White and green light emissions of flexible polymer composites under electric field and multiple strains

The development of energy harvesting white light sources converted from multiple stimuli especially mechanical strain requires the search of new types of phosphors and devices. In this work, various flexible composites of single-phase metal ion-doped ZnS mixed to polydimethylsiloxane (PDMS) matrix have been prepared. The synthesized composites possess flexible, durable, easy fabrication, and compatible with arbitrary substrate characteristics. With the advantages of the flexible composite, we have fabricated two kinds of light-emission devices, namely flexible electroluminescence device with graphene electrode, and piezo-phototronic luminescence device composed of polymer phosphor layer coated on the top of piezoelectric actuator. Such composite phosphors and the related hybrid devices are capable of responding to different types of external stimuli, including electric field, uniaxial strains of stretch and mechanical writing, and piezoelectric biaxial strain, resulting in the observed white and green light emissions by the naked eyes. The stimulus excitation, resultant luminescence spectra, and their relations with color properties (color coordinates, color temperature, etc.) of the white light emission have been investigated. The observation is ascribed to the non-central symmetric crystal structure of wurtzite-type ZnS material, inherently producing piezoelectric effect under strain. Energy transitions of the observed white light emissions may originate from the donor-acceptor pairs recombination between AlZn→CuZn, as well as the radiation transition between4T1and6A1of Mn2+ion. Our results show promise in constructing new types of flexible light source for applications in sustainable energy and so on.