What a difference 34 years can make. When the Carter Administration installed solar panels at the White House back in 1979, photovoltaic cells were space age technology that most households could not afford, aside from the rare DIY-er. Now the price of solar power has been sinking like a stone, thanks partly to the introduction of low cost materials and inexpensive thin film fabrication methods. Switzerland’s Empa, the Swiss Federal Laboratories for Materials Science and Technology, has just come out with a low cost thin film solar cell breakthrough that demonstrates both in the form of a new high efficiency copper-doped cadmium telluride (CdTe) solar cell.
Low Cost Materials For High Efficiency Solar Cells
The new Empa solar cell boasts an efficiency of 11.5 percent, which might not sound like a big deal compared to last year’s announcement of a 44 percent efficiency mark by the company Solar Junction, but we’re talking about two distinctive technologies. The key takeaway is that today there are multiple paths to affordable solar cells. One of them is finding the most efficient way to collect and convert solar energy, another is finding the cheapest way to do it, and a third way is to find a balance between the two.
For its solar cell, the Empa team has set a near-term goal of achieving 15 percent efficiency and they’re looking at the potential for 20 percent or even greater, so the 11.5 percent rate is just the beginning.
They started with CdTe, which is already the least expensive of its kind in terms of production methods, and they focused on one obstacle to lowering the cost even farther. The problem with CdTe is that conventional production methods involve “growing” cells on a rigid glass sheet, and using an expensive transparent foil to enable sunlight to pass onto the CdTe layer.
The solution is to flip the cell around and have sunlight come in through the CdTe side, which opens up a whole new field of possibilities for flexible, low cost materials, including metal foil.
However, that left another obstacle. Where the record for CdTe solar cells on glass has reached 19.6 percent in the lab, the previous high for metal foil was less than eight percent.
The answer to that particular problem is to tweak or “dope” the semiconductor layer with a relatively cheap metal like copper, but that opens up another obstacle. Ayodha Nath Tiwari, head of Empa’s laboratory for Thin Films and Photovoltaics, explains:
“People have tried to dope CdTe cells in substrate configuration before but failed time and again.”
The basic problem is that CdTe is “notoriously hard to dope,” mainly because too much extra metal is almost as bad as adding too little, but it seems that nanoscale advances in lab technology saved the day. The team achieved precise control over the amount of copper added to the CdTe layer by using a high-vacuum evaporation process followed by a penetrative heat process, which they fine tuned to atomic level proportions.
The result was a monolayer of copper atoms, which enabled the rate of 11.5 efficiency on metal foils. When the team compared the same technique to CdTe on glass, their best value was a comparable 13.6 percent.
Another Path To Low Cost Solar Power
That brings us to yet another pathway to mainstreaming solar power. While glass-based solar cells have higher efficiency, the flexibility of thin film solar cells opens up a whole new range of applications, enabling the introduction of solar power where conventional glass panels are unfeasible.
That means, for example, integrating transparent thin film solar cells into window glass, so who knows, maybe some day in addition to solar panels at the White House you’ll have solar windows for every office in the U.S. House of Representatives and the Senate, too.
With that in mind, keep your eye on the transparent thin film solar cell under development by the company New Energy Technologies in partnership with the National Renewable Energy Laboratory (wait for it…hey, we built this!).