Cost-effective solid-state electrolytes with improved conductivity and device performances have received much research interest in (photo)-electrochemical cells, including solar cells, secondary batteries, and so on. In this work, a series of organic ionic crystals (OICs) containing imidazolium salts modified with bulky fluorenyl and diphenylmethyl substituents and BF4 anions were synthesized and applied into quantum dot-sensitized solar cells (QDSSCs) as the matrix of solid-state electrolytes. The performance of QDSSCs employing these new OICs was evaluated by electrochemical impedance spectroscopy, Tafel plots, and incident monochromatic photon-electron conversion efficiency traces. A champion device applying imidazolium salts with nonplanar diphenylmethyl substituents and BF4 anions afforded a highest power conversion efficiency of 5.69% under AM 1.5 (100 mW.cm(-2)) irradiation.