Analysis of Carrier Transients in Double-Layer Organic Light Emitting Diodes byElectric-Field-Induced Second-Harmonic Generation Measurement

Dai Taguchi, Le Zhang, Jun Li, Martin Weis,† Takaaki Manaka, and Mitsumasa Iwamoto*Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 S3-33, O-okayama, Meguro-ku,Tokyo 152-8552, Japan

ReceiVed: May 22, 2010; ReVised Manuscript ReceiVed: July 24, 2010

By using time-resolved electric-field-induced optical second-harmonic generation (EFISHG) measurements, wedirectly probed carrier transients in double layer (R-NPD/Alq3) organic light emitting diodes (OLED) (R-NPD,N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)).Results showed that carrier transients are comprised of two relaxation processes. One is charging on electrodes ina single exponential form exp(-t/τRC) with a time constant of τRC ) RsC0 (Rs, connecting lead resistance; C0,device capacitance), independent of applied voltage. The other one is charging at the R-NPD/Alq3 interface in astretched form exp(-(t/τ)) ( ) 1.6 ( 0.1) with a relaxation time, τ, proportional to V-2.1 (OLED with Al cathode)and V-2.6 (LiF/Al cathode). The Maxwell-Wagner model analysis well accounts for the two relaxation processes.We conclude that analysis of the carrier transients, directly probed by EFISHG, is effective for understanding thecarrier mechanism leading to electroluminescence in OLEDs.

Introduction

Organic light emitting diodes (OLEDs) have attracted muchattention for practical use in electronics.1,2 Manufacturingtechniques such as the printing method are envisioned forrealizing a low-cost and large area OLED display.3 Basically,OLED is an injection-type device with two different electrodeswhich are separated by an organic multilayer. Assuming theenergy diagram of the device, much research has been carriedout for improving the device performance. Ideas for using lowwork function electrodes4 and depositing dipolar layers on thesurface of electrodes5 have been proposed. Also ideas of utilizinga multilayer system comprised of hole-, electron-transport, andelectroluminescent layers, etc., have been presented.5,6 All theseideas are still key in frontier technologies, e.g., white OLEDs(WOLED) for lighting.7,8 However, owing to ambiguities ofenergetics at the organic-metal and organic-organic interfaces,OLED device physics is still not fully understood. To clarifythe details, we need to pay attention to carrier injection fromelectrodes, carrier transport across active layers, and chargeaccumulation and recombination at the interface leading toelectroluminescence (EL). Transient electroluminescence (TEL)9

measurement is available for exploring carrier behaviors, whileit cannot probe carrier dynamics prior to electroluminescence(EL). Electrical measurements such as the time-of-flight (TOF)10

can be candidates of other measuring methods, while multilayerand multispecies (electrons and holes) carrier transport com-plicate our analysis. A method that can directly probe carriermotion prior to EL will provide a potential way to make a clearcarrier mechanism in OLED devices. Electric-field-inducedoptical second-harmonic generation (EFISHG) probes directlythe carrier motion, e.g., carrier migration across the field-effecttransistor (OFET) channel.11 Recently, a modified version ofadvanced EFISHG measurement has found a way to investigatecarrier behavior in multilayer OLEDs.12 The charging process

leading to EL enhancement was successfully probed.13 However,a detailed analysis of the transients as well as carrier mechanismis still not complete.

In this paper, we studied carrier transients of double-layerOLEDs using EFISHG measurements. Applying voltage to theOLED induces charges on anode (+Qm) and cathode (-Qm)electrodes, and at the multilayer interface (Qs). Charges Qm andQs generate the electric field change in OLED devices and thuscould be probed as EFISHG transients. Results of EFISHGmeasurements showed that carrier transients are comprised oftwo relaxation processes. One is the charging (Qm on electrodesin a single exponential form exp(-t/τRC) with a time constantof τRC ) RsC (Rs, connecting lead resistance; C, devicecapacitance), independent of applied voltage. The other one isthe charging Qs at the R-NPD/Alq3 (R-NPD, N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)) interface in a stretched formexp(-(t/τ)) ( ) 1.6 ( 0.1) with a voltage-dependent relaxationtime τ. A Maxwell-Wagner (MW) model analysis well-accounted for the results and showed that (i) injected chargepropagation toward the double-layer interface caused thestretched form charging and (ii) injected carriers were respon-sible for the applied voltage dependence of the relaxation time.

