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A non-toxic, solution-processed, earth abundant absorbing layer for thin-filmsolar cells†‡

Kyoohee Woo, Youngwoo Kim and Jooho Moon*

Received 8th August 2011, Accepted 4th October 2011

DOI: 10.1039/c1ee02314d

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) has attracted significant attention in the past few years as

a next generation absorber material for the production of thin film solar cells on large scales due to the

high natural abundance of all constituents, tunable direct band gap energy ranging from 1.0 to 1.5 eV,

and large absorption coefficient. In addition, to address the issue of expensive vacuum-based processes,

non-vacuum solution-based approaches are being developed for CZTS absorber layer deposition.

Here, we demonstrate the fabrication of a high quality CZTS absorber layer with a thickness of 2.8–3.0

mm and micrometre-scaled grains (1–2.5 mm) using air-stable non-toxic solvent-based inks. Our

approach for the fabrication of CZTS absorber, reported here, will be the first step in achieving low-

cost and large area solar cells with high efficiency.

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is a very promising

material for use as a low cost absorber alternative to other

chalcopyrite-type semiconductors based on Ga or In, because it

is only composed of abundant and economical elements.1–5 In

addition, CZTS has a direct band gap energy of 1.0–1.5 eV and

a large absorption coefficient of over 104 cm�1, properties similar

to those of Cu(In,Ga)Se2 (CIGS), which is regarded as one of the

best absorber materials for sustainable and highly efficient solar

cells.6–8 Typically, metal chalcogenide films such as CIGS and

CZTS are deposited by evaporation or sputtering techniques that

rely on vacuum environments.9–11 However, this vacuum depo-

Department of Materials Science and Engineering, Yonsei University, 50Yonsei-ro, Seodaemun-gu, Seoul, 120-749, Republic of Korea. E-mail:jmoon@yonsei.ac.kr; Fax: +82 2 312 5375; Tel: +82 2 2123 2855

† Electronic supplementary information (ESI) available: A detaileddescription of the experimental methods, surface SEM image ofas-prepared CZTS film, component depth profile of the CZTS filmwith the Cu-poor and Zn-rich composition for cell fabrication. SeeDOI: 10.1039/c1ee02314d

‡ The paper was presented in part at the International ChemicalCongress of Pacific Basin Societies (Pacifichem 2010), in Honolulu,Hawaii, USA, December 15–20, 2010.

Broader context

Solution processing for chalcogenide absorber materials in thin fi

materials have advantages including suitability for use in large-area

communication, we present a facile route to fabricate a Cu2ZnSnS

which commercially available precursor particles such as Cu2S, Z

efficiency of 5.14% under AM 1.5 illumination, the use of the n

convenient access to fabricate high quality CZTS absorber layers a

film solar cells.

5340 | Energy Environ. Sci., 2012, 5, 5340–5345

sition process suffers from relatively low throughput, low

material utilization, and difficulties associated with large-scale

production.12,13 In this regard, solution-based deposition

methods are being developed because they have advantages

including suitability for use in large-area substrates, high

throughput, and efficient materials usage.14–16 Various solution-

based approaches for the fabrication of CZTS thin films have

been reported including sol–gel17,18 and nanocrystal dispersion

processes,4,19 but they face some limitations. The sol–gel method

is vulnerable to contamination by carbon, oxygen, and other

impurities from precursors or starting solutions, which inevitably

leads to the formation of a porous structure with small grain size

due to significant shrinkage. The nanocrystal dispersion method

requires the complex synthesis of nanocrystals, and it is difficult

to achieve dense organic residue-free thick films from the

dispersion of nanocrystals capped with stabilizing molecules.

Recently, Todorov et al. reported the fabrication of CZTS thin

film solar cells with 9.6% power conversion efficiency (PCE)

using a hydrazine-based hybrid slurry approach.20,21 However,

hydrazine is a highly toxic and very unstable compound that

requires extreme caution during handling and storage.

lm solar cells is an attractive area of research because these

substrates, high throughput and efficient materials usage. In this

4 (CZTS) absorber layer using non-toxic solvent-based ink in

n, Sn, and S are dispersed. With our first cells exhibiting an

on-toxic precursor ink in a scalable coating process provides

t low cost and contributes to the large-scale deployment of thin

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Furthermore, due to the reactive nature of this solvent,22 all

processing for slurry and film preparations must be performed

under inert atmospheric conditions, and thus hydrazine would

not be easily adaptable for large-scale solar cell fabrication. With

these considerations, it is highly desirable to develop a robust,

easily scalable and relatively safe solution-based process for the

fabrication of a high quality CZTS absorber layer.

