Recoverable Photodegradation of Light-Emitting Polymers
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NTRODUCTION Considerable attention has recently been paid to the organic light-emitting diodes (OLEDs) 1-5 based on light-emitting polymers , due to their potential applications.6,7 State of the art diodes are planar structures in which a single electroluminescent layer of poly (p-phenylene vinylene), PPV, or derivatives , is sandwiched between high and low work function electrodes such as indium tin oxide (ITO) (work function φ∼4.4-4.9 eV) and Ga, A l, or Ag (φ-2.9-4.3 eV) or conducting polymers . One main challenge faced by the OLEDs is the stability. The photochemical stability of the active material under ambient illumination is an important requisite for the prevention of performance decay. Organic polymeric materials , by their nature , are prone to the effects of various environmental parameters , such as heat, light, radiation, and water. A good understanding of degradation in light-emitting polymers is important to the improvement of stability of these materials and related devices . In this paper, we present a photodegradation study of light-emitting polymers . W e found that the photoluminescence intensity of our samples decreases very fast initially, and then tends to saturate on the order of several tens of minutes . More interestingly , it was found that the degradation induced by laser beam can be self-healed. By stopping the exposure of the sample to the laser beam, the photoluminescence intensity recovers gradually without any treatment. The mechanism causing this phenomenon is discussed.
EXPERIMENT AND METHODOLOGY The photoluminescence (PL) and photodegradation study on light-emitting polymers were conducted with a conventional PL measurement system at room temperature. Figure 1 shows the chemical structure of one sample PMH3 used in this work. A He-Cd laser operating at a wavelength λ = 325 nm and a power density of 2.0 W/cm2 at the sample surface was used for the excitation of PL as well as the photodegradation of the sample.
OEH
N N
O
n
PMH3
O
EH = 2-ethylhexyl Figure 1. Chemical structure of the PMH3 sample used in this work.
The luminescence signal was dispersed in a 0.75 m monochromator and detected by a GaAs photomultiplier using standard phase-sensitive lock-in amplification. In order to monitor the evolution of the PL spectrum with exposure time to the laser beam, the PL spectra were taken at certain exposure time. When the PL tended to saturate, the laser beam illuminated on the sample was blocked for certain time to test the recovery of degraded PL.
RESULTS AND DISCUSSION Figure 2 shows a set of PL spectra measured at different exposure time to the HeCd laser beam. It is seen that the PL intensity decreases very fast at the initialstage of the laser illumination, then the decrease becomes slower and slower, and finally it gets to saturate. It is also seen that the shape of the spectrum does not change with the exposure. Figure 3 shows the evolution of the integrated PL intensity over the spectrum with the exposure time. The decay of PL intensity can be described by a second o
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