Theoretical possibilities of InxGa1−xN tandem PV structures

Abstract

We designed a model of In x Ga 1− x N tandem structure made of N successive p–n junctions going from two junctions for the less sophisticated structure to six junctions for the most sophisticated. We simulated the photocurrent density and the open-circuit voltage of each structure under AM 1.5 illumination in goal to optimize the number of successive junctions forming one structure. For each value of N, we assumed that each junction absorbs the photons that are not absorbed by the preceding one. From the repartition of photons in the solar spectrum and starting from the energy gap of GaN, we fixed the gap of each junction that gives the same amount of photocurrent density in the structure. Then we calculated the current density accurately and optimized the thicknesses of p and n layers of each junction to make it give the same output current density. The evaluation of n i : the intrinsic concentration permitted to calculate the saturation current density and the open-circuit voltage of each junction. Assuming an overall fill factor of 80%, we divided the output peak power by the incident solar power and obtained the efficiency of each structure. The numerical values for In x Ga 1− x N were taken from the relevant literature. The calculated efficiency goes from 27.49% for the two-junction tandem structure to 40.35% for a six-junction structure. The six-junction In x Ga 1− x N tandem structure has an open-circuit voltage of about 5.34 V and a short circuit current density of 9.1 mA/cm 2 .