Determination of energy band diagram and charge carrier mobilityof white emitting polymer from optical, electrical andimpedance spectroscopy

M.A. Mohd Sarjidan n, H.A. Mohd Mokhtar, W.H. Abd. Majid n

Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 20 August 2014Received in revised form8 October 2014Accepted 7 November 2014Available online 15 November 2014

Keywords:White polymer light emitting deviceEnergy band diagramCharge carrier mobilityOpticalElectricalImpedance spectroscopy

a b s t r a c t

A single-layer white polymer light-emitting device (WPLED) has been fabricated using spin coatingtechnique. The device was constructed as ITO/PEDOT:PSS(50 nm)/SPW-111(50 nm)/LiF(1 nm)/Al(100 nm). Indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS) are used as the transparent anode. SPW-111 is fabricated as a white emissive layer and lithiumfluoride (LiF) and aluminum (Al) are used as reflecting cathode. Energy band diagram of the device wasestimated from a combination of ultraviolet–visible (UV–vis) and current–voltage (J–V) analyses. Chargecarrier mobility (μ) of PLED was evaluated using negative differential susceptance (�ΔB) method fromimpedance spectroscopy (IS) analysis. The calculated μ of the SPW-111 device is in the magnitude of10�6 cm2/V/s.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

White light-emitting devices (LEDs) based on polymer materi-als have been studied recently for display and lighting application.Polymer LEDs (PLEDs) have many advantages such as quickresponsiveness, low energy consumption, better viewing angle,light in weight, possible to fabricate on a large and flexiblesubstrate and also cheap in cost. White emission from the devicescan be a combination of two or three emissive materials, whichexhibit a blue and red [1] or blue, green and red emission [2],respectively. They can be fabricated either as a blend system [3] ormultilayer structure [4] devices. In order to simplify the fabricationmethod of white PLEDs, several groups of researchers attemptedto synthesis the white-emissive materials [5–8].

Cyclic voltammetry (CV) studies were commonly carried out inorder to investigate the high occupied molecular orbital (HOMO)and lower unoccupied molecular orbital (LUMO) energy levels ofemissive polymer materials [9,10]. This electrochemical analysisrequires the sample in a solution form, which is less favorable fornon-dissolvable materials in common solvents. Alternatively, theenergy band diagram of the polymer can be estimated from the

combination of UV–vis and J–V analyses in a thin film and deviceform, respectively.

Another important parameter in PLEDs is a charge carriermobility (μ) which plays a significant role to determine theefficiency of the devices [11]. Carrier mobility of PLEDs is usuallyobtained from time-of-flight (TOF) analysis [12,13]. However, thistechnique requires sophisticated and expensive equipment. Thus,impedance spectroscopy (IS) becomes an option that can beutilized to study charge carrier mobility of organic materials [14]which is much simpler and cheaper compared to that of theTOF setup.

In this work, the performance of the single-layer polyfluorene(PFO) based white emitting PLED was investigated. SPW-111 hasbeen used as white emitting polymer. The white emission andstability of SPW-111 PLEDs have been reported by other [15]. Themain focus of this work is to report a simple technique that can beused to obtain band energy diagram and carrier mobility oforganic semiconducting materials. Photoluminescence (PL) andelectroluminescence (EL) spectroscopies were carried out to studythe emission characteristics of SPW-111 PLED under photon andelectrical excitation, respectively. Optical and J–V curve analyseswere manipulated to estimate the energy levels of the device. ISanalysis was performed to obtain the charged carrier mobility ofthe photo-emissive SPW-111. As far as we know, there is no reportpublished on energy levels and charged carrier mobility of theSPW-111 using these approaches.

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journal homepage: www.elsevier.com/locate/jlumin

Journal of Luminescence

http://dx.doi.org/10.1016/j.jlumin.2014.11.0070022-2313/& 2014 Elsevier B.V. All rights reserved.

n Corresponding authors. Tel.: þ603 79674147; fax: þ603 79674146.E-mail addresses: mohd.arif@um.edu.my (M.A. Mohd Sarjidan),

q3haliza@um.edu.my (W.H. Abd. Majid).

Journal of Luminescence 159 (2015) 134–138

2. Experimental method

Pre-patterned ITO coated glass substrate (ITO thickness100 nm, 20 Ω/sq.) was obtained from Ossila Ltd. PEDOT:PSS waspurchased from Heraeus Clevios GmbH. The Polymer White(Livilux-SPW-111) was purchased from Merck.

