Alternating-Current and Direct-Current Responsesof Light-Emitting Devices Basedon Decacyclene Langmuir-Blodgett Films

S. Das, A. Chowdhury, and A. J. Pal1)

Indian Association for the Cultivation of Science, Department of Solid State Physics,Jadavpur, Calcutta 700 032, India

(Received by J. R. Leite August 15, 2000; in revised form December 8, 2000;accepted March 1, 2001)

Subject classification: 68.47.Pc; 73.40.Gk; 73.50.Gr; 73.61.Ph; 78.60.Fi; 78.60.Qn; S12

Light-emitting devices have been fabricated with Langmuir-Blodgett films of decacyclene. The de-vices have been operated under both direct-current (dc) and alternating-current (ac) modes. Fre-quency response of luminance of such devices under ac voltage has been studied, and moderatelyhigh-frequency electroluminescence has been observed from these devices. Light output character-istics, charge injection, and operation mechanism under the two modes have been compared andfound to be different. Fowler-Nordheim tunneling mechanism has been found to be applicable inthe dc case, and space-charge-assisted electron injection lowered the tunneling barrier ac modes.The transient characteristics of luminance have been studied which supported the role of spacecharges in device operation.

1. Introduction

The use of conjugated organic and polymeric materials for emissive devices has becamea subject of extensive research due to the encouraging results obtained during the lastdecade [1]. The light emission from organic light-emitting devices (LEDs) is based oninjection of both electrons and holes. The electrons and holes drift towards each otherunder an applied electric field, form excitons, and recombine to radiatively emit lightcharacteristic of the emitting material. In general, the work function of the metal elec-trodes and hence, the barrier heights of the charge carriers allow light emission onlyunder forward bias. Recently, alternating current (ac) LEDs have attracted attention,which show somewhat different operation mechanism than the dc ones [2–5].Although spin-casting and vacuum evaporation are the most common techniques for

device fabrication, layer-by-layer self assembly [6] and Langmuir-Blodgett (LB) [5] filmdeposition techniques have some added advantages. These two techniques allow one tobuild up LED structures layer by layer, and therefore enable one to fabricate hetero-structure devices. Moreover, due to the high degree of order and control of film thick-ness, the LB film deposition technique is very suitable for making model systems fordevice characterization [7].In this work, we have used decacyclene as an emitting material which is known to

form stable LB film in pure phase, also when mixed with fatty acids. We have fabri-cated LEDs with different molar concentrations of the emitting material. The devicecharacteristics under dc and ac modes have been studied to compare the device opera-tion in both cases.

1) e-mail: sspajp@mahendra.iacs.res.in

phys. stat. sol. (a) 185, No. 2, 383–389 (2001)

2. Experimental

The active semiconductor in the present study was decacyclene, whose molecular struc-ture is shown in the inset of Fig. 1. Both stearic acid (SA) and the dye were purchasedfrom Aldrich Chemical Co., and were used as received. To deposit LB films, decacycleneand SA were dissolved in chloroform. The molar ratios of decacyclene were 0.25, 0.8 and1.0 for the three different cases. Pressure–area (P–A) isotherms showed that the area permolecule varied from 0.22 to 0.11 nm2. LB films were deposited on indium tin oxide(ITO) coated glass substrates at a surface pressure of 25 mN/m. UV-visible absorptionand photoluminescence spectroscopy confirmed the presence of decacyclene in LB films.To fabricate LEDs, aluminum (Al) electrodes were vacuum-evaporated on top of LBfilms at a pressure below 10––5 Torr. A mechanical shutter was used to protect the emittinglayer from excess heat before and after metal evaporation. Each LED structure had atypical area of about 3 mm2, which was defined by a mask during Al evaporation. Devicetesting was PC controlled using a GPIB interface and performed in nitrogen environment.For dc LED studies, the current voltage (I–V) characteristics were measured using aHewlett-Packard 34401A multimeter (or a Keithley 486 picoammeter) and a Yokogawa7651 dc voltage source. The luminous output was measured using a calibrated siliconphotodiode coupled to a Keithley 617 electrometer. For ac and transient measurements,sinusoidal voltage or rectangular voltage pulses were applied from a Hewlett-Packard HP3245 Universal Source. An Electron Tubes 9256B photomultiplier detected the time re-sponse of light output. A Hewlett-Packard HP 54600B two-channel real-time storageoscilloscope simultaneously digitized the applied voltage and EL responses.

