Since the early days of terrestrial photovoltaics, many have thought that “first
generation” silicon wafer-based solar cells eventually would be replaced by a
“second generation” of inherently much less material intensive thin-film
technology, probably also involving a different semiconductor. Historically,
cadmium sulphide, amorphous silicon, copper indium diselenide, cadmium
telluride and now thin-film silicon have been regarded as key thin-film candidates.
Since any mature solar cell technology must evolve to the stage where cost is
dominated by that of the constituent material, be it silicon wafers or glass sheet, it
seems that high power output per unit area is the key to the lowest possible future
manufacturing costs. Such an analysis makes it likely that photovoltaics, in its
most mature form, will evolve to a “third generation” of high-efficiency, thin-film
technology. By high-efficiency, what is meant is energy conversion values double
or triple the 15-20% range presently targeted, closer to the thermodynamic limit
upon solar conversion of 93%.
Tandem or stacked cells provide the best known example of how such high
efficiency might be achieved. In this case, conversion efficiency can be increased
merely by adding more cells of different bandgap to a stack, at the expense of
increased complexity and spectral sensitivity. However, as opposed to this “serial”
approach, better-integrated “parallel” approaches are possible that offer similar
efficiency to even a stack involving an infinite number of such tandem cells.
These alternatives will become increasingly feasible with the likely evolution of
materials technology over the decades to 2020. This book discusses a range of
these options systematically as well as paths to practical implementation. By
clearly defining these options and identifying their strengths, weaknesses and
areas where further work is required, their development may be accelerated.