n/p/n tunnel junction InGaAs Monolithic Interconnected Module (MIM)

The Monolithic Interconnected Module (MIM), originally introduced at the First NREL thermophotovoltaic (TPV) conference, consists of low-bandgap indium gallium arsenide (InGaAs) photovoltaic devices, series interconnected on a common semi-insulating indium phosphide (InP) substrate. An infrared reflector is deposited on the back surface of the substrate to reflect photons, which were not absorbed in the first pass through the structure. The single largest optical loss in the current device occurs in the heavily doped p-type emitter. A new MIM design (pat pend.) has been developed which flips the polarity of the conventional MIM cell (i.e., n/p rather than p/n), eliminating the need for the high conductivity p-type emitter. The p-type base of the cell is connected to the n-type lateral conduction layer through a thin InGaAs tunnel junction. 0.58 eV and 0.74 eV InGaAs devices have demonstrated reflectances above 90% for wavelengths beyond the bandgap (>95% for unprocessed structures). Electrical measurements indicate minimal voltage drops across the tunnel junction (<3 mV/junction under 1200 K-blackbody illumination) and fill factors that are above 70% at current densities $(Jsc)$ above 8 A/cm² for the 0.74 eV devices.