Elsevier

Microelectronics Reliability

Volume 49, Issue 12, December 2009, Pages 1503-1507
Microelectronics Reliability

Stability of the JV characteristics of (BEHP-PPV)-co-(MEH-PPV) based light-emitting diodes

https://doi.org/10.1016/j.microrel.2009.07.053Get rights and content

Abstract

This paper reports main characteristics of copolymer (BEHP-PPV)-co-(MEH-PPV) based PLEDs with PEDOT:PSS as anode and Mg as cathode. Polymeric light-emitting diodes, PLEDs, were prepared by spin-coating of the polymeric layers. Barrier at the anode is around 0.2 eV corresponding to a copolymer HOMO of around 5.3 eV. The energy gap is around 2.3 eV. Photoemission peak is at 547 nm with a refractive index of 1.44. The onset of visible electroluminescence was around J = 100 A/m2. The study of the stability of JV curves showed that for current densities between 620 and 3100 A/m2, Joule heating effect associated causes segregation of the PSS from the bulk PEDOT:PSS that increases the anode–copolymer barrier height. For current densities higher than 1500 A/m2 and for longer operation times, a delamination process in the cathode metal is observed associated to the formation of bubble-like structures that appear as dark spots, leading to final failure of the LED.

Introduction

Recently, much attention has been paid to the study of the emissive properties of polymer materials, due to the advantages they present regarding their relatively simple method of deposition, as well as the possibility of depositing them on large areas and flexible substrates [1], [2], [3], [4], [5]. In the case of emissive properties, polymers present another advantage for PLEDs. Since electroluminescence can be obtained efficiently at relatively low voltage range, it is possible to use only one active layer, instead of the multilayer structures required when using oligomers [5]. Electroluminescence in polymer materials was first reported in poly(para-phenylenevinylene), PPV [1], and since then, PPV and its derivatives continue to be widely used in the fabrication of light-emitting diodes due to their excellent luminescent and mechanical properties. Organic polymers have the distinct advantage of being easily processed by spin-coating or inkjet printing. Despite these advantages, unsubstituted PPV is insoluble and intractable, whereas some high molecular weight PPV derivatives are also difficult to dilute in common solvents [6]. Another adverse property of some PPV derivatives is their sensitivity to air and light [7]; consequently, the semiconductor in organic light-emitting diodes (OLEDs) tends to oxidize easily and diodes have shorter lifetime [8]. In order to overcome these problems, the synthesis and characterization of new materials with better properties is necessary.

Among light-emitting polymers, poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV, has been reported with highly satisfactory characteristics [2], [3], [4], [5] to be used as active layer in PLEDs. It presents a maximum in photoluminescence at around 590 nm and can be better diluted in toluene and xylene. During its preparation, solutions must be manipulated in dark, since the properties of the polymer may be affected by light exposure due to photooxidation [6].

Poly[2-(2′,5′-bis(2″-ethylhexyloxy)phenyl)-1,4-phenylenevinylene], BEHP-PPV, is another polymer that has been characterized as active layer for PLEDs. The maximum of its photoluminescence spectra lies around 537 nm and it is reported to be more stable to photooxidation than MEH-PPV [7]. The HOMO and LUMO for BEHP-PPV have been reported to be around 5.1 and 2.7, which makes this material better to couple with PEDOT:PSS than MEH-PPV [9].

Recently a new copolymer PPV derivative has become commercially available, the poly{[2-[2′,5′-bis(2′′-ethylhexyloxy)phenyl]-1,4-phenylenevinylene]-co-[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]}, (BEHP-PPV)-co-(MEH-PPV), which chemical structure is shown in Fig. 1. This material has been reported to be more resistant to photodegradation during the preparation of the polymer solution for spin-coating and presents higher solubility in chloroform, THF, and xylene at room temperature, compared to its component polymers [10], although we have not found reports of polymeric organic diodes (PLEDs) using it.

In polymer light-emitting diodes, a very important issue is the potential barrier formed at the electrode–polymer interface to guarantee efficient carrier injection [11]. Barriers formed by poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene), MEH-PPV, with different metals as cathodes and organic conductors as anodes have been studied in much detail [4], [5], [12].

Among transparent polymeric anodes, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, which workfunction is around 5.1 ± 0.1 eV results convenient to use with PPV derivatives having HOMO levels around that value. One of the problems of this material, however, is its relatively low conductivity, for which reason it is commonly used on top of an Indium Tin Oxide film, ITO, layer, although some new preparations of this material, as well as several post-treatments, have been reported to significantly increase its conductivity [13], [14], trying to avoid the use of the ITO layer.

In this paper we analyze the electrical behavior of PLEDs fabricated employing (BEHP-PPV)-co-(MEH-PPV) as the active layer and PEDOT:PSS treated in ethylene glycol, in order to increase its conductivity as anode contact. Diodes were prepared using Mg as cathode. (BEHP-PPV)-co-(MEH-PPV) layers, as well as barriers formed with PEDOT:PSS, were characterized to determine or confirm previously reported basic parameters of the polymer [15], [16]. Current voltage JV characteristics of (BEHP-PPV)-co-(MEH-PPV) based PLEDs are presented and their stability under different operating conditions is studied and discussed.

Section snippets

Experimental part

(BEHP-PPV)-co-(MEH-PPV) from Sigma–Aldrich [10] in powder form was diluted in chloroform in a 10 mg/ml proportion for 20 min in an ultrasonic agitator, followed by filtration through a 10 μm prefilter and a 0.45 μm filter, before being deposited by spin-coating to obtain film thickness between 70 nm and 130 nm. Spin-coating at 2000 rpm for 20 s at room temperature was used to obtain films of 80 nm.

The thickness and refractive index of the (BEHP-PPV)-co-(MEH-PPV) layers were determined by variable angle

Results and analysis

Typical current density vs. electric field, JF, characteristics of PLED structures with PEDOT:PSS layers deposited on ITO and post-treated in EG solutions are shown in Fig. 4. Current onset was always around 1 × 108 V/m, while series resistance could vary from several units to several hundreds Ω, depending on the diode structure.

As already mentioned, conduction in PLEDs depends on the injection properties of the metal contacts, where Fowler–Nordheim tunneling should take place. The dependence of

Conclusions

Copolymer (BEHP-PPV)-co-(MEH-PPV) based PLEDs with PEDOT:PSS as anode and Mg as cathode were prepared by spin-coating of the polymeric layers. Barrier at the anode is around 0.2 eV corresponding to a HOMO for the copolymer of around 5.3 eV. The energy gap is around 2.3 eV for a LUMO around 3 eV. Photoemission peak is at 547 with a refractive index of 1.44. The onset of visible electroluminescence was around J = 100 A/m2 with the emission peak at around 547 nm. Emission of the diodes was visible

Acknowledgements

We want to thank Olga Gallegos and Mario Avila for the fabrication of the samples. This work was supported by CONACYT Project 45689 in Mexico.

References (19)

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