2 research outputs found
Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> Films
Halide perovskite-based solar cells
still have limited reproducibility,
stability, and incomplete understanding of how they work. We track
electronic processes in [CH<sub>3</sub>NH<sub>3</sub>]ĀPbI<sub>3</sub>(Cl) (āperovskiteā) films <i>in vacuo</i>, and in N<sub>2</sub>, air, and O<sub>2</sub>, using impedance spectroscopy
(IS), contact potential difference, and surface photovoltage measurements,
providing direct evidence for perovskite sensitivity to the ambient
environment. Two major characteristics of the perovskite IS response
change with ambient environment, viz. -1- appearance of negative capacitance <i>in vacuo</i> or post<i>-vacuo</i> N<sub>2</sub> exposure,
indicating for the first time an electrochemical process in the perovskite,
and -2- orders of magnitude decrease in the film resistance upon transferring
the film from O<sub>2</sub>-rich ambient atmosphere to vacuum. The
same change in ambient conditions also results in a 0.5 V decrease
in the material work function. We suggest that facile adsorption of
oxygen onto the film dedopes it from n-type toward intrinsic. These
effects influence any material characterization, i.e., results may
be ambient-dependent due to changes in the materialās electrical
properties and electrochemical reactivity, which can also affect material
stability
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>3ā<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