There is no dearth of ideas on new ways to make solar cells, but these are not yet practical. Solar power has a great advantage in the development stage over other technologies such as wind, nuclear, or fusion. New ideas can be explored on a small scale. No large machines or wind turbines have to be constructed. Experimental solar cells can be as small as 1 cm2. This means that new ideas can be developed profitably by small companies, thus shifting the research burden to the commercial sector. Large, government-funded installations are still needed for commercial viability, but not for testing new ideas. These ideas fall into the cate­gory of Generation III solar cells, as shown in Fig. 3.44.

In this graph, the efficiency of solar cells is plotted against their cost per square meter and per peak watt. The three elliptical areas are where Generations I, II, and III lie. Generation I comprises the single-junction silicon cells, costing more than $3.50 per peak watt and achieving efficiencies no higher than 18%. Generation II contains the thin-film and organic cells, which are much cheaper but have low efficiencies. Generation III includes multijunction cells with efficiencies above 40% and new ideas which are still in the thinking stage. The efficiencies of these solar cells can go above the 31% of the theoretical maximum known as the

Shockley-Queisser limit. The limit applies to single-junction cells in unconcen­trated sunlight whose photons produce only one electron each and whose excess energy is lost as heat. Generation III cells go higher by violating these conditions. For instance, concentrating the sunlight can give more than one electron per pho­ton, and new nanomaterials can capture the excess energy as current [16].