Vibrational properties of metastable orthorhombic Bi2Se3.

Chalcogenide materials are being considered as some of the most promising systems for energy harvesting and energy conversion. Among them, the orthorhombic family of compounds X2Y3 (with X = Bi, Sb and Y = S, Se) has attracted special attention due to its interesting atomic structure and thermoelectric and optical properties. While Bi2S3 and Sb2Se3 have already been applied to solar cells, practical application of the new metastable Bi2Se3 is still a challenge due to the lack of data and knowledge on its properties. Here, the vibrational and structural properties of the orthorhombic metastable phase of Bi2Se3 are investigated by using Raman spectroscopy and ab initio calculations. We perform Raman spectroscopy measurements along with in situ thermal treatment on samples grown by electrochemical deposition. We show that by increasing the temperature an improved crystallization occurs in the orthorhombic structure, followed by recrystallization to the usual rhombohedral phase. Our results point out specific Raman modes of the orthorhombic phase. First principles computational results based on the density functional theory support the experimental data and describe three singlet Raman active vibrational modes, such as B(2)3g, B(2)2g and A(6)g.

Photoelectrochemistry of Self-Limiting Electrodeposition of Ni Film onto GaAs.

Gallium arsenide (GaAs) provides a suitable bandgap (1.43 eV) for solar spectrum absorption and allows a larger photovoltage compared to silicon, suggesting great potential as a photoanode toward water splitting. Photocorrosion under water oxidation condition, however, leads to decomposition or the formation of an insulating oxide layer, which limits the photoelectrochemical performance and stability of GaAs. In this work, a self-limiting electrodeposition method of Ni on GaAs is reported to either generate ultra-thin continuous film or nanoislands with high particle density by controlling deposition time. The self-limiting growth mechanism is validated by potential transients, X-ray photoelectron spectroscopy composition and depth profile measurements. This deposition method exhibits a rapid nucleation, forms an initial metallic layer followed by a hydroxide/oxyhydroxide nanofilm on the GaAs surface and is independent of layer thickness versus deposition time when coalescence is reached. A photocurrent up to 8.9 mA cm-2 with a photovoltage of 0.11 V is obtained for continuous ultrathin films, while a photocurrent density of 9.2 mA cm-2 with a photovoltage of 0.50 V is reached for the discontinuous nanoislands layers in an aqueous solution containing the reversible redox couple K3 Fe(CN)6 /K4 Fe(CN)6 .

Bias-dependent effects in planar perovskite solar cells based on CH3NH3PbI(3-x)Clx films.

A unique bias-dependent phenomenon in CH3NH3PbI(3-x)Cl(x) based planar perovskite solar cells has been demonstrated, in which the photovoltaic parameters derived from the current-voltage (I-V) curves are highly dependent on the initial positive bias of the I-V measurement. In FTO/CH3NH3PbI(3-x)Cl(x)/Au devices, the open-circuit voltage and short-circuit current increased by ca. 337.5% and 281.9% respectively, by simply increasing the initial bias from 0.5 V to 2.5 V.