The 2016 paper in Nature Energy read "CdTe solar cells with open-circuit voltage breaking the 1 V barrier."
Not a headline most people would find inspiring.
However, if you happened to be a scientist working on improving the efficiency of cadmium-tellurium based photovoltaic solar cells, it was a significant accomplishment. Certainly the sort which results in a few awards and recognition.
More than 90 per cent of photovoltaic cells are constructed from silicon wafers.
These are not easy to make at the purity level needed to work effectively.
There is a lot of embedded energy involved. Purifying silicon to the level of seven 9s (99.99999 per cent pure) requires clean rooms, scrupulous techniques and a lot of patience. It is also very energy expensive.
By switching to other elements, photovoltaic systems can be generated which should be a lot more carbon friendly and have smaller footprints. A CdTe solar cell should be easier to manufacture but it had a drawback.
The practical limit was thought to be 900 millivolts or 0.9 V.
This would mean connecting over 130 cells in series to generate the 120 volts in our present power grid. That is a lot of cells and a large amount of area if we want to draw the sort of amperage used in a typical household.
Hitting the 1.0 V mark means fewer cells - not many but enough. It also means there is the possibility of pushing the voltage a little higher. At 1.5 volts, it reduces the number of cells to just 80 which is still a lot but much more reasonable than 130-plus.
Of course, there will still need to modifications and refinements to even get close to that sort of voltage. It may not, in fact, be feasible as physical limits and materials science may not be able to modify the material enough.
To make the CdTe solar cells described in the paper, scientists at the Department of Energy's National Renewable Energy Laboratory (NREL) and Washington State University had to grow crystals using a technique called "melt growth."
It allows for precise control over purity and composition.
The researchers mixed the materials inside a clean room and vacuum seal the slurry in a specialized reaction vessel.
The vessel is heated to above 1100 C and cooled very, very slowly from the bottom up at a rate of about 1 millimeter per hour.
The result is a single crystalline structure which can then be sliced in wafers and polished for use in solar cells.
It is energy expensive as the cooling process is time consuming and the temperatures must be maintained in the bulk of the material as the crystal is slowly pulled from the mixture. And there is the whole second step involved in actually fitting the crystal into an appropriate structure to allow for the creation of a solar cell.
On top of this, there is the energy involved in the remaining components of the circuitry. Copper wires and silicon chips all require massive amounts of both energy and water in their making.
There are the costs associated with mining both the cadmium and the tellurium. Neither are particularly abundant and cadmium is well recognized as a toxic heavy metal.
Tellurium has interesting effects on the human body as well.
The net result could well be more efficient and cheaper solar cells but they still have a carbon footprint as well as a physical footprint on the land and water.
It is estimated that solar photovoltaic cells based on silicon cost about 46 g of carbon dioxide equivalents for every kilowatt-hour of useable energy. More if the cell is not used for its full lifetime.
Lowering that cost by 25 per cent would make solar photovoltaics more appealing as an alternative energy supply. However, it would not achieve the 12 g of carbon dioxide equivalents achieved by either wind powered turbines or hydroelectric dams.
On the other hand, a photovoltaic cell beats the 469 g of carbon dioxide equivalents generated by a gas-fired plant or the 230 g generated by a biomass-based facility.
The difficulty with virtually any method for generating electricity is it costs energy in the first place. With solar cells, the manufacturing process is very energy intensive but the maintenance is very light. This affords certain advantages such as ease of use and portability.
On the other hand, photovoltaics only work when the sun is shining, which puts a severe damper on their usefulness. In theory, they could be used to charge up an electric vehicle during the day but then we need to consider all of the embedded energy in putting together the battery system required to store the energy for use at night.
Trying to understand alternative energy production requires deep life cycle analysis as it is interconnected in many different ways.
