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Piezoelectric ZnO nanorods grown on a fl exible substrate are combined with the p-type semiconducting polymer PEDOT:PSS to produce a p-n junction device that successfully demonstrates kinetic-to-electrical energy conversion. Both the voltage and current output of the devices are measured to be in the range of 10 mV and 10 μ A cm − 2 . Combining these fi gures for the best device gives a maximum possible power density of 0.4 mW cm − 3 . Systematic testing of the devices is performed showing that the voltage output increases linearly with applied stress, and is reduced signifi cantly by illumination with superband gap light. This provides strong evidence that the voltage output results from piezoelectric effects in the ZnO. The behavior of the devices is explained by considering the time-dependent changes in band structure resulting from the straining of a piezoelectric material within a p-n junction. It is shown that the rate of screening of the depolarisation fi eld determines the power output of a piezoelectric energy harvesting device. This model is consistent with the behavior of a number of previous devices utilising the piezoelectric effect in ZnO.

The optimisation of the interdigitated electrode (IDE) design for active fibre composites was performed using finite element analysis. The effect of the IDE geometry (electrode width and spacing) and electroceramic substrate thickness on the developed strain for bulk PZT substrates was modelled. The modelling results show that the highest strain is generated when the electrode width equals half the substrate thickness and for thin substrates the electrode finger spacing can be reduced to enable lower driving voltages. Approximately 80% of the maximum d 33 strain can be achieved with an electrode separation to substrate thickness ratio greater than 4. The results present simple coherent guidelines for the optimisation of electrode geometry for piezoelectric actuators and active fibre composites.

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Piezoelectric energy harvesting cantilevers provide a simple, compact low cost construction method for energy harvesting from vibrational sources. Beam theory predicts a linear distribution of strain along the length of the beam, but the conversion of this strain to electrical energy is dependent on the coverage of the beam with active material. In this paper, we demonstrate how re-distribution of charge within the piezoelectric leads to losses that can be as high as 25% of the potential generated power. Reducing the area coverage of the piezoelectric is shown to significantly improve cantilever power output, with the optimum coverage being 2/3.