Here, we devise for the first time a non-toxic solvent-based

process for the fabrication of a dense CZTS absorber layer. The

slurry (or ink) employed for CZTS deposition is a commercially

available powder mixture of Cu2S, Zn, Sn, and S dispersed in

ethanol that is safe and easy to use. As an environmentally

benign solvent, ethanol was selected because it can evaporate

quickly and thus may minimize residual carbon- or oxygen-

containing impurities in the film. The slurry composition was

controlled to have the atomic ratio of Cu : Zn : Sn : S ¼2 : 1 : 1 : 4. Our simple slurry approach may encounter phase

segregation and the presence of unreacted species or other

unwanted intermediate compounds in the final film because the

precursor particles of Cu2S, Zn, and Sn are insoluble, unlike

hydrazine. In other words, to achieve our goal, the precursor

powders should be well-dispersed in the solvent and must be

reactive enough to be converted into CZTS granular films during

thermal treatment. We employed a milling process to grind

precursor powders to nanosize particles and to obtain homoge-

neous well-dispersed slurry. The large surface areas of finely

milled precursor particles can induce material transfer and

interparticle densification. In addition, some of the precursor

particles retain the low melting points of Zn (420 �C) and Sn

(231 �C), which will bring about reactive liquid-phase sintering

between constituent particles and/or intermediate compounds at

temperatures above 500 �C, even in the presence of Cu2S with its

high melting point (1130 �C).The thermal behavior of the CZTS precursor ink-containing

powder mixture (Cu2S, Zn, Sn, and S) was analyzed by thermo-

gravimetry coupled with differential scanning calorimetry (TG-

DSC) under a nitrogen atmosphere (150 cm3 min�1) as shown in

Fig. 1a. A weight loss of �2.7% accompanying the exothermic

peak at 200 �C is ascribed to the partial sublimation of sulfur,

while the endothermic peak at 480 �C likely results from the

crystallization of CZTS. The TG-DSC data indicate that the

precursor inks might be converted into the CZTS phase by

annealing at temperatures around 500 �C, lower than the glass

transition temperature (Tg) of the soda lime glass (SLG) that is

typically used to fabricate the CZTS thin film solar cells. The

phase development of the precursor films during annealing is

presented in Fig. 1b. Sharp peaks at 2q ¼ 28.45�, 47.3� and 56.2�

can be attributed to the diffraction of the (112), (220) and (312)

planes of kesterite structure CZTS (JCPDS no. 26-0575),

respectively, suggesting the formation of a CZTS crystalline

phase at temperatures ranging from 500 to 530 �C. Fig. 1c

presents SEM images showing the microstructural evolution of

the precursor films annealed under N2 + H2S (5%) atmosphere in

a tubular furnace at temperatures ranging from 400 to 530 �C for

30 min. The particle size in the as-prepared granular film was

smaller than �150 nm (see ESI, Fig. S1†). As the annealing

temperature increased, the films were gradually densified, while

the grain size increased. When annealed at over 530 �C, a rela-

tively dense structure with large grains (1–2.5 mm) and occasional

This journal is ª The Royal Society of Chemistry 2012

voids developed. Microstructural observations support the use of

particle mixture-based ink for the production of the solution-

processed absorbing layer.

It should be noted that three XRD diffraction peaks at around

2q ¼ 28.6�, 47.5� and 56.3� overlap with those of Cu2S and ZnS,

so the crystallization of CZTS cannot be confirmed solely by

XRD analysis. Therefore, Raman spectroscopy was utilized to

obtain further insight into the phase identification, and the

results of the CZTS films as a function of the annealing

temperatures are shown in Fig. 2. The as-prepared precursor

films exhibited a strong peak at 473 cm�1 as well as small peaks at

around 260 cm�1, which correspond to precursor components

such as Cu2S and ZnS. These undesirable phases disappear

completely when annealed at 530 �C, which is in good agreement

with the XRD results. For the sample annealed at 530 �C, peakswere observed at 251, 287, 338, and 368 cm�1; all of these peaks

can be assigned to kesterite CZTS.23,24 Raman analysis indicates

that the large surface area of the finely milled precursor particles

and the low melting points of Zn (420 �C) and Sn (231 �C)promoted the crystallization of CZTS at the temperature of

530 �C and allowed for the formation of a dense absorbing layer

by a reactive liquid phase sintering.

A cross-sectional image of a film annealed at 530 �C is shown

in Fig. 3a, in which a uniform dense structure without significant

large pores and/or cracks can be observed. It should be noted

that such a relatively thick (�2.9 mm) film can be achieved only

by three consecutive spin-coatings. The surface composition of

the CZTS film was determined by electron probe microanalysis

(EPMA) as shown in Fig. 3b and Table 1. The surface compo-

sition of the film was relatively uniform and the average

composition was close to the starting precursor composition

(25.0 at.% Cu, 12.5 at.% Zn, 12.5 at.% Sn, and 50.0 at.% S). We

also confirmed that the impurity levels of carbon and oxygen in

the film prepared under atmospheric conditions were about 3%,

which is lower than that (>5%) of the chalcogenide films fabri-

cated by other solution-based approaches.25,26 In addition,

considering that the oxygen of the SLG substrate could be

detected by EPMA, these impurity levels may be regarded as

negligible. Fig. 3c shows the compositional depth profile of the

CZTS films annealed at 530 �C for 30 min. No significant

compositional variation can be observed across the film. The

average composition across the films was also similar to the

starting precursor composition. This means that the precursor

particles are homogeneously well-dispersed in the ink, resulting

in CZTS phase formation with uniform composition after heat

treatment even though four individual precursor particles are

involved.

