Theoretical and experimental evaluation of a high efficiency graded band-gap n/p AlxGa1-xAs-GaAs solar cell

Theoretical and experimental analyses show that the higher energy spectral response of an n/p AlxGa1-xAs-GaAs graded band-gap solar cell is substantially greater than that of a similar n/p AlxGa1-xAs-GaAs heteroface cell, particularly for wavelengths below 0.5 µm. The theoretical analysis includes all practical energy loss mechanisms such as photon reflection; surface, bulk, and junction recombination currents; and series resistance. It predicts air mass zero (AMO) efficiencies of 16.7% for standard transport parameters and a surface recombination velocity (S) of cm/sec and 17.7% for S = cm/sec. Small, preliminary n/p graded band-gap structures have been fabricated using a new LPE, melt mixing growth technique to obtain the graded composition AlxGa1-xAs layer approximately 0.5 µm thick. Comparison of the measured spectral responses of the graded band-gap cell with similar n/p heteroface structures (x = 0.85) demonstrates substantial improvement provided by the graded cell for wavelengths between 0.60 µm and 0.35 µm . Preliminary electrical measurements performed in a calibrated AMO solar simulator on structures with a Si3N4anti-reflection film indicate maximum power conversion efficiency of 13.6% (for zero contact area). This efficiency corresponds to measured short-circuit current density, JSC, of 27.7 ma/cm², open-circuit voltage, VOC, of 0.88 volts, and fill factor, FF, of 0.76. Individual values of JSCVOC, and FF as high as 28.2 ma/cm², 0.95 volts, and 0.78 have been measured, each for a separate cell.