Optimal Sunlight Harvesting in Photovoltaics and Photosynthesis

Materials employed to harvest sunlight are commonly recognized to be at a premium when their optical absorption peaks in the visible, extends to the infrared, is panchromatic, and is matched to the solar spectrum. By contrast, natural photosynthetic absorbers such as chlorophylls and carotenoids display absorption spectra with narrow peaks for yet-unknown evolutionary reasons. Beyond such general observations, a rigorous treatment of sunlight harvesting optimization is still lacking. In this work, we provide a quantitative analysis of optimal solar energy harvesting in materials. We derive optimal absorption spectra as a function of absorber thickness, elucidate the concept of solar-matched absorption and its applicability limits, and define a procedure to rank photovoltaic materials for sunlight harvesting. In addition, we suggest a possible explanation for why absorption in plant photosynthetic pigments occurs in narrow energy windows.