Application of Nanostructured Materials for Highly Efficient Flexible Energy Harvesters

Abstract

With the energy crisis and environmental pollution by fossil fuels, the development for clean and renewable energy sources has attracted considerable interest in the past decade. In most cases, it has been focused on the energy resources for large-scale power generation, such as hydraulic power, wind power, and nuclear plants. More recently, in addition to alternatives energy sources for the large scare power generation, energy harvesting from ambient energy sources including solar, thermal and mechanical energy for small-scale mobile application has been actively investigated due to recent progress of power consumption efficiency. In this study, we prepared highly efficient energy harvester using nanostructured materials. Among the variety of energy sources, we focused on the light and mechanical energy and fabricated solar cells and piezoelectric nanogenerators. These energy sources are always available in environment and easy to utilize. Also, power generation characteristics of assembled devices were investigated. First, we fabricated flexible dye-sensitized solar cells (DSSCs) using 0-D TiO2 aggregates. Hierarchically structured TiO2 (HS-TiO2) was prepared on a flexible ITO-PEN (polyethylene naphthalate) substrate via electrospray deposition using a commercially available TiO2 nanocrystalline powder in order to fabricate flexible DSSCs under low-temperature (<150°C) conditions. The cell efficiency increased when using flexible ITO-PEN substrates post-treated by either a mechanical compression treatment or a chemical sintering treatment using titanium tetrabutoxide (TTB). The mechanical compression treatment reduced the surface area and porosity of the HS-TiO2

however, this treatment improved the inter-particle connectivity and physical adhesion between the HS-TiO2 and ITO-PEN substrate, which increased the photocurrent density of the as-pressed HS-TiO2 cells. The electron diffusion coefficients of the as-pressed HS-TiO2 improved upon compression treatment whereas the recombination lifetimes remained unchanged. An additional chemical sintering post-treatment involving TTB was tested for its effects on DSSC efficiency. The freshly coated TiO2 submitted to TTB hydrolysis in water at 100°C yielded an anatase phase. TTB treatment of the HS-TiO2 cell after compression treatment yielded faster electron diffusion, providing an efficiency of 5.57% under 100 mW cm-2, AM 1.5 global illumination. Long-term stability of flexible DSSCs is also important for real application. We examined the influence of dye binding mode on long-term stability of room-temperature fabricated TiO2 photoelectrodes (R-TiO2). A surface OH group-rich R-TiO2 photoelectrode was prepared by electrospray method and was found to exhibit poor long-term stability (34.8% of its initial efficiency after 1000 h at 60°C under illumination of 100 mWcm-2) due to the desorption of adsorbed dye molecules from the R-TiO2 surface. We found that large amounts of N719 dye on the R-TiO2 surface were weakly anchored with one carboxylic acid as a result of rapid adsorption of dye by excess surface OH groups on R-TiO2 surface. The loss of weakly adsorbed N719 from the R-TiO2 surface could be suppressed by the addition of stearic acid (SA) to the dye solution as co-adsorbents during the dye anchoring process (RS-TiO2). The competitive adsorption of SA and N719 on the surface Ti-OH groups slowed the adsorption rate of N719, which decreased the amount of weakly bound N719 present on the RS-TiO2 surface. The RS-TiO2 device exhibited a high resistance to dye desorption and displayed enhanced long-term stability (70.1% of its initial efficiency after 1000 h at 60°C under illumination of 100 mW cm-2). Also, we prepared the co-sensitized TiO2 photoelectrode using mixed dye solution, including highly efficient organic dye, JH-1, and near IR squaraine dye, SQ2, to make panchromatic adsorption of TiO2 photoelectrode. Afterward, the use of mixed dye of JH-1 and SQ2 on HS-TiO2 photoelectrode, the efficiency of 6.31% was achieved for flexible DSSCs, which is higher than pure JH-1 sensitized DSSCs. We found that increased total amount of adsorbed dye on TiO2 surface induce higher photocurrent density, resulted in higher PCE of co-sensitized photoelectrode. Finally, we fabricated flexible piezoelectric nanogenerator using 1-D piezoelectric nanofibers. An electrospun nanofiber-based PbZr0.52Ti0.48 (PZT) textile was used as a flexible piezoelectric nanogenerator, and its performance parameters were investigated. The fiber orientation of electrospun PZT textile was controlled using a multi-pair metal wire or metal mesh. Flexible piezoelectric nanogenerators were assembled by forming a composite of the textile and a polydimethylsiloxane (PDMS) matrix sandwiched between two flexible ITO-PEN substrates. The assembled nanogenerator could generate an electrical output of 1.1 V at 1.4 uA at a thickness of 80 um and an area of 8 cm2 under bending strain. The piezoelectric voltage depended on the thickness of the PZT textile, whereas the piezoelectric current depended on both the thickness and area of the PZT textile. We found that the electrical output of the device was significantly influenced by the orientation of the PZT fiber and the bending direction. The output voltage and current were strain-dependent, whereas the total integrated charge was strain rate-independent. The characteristics of the flexible nanogenerator quantified the device performance.