The optical absorption coefficient (a) is obtained from the

measured spectral transmittance (Tl) and reflectance (Rl) data

using the following formula:

a ¼ 1/t ln[(1 � Rl)2/Tl] (1)

where t is the thickness of the film. The nature of the optical

transitions and the optical band gap (Eg) of the film are obtained

from eqn (2):

a ¼ A(hn � Eg)n/hn (2)

Energy Environ. Sci., 2012, 5, 5340–5345 | 5341

Fig. 1 (a) TG-DSC analysis of the ethanol-based CZTS precursor ink. This analysis was performed under a nitrogen atmosphere. (b) XRD analysis of

the CZTS film as a function of the annealing temperatures. Enlarged graphs in the 2q-angle range from 28� to 33� are displayed to show low-intensity

peaks. (c) Microstructure evolution of the CZTS film as a function of the annealing temperatures ranging from 400 to 530 �C. The precursor films were

annealed under N2 + H2S (5%) atmosphere in a tubular furnace.

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where A is a constant. The exponent n can take values of 2, 1/2, 3

or 3/2 for indirect-allowed, direct-allowed, indirect-forbidden or

direct-forbidden transitions, respectively. The values of a are

found to obey eqn (2) for n ¼ 1/2 , indicating that the optical

transitions are direct-allowed in nature. Therefore, the Eg of

CZTS with direct transitions can be determined by applying

eqn (3):

(ahn)2 ¼ A(hn � Eg) (3)

Fig. 2 Raman spectra of the CZTS films as a function of annealing t

5342 | Energy Environ. Sci., 2012, 5, 5340–5345

The optical band gap is determined by extrapolating the linear

region of the plot (ahn)2 versus hn and taking the intercept on the

hn-axis. Fig. 3d presents (ahn)2 versus hn plots of the CZTS films

annealed at temperatures of 500 and 530 �C. The direct optical

band gap energy is found to be 1.44 eV for the CZTS film

annealed at 530 �C. This Eg value measured from our CZTS films

is similar to the band gap of bulk CZTS reported by others. In

contrast, for the sample that was heat treated at 500 �C, the bandgap values of the CZTS films increased to 1.66 eV. This increase

emperatures. Detailed graphs indicate traces of unreacted phases.

This journal is ª The Royal Society of Chemistry 2012

Fig. 3 (a) Cross-sectional image of the film annealed at 530 �C. (b) Composition mapping at the surface of the CZTS films annealed at 530 �C by

EPMA. (c) Component depth profile by Auger electron spectroscopy. (d) Band gap energy of the CZTS films annealed at 500 and 530 �C.

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might be due to the presence of Cu2S, which has the direct optical

band gap in the range 1.7–2.16 eV.27,28

We fabricated thin film solar cells using non-toxic solvent-

based ink. The CZTS precursor layer of �2.9 mm in thickness

was formed by three spin coatings. The precursor film was dried

at 80 �C followed by annealing under N2 + H2S (5%) atmosphere

at 530 �C for 30 min. Chen et al. reported that Cu-poor and Zn-

rich conditions improve the efficiency of the CZTS solar cells

because a Cu-poor composition enhances the formation of Cu

vacancies, which gives rise to shallow acceptors in the CZTS,

while a Zn-rich condition suppresses the substitution of Cu at Zn

sites, which results in relatively deep acceptors.29 Therefore, our

film composition for the cell performance measurement was

selected to include Cu-poor and Zn-rich compositions (approx-

imately Cu/(Zn + Sn) ¼ 0.8 and Zn/Sn ¼ 1.2) (see ESI, Fig. S2†).

Table 1 Composition ratios at the surface of the CZTS films annealed at 53

Composition ratio (before heat treatment) Composition ratio

Cu/(Zn + Sn) Zn/Sn S/metal Cu/(Zn + Sn)

1.06 0.84 1.0 0.99

This journal is ª The Royal Society of Chemistry 2012

The sintered CZTS absorber films were processed into photo-

voltaic devices following standard procedures, including the

chemical bath deposition of CdS (�50 nm), DC sputtering of

i-ZnO (�50 nm), RF sputtering of ITO (�250 nm), and thermal

evaporation of a patterned Ni/Al grid as the top electrode, as

shown in Fig. 4a. Finally, the samples (2 � 2.5 cm2) were

mechanically scribed into the cells with a total area of 0.25 cm2.