3. Results and Discussion

The current density–applied voltage characteristic of an LED based on 16 LB layers ofdecacyclene is shown is Fig. 1. The figure shows that the device exhibited a reverse

384 S. Das et al.: AC-Current and DC-Current Responses of Light-Emitting Devices

Fig. 1. Device current–voltage characteristics of an LED based on LB films of decacyclene. Theinsets show the chemical structure of decacyclene and log (J/V2) versus 1/V plot under forward (*)and reverse bias (*) to support tunneling mechanism. The straight lines are the best fit to themodel in the high field region

rectification ratio of 10 at a voltage of 4.5 V. To study charge injection mechanism inthe high field region, where luminance is generally observed, we have considered tem-perature-independent Fowler-Nordheim (FN) tunneling mechanism [8, 9]. The mecha-nism presumes the injection of charge carriers directly into the bands of the semiconduc-tor. Plots of log (J/V2) versus 1/V for both forward and reverse bias directions areshown in the inset of Fig. 1. Here J and V represent device current density and appliedvoltage, respectively. The plots show a linear behavior in the high field regime, whichsuggests that tunneling mechanism is operative in these devices. The slope of the plotwas higher under forward bias, suggesting higher barrier heights for holes as comparedto reverse bias case. This further explains the reverse rectification in the J–V plot. Thedeviation from the linear behavior in the low field region indicates additional contribu-tion in current due to thermionic emission.Luminance was observed in both bias directions. Figure 2 shows luminance versus

applied voltage plots separately under the two bias directions. The plots show that atany particular voltage, higher luminance was observed under reverse bias than underforward bias. The luminance–current density plots (inset of Fig. 2), on the other hand,shows that the device efficiency was higher under the forward bias. At any particulardevice current, the luminance was about one order higher in the forward bias. The plotsfor forward and reverse bias cases showed a sharp turn-on of EL at current densities of30 and 125 mA/cm2, respectively. Higher turn-on current under reverse bias shows thatthe minimum electron injection rate required for light emission is achieved at a higherdevice current as compared to the forward bias case. Beyond the turn-on current, theluminance–current density plots showed a linear dependence in log–log scale with aslope of unity. Such linear dependence can be explained by considering FN tunnelingmechanism. Since holes are considered to be the majority carriers in these materials, allthe electrons are expected to recombine before reaching the other electrode. Therefore,luminance–current density plot becomes equivalent to a plot of electron current versushole current [7]. Again, both electron and hole injection in the device are voltage depen-dent. Considering a similar dependence for both hole and electron injection, the ob-served linear behavior of luminance–current density plots is expected.

phys. stat. sol. (a) 185, No. 2 (2001) 385

Fig. 2. Luminance–applied voltageand luminance–device current (in-set) characteristics of an LEDbased on decacyclene LB films un-der forward and reverse bias. Thearrows indicate the turn-on currentin each case. The horizontal linerepresents the background signallevel and the other lines are bestfits to the experimental pointsabove turn-on. The symbols havethe same meaning as in Fig. 1

We have studied the EL response of the devices under sinusoidal ac voltage. The con-centration of the active material in the emitting layer, and amplitude and frequency ofthe applied voltage have been varied to study a wide range of responses. Figures 3a and bshow typical EL responses at two different frequencies of the applied ac voltage from adevice with decacyclene LB films. The driving voltage, which was 8 V (p–p) in both casesis also shown in the figures. The figures show that luminance was observed in both biasdirections. The ratio between the EL during forward and reverse bias half-cycles de-pended on the frequency of the ac voltage. At low frequencies, the luminance was stron-ger during forward bias half-cycles, while luminance under reverse biased half-cycles wasstronger at higher frequencies. The EL and applied voltages were in perfect phase at lowfrequencies, while the EL started to lag the voltage signal at higher frequencies.The frequency responses of EL for the devices with decacyclene LB films separately

under forward and reverse bias half-cycles are presented in Fig. 4. Responses under differ-ent applied voltages are shown in the figure. The luminance levels in both biased half-cycles increased with increase in amplitude of the applied voltage. Luminance has beenobserved up to a frequency of around 50 kHz. With an increase in frequency, luminance

