ALGAAS/GAINP HETEROJUNCTION TUNNEL-DIODE FOR CASCADE SOLAR-CELL APPLICATION

A p+-AlGaAs/n+-GaInP heterojunction tunnel diode with band gap E(g) almost-equal-to 1.9 eV was fabricated by the atomic layer epitaxy growth. Doping levels of 1 x 10(20) cm-3 and 5 x 10(19) cm-3 were achieved in the p and n side of the diode using carbon and selenium, respectively. The diode can be used to interconnect the high and low band-gap cells in the AlGaAs/GaAs cascade solar cell structure. For forward current of 20 A/cm2, which is the expected current density at 1000 suns operation, there is only approximately 20 mV voltage drop across the tunnel junction. When annealed at 650 and 750-degrees-C to simulate the growth of the top cell, the diode was still suitable for 1000 suns operation. This is the first reported tunnel diode fabricated in high band-gap material systems that can be used as the connecting junction in the cascade solar cell structure operating at 1000 suns.