solar cells (OSCs), organic thin fi lm transistors (OTFTs), organic memory devices (OMDs), etc. [4][5][6][7][8] Interestingly, liquid or gel-type electrolytes have been still used for sophisticated energy storage devices including batteries and capacitors, [9][10][11][12][13][14] even though a couple of inorganic solid-state electrolytes such as thio-LISICON (Li 3.25 Ge 0.25 P 0.75 S 4 ), Li 2 NaPO 4 (nalipoite structure), and a-60Li 2 S·40SiS 2 have been developed. [15][16][17] Here we note that such inorganic electrolytes cannot be used for fl exible devices because of their intrinsic brittleness. In the case of liquid electrolytes such as aqueous solutions, [ 18,19 ] organic solutions, [ 20,21 ] and ionic liquids, [ 22,23 ] there are essential limitations for miniaturizing devices because of tricky sealing processes to avoid leakage of electrolyte solutions that are normally toxic and corrosive.Compared to the liquid electrolytes, gel-type electrolytes deliver improved processability but they do still contain liquid components so that they bear potential disadvantages as liquid electrolytes do. To date, most reports claiming "solid-state electrolytes" have employed gel-type electrolytes such as proton conducting polymer gel electrolytes, [24][25][26] lithium ion gel polymer electrolytes, [27][28][29] and alkaline gel polymer electrolytes. [30][31][32] Hence a perfect solid-state electrolyte without including any liquid components should be developed to achieve real ultrathin/ fl exible devices in the coming fl exible electronics era. In addition, the perfect solid-state electrolyte is required to have solution processability using water as a solvent in consideration of environmental issue for using toxic solvents in the conventional fabrication processes. In particular, a well-defi ned small molecular electrolyte, which is of course highly reproducible in the large scale synthesis, is practically desirable for device miniaturization toward a nanoscale energy storage device that needs only few molecules in the thickness direction of the electrolyte nanolayers.In this work, we attempted to design a small molecular metal-chelate complex that has three sulfonic acid groups per molecule in order to maximize solubility in water and secure proton ions for energy storage. Taking into account easy synthesis for possible mass production, one-step synthesis protocol was adopted on the basis of condensation reaction between aluminum triisopropoxide (AltiP) and 8-hydroxyquinoline-5-sulfonic acid (HQSA). The resulting metal-chelate A small molecular metal-chelate complex, tris(8-hydroxyquinoline-5-sulfonic acid) aluminum (AlQSA 3 ), that has three sulfonic acid groups per molecule leading to an excellent solubility in water is reported as a liquid-free perfect solid-state electrolyte for fl exible fi lm-type all-solid-state energy storage devices. The AlQSA 3 material is synthesized by one-step reaction of aluminum triisopropoxide and 8-hydroxyquinoline-5-sulfonic acid. The aqueous solutions of AlQSA 3 are applied to fabric...