High efficiency III–V solar cells

Abstract

Solar cells based on single junction or multijunction architectures with compound group III–V semiconductor absorbers have very high efficiencies. A review of spectroscopic ellipsometry characterization of component III–V semiconductors (III: Al, Ga, In; V: As, P) in the near infrared to ultraviolet range is provided. Variations in complex dielectric function spectra over the near infrared to ultraviolet have been tracked as functions of composition in ternary and quaternary alloys, compressive or tensile stress reflected in blue-shifting or red-shifting of critical point features, and limitations in the mean free path of carriers detected in critical point broadening. Extensions of ellipsometry measurements to longer wavelengths show sensitivity to free carrier absorption and infrared active phonon modes related to chemical bonding and lattice vibrations. When free carrier absorption is detected, the Drude model can be applied to deduce some electrical transport properties (resistivity, carrier mean scattering time). Measurements collected as functions of applied magnetic field, noted as the optical Hall effect, can yield carrier concentration, mobility, and effective mass. Surface oxidation effects are noted and pathways to either eliminating this contribution to measured ellipsometric spectra or incorporating it into the data analysis procedure are discussed. In situ real time spectroscopic ellipsometry is reviewed for processing monitoring and control. Future outlooks include extension of complex dielectric function databases to incorporate composition, stress, and defects simultaneously as well as accounting for systematic variations in phonon modes and free carrier absorption (due to doping type and concentrations). The challenge for future research is to successfully characterize complete III–V solar cells by spectroscopic ellipsometry in a manner similar to that already done for thin film photovoltaics based on polycrystalline or otherwise disordered materials.