Two newly synthesized ultra-small copper nanoclusters, [Cu₃(μ₃-H)(μ₂-dppy)₄](ClO₄)₂ (1) and [Cu₄(μ₄-H)(μ₂-dppy)₄(μ₂-Cl)₂](ClO₄) (2) (dppy = diphenyl-2-pyridylphosphine), have been shown to exhibit ultrabright yellow and yellow-green room-temperature phosphorescence (RTP) emission, with high quantum yields of 71.8% and 63.5%, respectively. Therefore, nanocluster 1 has been applied for the first time as a single component phosphor for yellow and white light-emitting diodes (LEDs) with favourable characteristics.