Symko-Davies was referring to the Shockley-Queisser limit, which postulates that the efficiency of silicon solar cells can't exceed 29%; that is, no more than 29% of the photons that hit the cell can be converted into electricity. Modern monocrystalline solar cells don't achieve much higher than 22% conversion efficiency due to practical considerations such as reflection off the cell and light blockage from the thin wires on its surface. That's why analysts are enthusiastic about the TetraSun cell, which comes in at 21% efficiency even as copper replaces silver to lower the cost.
TetraSun had a unique idea, but NREL's measurements and characterization capabilities made it practical. "As the margins go down with silicon, the cost of every component becomes significant, especially when you're talking about square miles of this material," said NREL Principal Scientist Mowafak Al-Jassim. "We're trying to make enough of these solar panels to generate gigawatts of power. That's a lot of silver. We needed to replace silver with an equally good conductor, but one that was much cheaper."
Detective Work: Finding Defects, Switching Recipes
Copper is a good conductor and connector, but unlike silver, copper doesn't like to stay where it's put. Researchers had to find a way to control the diffusion of the copper so it wouldn't shunt off and short out the cells and modules. Al-Jassim's role was to develop the means to characterize the new contacting scheme that uses copper. He turned to scanning capacitance microscopy to investigate and optimize the electrical properties of the contacts.
As NREL and TetraSun perfected the technique, the partnership between the national lab and the private company was akin to long-distance chess, with e-mails and packages traveling back and forth. TetraSun would send a sample, and NREL would examine the uniformity and continuity of the copper on the device.
|Contact: David Glickson|
DOE/National Renewable Energy Laboratory