Layer-by-layer assembly of conducting membranes for photoelectrochemical cells

Abstract

Spray-assisted layer-by-layer (Spray-LbL) assembly is used to achieve vertical transfer of silicon microwire arrays into an ion-conducting, ultrathin polymer membrane. The choice of LbL platform and the properties of the silicon surface control the film morphology, generating either a conformal coating around each wire or a bridging film across the top of the array. Multilayer transfer printing is used to merge together separately assembled free-standing membrane/microwire assemblies into a single functional film. This technique offers an attractive option relative to traditional materials for microfabrication of Si devices such as solar-driven water splitting systems, capacitors, or electrochemically active electrodes. Transparent mixed conducting polymer films with conductivity above 0.1 mS/cm are highly desirable for photoelectrochemical cell membrane applications. Mixed conducting polymer composite PEDOT:sPPO was incorporated into LbL films and the composition ratio of the film components was varied to generate a series of films with tunable transparency and electrical and protonic conductivities. The visible light transmission properties are excellent: 1.1 [mu]m thick films with 150 mS/cm electrical conductivity have 80% transmission of light in the visual range. The electronic and ionic conductivities are inversely related, as one can be increased at the expense of the other. The highest ionic conductivity recorded was 4 mS/cm for 4.6 [mu]m thick films with 2 mS/cm electrical conductivity. Electron microscopy was used to provide insight into the effect of film morphology on electrical conductivity, and temperature dependent impededance spectroscopy and ion exchange capacity measurements yielded insight into the ionic conductivity changes.