1-eV solar cells with GaInNAs active layer

We demonstrate working prototypes of a GaInNAs-based solar cell lattice-matched to GaAs with photoresponse down to 1 eV. This device is intended for use as the third junction of future-generation ultrahigh-efficiency three- and four-junction devices. Under the AM1.5 direct spectrum with all the light higher in energy than the GaAs band gap filtered out, the prototypes have open-circuit voltages ranging from 0.35 to 0.44 V, short-circuit currents of 1.8 mA/cm(2), and fill factors from 61% to 66%. The short-circuit currents are of principal concern: the internal quantum efficiencies rise only to about 0.2. We discuss the short diffusion lengths which are the reason for this low photocurrent. As a partial workaround for the poor diffusion lengths, we demonstrate a depletion-width-enhanced variation of one of the prototype devices that trades off decreased voltage for increased photocurrent, with a short-circuit current of 6.5 mA/cm(2) and an open-circuit voltage of 0.29 V. (C) 1998 Elsevier Science B.V. All rights reserved.