A Flexible Nanocrystal Photovoltaic Ultraviolet Photodetector on a Plant Membrane

and increases the light absorption of the active material, which improves the overall device performance. Furthermore, paper is able to act as a good interspacing layer to prevent short-circuit problem commonly encountered in fabricating vertical devices with NCs on fl exible substrates. People have long used paper as a dielectric layer for energy storage devices [ 29,30 ] and fi eld effect transistors. [ 25,31 ] Based on these rationales, we have fabricated CdSe quantum dot photodetectors on tracing papers and achieved consistent results, [ 32 ] yet the large thickness of conventional papers deteriorates the carrier transportation and reduces the optical transparency as well. The high density of fi bers in a paper also reduces the concentration of quantum dots that can be incorporated into the structure, which further reduces the device performance. On the other hand, Mother Nature has supplied us with a variety of thin and porous cellulose structures with good transparency in the form of plant membranes, such as reed inner fi brous structures or leaf-structuring membranes. In this communication, UV photodetectors are fabricated, as far as we know, for the fi rst time on a reed plant membrane ( Figure 1 a). The material is commercially available as the vibration membrane for Chinese bamboo fl utes. At a thickness of ≈5 μm when compressed, the membrane is signifi cantly thinner than most of the commercial papers (>30 μm) and offers much better transparency. We designed a vertical structure for the device that consists of a ZnO NC-embedded reed membrane sandwiched between gold and aluminum electrodes, as shown in Figure 1 b. Though lateral photoconductor structure is typically of interest due to its simplicity and potentially high optical gain, ZnO NC-based fl exible UV photoconductors in general suffer from slow response [ 15–18,33–35 ] due to the large number of trapping states and slow carrier mobilities compared to bulk semiconductor material. By using the cellulose structure as an interspacing layer with the active material embedded, a Schottky junction structure can be readily achieved between ZnO and Au as shown in Figure 1 c, which improves the response speed. While thin-fi lm Au and indium tin oxide (ITO) can both serve as transparent electrodes and have high work functions that can result in Schottky junctions interfacing with ZnO, ITO becomes opaque at UV region and it also has poor fl exibility due to the ceramic-like crystal structure. Given these plus further consideration in fabrication simplicity, we chose Au as the transparent electrode for our device despite that ITO is currently the most widely used transparent electrode material. Furthermore, Au also shows good adhesion on the cellulose A Flexible Nanocrystal Photovoltaic Ultraviolet Photodetector on a Plant Membrane