.15 cm2.

. Results and discussion

In previous paper [8–10], researchers provide an in-depth anal-sis of the complex admittance that describes the carrier dynamicsnside an organic film sandwiched by two electrodes. It is foundhat the average transit time �dc of carriers can be extracted from

plot of the negative differential susceptance −�B = −ω(C − Cgeo)gainst the frequency f = ω/2�, where C and Cgeo are, respectively,he frequency dependent and geometric capacitances of the organiclm, ω is an angular frequency. Previous research [9] shows that−1tr , the position at which the maximum in −�B plot occurs, iselated to �dc = 0.56 �tr. Charge mobility � is then determined byq. (1) from the transit-time, �dc:

= L2

0.56 Vdc�tr(1)

here L is the thickness of an organic thin film, and Vdc is thepplied bias voltage. Fig. 1(a) and (b) shows the frequency depend-nce of capacitance (C) in device A and B for different values ofhe applied dc bias and Fig. 2 shows the corresponding frequencyependence of conductance (G). As the bias is increased the capac-

tance becomes frequency dependent drastically due to carriernjection. The excess capacitance observed at low frequencies cane attributed to electron trapping. The capacitance now shows ainimum at a distinct frequency, which is shifted to higher values

s the bias increases. The position of the minimum point is deter-ined by the average carrier transit time. These contributions to C

re more clearly visualized by plotting the differential susceptance�B = −ω(C − Cgeo). Fig. 3(a) and (b) shows the corresponding fre-uency dependence of −�B spectra for device A and B, respectively.learly, −�B spectra exhibit peaks which move to higher frequen-ies with an increasing dc bias, and which corresponds to the carrierransit time. The charge-carrier mobility � values calculated fromhe maximum frequencies by Eq. (1) are shown in the plot of Fig. 4s a function of the square root of the electric field E, estimateds Vdc/L. Fig. 4 shows a plot of ln � vs. E1/2 for device A and B.

he linear trend indicates that the experimental data follow theoole–Frenkel law form, � = �0 exp(�

√E), where E is the electric

eld and �0 and � are temperature dependent parameters. Lin-ar fittings lead to � = 1.9793 × 10−3(cm V)1/2 for device A and

Fig. 1. Frequency dependence of capacitance in device A (a) and device B (b) at DCbias voltage of 0.5, 1.0 and 1.5 V.

� = 4.3325 × 10−3(cm V)1/2 for device B. According to the previ-ous studies [13], the value of � also increases with the disorder orlevel spread, so-called dispersion parameter. Apparently, the slopefor device B is larger than that for device A. We speculate that thevalue of � was not only relevant for the disorder of material andtemperature, but also for the structure of devices such as thickness[14] and electrode modifications. We assume that the enhance-ment in injection efficiency of A1 contacts to PFO-DBT15 could beobtained by interjecting an ultrathin layer of LiF between the filmand the A1 contact. The reason why the slope for device B is largerthan that for device A is that device B had both electrons and holespropagating. Therefore it becomes more sensitive for applied biascompared with device A. In order to analyze the effect of the devicesstructure on the value of � , further experiments need to be car-ried out in the future. Device A is a hole-only device, which onlyholes are injected from an ITO electrode when applied bias. Butdevices B is a dual-carrier device, which holes and electrons areinjected from anode and cathode when applied bias. The charge-carrier mobility in device B with CsF buffer layer electrode wasobviously higher than that in device A without CsF. As reported byChan et al. [5], the device with a CsF buffer layer cathode showedgood performance due to a reduction in the cathodic barrier. The

mobility calculated by AS is much smaller than that of other poly-fluorene derivatives reported elsewhere [15], but we will considermainly the effect of CsF buffer layer on charge-carrier mobility in

R. Zheng et al. / Synthetic Metals 162 (2012) 1919– 1922 1921

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Fig. 3. Frequency dependence of −�B = −ω(C − Cgeo) in device A (a) and in device

will be achieved.To further verify our explanation, we use scanning electron

microscopy (SEM) and energy dispersive spectrometer (EDS) toanalyze the cross-section of device A. Fig. 5(a) and (b) shows

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ig. 2. Frequency dependence of conductance in device A (a) and device B (b) at DCias voltage of 0.5, 1.0 and 1.5 V.

he device with the CsF/Al cathode in this article. In early paper, aheoretical study by Pitarch et al. [16] showed that a dual injectioniode is expected to behave identically to a single-carrier device.hey indicated that carrier mobility in a dual-carrier device equaledo the sum of hole and electron mobilities, namely the effective

obility. Furthermore, a experimental study by D. Poplavskyy et al.10] demonstrated that only the effective mobility, i.e. the sum ofole and electron mobilities, could be measured in a dual injec-ion device by AS, and electron and hole transports could not beeparated in dual-carrier diodes. Thus, according to the concep-ion of effective mobility in dual-carrier device, we suggest that thextra increment of charge-carrier mobility in device B compared toevice A might be predominantly due to electrons injection. Thenode of device B is not be modified compared with device A, soe believe that the element of effective mobility, which due toole transport in device B, has been unchanged and the changes offfective mobility should be stemmed from modifying the cathode.his is expected when the electron injection barrier decreases andllows for additional electrons to be injected, hence, the incrementf charge-carrier mobility is attributed to the transit of electronshrough the organic/metal interface, resulting in a increase of the

ffective mobility in device B with CsF buffer layer electrode. Weelieve that with the release of low-work-function Cs metal atomst the organic/metal interface, leading to an increase in the effectiveobility because of the response of the electrons participating in

B (b). −�B is a negative differential susceptance, ω is an angular frequency, and Cgeo

is a geometrical capacitance. fmax is the frequency of maximum value at the −�B vs.frequency curves. The driving voltages are 0.5, 1.0 and 1.5 V.

the current transport, thus a better balance of carriers in the device

E1/2

(Vcm-1 )

Fig. 4. Electric field dependence of charge-carrier mobility measured in −�Bmethod.

1922 R. Zheng et al. / Synthetic Metals

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Advanced Materials 14 (2002) 228.[16] A. Pitarch, G. Garcia-Belmonte, J. Bisqnert, Proceedings of SPIE 5519 (2004)

ig. 5. (a) SEM cross-section images of Al/CsF/PFO-DBT15, EDS reveal the distribu-ion of Cs content within PFO-DBT15 layer. (b) EDS reveal the distribution of elementind within PFO-DBT15 layer.

he cesium atom diffusion in polymer emitting layer. SEM andDS analysis on PFO-DHTBT15/CsF/Al reveal CsF doping into PFO-BT15, in good agreement with the findings by Hsieh et al. [17].

[

162 (2012) 1919– 1922

4. Conclusion

In summary, the details of electrical characteristics of thePFO-DBT15 with CsF buffer layer were investigated by employ-ing AS. The changes of effective mobility turn out to be sensitivefor modifying the cathode and can be used as a probe for thecarrier balance and charge carrier injection behavior in OLEDs.The SEM and EDS verify the diffusion process of CsF in polymerlayer. AS is a well-established method for investigating organicsemiconductor devices and complement the shortage of I–V mea-surement.

Acknowledgements

The authors acknowledge the financial support from theNational Natural Science Foundation of China (Grant No.51010003).

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