Exploring the potential of quantum wells for efficiency enhancement in photovoltaic cells

A quantum-well suparlattice cell, in which In0.13Ga0.86As (4.7 nm) / GaAs0.57P0.43 (3.1 nm) strain-balanced quantum wells are inserted in the intrinsic region of a GaAs pin cell, has been implemented by metalorganic vapor-phase epitaxy (MOVPE) and has exhibited an enhanced short-circuit current density, with an increment of 3.0 mA/cm2 and a minimal drop in open-circuit voltage (0.03 V) compared to a pin cell without the superlattice. The collection efficiency of photocarriers, 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 quantum-well cell with thicker wells and barriers. This carrier collection efficiency is was above 0.95 for the superlattice cell, regardless of a wavelength 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.