Solid-state polymer light-emitting electrochemical cells

ELSEVIER

Synthetic Metals80(1996)131-136

Solid-state polymer light-emitting electrochemical cells Qibing Pei, Yang Yang UMAX

Corporation,

6780 Cortona

Drive, Santa Barbara,

CA 93117, USA

Received 6 March 1996; accepted 15 April 1996

Abstract Solid-statepolymerlight-emittingelectrochemical cellshavebeendevelopedusingthin filmsof conjugatedpolymersblendedwith solid electrolytes.The cellscontainthreeparts:the polymerblendfilmsasthe active medium,andtwo contacteiectrodes:indium-tin oxide and aluminum.Whenexternallybiased,theconjugatedpolymersarep-dopedandn-dopedon oppositesidesof the polymerlayer,anda dynamic light-emittingp-n junction is formedbetweenthe dopedregions.Theadmixedsolidelectrolyteprovidesthedopantcounterionsandtheionic conductivity necessary for the doping.The p-n junction is dynamicandreversible,with aninternalbuilt-in potentialcloseto thebandgapof the redox-activeconjugatedpolymers,Orange,greenandbluelightsemittedfrom thep-n junctionhavebeenobtainedwith turn-onvoltage lessthan3 V andexternalquantumefficiencyhigherthan2% photons/electron. In addition,a two-colorlight-emittingelectrochemical cell hasalsobeenfabricatedbasedon abilayerstructureconsistingof two differentluminescent polymers.The light-emittingp-n junctioncanbe switchedfrom onelayer to anotherunderdifferentbiasconditions. Keywords:

Electrochemical cells; Luminescence

1. Introduction

LEDs are constructed by sandwiching a layer of conjugated

Conjugated polymers have attracted much attention since the discovery by Heeger, MacDiarmid, Shirakawa and their co-workers in the late 1970s that these polymers could be

doped such that their electric conductivity dramatically increased, from insulator to metal. They are semiconductors in the neutral non-doped state, and become highly conductive upon oxidation or reduction. Counterions are incorporated into the polymers during the oxidation-reduction process which is therefore a doping process; p-doping for oxidation and n-doping for reduction. This reversible electrochemical doping, and the corresponding changes of the electrical, optical and mechanical properties of the material, have been exploited for a variety of apphcations, such as rechargeable batteries, electrochromic windows, electrochemical ‘artificial muscles’ andsensors[ I-51. All of theseapplicationsinvolve the use of a layer of conjugated polymer as the working electrode in an electrochemical cell configuration. On the other hand, semiconducting conjugated polymers have been exploited for use in electronic and optical devices; thesepolymers have been successfullyused asactive mate-

polymer between a pair of electrodes. When applying a sufficiently high electric field between this pair of electrodes, electrons are injected from the cathode and holes are injected from the anode into the polymer layer. If the polymer is

fluorescent, part of the injected electrons and holes recombine radiatively, yielding electroluminescence (EL). Recently, we invented a new type of light-emitting devices: the polymer light-emitting electrochemical cells (LECs) which combine the novel electrochemicalproperties of con-

jugated polymers with the ionic conductivity of solid electrolytes [ 1I 1. Efficient orange, green and blue LECs which emit bright light at less than 4 V have been fabricated.

2. Mechanism of operation of the solid-state polymer

LECs Fig. 1(a) is a schematic diagram of a LEC. Between the two contact electrodes (indium-tin oxide (ITO) and Al) is a layer of a conjugated polymer, poly( l,bphenylenevinylene) (PPV), admixed with a complex of poly(ethylene

rials in field-effect transistors, photovoltaic cells, light-emit-

oxide) (PEO) and lithium trifluoromethanesulfonate,

ting diodes and polymer grid triodes [&lo]. Among these devices,the polymer-based light-emitting diodes (LEDs) are

a sufficiently high voltage is applied between the IT0 and Al, chargeis injected from the electrodes into PPV. Assuming

especially attractive for use in display technology. Polymer

that the IT.0 electrodeis wired asthe anodeandthe Al elec-

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When

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PPV and PEO(LiCF3S03)

II

(a)

(bi

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IT0

CC)

Fig. 1, Schematic diagram of the electrochemical processesin a LEC: (a) the cell before applying a voltage; (b) doping opposite sides as n-type and p-type; (c) charge migration and radiative decay; 0, oxidized molecule; 0, reduced molecule; 8, anion; $, cation; 0, hole; l , electron; *, photon.