Experimental Section

Sample Preparation. We prepared double-layer OLEDs withan anode/R-NPD/Alq3/cathode structure (anode, indium-zincoxide (IZO) or indium-tin oxide (ITO); R-NPD, N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)); cathode, Al or LiF/Al).R-NPD, Alq3, and cathode material (Al or LiF/Al) weresuccessively deposited onto ITO- and IZO-coated glass sub-strates using a vacuum evaporation technique. A single-layerdevice (IZO/R-NPD/Al) was also prepared in the manner similarto the double-layer OLED devices. The electrical resistance ofthe connecting lead and the capacitance of the prepared deviceswere measured with an impedance analyzer.

* To whom correspondence should be addressed, iwamoto@pe.titech.ac.jp.† Present address: Institute of Physics, Slovak Academy of Sciences,

Dubravska cesta 9, 845 11 Bratislava 45, Slovak Republic.

J. Phys. Chem. C 2010, 114, 15136–1514015136

10.1021/jp104712m 2010 American Chemical SocietyPublished on Web 08/12/2010

Transient SHG Measurement. In EFISHG measurements,we selectively probed the electric field in the R-NPD layer, E1

(see Figure 1a), by choosing an appropriate laser beam wave-length.12 Here E1 ) Em + Es, Em and Es are the Laplace fieldgenerated from charges (Qm on electrode and space charge fieldfrom charges Qs accumulated at the interface, respectively. Inother words, EFISHG directly probes the electric field causedby Qm and Qs. Figure 1b illustrates an equivalent circuit of adouble-layer OLED modeled as the Maxwell-Wagner effectelement. A set of parallel capacitance Ci and conductance Gi

values (i ) 1 and 2 represent R-NPD and Alq3) stands for eachorganic layer. Charging (Qm on electrodes takes a relaxationtime τRC ) RsC (C ) C1C2/(C1 + C2)). On the contrary, chargingQs at the double-layer interface takes a relaxation time τMW )(C1 + C2)/(G1 + G2) (τRC , τMW). τRC and τMW are both voltage-independent constants under assumption that R-NPD and Alq3merely function as dielectrics with a set of constants Ci and Gi.After a voltage V is applied to the double-layer OLED, charges+Qm and -Qm, respectively, appear on anode and cathode witha relaxation time τRC. Qm generates an electric field in R-NPD,and it is given by

with

At t > τRC, E1(t) ) Em = CV/ε1ε0. Afterward, i.e., t . τRC, holesand electrons, respectively, are injected into the R-NPD and

Alq3 layers from the opposite electrodes, and finally charge Qs

accumulates at the interface. Charge Qs is a source of the spacecharge field, and results in E1(t) as

where d1 is the thickness of an R-NPD layer. Here

with

Equation 3 suggests that charge Qs * 0 accumulates at theinterface where the adjacent two materials have differentrelaxation times, τ2 * τ1 (Maxwell-Wagner effect).14 The firstterm of eq 2 is the Laplace field by (Qm and the second termis the space charge field by Qs. In this way, both (Qm and Qs

contribute to the formation of the electric field, thus to theEFISHG transients. Note that the time constant τRC and τMW

(τMW > τRC) are both independent of the applied voltage.Figure 1a portrays experimental arrangement for the EFISHG

measurements,12 a pulsed laser beam was used as a probing light,which was generated using an optical parametric oscillatorpumped with the third-harmonic light of a Q-switched Nd:YAGlaser (repetition rate 10 Hz, average power 1 mW, duration 4ns). p-Polarized laser pulses impinged onto the sample surfaceat an incident angle of 45°, where the spot size was smallerthan the device area (∼4 mm2). In the presence of an electric-field E(0) (* 0), a nonlinear polarization wave is induced(EFISHG15) as P(2ω) ) ε0(3)lE(0)E(ω)E(ω), where ε0 is thepermittivity of vacuum, (3) is the third-order nonlinear suscep-tibility, and E(ω) is the electric field of the laser beam. P(2ω)generates propagating second-harmonic light, and we measuredits intensity Ish ∝ |P(2ω)|2 using a photomultiplier tube.Consequently, the square root of EFISHG light intensity (Ish

