Effects of in situ annealing on GaInNAs solar cells

GaInNAs solar cells grown by metal-organic, chemical vapor deposition (MOCVD) are known to demonstrate dramatic changes in performance upon annealing. GaInNAs necessarily experiences some in situ annealing when integrated into a multijunction cell, as when the upper four junctions of a six-junction (e.g., AlGaInP/GaInP/AlGaInAs/GaInAs/ GaInNAs/Ge) cell are grown on top of the GaInNAs subcell. Therefore, understanding the changes that occur during these inadvertent, in situ anneals is necessary to design an MOCVD growth process for high-performance six-junction cells. These six-junction cells have the potential for > 50% efficiency under the concentrated terrestrial spectrum and > 40 % under the air mass zero (AM0) spectrum. This paper shows how an in situ anneal at 650°C can cause movement of the junction in the GaInNAs, first improving and then ruining the performance of the cell. Similarly, in situ annealing depends on the annealing temperature, showing, for the conditions studied, optimal performance for an anneal at 675°C. Higher temperatures resulted in improved material quality, but the junction did not move, resulting in inferior performance compared with the samples annealed at lower temperatures. The performance of the best GaInNAs cells is summarized showing background carrier concentrations as low as 2 × 10¹⁵ cm ^-3, depletion widths as wide as ∼ 0.6 μm, and AM0 photocurrents for operation under GaAs approaching 12 mA/cm².