RESUMEN
Among inorganic, Earth-abundant, and low-toxicity photovoltaic technologies, Sb2Se3 has emerged as a strong material contender reaching over 10% solar cell power conversion efficiency. Nevertheless, the bottleneck of this technology is the high deficit of open-circuit voltage (V OC) as seen in many other emerging chalcogenide technologies. Commonly, the loss of V OC is related to the nonradiative carrier recombination through defects, but other material characteristics can also limit the achievable V OC. It has been reported that in isostructural compound Sb2S3, self-trapped excitons are readily formed leading to 0.6 eV Stokes redshift in photoluminescence (PL) and therefore significantly reducing the obtainable V OC. However, whether Sb2Se3 has the same limitations has not yet been examined. In this work, we aim to identify main radiative carrier recombination mechanisms in Sb2Se3 single crystals and estimate if there is a fundamental limit for obtainable V OC. Optical transitions in Sb2Se3 were studied by means of photoreflectance and PL spectroscopy. Temperature, excitation intensity, and polarization-dependent optical characteristics were measured and analyzed. We found that at low temperature, three distinct radiative recombination mechanisms were present and were strongly influenced by the impurities. The most intensive PL emissions were located near the band edge. In conclusion, no evidence of emission from self-trapped excitons or band-tails was observed, suggesting that there is no fundamental limitation to achieve high V OC, which is very important for further development of Sb2Se3-based solar cells.
RESUMEN
Controlled heating experiments in an inert environment have been performed on WS2 monolayers, in order to clarify the conflicting reports on the high-temperature photoluminescent response of 2D TMDs. We find that in contrast to some previous results on both WS2 and MoS2, the photoluminescent intensity shows a consistent reduction above room temperature. This is accompanied by an almost linear redshift of the peak maximum, and a nearly linear increase in the peak width, which is attributed to an enhanced interaction with optical phonons. Moreover, by fitting the photoluminescence integral intensity with an Arrhenius type dependence, we demonstrate that the center of the WS2 monolayer flake starts to undergo irreversible degradation at a temperature of 573 K in an inert environment. Regions close to flake edges in contrast, with a more intense room temperature PL response, remain stable. The macroscopic PL signal is largely recovered in these regions following subsequent cooling to room temperature.
RESUMEN
Photovoltaics is a promising technology to produce sustainable energy, thanks to the high amount of energy emitted by the sun. One way of having solar cells with low production costs is to apply thin-film technology and with earth-abundant raw materials. A keen interest is arising in kesterite compounds, which are chalcogenides composed of abundant and non-toxic elements. They have already achieved excellent performance at the laboratory level. Here, we report the synthesis and characterization of mixed chalcogenides based on copper, zinc, iron, and tin. Solutions have been studied with different zinc and iron ratios. The distortion of the elementary cell of kesterite increases with the addition of iron until a phase transition to stannite occurs. The process of synthesis and deposition proposed herein is cheap and straightforward, based on the sol-gel technique. These thin films are particularly attractive for use in cheap and easily processable solar cells. The synthesized layers have been characterized by X-ray diffraction, UV-Vis absorption, and Raman, X-ray photoelectron, and energy-dispersive X-ray spectroscopy measurements.