0.5),proportional to the electric field E(0)16 in the device thicknessdirection, was obtained. (3) is a material-dependent parameterwith ω and allows us to probe the electric field in each layer.In order to probe the electric field change in the R-NPD layeronly, we chose a laser wavelength at λω ) 820 nm,12 andrecorded EFISHG light intensity at λ2ω ) 410 nm with applyingsquare-wave voltage (frequency 10 Hz, duty time 50 ms). Notethat the Alq3 layer generates no EFISHG at the wavelengthλ2ω ) 410 nm. In transient EFISHG measurements, Ish ∝ |E1(t)|2

was recorded with time.Resulting EFISHG transients were analyzed using a filtering

method.17,18 For the analysis, we assumed a trial transientfunction with a form s(t) ) s1 exp(-(t/τ)) + s0 (s1 and s0,constants; , stretching exponent; τ, relaxation time), and definedthe first-order filtering function F1(t1) ) s(t1) - s(2t1). Simplecalculation shows that F1(t1) takes a maximum at tmax ) (2 -1)/( ln 2)-1/τ, where F1(tmax) ) g()s1 with g() ) (1 -2-)2-/(2-1). In our analysis of the EFISHG transients, thetransient Ish

0.5 was s(t), and we determined filtering parameters, and τ, using experimental values of tmax and F1(tmax1).

Figure 1. (a) Experimental arrangement for the EFISHG measure-ments. (b) Equivalent circuit of OLED based on the Maxwell-Wagnereffect model.

E1(t) )Qm

ε1ε0(1)

Qm ) CV(1 - exp(-t/τRC))

E1(t) )CVε1ε0

- 1d1

Qs

C1 + C2(2)

Qs ) QMW(1 - exp(-t/τMW)) (3)

QMW )G1G2

G1 + G2V(τ2 - τ1), τ1 )

C1

G1, τ2 )

C2

G2

Carrier Transients in OLEDs J. Phys. Chem. C, Vol. 114, No. 35, 2010 15137

Results and Analysis

Both charging (Qm on the electrode and charging Qs at theinterface are responsible for the EFISHG transients. First, wediscuss the EFISHG transients in the single-layer IZO/R-NPD/Al device, where Qm mainly contributes to the EFISHGtransients. Then we discuss the results of the double-layerOLEDs.

Single-Layer Device (IZO/r-NPD/Al). Parts a, b, and c ofFigure 2, respectively, show EFISHG transients, Ish, Ish

0.5, andF1(t1) ) Ish

0.5(t1) - Ish0.5(2t1) of the single-layer IZO/R-NPD/Al

device. F1(t1) gives a peak at tmax1 ) (4.3 ( 1.7) × 10-7 s withF1(tmax1) ) (0.25 ( 0.1)δIsh

0.5 (δIsh0.5, change in Ish

0.5, see Figure

2b). Using these values, we obtained ) 1 and τ ) 6.2 ( 2.5× 10-7 s as fitting parameters. This result suggests that thetransient is a single relaxation process due to charging onelectrodes, in a manner as represented by eq 1 (Qm charging,see Figure 2d). That is, the charging process is modeled usingan equivalent circuit, comprised of a series Rs () 1.2 kΩ) andC0 () 4.3 × 10-10 F), where τRC ) RsC0 ) 5.2 × 10-7 s (seeeq 1). To confirm this charging process, we carried out thetransient EFISHG measurements at various applied voltages andthen analyzed the results using the filtering method. The tmax1

was independent of applied voltage, as shown in Figure 3a, andthe estimated value was 1. This result well supported ourmodel, charging on electrode. To further confirm this chargingprocess model, we carried out the same experiments and analysisby changing the series resistance to Rs ) 9.3 and 100 kΩ. Thefiltering analysis showed that tmax1 changes on satisfying therelation τRC () RsC0) ) τ() tmax1/ln 2), as shown in parts b andc of Figure 3. Results again supported our model.

Double-Layer Device (Anode/r-NPD/Alq3/Cathode). Partsa and b of Figure 4 show EFISHG transients Ish

0.5(t) at V ) 15and 10 V, and F1(t1) ) Ish

0.5(t1) - Ish0.5(2t1) for the double-

layer R-NPD/Alq3 device. F1(t1) gives peaks at tmax1, tmax2 andtmax2′. The tmax1 was voltage-independent, and it was (4.4 ( 1.2)× 10-7 s. We found ) 1, and the relaxation time tmax1/ln 2was in good agreement with τRC ) RsC ) 7.9 × 10-7 s (Rs )1 kΩ, C ) 7.9 × 10-10 F, C ) C1C2/(C1 + C2), C1 and C2 are

Figure 2. EFISHG measurement of the single-layer device: (a) SHGintensity Ish after applying square-wave voltage, (b) electric field inR-NPD layer E1 ∝ Ish

0.5, (c) filtering result Ish0.5(t1) - Ish

0.5(2t1). (d)Model of charging (Qm on the electrodes.

