Cascade Solar Cells
Most modern solar cells have a pn-transition. In this element of free charge carriers are created only those photons whose energy are greater than or equal to the width of the gap. In other words, the photoelectric response element Unijunction restricted part of the solar spectrum, whose energy above the band gap, and lower energy photons are not used. To overcome this limitation enables multilayer structure of two or more solar cells with different band gap.
Such elements are called multijunction, cascade or tandem. Because they operate with a much greater part of the solar spectrum, the photoelectric conversion efficiency is higher. In a typical multijunction solar cell photovoltaic cells are isolated each other in such a way that sunlight falls on the first item with the highest band gap, while photons are absorbed with the utmost energy. Missed the upper layer of the photons penetrate into the next element with a smaller band gap, etc. The main direction of research in the field of cascade elements associated with the use of gallium arsenide as one or several components. The conversion efficiency of such solar cells reaches 35%.
In addition to the cascade elements are widely used amorphous silicon based alloys (a-Si1-xCx: H, a-Si1-xGex: H), and CuInSe2. On Fig.15 shows the cascade battery in which the upper part of the structure is based on the GaInP c n-AlInP as a window, followed by a tunnel diode on GaAs for the passage of vehicles between the elements and the lower part of the GaAs. Very promising cascading battery, consisting of three elements with different band gap. Upper layer that absorbs shorter wavelengths of the solar spectrum, formed from an alloy based on a-Si: H with the width of optical gap of 1.8 eV. For the middle element as the i-type layer of the alloy used in a-SiGe: H with germanium content ~ 10-15%. The width of the optical gap of this layer (1.6 eV) is ideal for the absorption of the green region of the solar spectrum. The lower part of the solar cells absorbs wavelengths of the spectrum, for this using i-layer a-SiGe: H with germanium concentration of 40-50%. Unabsorbed light is reflected from the rear contact on the basis of Ag / ZnO. All three elements of the cascade solar cell linked heavily doped layers, forming a tunneling between neighboring elements.