*Marlon G. CummingsLab Manager, Aspuru-Guzik GroupMallinckrodt
M136Department of Chemistry and Chemical BiologyHarvard University12 Oxford
StreetCambridge, MA 02138617-496-9964617-496-9411
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---------- Forwarded message ----------
From: Catherine M Bourgeois <cmbourg(a)mit.edu>
Date: Tue, Mar 8, 2016 at 9:28 AM
Subject: [Aspuru-Guzik Group List] Excitonics/Perovskites Seminar: Eric
Hoke, TOMORROW: Wed. Mar 9 at 4:30 pm
To: efrc-all <efrc-all(a)mit.edu>du>, perovskite-seminars <
perovskite-seminars(a)mit.edu>
Please post and forward to your groups.
*CENTER FOR EXCITONICS Seminar Series:*
*Perovskite Seminar:*
Reversible and irreversible ion migration processes in lead halide
perovskites for photovoltaics
March 9, 2016 at 4:30 PM/36-462
*Eric Hoke *
*Standord University, Draper Laboratory*
[image: hoke-eric]
Lead hybrid perovskites are a promising family of photovoltaic absorber
materials that have achieved power conversion efficiencies of over 20%.
Lead halide perovskites are ionic materials with a low lattice energy which
are unusual properties for a photovoltaic material. It has been conjectured
that this ionic character may be responsible for the material’s
defect-tolerant optoelectronic properties that have enabled its success as
an efficient photovoltaic material. However, lead halide perovskites are
highly susceptible to ion migratory processes which pose a significant
challenge to the commercialization of stable devices.
In this talk, I will present experimental evidence for a variety of ionic
migration processes in perovskite thin films resulting from optical or
electrical excitation. We observe from in-situ XRD measurements that
bromine-rich (0.2<x<1) CH3NH3Pb(BrxI1-x)3 and other mixed-halide
perovskites undergo a structural transformation under photoexcitation,
segregating into two distinct perovskite phases over the course of minutes.
The optical properties are also changed by this transformation resulting in
the appearance of intense photoluminescence and absorption features from a
lower bandgap phase. The perovskite surprisingly reverts back to its
original structure and starting optical spectra when left in the dark. We
suggest that photoexcitation induces halide segregation, resulting in
iodide-rich domains. These lower bandgap domains act as traps and are
likely responsible for the poor open circuit voltages generated by large
bandgap, bromine-rich (0.2<x<1) CH3NH3Pb(BrxI1-x)3 solar cells.
Perovskite solar cells frequently exhibit hysteretic behavior, which is
greatly aggravated by the presence of moisture. The influence of electric
field and moisture on perovskite films was investigated using a combination
of photocurrent, optical, Raman, and Auger mapping techniques on lateral
thin-film devices. These studies suggest that irreversible field-induced
degradation in air occurs via a hydrated phase, in which the organic cation
is loosely bound and can drift in response to an electric field, finally
degrading the material to PbI2.
*Eric Hoke* was recently the Associate Director of the Center of Advanced
Molecular Photovoltaics at Stanford University. He completed his PhD from
Stanford University in 2012 as a Fannie & John Hertz fellow and National
Science Foundation Graduate Research fellow where he studied light
harvesting mechanisms and degradation processes in organic photovoltaics
and dye sensitized solar cells. Dr. Hoke recently joined Draper Laboratory
as a member of Senior Technical Staff where he is developing imaging
systems.
*This talk is part of the Perovskites Seminar Series organized by Sam
Stranks and sponsored by the Center for Excitonics. For more info contact
Sam:* *stranks(a)mit.edu <stranks(a)mit.edu>*
*Light refreshments will be served*
*The Center For Excitonics Is An Energy Frontier Research Center Funded By
The U.S. Department Of Energy, Office Of Science And Office Of Basic Energy
Sciences*
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