An organic light-emitting device comprising a first electrode, a second electrode and at least one light-emitting layer between the first and second electrodes wherein the device comprises a plurality of light-emitters that together provide a source of white light; wherein: a first light-emitting layer comprises a host material and a first light-emitter of the plurality of light-emitters that emits light having a peak photoluminescent wavelength in the range of 580-605 nm; a LUMO of the first light-emitter is the same as or closer to vacuum than a LUMO level of the host material, the first light-emitter is a phosphorescent emitter of formula: ML 1 q L 2 r L 3 s wherein M is a metal; L 1, L 2 and L 3 each independently represent a coordinating group; q is an integer; r and s are each independently 0 or an integer; the sum of (a. q) + (b. r) + (c.s) is equal to the number of coordination sites available on M, wherein a is the number of coordination sites on L 1, b is the number of coordination sites on L 2; and c is the number of coordination sites on L 3; L 1 is a ligand of formula; wherein Ar 6 and Ar 7 each independently represent an optionally substituted aromatic or heteroaromatic group comprising an atom capable of coordinating to M, d is at least 1 and X in each occurrence is selected from the group consisting of O, S, NR 7 and -CR 7 2 - wherein R 7 in each occurrence is H or a substituent; the host material is blended with the first light-emitter and any other of the plurality of light-emitters present in the first light-emitting layer; and the host material has formula: wherein Ar 1, Ar 2 and Ar 3 are independently in each occurrence an optionally substituted aryl or heteroaryl group, and z independently in each occurrence is at least 1, optionally 1, 2 or 3. a first light-emitting layer comprises a host material and a first light-emitter of the plurality of light-emitters that emits light having a peak photoluminescent wavelength in the range of 580-605 nm; a LUMO of the first light-emitter is the same as or closer to vacuum than a LUMO level of the host material, the first light-emitter is a phosphorescent emitter of formula: ML 1 q L 2 r L 3 s wherein M is a metal; L 1, L 2 and L 3 each independently represent a coordinating group; q is an integer; r and s are each independently 0 or an integer; the sum of (a. q) + (b. r) + (c.s) is equal to the number of coordination sites available on M, wherein a is the number of coordination sites on L 1, b is the number of coordination sites on L 2; and c is the number of coordination sites on L 3; L 1 is a ligand of formula; wherein Ar 6 and Ar 7 each independently represent an optionally substituted aromatic or heteroaromatic group comprising an atom capable of coordinating to M, d is at least 1 and X in each occurrence is selected from the group consisting of O, S, NR 7 and -CR 7 2 - wherein R 7 in each occurrence is H or a substituent; the host material is blended with the first light-emitter and any other of the plurality of light-emitters present in the first light-emitting layer; and the host material has formula: wherein Ar 1, Ar 2 and Ar 3 are independently in each occurrence an optionally substituted aryl or heteroaryl group, and z independently in each occurrence is at least 1, optionally 1, 2 or 3.