Abstract
ABSTRACT A quantum-well suparlattice cell, in which In 0.13 Ga 0.86 As (4.7 nm) / GaAs 0.57 P 0.43 (3.1 nm) strain-balanced quantum wells are inserted in the intrinsic region of a GaAs pin ce ll, has been implemented by metalorganic vapor-phase epitaxy (MOVPE) and has exhibited an enhanced short-ci rcuit current density, with an increment of 3.0 mA/cm 2 and a minimal drop in open-circuit voltage (0.03 V) compared to a pin cell without the superlattice. The collection efficiency of photo-carriers, which are generated in a cell upon the irradiation of monochromatic light, to an external circuit has been evaluated for both the superlattice cell and a conventional quan tum-well cell with thicker wells and barriers. This carrier collection efficiency is was above 0.95 fo r the superlattice cell, regardless of a wa velength and an external bias, while the value for the quantum-well cell degraded to be below 0.8 at a large forward bias, which evidenced superior carrier transport with the help of tunneling through the thin barriers. With such a fast electron-hole separation in the superlattice, photo-current generation by two-step photon absorption has been observed, using the electron ground state of the superlattice as an intermediate band. Keywords: quantum well solar cell, InGaAs, GaAsP, superlattice, carrier collection efficiency , intermediate-band solar cell 1. INTRODUCTION Multi-junction solar cells have attracted much interest because of their high conversion efficiency under concentrating light, which is promising for space applica tion, low cost-per-kWh system and sola r power plant. One factor which limits the efficiency of III-V/Ge multi-junction solar cells, the most famous multi-junction structure, is the current mismatch among each subcell, specifically, low current density in GaAs junction (middle cell). Extending the absorption edge of a GaAs cell by employing InGaAs layer is a strategy for better current balancing and higher performance in a multi-junction solar cell.Insertion of narrow-gap InGaAs quantum wells (QWs) to GaAs p-i-n solar cells can enhance short-circuit current density (J
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