386 S. Das et al.: AC-Current and DC-Current Responses of Light-Emitting Devices

Fig. 3. EL and sinusoidal ac voltageas a function of time for deviceswith LB films of decacyclene drivenby 8 V peak-to-peak a) 30 Hz andb) 10 kHz sinusoidal voltage

under forward-biased half-cycles decreased. Luminance under reverse bias, on the otherhand, remained constant at low frequencies, but decreased at higher frequencies.It is interesting to compare the ac luminance results with that under dc voltage. Fig-

ure 2 shows that luminance at any particular voltage was stronger under reverse bias,while Fig. 3a shows that luminance was observed predominantly under forward-biasedcycles of ac voltage. A comparison of dc luminance with frequency response of ac lumi-nance (Fig. 4) suggests that charge injection and operation mechanisms must be differ-ent under the two types of driving voltages.The operation mechanism of LEDs under alternating voltage has recently been inter-

preted by considering the accumulation of space charges at the metal–semiconductorinterfaces [5, 10]. Under forward-biased half-cycles, holes form space charges near theelectron injecting electrode, and enhance the field near the interface to ease electroninjection. Excitons are formed and radiatively decay as EL. Under reverse bias half-cycles, the holes left near the Al electrode from the preceding forward-biased half-cyclemove towards the electron injecting electrode (ITO) to increase field and assist elec-tron injection. The injected electrons in-turn form excitons to decay as luminance. Thespace charges therefore have directly assisted in observing luminance under ac voltage.The frequency response of EL can also be understood in terms of space charges. As

the frequency increases beyond a certain value, the duration of each half-cycle becomesless than the time required for charge carriers to transit through the device and spacecharge formation. This will lead to a decrease in the density of space charges at higherfrequencies, and hence result in lower luminance (Fig. 4). The difference in EL re-sponses under forward and reverse bias modes may be due to dissimilar density oftrapped sites near the two electrodes, where the space charges form.We also have studied the frequency response of luminance from devices with differ-

ent concentrations of the emitting semiconductor (Fig. 5). Stearic acid has been used asinert matrix of the emitting layer. In devices with lower concentration of decacyclene inthe LB films, the forward and reverse bias EL responses were nearly identical. Suchbehavior is in contrast to the responses in devices with LB films of decacyclene only.We earlier have shown that the charge carriers in devices with low concentration of

phys. stat. sol. (a) 185, No. 2 (2001) 387

Fig. 4. Frequency response of peakluminance intensity for devices withLB films of decacyclene during for-ward (full symbols) and reverse(open symbols) biased half cycles.The driving voltages were 6 (cir-cles), 7 (triangles), and 8 V(squares) peak-to-peak

emitting semiconductor, accumulated carriers remain localized within the insulatingmatrix and do not take part in transport processes [11] in the voltage range of ourstudy. This could equalize the densities of trapped sites near the two electrodes andhence result in a similar frequency response under the two bias cycles. The absoluteintensity cannot correspond here, as the applied field might be different in each of thedevices.Transient EL characteristics of the devices have been studied, which further sup-

ported the contribution of space charges in device operation under ac voltage. We havestudied transient EL response of LEDs based on decacyclene LB films under a se-quence of two rectangular voltage pulses. The amplitude and width of voltage pulses,and the delay between them have been varied to study a wide range of responses.Figure 6 shows a typical EL response, along with the voltage pulse. During the firstpulse, an instantaneous EL was observed followed by a peak. The peak EL decayed toa steady state level. The peak part was absent during the second (and subsequent)pulse. Steady state EL was the same during both pulses. With an increase in delaybetween the pulses, the peak part of EL appeared in the second pulse and finally theEL responses in the two pulses equalized.Transient EL responses can be understood in terms of space charges in the devices