trode as the cathode, the PPV polymer near the IT0 is initially oxidized, the polymer near Al is reduced (Fig. 1 (b) ) . Simultaneously, counterions from the electrolytes move to compensate the charges on the oxidized and reduced polymer chains. Since both p-doped and n-doped PPV are good electronic conductors, thePPV/electrodeinterfacesconsequently become near ohmic contacts. In general, the current passing through the LEC is made up of two components; an ionic one and an electronic one. Once a sufficiently high voltage is applied, oxidation and reduction will occur and ions will move (i.e., the corresponding doping levels will adjust) until the cell reaches an electrochemical equilibrium [ I2]. After reaching the electrochemical equilibrium, and forming a p-n junction at an applied voltage greater than V,,, the ionic contribution to the current goes to zero. On the other hand, the electronic contribution to the current continues: under the influence of the applied voltage the holes in the v-band of PPV (p-type carriers) propagate from the anode toward the cathode, and the electrons in the #-band (n-type carriers) propagate from the cathode toward the anode. These holes and electrons meet in the compensated volume between the n-type doped and p-type doped regions; this compensated volume defines the electrochemically induced p-n junction. Within this p-n junction, the p-type and n-type carriers recombine to form neutral charge carrier pairs which radiatively decay to the ground state. The electrochemically induced p-n junction is dynamic and reversible. The direction of the p-n junction can be reversed simply by wiring IT0 as the cathode and AI as the anode. Alternatively, the junction can be removed by discharging the previously charged electrochemical cell; this discharge can be carried out either with the two contact electrodes shorted or with the cell under open circuit conditions. In the latter case, the discharge current flows internally through the semiconducting polymer: under short circuit conditions, the discharge current flows primarily in the external circuit (since the external impedance can be made much less

than the internal resistance of the conducting polymer layer). Because the electrical conductivity of the conducting polymer increases by many orders of magnitude upon p-type or n-type doping, most of the external bias is applied across the p-n junction. Therefore, the minimum bias to maintain the p-n junction equals the built-in potential difference of the p-n junction, which is ideally equal to the LUMO-HOMO bandgap of the redox material divided by the elementary charge (E,/e) . A bias voltage higher than E,le will initiate the flow of electrons and holes into the p-n junction with concurrent light emission. In principle, the turn-on and operating voltages of LECs are not sensitive to the total thickness of the polymer layer. Nevertheless, the formation of the p-n junction relies on the redistribution of counterions. Thisredistribution could be fast or slow depending on the ionic conductivity of the polymer layer. Unlike polymer LEDs which utilize the semiconductor properties of non-doped conjugated polymers and electrochemiluminescent cells (batteries) which utilize the redox properties of luminescent materials [ I3 1, LECs utilize both the semiconductor properties and the redox properties of conjugated polymers. The high electronic conductivity of conjugated polymers after doping is also essential to the operation of LECs. Polymer LEDs have no ionic species in the polymer layer to compensate for the charges on the polymer chains. Although the semiconducting polymer layer is oxidized on one side (holes are injected) and reduced on the other side (electrons are injected), doping does not take place, The electrons and holes are injected by tunneling through the energy barriers formed at the electrode/polymer interfaces. Consequently, efficient and balanced dual-charge injection is essential in producing bright and efficient light emission. In order to satisfy this requirement, low work-function cathodes and high work-function anodes are usuaily required to match, respectively, the energy levels of LUMO and HOMO of the luminescent polymer [ 14,151.

