Design constraints of the quantum-dot intermediate band solar cell

The design constraints of the quantum-dot intermediate band solar cell are reviewed. They involve the determination of the size of the dots, their spacing, regularity, doping and the materials themselves for their manufacture. The design constraints derived from the elimination of the photon recycling and the assumption of infinite mobility for describing the ideal operation of this cell are specifically analyzed in this paper. The suppression of the hypothesis of photon recycling, that causes a typical drop in the limiting efficiency from 63.2% to 58.3%, causes the thickness of the solar cell to become a parameter to be optimized. The value of the mobility required to cause an internal voltage drop lower than kT/e is discussed and a link is established with the density of states at the conduction and valence bands. It is also determined that the recombination at the barrier region can cause a dramatic drop in the limiting efficiency of the cell (up to 46.0% for an electron filling factor of the unit cell in the dot array of 52%) if the wave function of the electrons in the dots does not significantly penetrate the barrier region. In general, the optimum gaps of the cell must be also recalculated when a new working hypothesis is introduced. (C) 2002 Elsevier Science B.V. All rights reserved.