The band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells on a stainless steel substrate was investigated using photoelectron spectroscopy and inverse photoemission. By combining both techniques, the conduction and valence band offsets were independently determined. We find an unfavorable conduction band offset of −0.45 (±0.15) eV, accounting for the generally observed low open-circuit voltage and indicating the great importance of the buffer∕absorber conduction band offset for such devices. The surface band gap of the Cu(In,Ga)S2 absorber is 1.76 (±0.15) eV, being increased with respect to the expected bulk value by a copper depletion near the surface.
KCN etching of the CuxS surface layer formed during the production process of Cu(In,Ga)S2 thin film solar cell absorbers as well as subsequent H2O2∕H2SO4 etching of the Cu(In,Ga)S2 surface have been investigated using x-ray photoelectron spectroscopy, x-ray excited Auger electron spectroscopy, and x-ray emission spectroscopy. We find that the KCN etching removes the CuxS layer—being identified as Cu2S—and that there is K deposited during this step, which is removed by the subsequent H2O2∕H2SO4 oxidation treatment. When a CdS buffer layer is deposited on the absorber directly after KCN etching, a K compound (KCO3) is observed at the CdS surface.
The
Breakthrough Starshot Initiative aims to send a gram-scale
probe to our nearest extrasolar neighbors using a laser-accelerated
lightsail traveling at relativistic speeds. Thermal management is
a key lightsail design objective because of the intense laser powers
required but has generally been considered secondary to accelerative
performance. Here, we demonstrate nanophotonic photonic crystal slab
reflectors composed of 2H-phase molybdenum disulfide and crystalline
silicon nitride, highlight the inverse relationship between the thermal
band extinction coefficient and the lightsail’s maximum temperature,
and examine the trade-off between minimizing acceleration distance
and setting realistic sail thermal limits, ultimately realizing a
thermally endurable acceleration minimum distance of 23.3 Gm. We additionally
demonstrate multiscale photonic structures featuring thermal-wavelength-scale
Mie resonant geometries and characterize their broadband Mie resonance-driven
emissivity enhancement and acceleration distance reduction. More broadly,
our results highlight new possibilities for simultaneously controlling
optical and thermal response over broad wavelength ranges in ultralight
nanophotonic structures.
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