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3. Single-layer

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LECs

In the initial studies, we chose PPV and PEO as the electronic and ionic conductors, respectively. PPV is aconjugated polymer capable of both p-type and n-type doping, The conductivity of neutral (non-doped) PPV is less than 10-‘2S/ cm at room temperature [ 161. Both p-doped and n-doped PPV are highly conductive. Furthermore, pure PPV is fluorescent, with efficient green emission, while, in doped PPV, the luminescence is quenched [ 171. PEO is a known ionic conductor. When complexed with a lithium salt, especially with a plasticizing salt such as LiCFJOs, the ionic conductivity is as high as 10v6 S/cm at room temperature, increasing to 10-4S/cmat72”C [18]. Thin films of PPV + PEO( LiCF,SO,) blends were prepared by spin-casting from a co-solution of poly (xylylidene tetrahydrothiophenium chloride), PEO and lithium triflate ( 1: l:O. 18 by weight) in acetonitrile and subsequent converting at about 200 “C. The PPV + PEO (LiCF,SO,) blend films had an ionic conductivity of about 1 X 10F7 S/cm at 22 “C, and about 1 X low5 S/cm at 80 “C. The films also retained the characteristics of PPV: (i) In the non-doped pristine form, they were photoluminescent, emitting green light with a spectrum characteristic of PPV. (ii) Such films could be doped and became highly conductive (c~ > 1 S/cm) after being exposed to IZ. Although doped PPV is not fluorescent, the blend of PPVcontaining ionic species, but without oxidative or reductive doping, retains the fluorescence of pure PPV. Since the PPV + PEO(LiCF,SOs) films retain both the characteristics of the PEO (LiCF,SO,) complex and PPV, a phase-separated interpenetrating polymer network is probably formed in the blend films. While the PEO network provides the channels for ion transport, the PPV network provides the pathways for transport of electronic charge carriers. The interpenetrating polymer network is an attractive morphology for the active material in the polymer-based solid-state light-emitting electrochemical cells. When an external voltage was applied to an ITO/ PPV + PEO/Al sandwich structure, light was emitted from the polymer layer. Fig. 2 shows the typical current-lightvoltage characteristics of an ITO/PPV+PEO/Al cell under both forward and reverse bias conditions (forward bias is defined by wiring IT0 as the anode and Al as the cathode). The current-voltage curve is antisymmetric about the origin, in contrast to conventional semiconductor diodes where a large rectification ratio (ratio of forward to reverse current) of order 103-10’ is obtained. In the LEC, the rectification ratio is near unity. The light-voltage curve is symmetric about zero bias. The apparent threshold voltages for appreciable current injection and visible light emission are at 2.5 V for forward voltage scans sweeping from 0 to 4 V, and around - 2.6 V for reverse scans sweeping from 0 to - 4 V. These threshold voltages are very close to the optical bandgap of PPV (2.4 eV).

-4

-3

-2

-I

0

1

2

3

4

Bias (V) Fig. 2. Typical current-voltage (solid line) and light-voltage (dashed line) curves of an ITO/PPV + PEO(Li) /Al cell. The voltage scans from 0 to 4 V and from 0 to -4 V, respectively.

The threshold voltages slightly increase with thicker polymer films, but the increase is much less significant than that in conventional polymer LEDs where the threshold voltage is roughly proportional to the thickness of the EL layer. This small thickness dependence of the threshold voltage results from a combination of the limited ionic conductivity (lo-’ S/cm) and the series resistance of the lightly doped material. The low mobility of the counterions delays the formation of the light-emitting p-n junction in thicker films. This effect is clearly seen when the voltage scan is carried out at various sweep rates. The apparent threshold voltage increases with sweep rate, characteristic of an electrochemical process during linear sweep voltammetry, The expected hysteresisresulting from the low ionic mobility is also shown in the lightvoltage curve during a forward scan (sweep from 0 to 4 V) and a reverse scan (sweep from 4 to 0 V) . The threshold voltage shifts by about 0.2 V up field in the forward scan compared to that in the reverse scan. During the reverse scan, a peak is observed at about 3.5 V. The time interval from the beginning of the sweep to the peak position is about 10 s. This interval reflects the time scale to form the p-n junction in this system. The hysteresis can be reduced and made negligible by reducing the sweep rate. Fig. 3 displays the EL spectra of the green light emitted from devices operated at 4 and - 4 V. Both spectra are close to the photoluminescence (PL) spectrum of the polymer layer (PPV is the chromophore) , confirming the expected reversibleformation of the junction. Photons are generated by the radiative recombination of holes in the HOMO and electrons in the LUMO of PPV. Initial stress life results are promising: devices were continuously stressed at 4 and - 4 V (light intensity about 100 cd/m’) for 24 h, with a loss of emission intensity by less than 50%. At lower light output, the stress life is significantly longer. For example, at 3 and - 3 V bias (about 8 cd/m2), the light intensity decays by only 5% in 24 h. In addition,

