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Pratt Fellows Project Spring 2020
Porous Cu2BaSn(S,Se)4 as a Photocathode Using Non-Toxic
Solvents and a Ball-Milling Approach Pratt Fellows Project -
Class of 2020
Jiwoo Song, Advisor: Dr. David B. MitziDepartment of Mechanical
Engineering and Materials Science
Department of ChemistryDuke University
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Pratt Fellows Project Spring 2020
IntroductionWhy thin film?
While silicon-based solar cells have widely been implemented and
costs have been significantly reduced, solar technology still
encompasses less than 1% of American energy output, indicating that
further improvements are necessary. Thin film photovoltaics are
~100 times thinner than monocrystalline silicon photovoltaics,
allowing for a potentially lower material cost.
Why Cu2BaSn(S,Se)4 (CBTS)?
Current thin film photovoltaics materials utilize toxic (e.g.
Cd) or expensive (e.g. In) materials which are less suitable for
manufacturing and preclude terawatt deployment. All elements used
in CBTS are non-toxic and are commonly and cheaply found in the
earth's crust. Theoretical maximum efficiency is at 33%, which is
slightly higher than that of traditional silicon solar cells
(31%).
Why ball milling?
Commonly used processing and deposition methods in the lab are
neither cost effective or scalable, requiring either vacuum
environments or small substrate areas. Ball milling is a simple and
scalable way to generate a large amount of precursor particulates
with sufficient particle size (10-100 nm), requiring no additives
that could lead to impurity or toxicity.
W/$
Si(200 μm)
CBTS(2 μm)
Sputtering Ball Milling
Pure Solution
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Pratt Fellows Project Spring 2020
Experimental MethodsBall Milling
CBTS formed from ~10nm nanoparticles
Chakraborty et al., ACS Appl. Energy Mater. 2019
Woo et al. Solar Energy Materials & Solar Cells 128 (2014)
362–368
Deposition and Synthesis
Selenium atmosphere
Process Schematic
A photoelectrochemical device using CBTSSe as the absorber layer
was prepared by i) milling the precursors (Cu2S, BaS, Sn, S) with
1mm and 0.1mm ZrO2 balls in ethanol, ii) depositing the mixture
onto a Mo-coated soda lime glass substrate via spin-coating, iii)
annealing the sample on a hot plate under sulfur vapor, iv)
annealing the sample on a hot plate under selenium vapor, v)
precipitating CdS onto the porous film structure via chemical bath
deposition, and vi) depositing TiO2 via atomic layer deposition
using TiCl4 as the precursor
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Pratt Fellows Project Spring 2020
Characterization MethodsScanning Electron Microscopy X-Ray
Diffraction Photoluminescence
https://science.howstuffworks.com/scanning-electron-microscope2.htm
https://www.rigaku.com/techniques/x-ray-diffraction-xrdhttp://jss.ecsdl.org/content/4/12/P456/F2.expansion.html
- To determine grain morphology- Resolves to ~5 nm
- To determine lattice parameters- To detect impurities
- To determine band gap- To identify defects
https://science.howstuffworks.com/scanning-electron-microscope2.htmhttps://science.howstuffworks.com/scanning-electron-microscope2.htmhttps://www.rigaku.com/techniques/x-ray-diffraction-xrdhttp://jss.ecsdl.org/content/4/12/P456/F2.expansion.html
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Pratt Fellows Project Spring 2020
Thin Film OptimizationBall Milling Annealing under Sulfur Vapor
Annealing under Selenium Vapor
Top-down SEM images of as-prepared films without heat treatment
of stoichiometric CBTS precursors Cu2S, BaS, Sn, and S a) after
milling with 1.0mm balls for 24h, b) after milling with 1.0mm balls
for 24h then 0.1mm balls for 24h, c) after milling with 1.0mm balls
for 48h then 0.1mm balls for 72h. d) Cross-section SEM image of an
as-prepared film of stoichiometric CBTS precursors after milling
with 1.0mm balls for 48h then 0.1mm balls for 72h.
XRD scans of CBTS samples annealed under sulfur vapor at
different temperatures. The reference is included in the
bottom.
