Question

What is bottom cell?
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Answer 1

Answer:

The deficiency of excess current in the bottom cell of the lattice-matched triple-junction solar cell can be partly reduced by implementing quantum wells (QWs) into the middle cell. These can be realized by thin alternating layers of GaAsyP1−y and GaxIn1−xAs (Fig. 10A) [68]. QWs with bandgap energy lower than 1.4 eV extend the absorption of the middle cell toward longer wavelengths [69,70]. Thus the current of the middle cell is increased at the expense of a reduction of the excess current in the Ge bottom cell. The lowest transition energy in the QWs and the number of wells determine the current density, which is achievable for the middle cell. However, the open-circuit voltage of QW solar cells is lower than for a cell structure without wells [69]. This drawback can be compensated by the increased current density due to the QWs. Theoretical calculations showed that an overall gain in energy harvesting efficiency between 3%–9% relative to the lattice-matched triple-junction solar cell is possible [33]. In addition, QWs are seen as a possibility to tune solar cells for specific spectral conditions [34].

Answer 2

Explanation:

The minimum bandgap for the bottom cell is used to maximize the power density output for lattice matched configuration the second device consists of a lattice matched. 0.74 V top cell and a lattice mismatch bottom cell with a bandgap again chosen to allow current matching