In solar cells, light "knocks" electrons out of a semiconductor, leaving behind positively charged "holes." These two charge carriers are separated from each other and can be collected as current.

Tandem cells were developed to better exploit the entire spectrum of sunlight and increase solar cell efficiency.

Tandem cells are made of two different semiconductors that absorb different wavelengths of light.

Primary contenders for use in this technology are a combination of silicon, which absorbs mostly red and near-infrared light, and perovskite, which very efficiently uses visible light.

Monolithic tandem cells are made by coating a support with the two types of semiconductor, one on top of the other.

Source: https://www.sciencedaily.com