Solar and mechanical energy harvesting integrated in a hybrid nanostructure
Solar and mechanical energy are two of the most commonly available power sources for small-scale, portable electronics and wireless sensing networks in remote locations. However, their availability is not normally continuous in time. Hence, their integration can increase the flexibility of nanogenerator systems, so that they can provide power more reliably.1 On the other hand, integration of piezo-electric materials with solar cells can increase their complexity and total volume, as well as possibly lead to deteriorating performance of the two subsystems. At the same time, resistive touch sensors in cell-phone displays can decrease screen brightness and increase their total thickness and power consumption. The challenge is to find materials that extend the available functions of a device without compromising its original performance. In this context, we have achieved integration of a piezo-electric material—for both power generation and touch sensing—with a polymer solar cell.2
An array of zinc oxide (ZnO) nanorods allows harvesting of mechanical energy and may act as a variable-force touch sensor. Use of ZnO is advantageous, because it is a nontoxic piezo-electric semiconductor. The ZnO nanorods are part of the top electrode of the solar cell, thus forming a tightly integrated device (see Figure 1). The solar cell is comprised of a blend of an electron-donor semiconducting polymer and an electron-acceptor fulleride. These are poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. At 4.4%, the P3HT:PCBM polymer blend has one of the highest light-conversion efficiencies of organic cells,3,4 although this is rather lower than that achieved by inorganic semiconductors. However, organic solar cells offer an economic advantage, promising simpler, less expensive, larger-scale, and more environmentally friendly fabrication, as well as the possibility of creating flexible lightweight solar panels. They are thus more suitable for use in small-scale, portable, and remote electronics.
Figure 1(a) shows an outline of the device. The total thickness of the active layer in the hybrid structure was less than 500nm. The top electrode (100nm) consisted of a transparent polyethersulfon substrate coated with indium tin oxide (ITO), onto which the ZnO nanorods (50nm) were sputtered. This was followed by spin coating of the P3HT:PCBM polymer blend (250nm) and thermal evaporation of a molybdenum trioxide electron-blocking layer and a gold electrode (70nm). Under standard illumination conditions—AM 1.5G (airmass 1.5 global)—the device has an open-circuit voltage of 0.546V, the mean short current is 8.65mA/cm2, and the filling factor 22%, yielding a power-conversion efficiency of 1.04%. This is lower than the maximum value reported in the literature for P3HT:PCBM. However, efficiency optimization was not the focus of our work. Improvements could be achieved by solvent annealing—to avoid thermal damage to the rest of the structure5,6—and by employing a textured substrate.7
We found that the piezo-electric layer could act as a variable-force touch sensor, since it generated a 20mV signal under weak touch (4N) and approximately 40mV under stronger force (between 4 and 10N). This amounts to power generation of approximately 0.1W/cm2. Figure 2 describes the operation of the device. Under (i) room-light illumination of approximately 1mW/cm2, the output voltage is 0.2V. This decreases when the light is shaded (ii), followed by first (iii) a positive and subsequently (iv) a negative spike, generated by the piezo-electric layer.
In summary, we have described in detail the operation of a hybrid structure of a polymer solar cell incorporating an array of piezo-electric nanorods that acts as a variable-force touch sensor and allows harvesting of mechanical energy. This structure could be integrated with an energy buffer to allow harvesting of solar and mechanical energy for dependable, lightweight powering of small-scale electronics. Our research is now progressing toward substitution of the ITO electrode with graphene,8 which will allow a flexible geometry that would increase the ruggedness and range of potential applications.
Sang-Woo Kim is an associate professor. He received his PhD from Kyoto University (Japan). His recent research interests include nanomaterial-based piezo-electric energy harvesters, organic solar cells, graphene electronics, and nanostructured light-emitting devices.
Massimo Barbagallo received his PhD from the University of Cambridge (UK). He is currently a postdoctoral researcher. One of the subjects of his recent research is graphene-based electronics.
