We present a single layer, polymer white light emitting diode (WOLED) based on novel organic lanthanide complexes. The device is solution processed and possesses CIE coordinates of (0.33, 0.38), corresponding to a high color purity white light emission. The electroluminescence in this diode is found to originate from both the direct charge trapping on the lanthanide complexes and from the efficient diffusion-assisted long-range energy transfer between the conductive matrix and these emissive dopants. An expression for the steady-state transfer efficiency is given, the role of excitation diffusion in the system under study is quantitatively evaluated, and the values for the singlet exciton diffusion coefficients and diffusion lengths in the host are determined.