The polymer light-emitting electrochemical cells (LECs) were fabricated with MEH-PPV as the luminescent polymer and the ionic liquid of imidazolium salts as the supporting electrolyte. The imidazolium salts utilized include various 1-methyl-3-alkylimidazolium salts with the alkyl substituents of butyl (bmim), dodecyl (dmim), tetradecyl (tmim), or hexadecyl (hmim) and the anions of PF6- or BF4-, which possess different melting points from room temperature for [bmim(+)][PF6-] to 83 degreesC for [hmim(+)][PF6-]. The electroluminescent (EL) properties and the electronic structure of the LECs were characterized by current-voltage (I-V), light intensity-voltage (L-V), and ac impedance measurements. It was found that the phase compatibility between the conjugated polymer and the ionic liquid determines the performance of the light-emitting devices, and the concentration of the ionic liquid and the ionic conductivity of the polymer blend films also play an important role. The imidazolium salts investigated in this work are suitable for fabricating the LECs except for [bmim(+)][PF6-] which encounters phase separation problem with MEH-PPV. The LEC doped by [dmim(+)][BF4-] shows an EL external quantum efficiency of 0.2% at 4 V, which is comparable with that of the traditional LEC with PEO/Li+ salt as the polymer electrolyte. Room-temperature frozen p-i-n junction LEC was realized on the LECs based on MEH-PPV doped by [tmim(+)][PF6-] and [hmim(+)][PF6-] with the higher melting point. The frozen-junction LEC shows fast response, wide operating voltage window exceed 10 V and high EL performance. The external quantum efficiency of the LEC/[hmim(+)][PF6-] achieved 1.4% at 10 V. The electrochemical doping mechanism of the LECs was confirmed by the ac impedance measurement of the devices. The ionic liquids are very stable and insensitive to humidity, which could enable the fabrication and characterization of the LEC outside a drybox.