The polymer solution was prepared in chloroform with con-centration of 8 mg/ml. The 8 mg/ml was found to be the optimumlevel not only for the device performances but also for the devicelifetime of SPW-111 based PLED [15]. Solution was stirred for30 min and after that filtered through a 0.45 μm membrane filter.For the fabrication of device, ITO coated glass substrate wascleaned ultrasonically in a DeconTM solution and subsequentlycleaned with deionized water, acetone and isopropyl alcoholbefore dried under N2 flows. PEDOT:PSS (�50 nm), was spin-coated onto the pre-cleaned ITO substrate by 4000 rpm for 40 s.The device was baked at 120 1C in 10 min to removeresidual water.

The polymer solution was spin-coated onto the ITO at2000 rpm in 20 s to obtain �50 nm thick film. The device wasthen baked on a hot plate at 70 1C for 60 s to evaporate the solvent.Finally, lithium fluoride (LiF) (1 nm) and aluminum (Al) (�100 nm)was deposited as a cathode stacked layer by the vacuum evapora-tion (under�5�10�6 mbar) technique. The active area of thedevice is 4.5�10�6 m2. The device layout of the fabricated PLEDis given in Fig. 1.

Optical absorption and PL spectra of the SPW-111 wereobtained by using UV–vis–NIR spectroscopy (Perkin Elmer V750)and luminescence spectroscopy (Perkin Elmer LS50B), respec-tively. Electrochemical property of the polymer was investigatedby using cyclic voltammetry spectroscopy (DY2300 Series Biopo-tentiostat). EL spectra and current–voltage–luminance (J–V–L)relations of the white emitting PLED were measured by a Spectraand chroma meter (Konica Minolta CS-200), respectively, con-nected to a source measure unit (Keithley 236 SMU). Impedancespectroscopy of the devices was measured by using a precisionimpedance analyzer (Agilent 4294A). A stylus profiler (KLA TencorP-6) was utilized to determine the thickness of organic layer.

3. Results and discussion

3.1. Optical band gap determination and PL analysis

Fig. 2 shows the normalized absorption and PL spectra of SPW-111 thin film. The thin film shows a broad absorption peak in avisible region centered at 390 nm. This peak can be referred as Qband, which is attributed to the π-πn excitation between bonding

and anti-bonding molecular orbital of the materials. Opticalabsorption spectrum was further analyzed to determine theoptical band gap (Eg) of the thin films. The optical energy gapwas calculated by applying Tauc’s relations:

αhv¼ αoðhv�EgÞn ð1Þwhere, α and αo are the absorption coefficient and Tauc coefficient,respectively. h and v are Plank constant and velocity of light,respectively. The value of n in Eq. (1) determines the type ofabsorption transition, n¼1/2 and 2 for direct and indirect allowedtransitions, respectively. The absorption coefficient was calculatedfrom the absorbance data by using Beer Lambert’s formula:

α¼ 2:303d

A ð2Þ

where A and d are the transmittance spectra and thickness of theSPW-111 thin film, respectively. By assuming the SPW-111 has adirect band transition; n¼1/2 was substituted into Eq. (1) and theequation is constructed to form a linear relationship:

αhv¼ αoðhv�EgÞ1=2

ðαhvÞ2 ¼ αoðhv�EgÞ ð3Þ

From Eq.(3), a Tauc graph of (αhv)2 against hv was plotted asshown in the inset of Fig. 2. Extrapolation of the plot at (αhv)2¼0gives the value of optical energy gap, Eg. The energy gap of theSPW-111 was obtained to be Eg¼2.90 eV. From this value, the thinfilm was excited at 3.54 eV (λex¼350 nm) which is above the Egvalue in order for electrons from the highest occupied molecularorbital (HOMO) to jump to the lowest unoccupied molecularorbital (LUMO) to obtain PL spectrum as shown is Fig. 2. PLspectra of SPW-111 film exhibits a strong emission in the blueregion with 3 prominent peaks at 466, 494 and 520 nm. Thesepeaks are the combination of blue and green color which results ina white emission.

3.2. EL analysis and band diagram estimation

Fig. 3a shows the J–V–L characteristic of the fabricated PLEDwith applied voltage sweep from 0 to 10 V. The device exhibits alow turn on voltage (when L¼1 cd/m2) of 4 V and easily reaches1000 cd/m2 at 8.5 V. Injection properties of a charged carrier fromelectrodes into a polymer layer are usually referred as FowlerNordheim (FN) tunneling and Richardson Schottky (RS) thermionic

V

Al/LiF

SPW-111

ITO/PEDOT:PSS

Glass

Fig. 1. Device architecture of single layer PLED fabricated in this work.

