Measured efficiency of a ZnO nanostructured diode piezoelectric energy harvesting device

We used controlled bending of a ZnO/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) diode at known strain rates to measure the mechanical-to-electrical energy conversion efficiency. The mechanical energy input into the nanostructured diode was measured as 330 ± 2 nJ cm ^-2. The electrical energy output was calculated by integrating the product of the short-circuit current and open-circuit voltage over time. This gives a measured external efficiency of the device at a bending rate of 500 mm/min of 0.0067. The efficiency increased exponentially with bending rate, though this increase must slow as the mechanical coupling efficiency is approached, which gives a maximum possible efficiency of 23 for ZnO.