3 research outputs found
Broad Band Photometric Reverberation Mapping of NGC 4395
We present results of broad band photometric reverberation mapping (RM) to
measure the radius of the broad line region, and subsequently the black hole
mass (M), in the nearby, low luminosity active galactic nuclei (AGN)
NGC 4395. Using the Wise Observatory's 1m telescope equipped with the SDSS
g, r and i broad band filters, we monitored NGC 4395 for 9 consecutive
nights and obtained 3 light curves each with over 250 data points. The g and
r bands include time variable contributions from H and H
(respectively) plus continuum. The i band is free of broad lines and covers
exclusively continuum. We show that by looking for a peak in the difference
between the cross-correlation and the auto-correlation functions for all
combinations of filters, we can get a reliable estimate of the time lag
necessary to compute M. We measure the time lag for H to be
hours, comparable to previous studies using the line resolved
spectroscopic RM method. We argue that this lag implies a black hole mass of
M \Msun
Crystallization of Methyl Ammonium Lead Halide Perovskites: Implications for Photovoltaic Applications
Hybrid organic/lead halide perovskites
are promising materials
for solar cell fabrication, resulting in efficiencies up to 18%. The
most commonly studied perovskites are CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> where <i>x</i> is small. Importantly, in the latter system, the presence of chloride
ion source in the starting solutions used for the perovskite deposition
results in a strong increase in the overall charge diffusion length.
In this work we investigate the crystallization parameters relevant
to fabrication of perovskite materials based on CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub>. We find that the addition of PbCl<sub>2</sub> to the solutions
used in the perovskite synthesis has a remarkable effect on the end
product, because PbCl<sub>2</sub> nanocrystals are present during
the fabrication process, acting as heterogeneous nucleation sites
for the formation of perovskite crystals in solution. We base this
conclusion on SEM studies, synthesis of perovskite single crystals,
and on cryo-TEM imaging of the frozen mother liquid. Our studies also
included the effect of different substrates and substrate temperatures
on the perovskite nucleation efficiency. In view of our findings,
we optimized the procedures for solar cells based on lead bromide
perovskite, resulting in 5.4% efficiency and <i>V</i><sub>oc</sub> of 1.24 V, improving the performance in this class of devices.
Insights gained from understanding the hybrid perovskite crystallization
process can aid in rational design of the polycrystalline absorber
films, leading to their enhanced performance