300 400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

Nor

m. A

bs (a

.u)

Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

Nor

m. P

L (a

.u)

Fig. 2. Normalized absorption and photoluminescence spectra of SPW-111. Inset isthe Tauc plot. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

M.A. Mohd Sarjidan et al. / Journal of Luminescence 159 (2015) 134–138 135

emission. The FN tunneling model can be described mathematicallyby Eq. (4):

J ¼ AF2exp�KF

� �; ð4Þ

where A¼ q3=8πhϕ and K ¼ 8πð2mnÞ1=2ϕ3=2=3qh, here ϕ, mn, q andh are barrier height, effective mass of charge carrier, elementarycharge, and Planck constant, respectively. It is generally consideredthat the FN tunneling current is temperature independent. For theRS thermionic emission, the current has the form:

J ¼ Joexp βRSffiffiffiF

p� �; ð5Þ

where Jo ¼ AnT2exp �ϕ=kT� �

is the field-free injection current, andβRS ¼ e3=4πεrεo

� �1=2=kT is the RS coefficient. Here An is the Richard-

son constant (120 A/(cm2 K2) for mn¼mo), ϕ is the barrier height atthe electrode contacts, and k the Boltzmann constant. In this model,the diffusion effect is ignored. From Eqs. (4) and (5), the relationbetween J and F for FN and RS model have been simplified to linearrelation of ln J=F2

� �p�1=F and ln Jð Þp

ffiffiffiF

p, respectively. Fig. 3b

shows the plot of FN and RS from 5.5 to 10 V. It is interesting tonotice that linear relation can be observed for both plots with R2

values are closed to 1. This indicates that there are possibilities forboth mechanisms to have occurred in the device, which is similarlyreported on NPB/Alq3 heterojunction organic light-emitting devices[16]. By assuming that the injections of electron from cathode andhole from the anode are attributed to the FN [17] and RS [18] model,respectively, the barrier height at cathode and anode can bedetermined. From the slope, K, of ln(J/F2) versus 1/F plot, the barrierheight of the cathode is calculated to be 0.1 eV. Further analysis onJ–V curve at low voltage less than 4 V (Ohmic region) has deter-mined the saturation current density as Jo¼6.67�10�4 A/cm2. Thissaturation current describes the number of charge carriers who areable to overcome the barrier height [19]. At room temperature(T¼300 K), by using the relation of Jo ¼ AnT2exp �ϕ=kT

� �, the

barrier height at the cathode is calculated to be 0.6 eV.EL spectra of the white PLED at different applied voltage is

shown in Fig. 4a. The spectra exhibit 3 prominent peaks at 454,487 and 514 nm. The intensity of the EL spectra increases whenthe voltage was raised from 6 to 10 V. The normalized PL spectrumis plotted in the same axis as a comparison. It can be seen thatboth spectra exhibits almost identical peak position with a verysmall shift among each other. EL spectra showed a more intense

peak with broad shoulder in the green region (520 nm) whereasthe PL is more intense with a narrow peak in the blue region (466and 494 nm). This is related to the triplet exciton which is alsofound in the emissive polymer of PFO [20]. The CommissionInternationale de l’Eclairage (CIE) coordinates plot of EL and PL ofthe polymer are shown in Fig. 4b. It can be observed that the PLemission is more towards the blue region as a result of highluminescence intensity of 466 and 494 nm peaks. However, the ELof the PLED observed remained in the cool white region under theapplied voltage of 6 to 10 V.

HOMO and LUMO levels can be estimated from the energy gapand the potential barrier height value. This approach has beenused to determine the energy band diagram of ITO/PHF:rubrene/Alsingle layer white OLED [21]. LUMO level is calculated by addingthe value of work function of the cathode with barrier height ofelectron, while HOMO level is calculated by subtracting the valueof work function of the anode with barrier height of a hole. Fromliterature, the work functions of LiF/Al cathode and ITO/PEDOT:PSSlayers are �3.0 eV [22] and �5.2 eV [23], respectively. Thus, theLUMO and HOMO levels obtained are �2.9 eV and �5.8 eV,respectively. The energy gap of the estimated HOMO–LUMO levelsis 2.9 eV, which is proven by optical energy gap obtained from theTauc relation. The energy band diagram of the device is presentedin Fig. 5a. Barrier height diagram at polymer-LiF/Al interface isillustrated in the inset of Fig. 5a. These HOMO and LUMO valueswere close to the reported by Ref. [24] for PFO-based PLEDs.

