Inorganic Photovoltaic Solar Cells: Silicon and Beyond

(c) (d) Photovoltaic solar cells tap into sunlight—the most abundant renewable energy source on the Earth—for electricity. Our planet receives ~1.2×1017 W of solar power, while the rate of current worldwide energy consumption is ~10,000 times smaller at ~1.3×1013 W.1 This means that the Earth receives more solar energy in an hour than the total energy it consumes in an entire year. Solar energy alone has the capacity to meet all the planet’s energy needs for the foreseeable future. No other renewable energy sources have such a capacity, so they can only serve as auxiliary energy sources in our future energy mix. Other renewable energy sources include wind, hydroelectric energy, biomass, ocean currents, tides and waves, and geothermal energy. It is interesting that many of the other renewable energy sources originate from solar energy, except perhaps ocean tides and geothermal energy. The land requirement for solar cells is minimal. Covering 0.16% of the Earth’s surface with 10% efficient cells would provide ~2×1013 W of electricity, more than the current total energy demand of the planet.1 In sharp contrast to the huge amount of solar energy we receive, solar cells account for only 0.014% of the electricity the U.S. consumed in 2005, according to the U.S. Department of Energy.2 Worldwide, the contribution of solar electricity is higher due to many installed solar modules in Europe and Japan, but still an insignificant number. If by 2050 solar cells contribute to just 10% of the electricity we consume, the U.S. solar cell industry will have to grow ~1,000-fold in size, assuming that our demand for electricity will double by 2050. Both the potential and gap in solar energy utilization by solar cells are enormous. It should be noted that solar cells are just one of the several approaches for solar energy utilization. They convert solar energy into one of the most popular forms of energy in our life: electricity. The other popular form of energy in our life is gasoline as a fuel. Solar-to-chemical conversion through artificial photosynthesis is being developed to produce alternative fuels. Solar energy can also be converted into thermal energy by solar water heaters and into concentrated thermal energy for electricity generation.