Comparative study of dilute nitride and bismide sub-junctions for tandem solar cells

Theoretical models show that tandem solar cells can reach efficiency of more than 50 %. To achieve efficiencies closer to the projected outputs in a quad tandem solar cell, a 1.0 eV sub-junction with good optical and electrical properties is sought. The observed bandgap reduction when small amounts of Nitrogen or Bismuth is incorporated into GaAs makes these alloys (dilute nitrides and bismides) possible candidates for the tandem solar cells. In this work, the performance of dilute nitride and bismide solar cells of different designs which include p-i-n bulk and p-i-n multiple quantum wells (MQWs), and n-i-p-i structures are studied comparatively. Performance in this context refers to: the magnitude and quality of the dark current generated by the devices, the spectral response of the solar cells especially in the dilute nitride and bismide absorption regions, and efficiency responsivity to temperature and to elevated radiation. To achieve the latter objectives, dark and under the AM1.5G and spectral response measurements were performed on the solar cells. The results reveal that, in the dilute bulk or MQWs p-i-n and n-i-p-i solar the total dark current is nearly the dark current generated by dilute nitride or bismide material alone. The applicability of Sah-Noyce-Shockley model to bulk p-i-n dilute nitride solar cells dark current is demonstrated. The dilute nitride solar cells exhibit temperature coefficients of efficiency that are lower than that of the conventional GaAs p-n junction solar cell. MQWs solar cells show lowest sensitivity to temperature changes. The dilute nitride and bismide materials show similar diode characteristics with dark currents that are dominated by non-radiative recombination mechanisms.