Single Phase Formation of SnS Competing with SnS₂ and Sn₂S₃ for Photovoltaic Applications: Optoelectronic Characteristics of Thin-Film Surfaces and Interfaces

Tin monosulfide (SnS) is one of the most promising binary compounds for thin-film solar cells owing to its suitable optical properties and abundance in nature. However, in solar cells it displays a low open circuit voltage and power conversion efficiency owing to multiphases in the absorber layers. In this study, we investigated approximately 1.2-μm-thick SnS thin films prepared via a two-step process involving (1) the deposition of metal precursor layers and (2) sulfurization at 400 °C. To investigate the phase variations inside the thin films we employed a dimpling method to get a vicinal cross-section of the sample. Kelvin probe force microscopy, conductive atomic force microscopy, and micro-Raman scattering spectroscopy were used to characterize the local electrical and optical properties of the sample. We studied the distribution of the Sn–S polytypes in the film and analyzed their electrical performances for solar cell applications. The work functions of SnS and SnS₂ were determined to be 4.3–4.9 and ∼5.3 eV, respectively. The local current transport properties were also measured; they displayed an interesting transition in the conduction mechanism, namely from Ohmic shunt current at low voltages to space-charge-limited current at high voltages

Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic–Organic Lead Halide Solar Cells

We fabricated a mesoporous perovskite solar cell with a ∼14% conversion efficiency, and we investigated its beneficial grain boundary properties of the perovskite solar cells through the use of scanning probe microscopy. The CH₃NH₃Pb­(I_0.88,Br_0.12)₃ showed a significant potential barrier bending at the grain boundary and induced passivation. The potential difference value in the <i>x</i> = 0.00 sample is ∼50 mV, and the distribution of the positive potential is lower than that of the <i>x</i> = 0.12 sample. We also investigated the polarization and hysteretic properties of the perovskite thin films by measuring the local piezoresponse. Specifically, the charged grain boundaries play a beneficial role in electron–hole depairing and in suppressing recombination in order to realize high-efficiency perovskite solar cells

In Situ Observation of Dehydration-Induced Phase Transformation from Na₂Nb₂O₆–H₂O to NaNbO₃

We have monitored the phase transformation from a Sandia octahedral molecular sieve Na₂Nb₂O₆–H₂O to a piezoelectric NaNbO₃ nanowire through in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements at high temperatures. After dehydration at 288 °C, the Na₂Nb₂O₆–H₂O becomes significantly destabilized and transforms into NaNbO₃ with the increase of time. The phase transformation time is exponentially proportional to the inverse of temperature, for example, ∼10⁵ s at 300 °C and ∼10¹ s at 500 °C, and follows an Arrhenius equation with the activation energy of 2.0 eV. Real time TEM investigation directly reveals that the phase transformation occurs through a thermally excited atomic rearrangement due to the small difference of Gibbs free energy between two phases. This work may provide a clue of kinetic control for the development of high piezoelectric lead-free alkaline niobates and a deep insight for the crystallization of oxide nanostructures during a hydrothermal process

In Situ Observation of Dehydration-Induced Phase Transformation from Na₂Nb₂O₆–H₂O to NaNbO₃

We have monitored the phase transformation from a Sandia octahedral molecular sieve Na₂Nb₂O₆–H₂O to a piezoelectric NaNbO₃ nanowire through in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements at high temperatures. After dehydration at 288 °C, the Na₂Nb₂O₆–H₂O becomes significantly destabilized and transforms into NaNbO₃ with the increase of time. The phase transformation time is exponentially proportional to the inverse of temperature, for example, ∼10⁵ s at 300 °C and ∼10¹ s at 500 °C, and follows an Arrhenius equation with the activation energy of 2.0 eV. Real time TEM investigation directly reveals that the phase transformation occurs through a thermally excited atomic rearrangement due to the small difference of Gibbs free energy between two phases. This work may provide a clue of kinetic control for the development of high piezoelectric lead-free alkaline niobates and a deep insight for the crystallization of oxide nanostructures during a hydrothermal process