The current–voltage (I–V) characteristics for our best performing

CZTS solar cell measured in the dark and under AM 1.5 illu-

mination are shown in Fig. 4b. All device performance param-

eters were reported based on the cell area, excluding the shaded

areas (�11% of the total device area) by the Ni/Al finger elec-

trode. The as-fabricated device exhibited a total area efficiency of

5.14% [open-circuit voltage (Voc)¼ 0.517 V, short-circuit current

density (Jsc) ¼ 18.86 mA cm�2, fill factor (FF) ¼ 52.8%]. Fig. 4c

0 �C for 30 min by EPMA

(after heat treatment at 530 �C under N2 + 5% H2S)

Zn/Sn S/metal Atomic ratio % (Cu/Zn/Sn/S/O/C)

0.93 0.98 23.5 : 11.5 : 12.3 : 46.2 : 3.5 : 2.9

Energy Environ. Sci., 2012, 5, 5340–5345 | 5343

Fig. 4 (a) Cross-sectional image of the CZTS thin film solar cell. Inset

shows a photograph of the as-fabricated solar cell. The samples (2 � 2.5

cm2) were mechanically scribed into the cells with a total area of 0.25 cm2.

Note that the stoichiometry of the CZTS film was selected to yield Cu-

poor and Zn-rich compositions (approximately Cu/(Zn + Sn) ¼ 0.8 and

Zn/Sn¼ 1.2). (b) Current–voltage (I–V) characteristics of the CZTS solar

cell annealed at 530 �C for 30 min. The efficiency of the cell is 5.14%

under standard AM 1.5 illumination. (c) External quantum efficiency

(EQE) curve of the corresponding cell. The band gap of the absorber

layer is determined to be 1.51 eV by a plot of [E ln (1 � EQE)]2 vs. E, as

shown in the inset.

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shows the external quantum efficiency (EQE) of the corre-

sponding solar cell as a function of photon wavelength. The

maximum quantum efficiency of 74% is obtained for a photon

wavelength of 540 nm. The band gap of the absorber layer is

determined to be 1.51 eV by fitting a plot of [E ln(1 � EQE)]2 vs.

5344 | Energy Environ. Sci., 2012, 5, 5340–5345

E near the band edge, as shown in the inset of Fig. 4c. The band

gap energy of a CZTS film with the Cu-poor and Zn-rich

composition is larger than the stoichiometric film (1.44 eV). The

observed value is reasonable since the band gap energy of CZTS

shifts to higher energies as Cu/(Zn + Sn) decreases.30

Although the initial efficiency of our cell is lower than those of

the other solution-processed cells such as those produced by the

hybrid slurry method (9.6%) and nanocrystal dispersion

(7.2%),4,20 improvements in device performance are expected

with further studies to resolve several issues. A possible reason

for the low efficiency is the relatively thick CZTS absorber layer.

Although the devices made with thicker CZTS layers absorb

more light, the high intrinsic resistivity of the p-type CZTS

absorber layer itself and the charge carrier trap density inherent

in the thick layer can contribute to the increase in high series

resistance and low short circuit current that lead to losses in

efficiency,4,31 suggesting the fabrication process must be opti-

mized for the thinner film. In addition, fine-tuning the band gap

of the CZTS film through the replacement of S by Se and MgF2

antireflection coating on top of the device are currently underway

to improve the cell efficiency. We believe that resolving these

issues will allow us to produce large and low cost CZTS solar

cells with much higher efficiencies, which is highly desirable for

photovoltaic applications.

Conclusions

In summary, our simple solution-based deposition approach

employs a non-toxic solvent (ethanol)-based ink composed of

commercially available precursor particles. Our readily achiev-

able air-stable precursor ink, without the involvement of

complex particle synthesis, high toxic solvents, or organic addi-

tives, facilitates a convenient method to fabricate a high quality

CZTS absorber layer with uniform composition at the surface

and across the thin depth. Well-dispersed ink containing four

different finely milled precursor particles of low melting points

allows for the CZTS crystallization when annealed at 530 �C and

forms dense films with large grains (1–2.5 mm), possibly by

a reactive liquid-phase sintering between the constituent parti-

cles. The preliminary conversion efficiency and fill factor for the

non-toxic ink based solar cells are 5.14% and 52.8%, respectively,

although the processing details are not yet optimized. Our simple

and safe approach reported here represents the first step toward

realizing low-cost, large-area, high efficiency solar cells.

Acknowledgements

This research was financially supported by the Basic Research

Laboratory (BRL) Program through an NRF grant funded by

the MEST (No. 2011-8-2048). It was also partially supported by

the Second Stage of the Brain Korea 21 Project.

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