Inverted metamorphic InGaAsP/InGaAs dual-junction solar cells towards full solar spectrum harvesting

Abstract

An InGaAsP (1.04 eV)/InGaAs (0.54 eV) dual-junction solar cell, monolithically grown in an inverted configuration on an InP substrate, has been demonstrated. Five metamorphic compositionally graded buffers of InAs_xP_1−x are used to transition from the InP lattice constant to that of the In_0.74Ga_0.26As bottom cell. The InAs_0.45P_0.55 buffer, lattice-matched to In_0.74Ga_0.26As, has a surface crosshatch pattern with an rms roughness of 2.1 and 4.7 nm along the in-plane [110] and [10] directions, respectively, due to different lateral growth rates under high As pressure. Cross-section observations using transmission electron microscopy indicate a major reduction in the density of misfit and threading dislocations across the metamorphic InAs_xP_1−x structure. The formation of crystalline defects in the n⁺-InAsP buffers induces a strain relaxation of ∼95% in the bottom cell with a relaxed lattice constant of 5.9563 Å. The reverse saturation current and ideality factor of the bottom cell are increased because of enhanced nonradiative recombination at threading dislocations with a density of 2 × 10⁶ cm⁻². Under AM 1.5G illumination, the dual-junction solar cell with extended absorption wavelengths up to 2400 nm has a photovoltaic conversion efficiency of 5.49% with an open-circuit voltage of 0.70 V, a short-circuit current density of 9.86 mA cm⁻², and a fill factor of 80.12%.