Electroluminescence studies on self-assembled films of PPV and CdSe nanoparticles

Electrooptical and structural studies on self-assembled films composed of CdSe nanoparticles, poly(p-phenylene vinylene) (PPV), and different nonconjugated polyelectrolytes are reported. It is demonstrated by optical spectroscopy and X-ray reflectivity measurements that CdSe nanoparticles and PPV can successfully be incorporated into homogeneous ultrathin films by the self-assembly method. The surface roughness obtained from the X-ray measurements is 2.7 and 1.3 nm respectively for CdSe/PAH (PAH, poly(allylamine) hydrochloride) and PSS/PPV (PSS, poly(styrenesulfonic acid)) multilayer films. This allows us to stack a (PSS/PPV)*n film on top of a (CdSe/PAH)*n film to build up well-defined two-layer composite film devices. Electroluminescence studies show that pure (PSS/PPV)*n film devices exhibit green light emission but with a very short lifetime (several seconds) if operated in ambient air. During operation, the PPV emission shifts toward the blue, which indicates that the mean conjugation length of PPV is shortened due to oxidation. Oxidation of CdSe particles is also observed in (CdSe/PAH)*n devices during operation. However, the stability of CdSe particles is enhanced if they are combined alternately with PPV, and a (CdSe/PPV)*n device gives a broad, nearly white emission. The turn-on voltage of it is much smaller and better defined than that for a (CdSe/PAH)*20 device. This proves that PPV works like a charge-transportation layer rather than an emitting layer in the (CdSe/PPV)*n film device. In an ITO//PEI(CdSe/PAH)*10/(PSS/PPV)*10//Al two-layer composite film device the lifetime of PPV electroluminescence can be prolonged for at least 1 order of magnitude only after the device is first operated under backward bias (positive pole on Al electrode). This suggests that the oxygen within the film is removed by this operation due to oxidation of the particles. Afterward, this two-layer composite film device presents emission from PPV and CdSe, respectively, when the sign of the external voltage is changed.