III-V Materials for Photovoltaic Applications

Abstract

While there are only two group IV semiconductors, Si and Ge, the III-V material system offers a nearly infinite number of single-crystal semiconducting compounds to choose from. For example, there are nine binary compounds resulting from combining the group III elements Al, Ga, or In with the group V elements P, As, or Sb. From this group, high-performance photovoltaic cells have been made from GaAs, GaSb, and InP. Using ternary III-V compounds, high-performance photovoltaic devices have been made with AlGaAs, GalnAs, GaAsP, and InGaP2. All of the above-cited photovoltaic cells have respectable energy conversion efficiencies of over 15%.A primary strength associated with the III-V semiconductor material system lies therefore in the diversity of materials to choose from. Thus, a single-crystal III-V compound can be found with almost any bandgap ranging from 0.16 eV for InSb to 2.24 eV for GaP (or 3.5 eV for GaN). But not only can one choose a material with a particular bandgap, one can, more importantly, as a result of tetrahedral covalent bonding, also be assured that that material can be grown as a single crystal with either n- and p-type doping, and that junctions can be formed with nearly ideal rectifying characteristics. The implication this holds for solar cells of ideal diode characteristics is the achievement of theoretical limit open-circuit voltages and fill factors. An additional important feature associated with many of the III-V compounds is the direct-gap photon absorption which translates to a short optical absorption distance, with the result that excited-state carriers in a solar cell are created near the junction.