The alignment of peaks with the reference indicates the phase
purity of the samples.
Higher peaks at higher temperatures (>570oC) indicate that
increased crystallinity, and by extension grain size, can
occur.
SEM images of selenized CBTSSe samples with their respective XRD
scans below, corresponding to a,d) annealing at 570oC, b,e)
annealing at 600oC, c,f) annealing at 630oC.
Annealing at a temperature too low led to smaller grains
(0.3-0.6um) and the presence of CuSe and Ba impurities. Annealing
at a temperature too high led to preferred orientation of grains
and the generation of large voids, which allow for device shunt
paths.
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Pratt Fellows Project Spring 2020
Current Record:Sputtered
Proposed:Ball Milling
Comparison to Previous Methods Changing Stoichiometry
Changing Nucleation Site Availability Implementation in
Photoelectrochemical Cell
● Use of large barium particles from ball milled mix creates
large separated grains
● Using dissolved, molecular barium creates smaller, evenly
sized grains
● Photoelectrochem-ical cells are used to convert solar energy
into hydrogen gas
● Generated photocurrent of 5.5 mA/cm^2
● Use of large barium particles from ball milled mix creates
large separated grains
● Using dissolved, molecular barium creates smaller, evenly
sized grains
Ideal thin film:● Large grains (>1um)● Low thickness
(~1um)● No holes
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Pratt Fellows Project Spring 2020
Device FabricationDevice Structure Characterization of the
Growth of CBTSSe
Schematic of the Pt/TiO2/CdS/CBTSSe/Mo photocathode. Hydrogen
generation is achieved via light-induced charge separation at the
absorber layer (CBTSSe), leading to 1) water splitting. The two
generated electrons participate in the 2) reduction of protons to a
hydrogen molecule.
1) H2O + 2h+ → 2H+ + ½ O2, 2) 2H+ + 2e- → H2.
Above: SEM images showing the evolution of grain morphology
during the annealing process, from a) the as-prepared precursors
(Cu2S, BaS, Sn, S) after having been ball-milled, b) CBTS formed
after annealing under sulfur vapor, to c) CBTSSe formed after
annealing under selenium vapor.
Left: XRD scans of the same films. Initially the as-prepared
film appears amorphous, with the exception of the molybdenum peak
(highlighted at 40°2θ). Annealing under sulfur vapor followed by
selenium vapor resulted in a crystal structure that is in good
agreement with previously recorded CBTSSe values.
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Pratt Fellows Project Spring 2020
a) Top-down SEM image of the final filmb) Cross-section SEM
image of the final film (film thickness ~1.5 um)c) X-ray
diffraction (XRD) pattern of the sample with reference for
comparison; Mo peak highlightedd) Photoluminescence (PL) scan with
peak at 796 nm corresponding to 1.56 eV bandgape) Linear sweep
voltammetry (LSV) results showing a photocurrent density of 5.54
mA/cm^2 at 0V
b)Final Device Results
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Pratt Fellows Project Spring 2020
Thank you!
Please forward questions about this presentation to Jiwoo Song:
[email protected].
Acknowledgements
● Dr. David B. Mitzi is thanked for his guidance and patience
throughout the project.● Betul Teymur is thanked for her advice and
mentorship in the film synthesis of CBTSSe and
general lab procedures.● Dr. Yihao Zhou is thanked for his
assistance throughout the fabrication of the PEC devices for
this
project.● Dr. Carmen Rawls and the Pratt Fellowship Program at
Duke University are thanked for their
academic and financial support during the last 1.5 years of this
project.● The staff at the Shared Materials Instrumentation
Facility at Duke University are thanked for the
training on their characterization and deposition equipment.●
The Lord Foundation is thanked for their funding of the use of
equipment in the Shared Materials
Instrumentation Facility at Duke University.● The Department of
Mechanical Engineering and Materials Science and the Department
of
Chemistry at Duke University are thanked for their academic
guidance during my time at Duke.● And finally, to all of my
friends, family, and teachers who have supported my path into
and
through Duke, thank you!
mailto:[email protected]
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Pratt Fellows Project Spring 2020
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