This document describes different types of inks for producing Cu2ZnSnS4 (CZTS) thin films for solar cell applications. It discusses three main types of inks: 1) metals and chalcogenides dissolved in solvents, 2) inorganic metal salts and chalcogen sources dispersed in solvents, and 3) nanoparticles of metal oxides/chalcogenides dispersed in solvents. The document focuses on experiments using Type 2 inks containing metal salts and thiourea dispersed in water and organic solvent mixtures. Various ink formulations were tested and Ink 5 was found to produce crystalline CZTS films but with poor adhesion and porosity. Improving the film formation process is needed to enhance film properties

1. hot plate dry
300 oC
spin-coat
3000rpm, 30s
Mo Mo Mo
repeat to obtain desired thickness
Mo
sulfurization
in S vapor
500 oC, 1 hr
Film characterizations
XRD, SEM, EDS
apply ink
Spin
coating+
sulfurization
Sonication in
and wash with
EtOH, dispersed
in EtOH
Drop-cast+
sulfurization
+ EDS results
Cu1.6ZnSnS4
Impurities (atom%): N (8%), Cl (1%)
XRD available
+ EDS results
Cu1.6ZnSn0.8S2.8
Impurities (atom%): N (35%), C (35%)
XRD available
Very poor
adhesion
Ink 5
Ink DF5
4/2/2013 74/2/2013 74/2/2013 7
Methods
Ink Type 2: inorganic metal salts and chalcogen sources
dispersed in solvents (Example: CuCl, Zn(NO)2, SnCl2, thiourea dispersed
in non-toxic solvents)
Advantages: disadvantages:
non-toxic solvents reaction by-products
easy to formulate material dispersion
impurities
Adv. Energy Mater., 2011, 1, 732-735
Development of Cu2ZnSnS4 Inks for Solution Processable Thin Film Solar Cells
Feng Jiang and Anthony Muscat
muscat@email.arizona.edu
Department of Chemical and Environmental Engineering
University of Arizona, Tucson, AZ 85721
Introduction
Comparing to CuInGaS2 (CIGS), kesterite Cu2ZnSnS4 (CZTS) has been
considered as a more ideal absorber material for the next generation low-
cost solar cell, due to the lack of rear and expensive indium. With a direct
band gap of 1.0-1.5 eV and absorption coefficient larger than 104
cm-1
, CZTS
based thin film solar cell has reached reported efficiency up to 9.6%.
Although vacuum sputtering has been used to deposit CZTS film, it is
difficult to control the chemical stoichiometry during the deposition,
resulting undesired crystalline phases and impairing the cell efficiency.
Solution processable thin film fabrication not only opens the door for precise
control of film stoichiometry, but also creates the opportunity for being
adapted into large scale, continuous roll-to-roll solar cell manufacturing.
This work gives a quick review of three types of major ink formulations for
CZTS cells. Then we present our experiment results on one of these
formulations, which is based on having inorganic metal salts and chalcogen
sources dispersed in common organic solvents. Concentration, pH, and
precursor ratio were systematically changed to optimize the ink formulation
in order to obtain an adhesive and stoichiometric CZTS film.
Review on Inks
The requirements of an ink for thin film solar cell fabrication:
- contains metal chalcogenides or ready-to-react metal and chalcogen
sources
- has metal precursors close to the oxidation states of target material
- minimizes the impurity contents that is hard to remove
- has a good material dispersion (i.e. no severe agglomeration)
- uses non-toxic solvents with low boiling points
- has an adjustable viscosity that is adaptable to the deposition methods
- is easy to formulate and process
Why these requirements
- to fabricate a dense film with large grain size and small number of voids:
lower impurities in the ink; minimize reactions that impede crystallization;
accelerate solvent evaporation
- to incorporate the ink into mass production line without extra costs and
safety issues
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Methods
Ink Type 1: metals and metal chalcogenides dissolved
in solvents (Example: Cu2S, Zn, SnSe, S and Se dissolved in N2H4)
Prog. Photovolt: Res. Appl., 2012, 20, 6-11
Adv. Mater. DOI: 10.1002/adma.201201785
Advantages: disadvantages:
no carbon content toxic solvent
good material dispersion
no reaction by-product
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Equipments and Preliminary
results
Ink Types Major Equipments Preliminary results
Ink type 1 glove box dense, low-voids, large grain
CZTS film, cell efficiency
10.1%1
Ink type 2 N/A CZTS film with smaller
grains and some voids, cell
efficiency 4.1%2
Ink type 3 milling machine, vacuum
oven
dense, low-voids, large grain
CZTS and CIGS films, cell
efficiency exceeds 10%3
1. Prog. Photovolt: Res. Appl., 2012, 20, 6-11
2. Adv. Energy Mater., 2011, 1, 732-735
3. Thin Solid Films, 2003, 431-432, 53-57
Back to TOC
Reference:
1. Prog. Photovolt: Res. Appl., 2012, 20, 6-11
2. Adv. Energy Mater., 2011, 1, 732-735
3. Thin Solid Films, 2003, 431-432, 53-57
4/2/2013 84/2/2013 84/2/2013 8
Methods
Ink Type 3: nanoparticles of metal oxides/ chalcogenides
dispersed in solvents (Example: CuO, In2O3, Ga2O3 (for CIGS) and Cu2S,
Zn, Sn, and S (for CZTS) dispersed in non-toxic solvents)
Energy Environ. Sci., 2012, 5, 5340-5345
Thin Solid Films, 2003, 431-432, 53-57
Advantages: disadvantages:
non-toxic solvents material dispersion
low reaction by-products formulate process
no or low carbon content
Experiments on Inks Type #2
ink # Cu (conc.) Zn (conc.) Sn (conc.) Thiourea (conc.) Solvent (volume ratio)
1 Cu(NO3)2 0.8 M Zn(acetate)2 0.5 M SnCl2 0.5 M 4 M H2O:EG=1:1
2 Cu(NO3)2 0.8 M Zn(NO3)2 0.5 M SnCl2 0.5 M 6 M H2O:EG=1:2
3 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.5 M 6 M H2O:NH4OH:EG=7:4:9
4 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.5 M 6 M H2O:EtOH =5:1
5 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:NH4OH =3:16:1
6 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EG:NH4OH =3:16:1
7 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:NH4OH =3:36:1
8 Cu(NO3)2 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:NH4OH =3:16:1
9 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:IPA:NH4OH =3:16:1
10 Cu(NO3)2 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:NH4OH =3:16:1
11 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:HCl=3.8:16:0.2
12 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:NH4OH =3.5:16:0.5
13 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH=1:4
14 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M H2O:EtOH:NH4OH=1:1:8
15 CuCl 0.8 M Zn(NO3)2 0.5 M SnCl2 0.4 M 6 M EtOH:NH4OH=4:1
Ethylene glycol (EG) yielded CuS impurity phase, identified by the diffraction
peak at 15o
in the XRD spectra.
Ink 5 and 8 yielded CZTS crystalline film. However, ink 8 has a short
storage time and large aggregation formed after days of storage. Ink 5 is
stable and suitable with deposition process, such as spin coating. Again, EG
in Ink 6 yielded CuS phase. Inks without XRD were not stable and thus not
suitable for making films.
Problem: film made by ink 5 is very porous, containing small particles.
Tried to solve this issue by collecting particles and re-crystallizing them to
form a new film.
Outlook: ink #2 uses thiourea as S source, which
brings C content into the film. To avoid this issue, ink
type #3 could be a good candidate. Experiment on ink
type #3 is undergoing.
Film Fabrication Process Using Formulated Inks