Persistence in nonequilibrium surface growth

Persistence probabilities of the interface height in (1+1)- and (2+1)-dimensional atomistic, solid-on-solid, stochastic models of surface growth are studied using kinetic Monte Carlo simulations, with emphasis on models that belong to the molecular beam epitaxy (MBE) universality class. Both the initial transient and the long-time steady-state regimes are investigated. We show that for growth models in the MBE universality class, the nonlinearity of the underlying dynamical equation is clearly reflected in the difference between the measured values of the positive and negative persistence exponents in both transient and steady-state regimes. For the MBE universality class, the positive and negative persistence exponents in the steady-state are found to be $\theta^S_{+} = 0.66 \pm 0.02$ and $\theta^S_{-} = 0.78 \pm 0.02$, respectively, in (1+1) dimensions, and $\theta^S_{+} = 0.76 \pm 0.02$ and $\theta^S_{-} =0.85 \pm 0.02$, respectively, in (2+1) dimensions. The noise reduction technique is applied on some of the (1+1)-dimensional models in order to obtain accurate values of the persistence exponents. We show analytically that a relation between the steady-state persistence exponent and the dynamic growth exponent, found earlier to be valid for linear models, should be satisfied by the smaller of the two steady-state persistence exponents in the nonlinear models. Our numerical results for the persistence exponents are consistent with this prediction. We also find that the steady-state persistence exponents can be obtained from simulations over times that are much shorter than that required for the interface to reach the steady state. The dependence of the persistence probability on the system size and the sampling time is shown to be described by a simple scaling form.Comment: 28 pages, 16 figure

Optical Magnetometry

Some of the most sensitive methods of measuring magnetic fields utilize interactions of resonant light with atomic vapor. Recent developments in this vibrant field are improving magnetometers in many traditional areas such as measurement of geomagnetic anomalies and magnetic fields in space, and are opening the door to new ones, including, dynamical measurements of bio-magnetic fields, detection of nuclear magnetic resonance (NMR), magnetic-resonance imaging (MRI), inertial-rotation sensing, magnetic microscopy with cold atoms, and tests of fundamental symmetries of Nature.Comment: 11 pages; 4 figures; submitted to Nature Physic

Improving the triple-cation perovskite solar cells by two-step deposition methods with perovskite seeds

Abstract As of recent years, triple-cation perovskite solar cells have received immense attention due to its superior efficiency and better stability comparing to the classic single-cation perovskite solar cells such as MAPbI3 or FAPbI3. A triple-cation perovskite layer which has been used most recently is cesium-containing FAPbI3-based perovskite. One of decent approaches to fabricate the layer is spin-coating technique by using two-step deposition process in which mixed lead-halide and CsI precursor is firstly spin-coated onto a substrate, then organic cation solution is deposited on the lead-halide layer. In this work, the results show that the performance of the devices from this process is lower than expected that could be due to difficulty of cesium ion incorporation as a stabilizer for FAPbI3-based perovskite. Perovskite seeding growth is introduced to solve the problem where the process is slightly modified from conventional two-step deposition methods by adding small amount of perovskite seed precursor into PbI2 solution. The concentration of the perovskite seed in PbI2 solution was varied for 0, 7, 14 and 20% v/v. The highest average efficiency of 11.9% was obtained from 7% v/v seeding concentration. Furthermore, the device performance could be improved by using proper amount of chlorobenzene (CB) as an anti-solvent. The highest efficiency of 18.4% was achieved by using 30 µl of chlorobenzene.</jats:p

Influence of Cu-atomic ratio in the 3-stage deposition technique on the efficiency of CuIn_1-xGa_xSe₂ solar cells

Abstract The 3-stage co-evaporation technique is one of the deposition processes used to fabricate photon absorber layer in high efficiency CuIn1-xGaxSe2 (CIGS) solar cells. For this technique, the [Cu]/[III] ratio (y), where [III] refers to group-III elements, evolves from Cu-poor (y &lt; 1) to Cu-rich (y &gt; 1) in the 2nd stage and finally ends with slightly Cu-poor (y ~ 0.9) in the 3rd stage of the 3-stage process. Here, the highest values of [Cu]/[III] in the 2nd stage are intentionally varied from 1.0 to 1.5 by setting the deposition time of the pre-calibrated Cu and Se fluxes in the 2nd stage. The [Ga]/[III] ratio (x) is set at 0.37 during the 1st and 3rd stages in all devices. The influences of the Cu-atomic ratio are examined for the crystal grain growth, elemental depth profiles of the CIGS absorbers as well as the photovoltaic parameters and external quantum efficiency (EQE) of the CIGS solar cells. The optimal value of y = 1.3 is found to provide the highest efficiency CIGS device. The double-grading depth profile in the [Ga]/[III] ratio has also been observed despite the constant fluxes of group-III elements set during the whole process. The performances of the CIGS solar cells are investigated under AM1.5 condition and found to have open-circuit voltage (V OC) of 670 mV, short-circuit current density (J SC) of 33.2 mA/cm2, fill factor (FF) of 75.5% and the power conversion efficiency of 16.8% for the best CIGS device. The J SC of the device with y = 1.3 is relatively higher than other devices due to the increase of photo-generated currents in the short wavelength region as seen in the EQE spectrum.</jats:p

Fabrication of SnO₂ by RF magnetron sputtering for electron transport layer of planar perovskite solar cells

Abstract The requirements of electron transport layer (ETL) for high efficiency Perovskite solar cells (PSCs) are, for example, appropriate band energy alignment, high electron mobility, high optical transmittance, high stability, and easy processing. SnO2 has attracted more attention as ETL for PSCs because it has diverse advantages, e.g., wide bandgap energy, excellent optical and chemical stability, high transparency, high electron mobility, and easy preparation. The SnO2 ETL was fabricated by RF magnetron sputtering technique to ensure the chemical composition and uniform layer thickness when compared to the use of chemical solution via spin-coating method. The RF power was varied from 60 - 150 W. The Ar sputtering gas pressure was varied from 1 × 10−3 - 6 × 10−3 mbar while keeping O2 partial pressure at 1 × 10−4 mbar. The thickness of SnO2 layer decreases as the Ar gas pressure increases resulting in the increase of sheet resistance. The surface morphology and optical transmission of the SnO2 ETL were investigated. It was found that the optimum thickness of SnO2 layer was approximately 35 - 40 nm. The best device shows Jsc = 27.4 mA/cm2, Voc = 1.03 V, fill factor = 0.63, and efficiency = 17.7%.</jats:p