Sine-limiting microcell solar concentrators for space.

Microcell concentrating photovoltaics (µCPV) have the potential to improve performance and reduce the cost of solar power in space. Here, we introduce an ultracompact V-cone tailored edge ray (V-TERC) concentrator, rooted in nonimaging optics, which enables operation near the sine limit. Relative to previous space µCPV implementations, this concentrator design enables an approximate four-fold increase in concentration ratio for a given acceptance angle and specific power. We validate the design through ray tracing simulations and construction of a proof-of-concept system that consists of a 650 × 650 µm2 triple-junction microcell bonded to a 3.1 mm-thick prototype V-TERC optic. In outdoor testing on a sunny day, the system achieves a power conversion efficiency of 30% at a geometric gain of 137× with a specific power of 90 W kg-1 and an acceptance angle of ±4.5°. This is a record combination for µCPV to date and represents an important step toward increasing efficiency and lowering the cost of solar power in space.

Photoexcited Carrier Dynamics of In2S3 Thin Films.

Indium sulfide (In2S3) is a promising absorber base for substitutionally doped intermediate band photovoltaics (IBPV); however, the dynamics of charge carriers traversing the electronic density of states that determine the optical and electronic response of thin films under stimuli have yet to be explored. The kinetics of photophysical processes in In2S3 grown by oxygen-free atomic layer deposition are deduced from photoconductivity, photoluminescence (PL), and transient absorption spectroscopy. We develop a map of excited-state dynamics for polycrystalline thin films including a secondary conduction band ∼2.1 eV above the first, plus sulfur vacancy and indium interstitial defect levels resulting in long-lived (∼100 ns) transients. Band-edge recombination produces PL and stimulated emission, which both intensify and red-shift as deposition temperature and grain size increase. The effect of rapid conduction band electron relaxation (<30 ps) and deep defect levels on IBPV employing In2S3-based absorbers is finally considered.

Oxygen-free atomic layer deposition of indium sulfide.

Atomic layer deposition (ALD) of indium sulfide (In2S3) films was achieved using a newly synthesized indium precursor and hydrogen sulfide. We obtain dense and adherent thin films free from halide and oxygen impurities. Self-limiting half-reactions are demonstrated at temperatures up to 225 °C, where oriented crystalline thin films are obtained without further annealing. Low-temperature growth of 0.89 Å/cycle is observed at 150 °C, while higher growth temperatures gradually reduce the per-cycle growth rate. Rutherford backscattering spectroscopy (RBS) together with depth-profiling Auger electron spectroscopy (AES) reveal a S/In ratio of 1.5 with no detectable carbon, nitrogen, halogen, or oxygen impurities. The resistivity of thin films prior to air exposure decreases with increasing deposition temperature, reaching <1 Ω·cm for films deposited at 225 °C. Hall measurements reveal n-type conductivity due to free electron concentrations up to 10(18) cm(-3) and mobilities of order 1 cm(2)/(V·s). The digital synthesis of In2S3 via ALD at temperatures up to 225 °C may allow high quality thin films to be leveraged in optoelectronic devices including photovoltaic absorbers, buffer layers, and intermediate band materials.