<p>As global population continue to
rise, the demand for energy is slated to increase substantially. To combat
climate change, large amounts of renewable energy will be needed to feed this
growing demand. Of renewable energy sources, photovoltaics are well positioned
to meet this increasing demand due to the immense abundance of solar energy
incident on earth. However, existing energy intensive, low throughput, and
costly manufacturing techniques for photovoltaics may pose a barrier to
continued large scale implementation.</p>
<p>Solution processing has emerged as
a promising photovoltaics fabrication technique with high throughput, high
materials utilization, and lower cost than existing vacuum-based methods. Thin
film photovoltaic materials such as Cu(In,Ga)(S,Se)<sub>2</sub> and CdTe have
both been fabricated using various solution processing methods. Of the various
solution processing routes, colloidal metal chalcogenide nanoparticles have
demonstrated promise as a hydrazine-free route for the solution processing of
high efficiency Cu(In,Ga)(S,Se)<sub>2</sub> solar cells. However, conventional
solution processing with colloidal nanoparticles has long suffered from anionic
and carbonaceous impurities, stemming from legacy synthesis methods. The work
in this dissertation aims to solve these issues through the development of
novel synthetic methods, ligand engineering, and ultimately improved
scalability through slot-die coating.</p>
<p> Typical colloidal syntheses rely on the use of
metal salts as precursors such as metal halides, nitrates, acetates, and so forth,
where the anions may incorporate and alter the electrical properties of the
targeted nanomaterials. In this work, the recent advances in amine-thiol
chemistry and its unique ability to solubilize many metal containing species
are expanded upon. Alkylammonium metal thiolate species are easily formed upon
addition of monoamine and dithiol to elemental Cu, In, Ga, Sn, Zn, Se, or metal
chalcogenides such as Cu<sub>2</sub>S and Ag<sub>2</sub>S. These species were
then used directly for the synthesis of colloidal nanoparticles without the
need for any additional purification. The metal thiolate thermal decomposition
pathway was studied, verifying that only metal chalcogenides and volatile
byproducts are formed, providing a flexible route to compositionally uniform,
phase pure, and anionic impurity-free colloidal nanoparticles including
successful syntheses of In<sub>2</sub>S<sub>3</sub>, (In<sub>x</sub>Ga<sub>1–x</sub>)<sub>2</sub>S<sub>3</sub>,
CuInS<sub>2</sub>, CuIn(S<sub>x</sub>Se<sub>1–x</sub>)<sub>2</sub>, Cu(In<sub>x</sub>Ga<sub>1–x</sub>)S<sub>2</sub>,
Cu<sub>2</sub>ZnSnS<sub>4</sub>, and AgInS<sub>2</sub>. </p>
<p>However, further impurities from deleterious carbonaceous
residues originating from long chain native ligands were still a persistent
problem. This impurity carbon has been observed to hinder grain formation
during selenization and leave a discrete residue layer between the absorber
layer and the back contact. An exhaustive hybrid organic/inorganic ligand
exchange was developed in this work to remove tightly bound oleyalmine ligands
through a combination of microwave-assisted solvothermal pyridine ligand
stripping followed by inorganic capping with diammonium sulfide, yielding greater
than 98% removal of native ligands via a rapid process. Despite the aggressive
ligand removal, the nanoparticle stoichiometry remained largely unaffected when
making use of the hybrid ligand exchange. Scalable blade coating of the ligand
exchanged nanoparticle inks from non-toxic dimethyl sulfoxide inks yielded remarkably
smooth and crack free films with RMS roughness less than 7 nm. Selenization of
ligand exchanged nanoparticle films afforded substantially improved grain
growth as compared to conventional non-ligand exchanged methods yielding an
absolute improvement in device efficiency of 2.8%. Hybrid ligand exchange
nanoparticle-based devices reached total-area power conversion efficiencies of
12.0%.</p>
<p>While extremely effective in ligand removal, ligand exchange
pathways increase process complexity and solvent usage substantially, which may
limit the cost advantage solution processing aims to provide. Further synthesis
improvement was developed through a ligand exchange free, direct sulfide capped
strategy. Using sulfolane as a benign solvent, CuInS<sub>2</sub> nanoparticles
with thermally degradable thioacetamide ligands were synthesized using thermal
decomposition of isolated metal thiolates from Cu<sub>2</sub>S and In
precursors. Through gentle thermal treatment, these ligands decomposed into
non-contaminating gaseous byproducts leaving carbon free nanoparticle films
without the need for ligand exchange.</p>
<p>With the development of virtually contamination free
colloidal nanoparticle inks, focus was shifted to scalability using slot die
coating. Unlike typical lab-scale coating techniques such as spin coating, slot
die coating is a widely used industrial coating technique with nearly 100%
materials utilization, and high throughput roll-to-roll compatibility. A custom
lab-scale slot-die coater was used to rapidly proof coating conditions, which
were rapidly analyzed for uniformity using absorbance scanning in conjunction
with profilometry. A cosolvent chlorobenzene/dichlorobenzene ink was developed
to yield highly uniform, crack free thin films from non-ligand-exchanged
Cu(In,Ga)S<sub>2</sub> nanoparticles, which were finished into devices with
champion total are efficiencies of 10.7%. To the best of our knowledge, this
represents the first report of slot die coated Cu(In,Ga)(S,Se)<sub>2</sub>
photovoltaics. The methods presented in this work offer a pathway towards low
impurity, high efficiency, scalable solution processed Cu(In,Ga)(S,Se)<sub>2</sub>
photovoltaics to enable low cost renewable energy.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12724832 |
| Date | 09 September 2022 |
| Creators | Ryan Gupta Ellis (9175325) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Synthesis_and_Ligand_Engineering_of_Colloidal_Metal_Chalcogenide_Nanoparticles_for_Scalable_Solution_Processed_Photovoltaics/12724832 |
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