ZnO nanostructures – energy harvesting opportunities

We will take a walk through a variety of devices – basic diode, photodetector, nanogenerator and finally enhanced photovoltaic performance - that use ZnO nanostructures. We will see how the common, and simple structure, of a nanostructured diode can be used to enhance the performance of some interesting electronic and optoelectronic systems. Initially the talk will focus on our process developments including deposition of CuSCN as p-type layer and high pH synthesis of ZnO in aqueous environments. These developments show very clearly how the key materials properties are process dependent as we compare diodes with rectifications ranging from 24 to 20,000 for a nominally identical structure.  We will then move to discuss some of the more dramatic exploitations of the piezoelectric effect in ZnO. First I will show how it is possible to use a hybrid inorganic-organic p-n diode to scavenge energy from the environment in the form of vibrations being converted into electrical energy . I will then show how the energy conversion efficiency is limited by the electromagnetic coupling of the material by illustrating a series of results experimentally obtained that support the model of carrier drift and band bending at the interface .  Finally I will show that with careful design selection of a photovoltaic architecture it is possible to enhance the output of a hybrid – TCO:ZnO:P3HT:Au device by 50% from 1.2% ECE to 1.8 ECE using sound at 10kHz and 75 dB. This is associated with a significantly reduced, and measured, carrier recombination rate . Sound of this level is consistent with that found in a typical noisy office or mode of transport. When we test ZnO:PCBM:Au devices we see no significant enhancement in photovoltaic performance. We explain our findings in terms of carrier recombination at the interface of the ZnO due to the oscillating electric field induced in the space charge layer due to the piezoelectric effect of the ZnO. In the PCBM device carrier separation occurs at locations distinct from the ZnO interface and so there is negligible piezoelectric contribution in that design.