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300

400

500

600

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Wavelength(nm) Fig. 3. Optical absorption, PL and EL spectra of a thin layer of a blend of PPV and PEO with LiCF,S03.

since stable metals are used as the contact electrodes, and the fluorescent conjugated polymers are in their neutral undoped form, all components of the LECs are environmentally stable at the quiescent ‘off state. As a result, one anticipates intrinsically longer storage lifetime. Our initial ITO/PPV + PEO/ Al devices have been stored in an argon atmosphere for 15 months. After such long-term storage, the devices operate without loss of efficiency and do not exhibit the formation of black spots which typically limit the storage lifetime of polymer LEDs. Since the color of the light emitted by a LEC is determined by the energy gap of the semiconducting polymer in which the p-n junction is formed, the emission color can be changed in a straightforward way by substituting a different conjugated polymer in the active layer. LECs which emit orange light were fabricated by using a soluble derivative of PPV, poly (5 (2’-ethylhexyloxy) -2methoxy- 1,4-phenylenevinylene) (MEH-PPV), as the fluorescent redox species. A thin polymer layer containing a 1: 1 ratio (by weight) of MEH-PPV and PEO(LiCF,SO,) was used as the active polymer layer. The apparent threshold voltages for appreciable current injection and for detection of visible light emission are below 2.5 V for both forward bias scans (sweeping from 0 to 4 V) and reverse bias scans (sweeping from 0 to - 4 V). In a darkened room, light emission is observed for V> 2.1 V. The external quantum efficiency was as high as 2% photons/electron. LECs that emit blue light were fabricated by using a soluble derivative of poly(p-phenylene), poly [ 2-(3,6-dioxaheptyloxy ) - 1,4-phenylene] , as the fluorescent redox species. In this case, turn-on occurred at approximately 3 V, well below that needed for blue polymer LEDs made from the same material ( 15-20 V) where charge injection takes place by tunneling 1191. External quantum efficiencies in excess of 2% photons/electron have been obtained. 4. LECs in planar electrode configuration

Evidence for the formation of &hedynamic light-emitting p-n junction in LECs came from the direct observation of

Fig. 4. Optical microscope photograph of an operating polymer LEC in the planar electrode configuration. With 4 V bias applied across the pair of electrodes, the emission light from the dyanamically formed p-n junction can be clearly seen. The width of the junction is approximately 2 pm.

light emission of LECs fabricated in a planar electrode configuration [ 11,201, Fig. 4 shows an optical microscope photograph of the planar structure LEC during light emission. The device was prepared by casting a thin layer of PPV+PEO(LiCF$O,) on- top of a glass substrate precoated with two parallel rectangular gold eiectrodes. As predicted by the proposed model, the light-emitting junction is a narrow region, approximately 2 km wide, within the spacing between the two gold electrodes which is 15 pm wide. As shown in the photo, when the inter-electrode spacing is 15 pm, the p-n junction width is only about 2 pm. The ptype and n-type doped regions between the junction and electrodes can also be seen clearly. The junction is off-center and closer to one electrode, due to the asymmetry of p-type and n-type doping of the active conjugated polymer (possibly resulting from different mobilities for electrons and hoies, different densities of states versus energy in the r and v* bands, etc.). When the polarity of external bias changes, the position of this junction will also change and shift closer to the opposite electrode.

5. Double-layer LECs

Since the radiative recombination of electrons and holes only occurs within the light-emitting junction, the light-emitting species inside the p-n junction determines the emission color. For single-layer LECs, the emitted color is insensitive to the precise position of the junction. However, for LECs consisting of a polymer bilayer as the active medium, i.e. two layers of different luminescent polymers, the p-n junction can be placed in either one of &hetwo layers, depending on the bias. A polymer bilayer consisting of a PPV layer and a MEHPPV layer was prepared. IT0 and aluminum were chosen as

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usually consist of an EL polymer layer and electron and hole transporting layers. Heterojunctions are formed at the polymer interfaces. These heterojunctions confine the charge carriers at the multilayer interface and ‘force’ them to undergo efficient radiative recombination. The recombination zone is fixed once the multilayer LEDs are fabricated. On the other hand, the bilayer LEC described in this paper is a p-n junction device; thus, the interface between the MEH-PPV and PPV in the bilayer has no charge confinement effect. The position of the emission center is determined by the relative p-type and n-type dopabilities of the conjugated polymers in both layers. Fig. 5. Current-voltage (solid line) and light-voltage (dashed line) curves of a bilayer LEC under forward and reverse bias conditions. When forward biased, the LEC emits red-orange light from the MEH-PPV layer. When reverse biased, the LEC emits green light from the PPV layer.

