Light emitting Electrochemical Cell - LEC -

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What is a LEC

A Light-emitting Electrochemical Cell -LEC-, as first introduced by Pei et al. , is a light emitting optoelectronic device consisting of a polymer blend embedded in between two electrodes, at least one of the two being transparent (typically Indium Tin Oxide, ITO).
The polymeric blend is comprised of a luminescent polymer, a salt and an ion conducting polymer in which the salt dissolves to form ions. This composition determines the difference between LECs and polymer light-emitting diodes (LEDs).
LECs, are experiencing increasing scientific and industrial interest as solid-state sources of luminescence, thanks to their good electroluminescence efficiency, low driving voltages and ease of fabrication.

 

How is our LEC made

The luminescent polymer used in our devices is m-LPPP (methyl added Ladder-type Poly-Para-Phenylene) setting the emission to the blue color. Lithium triflate (LiCF 3 SO 3 ) and poly(ethylene-oxide)-PEO- have been chosen as salt and ion conducting polymer, respectively. Poly(ethylene oxide), MW 5,000000 and Lithium triflate were heated under high vacuum to remove any content of water before use.
The LECs were produced by spin coating the solution of m-LPPP, PEO and LiCF 3 SO 3 , in a weight ratio of 20:10:3, in cyclohexanone on an ITO coated glass substrate (film thickness of about 200 nm). The films were heated afterwards at T=60C under argon atmosphere to remove any content of solvent. Afterwards aluminum was evaporated as top electrode ( thickness approximately 50 nm).

 

Principles of operation

By applying a voltage between the two contacts, the current stays low untill a threshold voltage is reached. After this value of voltage, current increases neatly and so does the emitted electroluminescence. Typical I_V behavior is seen in the figure.

In dependence of the applied voltage three different operating regimes have been identified.
     In the first regime -below threshold- the current flow is very low and is determined by thermionic emission of carriers from the conductive electrodes into the active device over a relatively high barrier. Current increases with voltage due to the lowering of the potential energy barriers by Schottky effect, that is by the combined effect of the applied electric field, of the image charge and of the accumulation of mobile ions at the contact, this latter being a peculiarity of the LEC.

      At higher applied voltages, when the Fermi levels of the conductive electodes are aligned with the polymer valence and conduction bands, the current through the LEC strongly increases due to field induced tunnelling injection of charge carriers from the electrodes through the thin potential barrier at the interfaces into the active layer. Contemporary to this, the higly available amount of carriers of both sign enhances their ricombination and produces a strong increase in the emitted radiation. In our case, the threshold for strong injection is at about 2.7V.

      A third regime is observed when the applied voltage is increased above the electrochemical redox potential of the active conjugated polymer layer, where an electrochemical doping of the polymer occurs. The I-V curve tends to linear indicating that an ohmic regime is reached in which the applied external voltage drops across the polymeric bulk material.

The last three figures represent Energy band diagrams of the LEC based on m-LPPP with ITO and Aluminum contacts in the three described biasing conditions. The Fermi level of the intrinisic m-LPPP is assumed to be around mid-gap.

 

Why could it be an interesting device

A LEC shares the same fields of applications as LEDs - Light Emitting Diodes - but has some advantages over them. In a LED the active layer consists of the pristine electroluminescent polymer and the electrical characteristics are inevitably dependent on the nature of the interfaces between the polymer and the electrodes (relative work functions of the two electrode materials, use of charge transport layers, interface traps, etc.). Therefore the LEDs, especially the ones based on wide band gap polymers, are characterized by high turn on voltages. This can be a disadvantage for the successful application of conjugated polymers in flat panel displays and low power optoelectronics.

     LECs overcome this problem thanks to the presence of the mobile ions in the active layer, which strongly facilitate charge injection already at low applied voltages and makes it fairly independent of the potential barrier height and of the trap densities at the interfaces polymer/electrodes. The threshold voltage is practically given by the energy gap of the active polymer and therefore it is the lowest possible value for the chosen emitted radiation. This feature makes the LEC interesting in low power optoelectronic applications.

 

Carrier transport theory

 

 

 

 


 
 

Electrical model of LEC

The different regimes of operation of the LEC and the onset of a dominant diffusion capacitance above threshold are well mirrored in the impedance measurements (absolute value and phase relationship).

Below is a circuit model of the LEC based on the physical characteristics of the device as highlighted in the previous paragraphs:
R s is the series resistance of the wire contacts (about 80 Ohm in our case due to the high resistance-per-square of the ITO electrode);
C geom is the geometrical capacitance of the device (2.2nF);
R LEC is the internal resistance of the LEC;
C diff is its diffusion capacitance.

