Nanostructured p-n junctions for kinetic-to-electrical energy conversion

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.