What is it with photovoltaic (PV) solar power?

What is it with photovoltaic (PV*) solar power? When Bell Labs reported the first silicon photovoltaic cell in 1954, the New York Times were quick to broadcast the imminent end of our dependence on fossil fuels – the energy revolution was fast approaching!

The original Bell Labs cell reached 6%. Five decades later, the silicon solar cell recently passed the milestone of 25% efficiency, under the guidance of Professor Martin Green right here at the University of NSW. This approaches the 29% theoretical limit for these "First Generation" cells – the same ones you now find on your roof, albeit operating at closer to 17%. Meanwhile, less than three months earlier, the National Renewable Energy Laboratory (NREL) in Colorado set a world efficiency record of 40.8% using Multiple-Junction technology.

And yet we still wait for the revolution. There are, however, some encouraging signs. Demand is growing at greater than 20% annually. This graph from solarbuzz.com, using International Energy Agency data, shows the exponential growth in solar energy output. However, to continue growing at such a rate in the long term – once the solar market is large enough to provide serious competition for general market share, technological breakthroughs need to be made. So what is on the horizon?

The silicon solar cell charge is perhaps currently lead by SunPower, reporting an 85% revenue growth from 2007-8, and a forecast 2009 revenue of US$1.6 - $2 billion. Reflecting the maturity of first-generation silicon solar cells, their competitive advantage has come through improved module design (how the cells are stacked together) and innovative installation systems, that minimize efficiency losses between single cells and the final rooftop modules.

The "Second-Generation" is hot on their heels, however. First Solar and Nanosolar especially, are making waves with "thin-film" cells that are less efficient – at just over 10%, compared with 15-20% for silicon, but with much faster and cheaper manufacturing. They are thus pushing the magic US$1 per watt barrier – the price required to compete directly with coal and gas, even without them paying for emissions (and other forms of environmental damage), and are struggling for production to keep pace with orders.

Thus we have "high efficiency – high cost" (1st Gen) versus "low efficiency – low cost" (2nd Gen). The quest now begins for a "high efficiency – low cost" solar cell.
Martin Green and his counterpart Art Nozick at the NREL touched gloves in a Sydney conference last year, announcing competing visions for what they term "Third Generation" solar cells.

The theoretical 29% limit arises from the mix of infrared (IR), coloured and ultraviolet (UV) light that makes up the sunlight hitting the earth. UV and blue light carry more energy; Red light and IR carry less. Silicon solar cells are designed to collect light at one characteristic energy, of about 1.2 volts. Photons (light particles) with 1.2V of energy or greater can be absorbed by electrons in the silicon and converted to electricity, whereas those with less energy pass straight through.

The 1.2V is a trade-off between maximising this voltage, and minimizing the number of photons that are allowed through and lost. (The 40.8% world record cell was produced by stacking three 2nd-Gen cells, of different characteristic energies, one on top of the other – an elegant approach but unfortunately quite expensive to engineer.)

Green is now looking for ways to collect so-called "hot carriers" – electrons that have absorbed high energy (blue and UV) light, before they lose that energy (in the form of heat) and return to the characteristic energy of the cell.

Nozick on the other hand, is building on recent experiments that have found that nano-sized quantum dot particles can absorb a high energy photon in such a way as to give half its energy to each of two electrons – an alternate way of conserving the excess energy these photons.

These are ambitious projects. But both Green and Nozick have assembled multi-laboratory teams aiming thus to surpass the 29% limit, while delivering photovoltaic power at 20-50 cents per watt, and just maybe delivering, if perhaps not golden handshakes, a few sleepless nights to coal executives.

* Photovoltaic (PV) refers to the direct conversion of light (photo) to electricity (voltaic), i.e. solar cells. This is not to be confused with solar thermal power, where concentrated sunlight is used eg,, to boil water to drive a turbine.

Change 2 contributor Dr Miles Page is an Australian scientist who has been working at the international coalface of the emerging Energy Revolution. After receiving his PhD from Sydney University, Dr Page held senior research positions with the Atomic Energy Commission in Paris and the Max Planck Institute in Potsdam. He has spent the past 3 years in Israel researching Thin Film Solar Cells at the Weizmann Institute of Science and developing alternative Fuel Cells.