Three types of alkyl-substituted poly(N-alkyl-1-vinyl-imidazolium) iodides were synthesized and plasticized using succinonitrile as a solid plasticizer to develop a series of novel solvent-free plastic-polymer composite electrolytes. All these electrolytes appeared as a soft solid at room temperature and became sticky gel state at high temperature of 100 degrees C. Among the as-prepared plastic-polymer electrolytes, the SCN-PMVII (succinonitrile-poly(1-vinyl-3-methylimidazolium) iodide) electrolytes with a SCN content of 40-60 wt.% showed a room temperature conductivity of 1.0-1.6 mS cm(-1) and a photoconversion efficiency of >4.1%, which are comparable to those observed from liquid organic carbonate electrolyte and the DSSCs using the liquid electrolyte at the same experimental conditions. Also, the DSSCs assembled with the SCN-PMVII electrolytes maintained their photoconversion efficiency very steadily during aging test of SO days despite of being placed at 40 degrees C under 1 sun illumination or stored at 60 degrees C in an oven. Since these plastic-polymer electrolytes are solvent-free, highly conductive and electrochemically compatible, it is possible to use this type electrolyte for development of practical DSSCs. (C) 2010 Elsevier Ltd. All rights reserved.