Quantum-Dot Intermediate-band Solar Cell used as Bottom Cell

An attempt to apply quantum-dot (QD) intermediate band solar cell (IBSC) as bottom cell in 4-terminal tandem solar cell instead of a conventional bulk bottom cell is presented. Considering the inferior lattice qualities in QD cells, we introduce a parameter η
_IC
= 1 ~ 0.0001 to represent slow or fast conduction-band-to-dots non-radiative relaxation rate respectively corresponding to strong or weak phonon-bottleneck effects, aiming to develop a more general IBSC model. Our results exhibit a well-designed QD solar cell can more sufficiently utilize the below-1.41eV photons passing through the GaAs top cell, which would be a better candidate as bottom cell. Compared with the highest efficiency (11.15% absolute) achieved by the best bulk bottom cell (E
_g
= 0.93eV), QD bottom cell with E
_g
= 1.74eV and E
₁
= 1.04eV will raise an over 3% absolute of efficiency boost, when phonon bottleneck effects are running well (η
_IC
≥0.1). If use a GaAs-host QD bottom cell with E
₁
= 0.98eV, 1.5% absolute of efficiency add-on can still be achieved, which attributes to a significant voltage boost by the separation of quasi-Fermi levels between IB and CB. Even for those QD bottom cells with inferior lattice qualities (η
_IC
<; <; 1), where the phonon-bottleneck effect is broken, their efficiencies degenerate into the radiative limit of bulk bottom cell whose bandgap is equal to E
₁
.