A correlation between the performance of electrochemiluminescent (ECL) displays and properties of ECL gels consisting of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([AMI][TFSI], ionic liquid), poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP), polymer matrix), and 2,2′-bipyridyl-bis[2-(2′,4′-difluorophenyl)pyridine]iridium hexafluorophosphate (Ir(diFppy)2(bpy)PF6, ECL luminophore) is investigated. Since a majority of the conductive channel in the gel is an ionic liquid (IL), the physical properties of the gel depend strongly on those of the IL. Here we vary the length of the alkyl chain on the 1-position of the cation [AMI] with fixed anion [TFSI], so that the physical properties of the gel are systematically tuned. As the alkyl length of [AMI] increases, a higher operating voltage is required due to a lower conductivity of the gel (i.e., larger voltage drop). In addition, the device response time is also intimately connected to the gel. The [AMI][TFSI] having longer alkyl chains induces a slower response of the device, which is attributable to impeded ion diffusion due to higher viscosity.