Well below the threshold (curves indicated with “0V” in the figures), the device behaves like a capacitor, whose value is almost purely given by geometrical considerations and therefore equal to C geom . The linear dependence of the absolute value of the impedance in the low frequency range indicates that the internal resistance R LEC of the device is higher than 10MOhm.

The reduction of the internal resistance when carrier injection takes place reflects in the flat behaviour of the absolute value at low frequencies and in a corresponding angle shift in the phase plot. At very weak injection (V=2.2V) the value of R LEC is 100kOhm.

Well above the threshold (V=4.4V), the internal resistance R LEC drops to 5kOhm and the onset of the diffusion capacitance ( 6.5nF), which adds to C geom , is responsible of the shift to the left of the phase curve at high frequency.
Continuous lines in the figures are experimental values, marked points are the values of impedance obtained from the model by using the mentioned values for the circuit elements.

 

Temperature behavior

Hysteresys behavior

Contributors to the research

The major contribution to the electrical characterisation of the LEC devices, as made in the Electronic Department of Politecnico di Milano (Italy), has been given by Riccardo Sotgiu within his thesys work for the Laurea degree. At present, Dario Natali is continuing and extending his work.
A constant interest and support has been given by the Physics Department of Politecnico di Milano in the persons of Sandro De Silvestri, Guglielmo Lanzani, Giulio Cerullo and Mauro Nisoli.

The preparation of the devices, all material characterisations and a strong contribution to the analysis of the experimental electric measurements are due to the staff of the Institut fur Festkorperphysik of the Technische Universitat Graz (Austria), leaded by G. Leising. Among them I like to thank in particular the LEC specialists Stefan Tash and Franz Wenzl.

This work is supported by the Italian MURST under a "cofinanziamento" program issued in 1999.

 

Selected bibliography

From our group

M.Sampietro, R.Sotgiu, F.P.Wenzl, L.Holzer, S.Tasch and G.Leising
"Electrical characteristics of light emitting electrochemical cells based on a wide bandgap polymer"
Phys. Rev. B, Vol.61 , N.1, 266-271 (2000)  

L.Holzer, F.P.Wenzl, R.Sotgiu, M.Gritsch, S.Tasch, H.Hutter, M.Sampietro, G.Leising
"Charge distribution in light emitting electrochemical cells"
Synth. Met., 102 1022-1024 (1999)
and Proc. International Conference on Science and Technology of Synthetic Metals, ICSM-98, Montpellier (France) July 12-18, (1998).

M.Sampietro, R.Sotgiu, F.P.Wenzl, L.Holzer, S.Tasch and G.Leising
“Transport mechanism and electrical properties of LECs based on mLPPP active material”
SPIE's 44th Annual meeting conference proceedings , Boulder Colorado USA, July 1999

S.Tasch, L.Holzer, F.P.Wenzl, J.Gao, B.Winkler, L.Dai, A.W.H.Mau, R.Sotgiu, M.Sampietro, G.Leising, A.J.Heeger
"Light-emitting electrochemical cells with microsecond response times based on PPPs and novel PPVs"
Synth. Met. 102 , 1046-1049 (1999)
 

About LECs

Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269, 1086 (1995)

Q.Pei, Y.Yang, G. Yu, C. Zhang, A.J. Heeger, J. Amer. Chem. Soc. 118, 3922,  (1996)

Y. Cao, G. Yu, Y. Yang, A. J. Heeger, Appl. Phys. Lett. 68, 3218, (1996)

H. Campbell, D. L. Smith, C. J. Neef, J. P. Ferraris, Appl. Phys. Lett. 72, 2565, (1998)

Y. F. Li, J. Gao, G. Yu, Y. Cao, A. J. Heeger, Chem. Phys. Lett. 287, 83, (1998)

G. Yu, Y. Cao, C. Zhang, Y. F. Li, J. Gao, A. J. Heeger, Appl. Phys. Lett. 73, 111, (1998)

D. L. Smith, J. Appl. Phys. 81, 2869, (1997)

I. Riess, D. Cahen, J. Appl. Phys. 82, 3147, (1997)

J. C. deMello, N. Tessler, S. C. Graham, R. H. Friend, Phys. Rev. B, 57, 12951, (1998)

J. A. Manzanares, H. Reiss, A.J. Heeger, J.Phys Chem B 102, 4723, (1998)
 
 

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[24] J. Gao, G. Yu, A.J. Heeger, Appl. Phys. Lett. 71, 1293, ( 1997)
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