Using imidazole-type ancillary ligands, a new class of cationic iridium complexes (1-6) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2'-bipyridine)-type ancillary ligands, imidazole-type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue-green to red emitting cationic iridium complexes are obtained. Single-layer light-emitting electrochemical cells (LECs) based on cationic iridium complexes show blue-green to cells electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A(-1) are achieved for the blue-green-emitting, yellow-emitting, and orange-emitting devices, respectively. By doping the red-emitting complex into the blue-green LEC, white LECs are realized, which give warm-white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color-rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A(-1), and 10 lm W-1, respectively, which are the highest or white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.