The Photon Trap
The TCO layer does two different jobs, and must balance conflicting requirements. As the top electrode of the cell, the TCO is responsible for getting all those freshly minted quanta of electricity out of the cell and into the grid. It must make a low-resistance contact with the next layer lower down, so as not to generate heat and waste power. To do this job, a nice thick slab of copper would be perfect. Unfortunately, the other job of the TCO is to admit light into the cell and keep it there (hence the T in TCO) and copper slabs are inconveniently opaque. A sheet of high-purity glass would let in lots of light, but glass is a very good insulator so the cell wouldn’t produce any power.
Also, the term transparent is misleading. In order to increase the chance of a photon knocking out an electron (and thus generating that tiny quantum of power), the cell should be designed to capture the photon. To achieve this, we create a rough, textured surface to deflect and scatter the incoming light sideways as it passes through the TCO, trapping the photon so it bounces back and forth through the active layers for as long as possible. It would be better if the T in TCO stood for translucent.
The creation of the TCO layer is one of the most critical processes in producing a high-efficiency thin film solar cell. As shown in the micrograph, a quasi-pyramidal texture is being investigated to determine the best trade-off between contact resistance and light capture over a wide range of incoming wavelengths. New TCO films like this can raise the energy harvest of the cell by about 10%, a big return for a relatively small change to the process.
Applied currently offers a glass coating system that deposits TCO films. This application for the company’s glass coaters, which are widely used by the architectural and automotive glass industries, is opening up the solar PV market for our glass customers.
Of course, the TCO isn’t the only part of the cell being worked on to raise efficiency. Put a few of these innovations together and pretty soon you’re looking at some serious extra watts.