Gel electrolytes for partly covered photoelectrochromic devices

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This work comprises a systematic experimental investigation of the effect of gel electrolytes in the properties of partly covered photoelectrochromic devices.A series of gel electrolytes was fabricated with different concentrations of poly(methyl methacrylate), PMMA, (Mw 120.000), ranging from 0% w/w to 45% w/w, dissolved in acetonitrile with 0.5 M lithium iodide, 0.005 M iodine and 0.5 M 4-tert-butylpyridine. The ionic conductivity of the electrolytes was in the order of 10−5 S/cm, decreasing with increasing PMMA concentration.These electrolytes were incorporated in partly covered photoelectrochromic devices. It was found that with increasing PMMA concentration, the power conversion efficiency of the incorporated solar cells decreases (from 1.7% to 0.5%), mostly due to the reduction of the short-circuit current. Regarding coloration kinetics, the photocoloration efficiency, PhCE of the devices was nearly constant up to 35% w/w concentration of PMMA, decreasing with further increase in the PMMA concentration. Of the electrolytes tested, that with 35% w/w concentration of PMMA was favored as it leads to devices with satisfactory electrical and optical characteristics.The long-term stability of devices with the aforementioned electrolyte was tested. A gradual decrease of the coloration depth with storage time was observed, due to losses at the WO3/electrolyte interface. A 35 nm thin ZnS barrier on top of WO3 suppressed these losses, leading to a stabilized contrast ratio of 2.1:1, 137 days post fabrication. Devices exposed to insolation in open-circuit and in short-circuit conditions remained operational for more than 200 days, without the stability problems related to liquid electrolytes.