Figure 3. Filtering result of EFISHG transients (single-layer device):(a) voltage dependence; (b) Rs dependence. (c) Plot of τRC () RsC0) vsτ () tmax1/ln 2).

15138 J. Phys. Chem. C, Vol. 114, No. 35, 2010 Taguchi et al.

capacitances of R-NPD and Alq3 layers). Hence we concludethe transient at tmax1 was due to charging (Qm on electrodes(Qm charging, see Figure 4c), and it was in a single exponentialform. On the other hand, the tmax2 (and tmax2′) was voltage-dependent. The filtering analysis showed that the transientsaround tmax2 were in a stretched form with ) 1.6 ( 0.1,different from a single relaxation process than that observed att ) tmax1 ( ) 1). We will discuss the stretched form relaxationin the following section, Discussion.

Figure 5 shows the voltage dependence on the peaks tmax2 forthe double-layer device with LiF/Al- and Al-cathode. tmax2 wasinversely proportional to the applied voltage (∝V-n). For LiF/Al-cathode device, n ) 2.6, while for the Al-cathode device, n ) 2.1.In the following section, we will discuss the details of this slope.

In Figure 5, we also plotted the relationship between tmax1 and V,but we could not see the voltage dependence on tmax1. In moredetail, the response times tmax1/ln 2 ) 9.4 ( 1.1 × 10-8 s (OLEDwith Al-cathode) and 7.4 ( 1.1 × 10-8 s (LiF/Al-cathode) well-agreed with the RsC time constant τRC ) RsC ) 8.3 × 10-8 s (Rs

) 180 Ω, C ) 4.6 × 10-10 F). These results suggest that tmax1 wascaused by the electrode charging (Qm.

Discussion

As described in the Experimental Section, charge Qs isaccumulated at the interface between two layers with differentdielectric relaxation times, and it is written as a single relaxationprocess using eq 3. A simple calculation shows that Qs(t)satisfies the following differential equation

The first term of the right-hand term represents the release ofaccumulated charges from the interface, while the second termrepresents the accumulation of charges at the interface. That is,the process of interfacial charging Qs by the Maxwell-Wagnereffect is very similar to a dielectric polarization process due todipoles.19 Equation 4 is also valid when τMW varies with time,because the physics behind the MW effect should be the same.Considering the contribution of injected carriers to relaxation timeε/σ (ε, dielectric constant; σ, conductivity), we find that carriersinjected into film, sandwiched between two electrodes, should showdiffusion-like behavior and carrier front position x moves with time,on satisfying the relation x ∝ t1/2.20,21 Here it is assumed for timet < τMW* low injected carrier density and thus space charge fieldcaused by injected carriers does not significantly influence potentialfield profile. Hence, we may consider conductance Gi of double-layers changes approximately as

(0 < t < τMW*) along with evolution of carriers injected fromelectrode, and finally reaches Gi

s (τMW* < t). Here Gis is the

Figure 4. EFISHG measurement of the double-layer (OLED) device.(a) EFISHG transient Ish(t1)0.5. Inset magnifies details near tmax2 (V )15 V) and (b) Ish

0.5(t1) - Ish0.5(2t1). (c) Model of charging Qm on the

electrodes and Qs at the double-layer interface.

