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Analysis of the current-voltage characteristics of polymer-based organic light-emitting diodes (OLEDs) deposited by spin coating

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Author(s): Henry Alberto Méndez-Pinzón | Diana Rocío Pardo-Pardo | Juan Pablo Cuéllar-Alvarado | Juan Carlos Salcedo-Reyes | Ricardo Vera | Beynor Antonio Páez-Sierra

Journal: Universitas Scientiarum
ISSN 0122-7483

Volume: 15;
Issue: 1;
Start page: 68;
Date: 2010;
Original page

Keywords: Organic semiconductors | OLEDs | electroluminescent polymers | MDMO-PPV | PEDOT:PSS | Spin coating | HOMO | LUMO | carrier injection | thermionic emission

ABSTRACT
Polymer-based organic light-emitting diodes (OLEDs) with the structure ITO / PEDOT:PSS / MDMO-PPV / Metal were prepared by spincoating. It is known that electroluminescence of these devices is strongly dependent on the material used as cathode and on the depositionparameters of the polymer electroluminescent layer MDMO-PPV. Objective. In this work the effect of i) the frequency of the spin coater(1000-8000 rpm), ii) the concentration of the MDMO-PPV: Toluene solution, and iii) the material used as cathode (Aluminium or Silver)on the electrical response of the devices, was evaluated through current-voltage (I-V) measurements. Materials and methods. PEDOT:PPSand MDMO-PPV organic layers were deposited by spin coating on ITO substrates, and the OLED structure was completed with cathodesof aluminium and silver. The electric response of the devices was evaluated based on the I-V characteristics. Results. Diodes prepared withthinner organic films allow higher currents at lower voltages; this can be achieved either by increasing the frequency of the spin coater orby using concentrations of MDMO-PPV: Toluene lower than 2% weight. A fit of the experimental data showed that the diodes have twocontributions to the current. The first one is attributed to parasitic currents between anode and cathode, and the other one is a parallel currentthrough the organic layer, in which the carrier injection mechanism is mediated by thermionic emission. Conclusions. The results of thefitting and the energy level alignment through the whole structure show that PPV-based OLEDs are unipolar devices, with current mainlyattributed to hole transport.
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