Organic electroluminescent materials and devices

An organic light emitting device (OLED) comprising:

• an anode;
• a cathode;
• and
• an emissive region disposed between the anode and the cathode;
• wherein the emissive region comprises:
• a first compound S1;
• a second compound A1;
• and
• a third compound H1;
• wherein the first compound S1 is capable of phosphorescent emission at room temperature and is a sensitizer that transfers energy to the second compound A1;
• wherein the second compound A1 is an acceptor that is an emitter;
• and
• wherein the third compound H1 is a first host, and at least one of the first compound S1 and the second compound A1 is doped in the third compound H1;
• with at least one of the following conditions being true:
• (1) the third compound H1 comprises a boron atom;
• or
• (2) the first compound S1 is a metal complex comprising metal M, and a first ligand LA that comprises at least one moiety F;
• wherein the at least one moiety F is a fused ring system comprising two or more 5-10 membered carbocyclic or heterocyclic rings and is joined to the metal M through an anionic bond;
• wherein M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Pd, Zn, Ag, Au, and Cu;
• and with the proviso that if M is Pt or Pd, then the at least one moiety F is not a nonfused carbazole.