Figure 5. Voltage dependence of the peaks in filtering result, tmax1

and tmax2 (indicated in Figure 4) of LiF/Al- and Al-cathode OLED.

dQs

dt) -

Qs

τMW+

QMW

τMW(4)

Gi(t) ) Gi + Gis√t/τMW* ≈ Gi

s√t/τMW*

Carrier Transients in OLEDs J. Phys. Chem. C, Vol. 114, No. 35, 2010 15139

conductance caused by injected carriers and τMW* () (C1 + C2)/(G1

s + G2s)) is the Maxwell-Wagner relaxation time at the final

state. Hence, we may consider that relaxation time τMW of a double-layer system changes as

Substituting τMW(t) into τMW of eq 4, we obtain a rate equation ofQs with a time-dependent Maxwell-Wagner relaxation time asfollows

Here QMW* is accumulated charge at the interface in the limit tf∞. It should be noted here that a function Qs(t) ) QMW*(1 -exp(-(t/τ))) is a solution of eq 6 when ) 1.5 and τ ) τMW*.The fitting parameter obtained from tmax2 in Figure 4b was 1.6( 0.1 and in good agreement with the value 1.5 of Qs(t) thatsatisfies eq 6. This result suggested that tmax2 is assigned to carrieraccumulation process at the interface (Qs charging, see Figure 4c).

As shown in Figure 5, tmax2 is dependent on applied voltage V,suggesting tmax2 ∝ τMW* because of the saturation of the conductiv-ity Gs () G1

s + G2s) at t > tmax2. A steady-state current flows across

the double layer at t > tmax2, accompanying with electrolumines-cence. This means, a steady-state current flows but it accompanieselectron-hole recombination at the interface, where electrons andholes injected from opposite electrodes meet each other. Such asituation is very similar to the case of two-carrier currentsmechanism in insulators21 with accompanying carrier recombinationduring the transportation of electrons and holes injected fromopposite electrodes. Following the analysis of the two-carriercurrents mechanism with recombination in insulators, we couldformalize a current equation in a similar way as

Here Vj is the velocity of charge injected from electrodes, τj is thelifetime for the electron-hole recombination. t1 and t2 are transittimes of holes and electrons, respectively, crossing R-NPD andAlq3 layers. The physical meaning of eq 7 is as that in steady-state among the total accumulated charge QMW*, a part of chargeQm*τj/(t1 + t2) contributes to the electroluminescence; thus thisamount of charge is continuously supplied from electrodes at avelocity of Vj ) µ*E in the presence of the external electric field E) V/L. Here µ* is the effective mobility and L ) d1 + d2 is adouble-layer thickness. In eq 7, accumulated charge Qm* at theinterface is obviously proportional to external voltage V (see eq1). t1 and t2 are approximately given as t1 ) d1/µ1E1 and t2 ) d2/µ2E2 (µ1 and µ2: mobility), and they are inversely proportional toapplied voltage V; hence τj/(t1 + t2) is proportional to V. Takingthis into account, we find the current I flowing across the doublelayer, and it is proportional to V3. That is, the conductance G )dI/dV ∝ V2. Therefore, we could derive the relation tmax2 ∝ τMW*∝ 1/Gs ≈ 1/G ∝ V-2. The slope of the double-layer IZO/R-NPD/Alq3/Al device in Figure 5 is 2.1 and well agrees with the value,2.0, obtained using this model. The slope of the IZO/R-NPD/Alq3/

LiF/Al device was 2.6, a little bit higher than 2.1, but we mayargue in the same way. For the details we need to carry discussionwith consideration of the effect of the space charge field causingdeviation from ideal case, i.e., injection property and chargetrapping, and field dependence of material properties, i.e., electronand hole mobilities. However, this is our future task.

Conclusion

EFISHG measurements were employed for analyzing thecarrier transients in double-layer R-NPD/Alq3 OLED devices.Results evidently showed that the transient processes werecomprised of electrode charging process and interfacial chargingprocess. Analyzing the results using a filtering method verifiedthese twoprocesses,andwecouldshowthat theMaxwell-Wagnermodel analysis well accounted for the two processes. Finallywe concluded that the analysis using EFISHG measurementprovides us useful information concerning carrier mechanismin double-layer OLED devices.

Acknowledgment. This work is financially supported by aGrant-in-Aid for scientific research (A) (No. 19206034) from theJapanese Society for the Promotion of Science (JSPS) and a Grant-in-Aid for Young Scientists (B) (No. 22760227) from the Ministryof Education, Culture, Sports, Science and Technology (MEXT).

References and Notes

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JP104712M

τMW(t) ≈C1 + C2

G1(t) + G2(t)≈ τMW*τMW*

t(5)

dQs

dt) -

Qs

τMW(t)+

QMW*

τMW(t)(6)

I ≈ Qm* τt1 + t2

Vj ∝ V3 (7)

15140 J. Phys. Chem. C, Vol. 114, No. 35, 2010 Taguchi et al.