The electron affinity (EA) of an organic semiconductor is a measure of the electron transport level.Although reliable values of the EA are required for designing the device architecture of organic light-emitting diodes (OLED), there were no appropriate methods. Recently we have developed low-energy inverse photoemission spectroscopy which enables us to determine the EA of organic materials in solid with the precision required for research of OLED. Using this new technique, we precisely determined EA of typical OLED materials, TCTA, CBP, Ir(ppy)3, BCP, Alq3 and Liq as well as a newly developed dopant 4CzIPN. The obtained electron affinities are generally smaller by about 1 eV than the commonly believed values urging the reconsideration of the electron injection/ transport mechanisms in OLED. We also compare EAs determined by various experimental and calculation methods for 29 materials. The results show that the reduction potential gives a reasonable estimate rather than the optical gap and ionization energy.

The surface potential across Alq 3 [tris(8-quinolinolato)aluminum] evaporated films on a metal (Au, Al) electrode was measured by the Kelvin probe method and the results were discussed taking into account the orientational ordering of Alq 3 molecules, the presence of surface charges on Alq 3 films and the displacement of excess charge (electrons and holes) from the electrode into Alq 3 films. The very large surface potential established in as-deposited Alq 3 films was mainly due to the alignment of dipoles of Alq 3 molecules, whereas the small surface potential that remained in the films after photoirradiation was due to excess charge. In order to clarify the drastic decrease of surface potential by photoirradiation, the surface potential decay was examined.

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