A Gd-complex

tris-(dibenzoylmethanato)-mono-(bathophenanthroline) gadolinium [Gd(DBM)

3

bath] was used to tune emission colour in OLEDs.The Gd(DBM)

3

bath layer was inserted between the hole-transporting layer(

m

-MTDATA) and electron-transporting layer(TPBI).The ELcolor can be tuned from green at 525 nm to orange at 593 nm by increasing the thickness of Gd-complex layer.When the Gd-complex layer was thin enough

this layer became discontinuous.The discontinuity was increased when the thickness of this layer was further decreased.As a result

the exciplex from

m

-MTDATA/TPBIwas also formed besides the exciplex from

m

-MTDATA/Gd-complex.The shifts of spectra were due to the change of proportion for the two exciplexes.These results show that different emission color can be achieved by changing exciplex formation and its intensity in the same device configuration.Asimple technique for tuning the color of OLEDs by controlling the thickness of Gd-complex layer can be demonstrated.Besides

other complexes of trivalent rare earth with(β-diketone) may also play alike roles in such OLEDs.