the two electrodes, Both PPV and MEH-PPV layers were blended with an electrolyte consisting of PEO and lithium &late. The PPV layer was first spin-cast on top of IT0 from a co-solution of poly (xylylidene tetrahydrothiophenium chloride), PEO and lithium triffate ( 1:1:0.18 by weight) in acetonitrile and subsequently converted at 150-200 “C, The MEH-PPV layer was spin-cast onto the PPV layer in a dry box from a co-solution of MEH-PPV, PEO and lithium triflate ( 1: l:O. 18 by weight). The PPV and MEH-PPV layers were each about 1500 A thick. Al electrodes were evaporated on top of the bilayer films. The current-voltage characteristics of the two-color LEC are shown in Fig. 5. Under forward bias, the device emits orange light identical to the EL of MEH-PPV. The device turned on at approximately 3 V, higher than the bandgap of either MEH-PPV (Ea = 2.1 eV) or PPV (2.5 eV) . A small difference in electronegativity of the two polymers would account for the high turn-on voltage. Fig. 5 also shows the reverse bias current-voltage characteristics of the bilayer device. A much higher current density was observed under reverse bias. The device turned on at the bandgap energy of PPV, about 2.4 eV. Bright green EL from the PPV layer was observed in a lighted room with no EL from the MEH-PPV. The characteristic EL spectra from either MEH-PPV or PPV when the device is biased indicate that the light-emitting p-n junction is completely inside the MEH-PPV or PPV layer, respectively. Moreover, the junction width is much less than the thickness of the MEH-PPV or the PPV layer. This information confirms that the junction width is a function of the total thickness of the film (or the inter-electrode spacing in the case of the planar electrode configuration). While the junction width is about 2 p,m for a 15 pm anode-to-cathode spacing, the width is much less than 1500 A in the case of a 3000 A anode-to-cathode spacing. In fact, we have fabricated planar electrode configuration LECs with 10 and 30 Frn gaps, and the junction width was 1 and 2.5 pm respectively. Bilayer or multilayer device structures have been demonstrated in organic [21] and polymer [ 15,221 LEDs, which

6. Conclusions

Solid-state light-emitting electrochemical cells have been developed using a phase-separated blend of a conjugated polymer and solid electrolyte as the active medium. In the LECs, a light-emitting p-n junction is created in situ through simultaneous p-type and n-type electrochemical doping of the luminescent polymer on opposite sides of the polymer layer. During the doping process, the ionic species of the complex redistribute so that, in the established doping profile, cations dominate the n-doped regime and anions dominate the p-doped regime. The p-n junction is dynamic and reversible: its polarity may be reversed by applying a reverse bias on the electrodes. The chromophores inside the p-n junction determine the emission color of the device. By substituting the polymer chromophores, we have fabricated bright orange, green and blue light emitters with turn-on voltage close to the bandgap of the luminescent materials, and external quantum efficiency as high as 2% photons/electron. Furthermore, with a polymer bilayer, the junction can be formed inside either one of the two layers, depending on the polarity of the external bias. Consequently, this LEC emits two completely different colors of light. Such a dual-color LEC can provide a solution to simple, low-cost color displays.

Dedication

It is our pleasure to dedicate this work to Professor Alan Heeger on the occasion of his 60th birthday. The operation of the polymer light-emitting electrochemical cells discussed here involves most of the fundamentally important properties of conjugated polymers: electrochemicaliy active, dopable to become highly conductive, and semiconductive and luminescent at the pristine undoped state. Therefore, this work is built upon the research of conjugated polymers which Professor Heeger has co-initiated and nurtured. One of the authors (Q.P.) is grateful to Professor Heeger whose seminal articles have taught him the fundamental science of conducting polymers and, more recently, whose encouragement and support enabled this work to be carried out.

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Acknowledgements We are indebted to G. Yu, Y. Cao, C. Zhang, N. Colaneri and I. Parker for important discussionsand technical assistance. This research was supported in part by NSF SBIR Grant No. 9361656 and by the Office of Naval Research (Kenneth Wynne Program Officer).

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