[11, 12]. As the semiconductor is sandwiched between two dissimilar metal electrodes(ITO and Al), there are charge transfer processes to align the Fermi level within thedevice before any bias is applied. Equilibrium is reached when electrons and holes areaccumulated at the two opposite metal–semiconductor interfaces. Under forward bias(ITO positive) the accumulated holes near the ITO/alizarin interface drift towards Alelectrode and form excitons. The excitons radiatively decay to result in an instanta-neous EL. The excess holes further form space charges near the Al electrode and mod-ify (lower) the barriers for the electrons. Electron injection therefore becomes easierallowing the formation of a large number of excitons. This was manifested as peakduring the first voltage pulse. When a sequence of two voltage pulses is applied, the ELpeak is absent during the second pulse, since most of the accumulated space chargesrecombined during the first pulse. Our transient results therefore confirmed the role ofspace charges in device operation.

388 S. Das et al.: AC-Current and DC-Current Responses of Light-Emitting Devices

Fig. 5. Frequency response ofpeak luminance intensity for thedevices with LB films of decacy-clene in stearic acid matrix dur-ing forward (full symbols) andreverse (open symbols) biasedhalf cycles. The concentrations ofdecacyclene in the emitting layerwas 100% (circles), 80% (trian-gles), and 25% (squares). Thedriving voltages were 8 V peak-to-peak in all the case

In conclusion, we have shown that LEDs based on molecularly thin LB films of deca-cyclene can be operated under both dc and ac voltages. The frequency response of lumi-nance showed that ac light of 50 kHz can be obtained from the devices. Luminance hasbeen observed during both bias cycles of the ac voltage. Luminance during the two biascycles depended on the frequency of the applied voltage. At low frequencies, luminanceunder forward bias was stronger than that under reverse bias. Under dc voltage, on theother hand, forward-biased luminance was much higher than the reverse-biased one atany applied voltage. The charge injection mechanism has been found to be different in thetwo modes of operation. Under dc bias, Fowler-Nordheim tunneling mechanism has beenfound to be applicable at higher fields. The origin of luminance under ac voltage has beendiscussed in terms of space-charge-assisted electron injection in the devices. The transi-ent characteristics of EL supported the role of space charges in device operation.

Acknowledgements This work was supported by the Department of Science and Tech-nology, Government of India (Project No. SP/S2/M-11/94), and the Council of Scientific& Industrial Research (Project No. 03(0812)/97/EMR-II).

References[1] S. Miyata and H. S. Nalwa (Eds.), Organic Electroluminescent Materials and Devices, Gor-

don & Breach, New York/London 1997.[2] Z. Yang, B. Hu, and F. E. Karasz, Macromol. 28, 6151 (1995).[3] Y. Z. Yang, D. D. Gebler, L. B. Lin, J. W. Blatchford, S. W. Jessen, H. L. Wang, and A. J.

Epstein, Appl. Phys. Lett. 68, 894 (1996).[4] H. L. Wang, F. Huang, A. G. MacDiarmid, Y. Z. Wang, D. D. Gebler, and A. J. Epstein, Synth.

Met. 80, 97 (1996).[5] A. J. Pal, T. .stergard, R. .sterbacka, J. Paloheimo, and H. Stubb, IEEE J. Sel. Top. Quan-

tum Electron. 4, 137 (1998).[6] J.-K. Lee, D. S. Yoo, E. S. Handy, and M. F. Rubner, Appl. Phys. Lett. 69, 1686 (1996).[7] T. .stergard, A. J. Pal, and H. Stubb, J. Appl. Phys. 83, 2338 (1998).[8] R. H. Fowler and L. Nordheim, Proc. R. Soc. Lond. A 119, 173 (1928).[9] A. J. Heeger, I. D. Parker, and Y. Yang, Synth. Met. 67, 23 (1994).[10] R. .sterbacka, A. J. Pal, K.-M. Kallman, and H. Stubb, J. Appl. Phys. 83, 1748 (1998).[11] S. Das and A. J. Pal, Appl. Phys. Lett. 76, 1770 (2000).[12] A. Chowdhury and A. J. Pal, Synth. Met., 122 (2001) in press.

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Fig. 6. Transient EL responseof an LED based on LB filmsof decacyclene under a se-quence of two forward biasedvoltage pulses. The pulses(dashed lines) had a width of 3ms and an amplitude of 6.35 V.The separation between thepulses was 1 ms