Group III-nitride alloys as photovoltaic materials

The direct gap of the In_1-xGa_xN alloy system extends continuously from InN (0.7 eV, in the near IR) to GaN (3.4 eV, in the mid-ultraviolet). This opens the intriguing possibility of using this single ternary alloy system in single or multi-junction (MJ) solar cells. A number of measurements of the intrinsic properties of InN and In-rich In_1-xGa_xN alloys (0 < x < 0.63) are presented and discussed here. To evaluate the suitability of In_1-xGa_xN as a material for space applications, extensive radiation damage testing with electron, proton, and alpha particle radiation has been performed. Using the room temperature photoluminescence intensity as a indirect measure of minority carrier lifetime, it is shown that In_1-xGa_xN retains its optoelectronic properties at radiation damage doses at least 2 orders of magnitude higher than the damage thresholds of the materials (GaAs and GaInP) currently used in high efficiency MJ cells. Results are evaluated in terms of the positions of the valence and conduction band edges with respect to the average energy level of broken-bond defects (Fermi level stabilization energy <i>E_FS</i>). Measurements of the surface electron concentration as a function of <i>x</i> are also discussed in terms of the relative position of <i>E_FS</i>. The main outstanding challenges in the photovoltaic applications of In_1-xGa_xN alloys, which include developing methods to achieve p-type doping and improving the structural quality of heteroepitaxial films, are also discussed.