In order to confirm the estimated HOMO and LUMO values,CV analysis has been performed. 10 mg/ml of SPW-111 solutionwas prepared in chloroform. During the preparation of thesolutions, tetrabutylammonium perchlorat (TBAP) in anhydrouschloroform solution as an electrolyte was added to provide aconcentration of 0.1 M. The CV measurements of the polymersolutions were performed at 50 mV/s on potentiostat between�2.5 and 2.5 V. The measurements were carried out by using Ag/AgCl reference electrode and glassy carbon disk and platinum asthe working and the counter electrodes, respectively. Fig. 5bshows the CV plot of the white emission polymer. The HOMO andLUMO energy levels of the polymer can be calculated from theonset oxidation potential ðEoxÞon and the onset reduction poten-tial ðEredÞon by using the linear relation with a correction factor of4.4 eV [25]:

HOMO¼ � ðEoxÞonþ4:4

eV ð6Þ

Fig. 3. (a) Current–luminance–voltage characteristic of PLED. (b) FN and RS plots.

M.A. Mohd Sarjidan et al. / Journal of Luminescence 159 (2015) 134–138136

LUMO¼ � ðEredÞonþ4:4

eV ð7Þ

The ðEoxÞon and ðEredÞon values obtained from the plot are 1.1 and�1.5 V, respectively. Thus, from Eqs. (6) and (7), HOMO and LUMOvalues were calculated to be �5.5 and �2.9 eV, respectively. Theobtained LUMO value from CV analysis is exactly the same withthe estimated LUMO value from barrier height and band gapanalyses.

3.3. Charge carrier mobility measurement by impedancespectroscopy

In order to estimate charge carrier mobility through the singlelayer PLED, the negative differential susceptance �ΔB method hasbeen used, which was proposed by Martens et al. [26]. Noted that�ΔB ωð Þ ¼ �ðωðC�CgeoÞ, where C and Cgeo is capacitance andgeometry capacitance (without bias), respectively. According tothe transit time effect, the maximum value of �ΔB versusfrequency plot is constantly occurred at fmaxE0.72tt�1 [27], wheret is a transit time. By calculating the transit time, t, the charge

carrier mobility (μ) can be determined by following equation [28]:

μ¼ 4d2t�1t

3Vbiasð6Þ

Fig. 6 shows the capacitance over frequency ranging from 10k to10 GHz of the device measured at 0, 7 and 10 V. The device wasperturbed with an alternating current (AC) signal of 500 mV. The�ΔB versus frequency is plotted in the inset of Fig. 6. The value offmax (marked by downward arrow) is observed shifted to higherfrequency when the applied voltage is increased from 7 to 10 V.Please noted that the �ΔB method is only applicable when thebarrier height is less than 0.2 eV [28]. Charge carrier mobility wascalculated to be μ7¼2.91�10�6 and μ10¼2.54�10�6 cm2/V/s at7 and 10 V, respectively, from the measured transit time. Thesevalues are close to electrons mobility of PFO-based PLEDs measuredby transient EL method [29]. It is noted that the calculated mobilityincreases while the transit time decreases with rising voltage applied.This can be related to reduction of electroluminescence delay as thecharge carriers travel faster across the polymer layer which agreedwith the disordered structure in PLED [30].

Fig. 4. White emission properties of the SPW-111 film. (a) Normalized EL spectra of PLED at different voltage applied and PL spectrum under 350 nm excitation wavelength.(b) CIE coordinates of PL and EL spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. (a) Proposed energy band diagram of the fabricated device. Inset is illustration of barrier height of electron between LUMO level of SPW-111 and work function of LiF/Alelectrode. (b) CV plot of the polymer.

M.A. Mohd Sarjidan et al. / Journal of Luminescence 159 (2015) 134–138 137

4. Conclusion

Performance and device characteristics of SPW-111 white PLEDhas been evaluated. PL and EL spectra exhibit three emission peakswith a slight differ in shape due to triplet exciton recombination.Energy band diagram of ITO/PEDOT:PSS/SPW-111/LiF/Al device hasbeen proposed by utilizing the optical and electrical analyses. TheHOMO and LUMO levels of the SPW-111 were estimated to be�5.8 eV and �2.9 eV, respectively. Charge carrier mobility ofSPW-111 can be obtained by IS measurement. The carrier mobilityof materials is μ7¼2.91�10�6 and μ10¼2.54�10�6 cm2/V/s at7 and 10 V, respectively.

Acknowledgments

This work was supported by University of Malaya High ImpactResearch Grant (UM.C/625/1/HIR/MOHE/SC/06), University ofMalaya Research Grant (RP007A-13AFR), ScienceFund (SF019-2013) and Fundamental Research Grant Scheme (FP033-2013B).

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Fig. 6. Capacitance–frequency plot of PLED. Inset is �ΔB(ω) against frequency.

M.A. Mohd Sarjidan et al. / Journal of Luminescence 159 (2015) 134–138138