In order to compare the stability between small molecule light-emitting materials and polymer light-emitting materials

tris-(8-hydroxyquinoline)aluminum (Alq)(a typical small molecule light-emitting material)and poly(9

9-dioctylfluorene)(PFO)(a brand new polymer light-emitting material)were chosen as study cases in this work.Ultraviolet photoelectron spectroscopy(UPS)

X-ray photoelectron spectroscopy(XPS)

atomic force microscopy(AFM)

and photoluminescence spectrometer(PL) were used to investigate the ambient effect on the electronic structures

surface morphology

and PL properties of the Alq film and the PFO film. Alq films

with thicknesses of 6~8nm

were deposited by the thermal evaporation on to either Si substrates or indium tin oxide (ITO) coated glass substrates in a deposition chamber attached to a VG ESCALAB 220i-XL photoelectron spectroscopy system. PFO was spin coated from a toluene solution (1 5wt%) to form the films with thicknesses of 7~15nm on Au coated Si substrates or on ITO coated glass substrates. After first time measurements

the freshly prepared samples were put into air for storage at room temperature for further measurements. The relative humidity during storage was 60~70%

and the storage duration was from 1 hour to 28 days. We used the He Ⅱ excitation line(40.8eV) from a He discharge lamp for the UPS measurements and a monochromatised Al Kα excitation line(1486.6eV) for the XPS measurements. A clean Au film on Si substrate was used for the Fermi level (E

F

) and the binding energy calibration. Surface morphologies of the films were observed in air by Scientific Auto probe CP AFM. The PL properties of the films were obtained by Perkin Elmer LS-50B Luminescence Spectrometer. UPS results indicated that the electronic structure of Alq film were obviously changed upon 1-hour exposure to air while that of PFO film was slowly changed with increasing the storage time up to 28 days

but this change was hardly discernible after 1-week storage in air.XPS results indicated that Al bonding and N bonding in Alq molecule were easily influenced by H

2

O and O

2

while C-C bonding in PFO was not easily attacked by H

2

O and O

2

.AFM revealed that the surface morphology of Alq film was also obviously changed after exposure to air but that of PFO film remained basically unchanged.The PL peak intenstity of Alq film was quenched rapidly in the air while that of PFO film was decreased much slowly.Therefore

PFO is a rather stable light-emitting material used in polymer light-emitting devices.