The general rule when developing a new kind of solar technology is to expect progress to be slow. Take silicon solar cells, the most ubiquitous and recognizable form of photovoltaic generations today. When silicon panels were first built in the early 1950s, they could only turn about 6 percent of the light that hit them into electricity. More than 30 years later, that number had inched up to 20 percent, and today—30 years after that—they regularly perform in the mid 20s.

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