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Why does the band gap matter in a solar cell?

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Study how a series of gears in a waterwheel translates a stream's energy to a millstone — energy conversion Before the Industrial Revolution, power came from three main sources: humans, draft animals, and water.
The ingenuity people used in harnessing waterpower can be seen in this medieval-style mill. The waterwheel is turned by a stream and is connected to a shaft that leads into the building. At the other end of the shaft is a gear. The connection of a series of gears translates the power from the stream to a shaft that drives a millstone, which grinds flour from grain.
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In a solar cell, the band gap is the energy required to excite an electron from the valence band to the conduction band, where it can contribute to an electric current. This property is fundamental to the device's operation and efficiency for several reasons:1

Photon Absorption: Only photons with energy greater than the band gap can excite electrons to the conduction band. A smaller band gap allows the cell to utilize a broader range of the light spectrum, potentially increasing efficiency.1
Recombination Limits: The band gap cannot be arbitrarily small. If it is too narrow, the excited electrons and the holes they leave behind recombine too easily, preventing the maintenance of a sizable current.1
Theoretical Efficiency: The band gap defines the theoretical maximum efficiency of a solar cell. Because of this, scientists often "tune" or engineer the band gap of semiconductor materials to optimize performance for specific applications.21

Ultimately, the band gap represents a critical trade-off between capturing sufficient light energy and preventing the loss of charge carriers through recombination.1