US 5294869 and US 5294870
US patent 1994 by C.W. Tang et al. of Eastman Kodak company
Above patents are based on following papers, also first papers on OLEDs:
Organic Electroluminescent Diodes
C.W. Tang and S. A. VanSlyke
Appl. Phys. Lett. 1987, 51, 913
and
Electroluminescence of Doped Organic Thin Films
C.W. Tang, S.A. VanSlyke, and C.H. Chen
J. Appl. Phys. 1989, 65, 3610
and some recent papers:
Doped organic electroluminescent devices with improved stability
Jianmin Shi and C. W. Tang
Appl. Phys. Lett. 1997, 70, 1665Â