Virginia Tech is developing both organic and inorganic photovoltaics. Dr Abhiman Hande of Texas Micropower reported on integrated energy harvesting involving deposited sol-gel piezoelectrics on both oxide and metal films on silicon wafers and amorphous silicon photovoltaics. Thin film MEMS sensors incorporating piezoelectric harvesting film and flexible film substrates are of interest as well.
Dr Charlie Green of Powercast impressed with a presentation showing beamed RF harvesting radically improving life of wireless sensor networks. Burkhard Habbe described Micropelt's thin layer thermogenerators made by RF sputtering. These amorphous structures give a device only 35 micrometers thick where one thermal watt converts to 2mW of electricity. Target price is Euros 1.5/mm2.
Professor Scott Huxtable of Virginia Tech pointed out that traditional thermoelectric theory predicts that miniaturisation can be achieved with thermogenerators without loss of performance (figure of merit X temperature difference). However, experimental results with miniaturised thermoelectrics do not comply with this theory. He therefore defines an effective figure of merit that takes account of interfaces and parasitic conductances and this gives a more appropriate scaleability.
Entrance and exit conductances are important. Optimum miniaturised device performance can be achieved with an element length 10-200 micrometers. Experimentation is being carried out on devices made with nanoparticles, nanotubes and pulverise, press, sinter procedures.
Gurpreet Singh of Virginia Tech reported on nanotechnology solutions to sensors and power generation. He employs one dimensional nanostructures.
Thermoreflectance is used for the tricky thermal conductivity measurement. Romny Scientific is involved in this ongoing research.
Virginia Polytechnic CIMSS
Mohammad Amin Karami of Virginia Polytechnic CIMSS noted that miniaturisation to scale of piezoelectric cantilevers for energy harvesting is also misguided but MEMS versions can be achieved and optimised. He observed that, although capacitive and electrodynamic harvesting of vibration exists, piezoelectric harvesting receives by far the most attention these days.
We note that this is despite one of the most widely used vibration harvesters being from Perpetuum in the UK which uses electrodynamics having found piezoelectric vibration harvesting something of a dead end. In his theoretical study, Karami found good correlation with experimental results. Several design recommendations resulted including use of segmented electrodes with appropriate circuitry and the observation that optimum electrical load is extremely sensitive to both the short circuit and open circuit resonance frequency.
Piezoelectric constants of bulk materials are not a sufficient guide to their suitability for use in a given miniature piezoelectric harvesting device.
University of Texas
Piezoelectric thin films-based energy harvesting was the topic covered by Professor Bruce Gnade, Vice President of Research at the University of Texas, Dallas, Erik Jonsson School of Engineering and Computer Science. In the real world, he finds that a few tens of hertz is typically encountered vibration, so he has sought to harvest this. One of his cantilevers involved sol-gel deposited doped PZT 0.5 microns TiO2 0.05 microns SiO2 0.5 microns and he also investigated combined organic-inorganic piezoelectric.
A lead titanate layer assists nucleation during production, there is a zirconium dioxide interposer also deposited by sol-gel and wet etching is used so this is, as yet, a long way from printing, though many very thin films are involved. Good dielectric, ferroelectric and piezoelectric properties are achieved and the work is ongoing.
Professor Stephen Foulger of Clemson University discussed how his colloidal science might help the printed organic electronics industry, which he described as mainly concentrating on OLEDs at present but moving out into many other things. His original work involved colloidal PEDOT organic as an alternative to silver in RFID. Then they used colloids to make an OLED.
His so called hybrid particles used are 59nm across. Electrostatic assembly can be used to prevent particles touching, thus changing color. Long range ordering is being attempted and there has been work on electrostriction as a means of changing color in a device. He believes that all this will be relevant to a printed photovoltaic structure but that work has yet to commence.
In the audience were several users and potential users and they were far from being hooked on any one technology. The debate continues.