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We discuss lattice-mismatched (LMM) approaches utilizing compositionally step-graded layers and buffer layers that yield Ill-V photovoltaic devices with performance parameters equaling those of similar latticematched (LM) devices. Our progress in developing highperformance. LMM, InP-based GalnAshAsP materials and devices for thermophotovoltaic (TPV) energy conversion is highlighted. A novel, monolithic. multibandgap, tandem device for solar PV (SPV) conversion involving LMM materials .is also presented along with promising preliminary performance results.

Monolit hic Interconnected Modules (MIM) are under dev e lopment for thermophotovoltaic (TPV) energy conversion apphcations. MIM de vices are typifi ed by series-interconnected photovoltaic cells on a common , semi-insul ati ng su bstrate and generally include rear-surface infrared (lR) reflectors. The MIM arc hitecture is being implemented in InGaAsSb materials without semi-insulating substrates through the development of alternative isolation methodologies. Moti vations for developing the MIM structure include: reduced resistive losses , higher output power density than for systems utilizing fro nt surface spectral control, improved thermal coupling and ultimately higher system efficiency. Numerous design and material changes have been investigated since the introduction of the MIM concept in 1994. These developments as well as the current design strategies are addressed.

Advanced thermophotovoltaic (TPV) modules capable of producing > 0.3 W/cm 2 at an efficiency > 22% while operating at a converter radiator and module temperature of 1228 K and 325 K, respectively, have been made. These advanced TPV modules are projected to produce > 0.9 W/cm 2 at an efficiency > 24% while operating at a converter radiator and module temperature of 1373 K and 325 K, respectively. Radioisotope and nuclear (fission) powered space systems utilizing these advanced TPV modules have been evaluated. For a 100 W e radioisotope TPV system, systems utilizing as low as 2 general purpose heat source (GPHS) units are feasible, where the specific power for the 2 and 3 GPHS unit systems operating in a 200 K environment is as large as ~ 16 W e /kg and ~ 14 W e /kg, respectively. For a 100 kW e nuclear powered (as was entertained for the thermoelectric SP-100 program) TPV system, the minimum system radiator area and mass is ~ 640 m 2 and ~ 1150 kg, respectively, for a converter radiator, system radiator and environment temperature of 1373 K, 435 K and 200 K, respectively. Also, for a converter radiator temperature of 1373 K, the converter volume and mass remains less than 0.36 m 3 and 640 kg, respectively. Thus, the minimum system radiator + converter (reactor and shield not included) specific mass is ~ 16 kg/kW e for a converter radiator, system radiator and environment temperature of 1373 K, 425 K and 200 K, respectively. Under this operating condition, the reactor thermal rating is ~ 1110 kW t . Due to the large radiator area, the added complexity and mission risk needs to be weighed against reducing the reactor thermal rating to determine the feasibility of using TPV for space nuclear (fission) power systems.

Thermophotovoltaic (TPV) tandem converter technology is being explored in an effort to improve both the efficiency and power density of TPV systems. Inverted, tandem structures incorporate epitaxially grown 0.74-eV latticematched Gadn0.53As and 0.55-eV lattice-mismatched Gao.2alno.72As diodes. Adddionally, a strategy has been developed to allow voltage matching between these two subcells. Performance modeling calculations show that, under typical operating conditions, the 0.74/0.55eV tandem converter should outperform a 0.55-eV single junction converter by 15% on an efficiency basis and by 15% on a power density basis. This paper will present details regarding the design, growth, fabrication, and electrical, optical. and structural characterization of voltage-matched tandem TPV devices.