Altermatt Lecture:   The PV Principle


2.10:  Cell improvements

Your main contributions to photovoltaics may be that you improve solar cells to make solar electricity cheaper and hence fully competitive to conventional electric power. This goal may be achieved in various ways:

Many non-experts think that improvements in cell efficiency play a key role. The figure below [15] shows that this is not the case: from 1957 to 2003, the cost for producing PV electricity has fallen by a factor of about 1000, while the efficiency of sold cells increased from about 6% to mostly about 16%, which is a factor of only about three.

Still, cell efficiency matters, because high-efficiency cells require a small area per produced power and hence are cheap to assemble in modules and to mount on roof tops. A rule of thumb says that, for competitive power prices, cell efficiency should be at least 12%, otherwise the installation costs dominate.

Cells have become cheaper mainly due to cheaper production techniques. On the one hand, there has been a scale-up of the production volume, which has also lead to a scale-up of the supplier's market for silicon and for fabrication equipment. This has led to the rather steady decline in PV electricity prices, plotted over the cumulative production in the figure below. On the other hand, production costs decreased due to continuous and intense technological developments at all levels: Si production, cell processing, module assembly, mounting, and DC to AC inverters. This has determined how steep the PV electricity costs are declining in the figure below, e.g., the periods of slow innovation in the 1980s and early 1990s.

A key issue in present cell improvements is simplicity in design and production. This may be vital for improving cell efficiency without substantially increasing cost of ownership and production. For example, simplicity keeps the production yield at high levels: you will learn in the lesson "Industrial Si cells" that most of today's cells are fabricated in about 8 processing steps. Still, if every step has a yield of 0.99, the overall yield is 0.998 = 0.92, and the yield decreases with more processing steps.

PV prices vs production

Figure: The cost of PV electricity vs PV production. Figure from [15].


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