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CZTS
based Solar Cells- A review.

 

Vinay
Tikkiwal, Deeksha Chandola, Priyanka Kwatra

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Abstract

 

CZTS (Cu2ZnSnS4)
has shown great promise in the preparation of absorption layer for low cost
thin film solar cells because of its ideal band gap, low toxicity and abundance
of the constituents. Several physical and chemical based methods are used to
fabricate CZTS thin films. Sputtering is one of the more prominent methods of
fabrication employed for preparing these CZTS based films. Sputtering offers
better control over the film properties as compared to other methods of
fabrication. Experimental methods using sputtering have led to successful
fabrication of CIGS solar cells, commercially as well. This work summarizes the
developmental work of CZTS films via the sputtering route and the performance
of the solar cells based on them.

 

Keywords:
CZTS, Sputtering, Thin Films, Solar Cells.

 

 

1.
Introduction

Shortage of non-renewable energy sources and pollution of
the earth has been the main cause of concern since the 21st century 1. It is
estimated that the world’s energy demand will be close to 28 Terawatt by 2050
2. In order to meet the high demand levels renewable energy sources must be
exploited. Solar energy is an economic and efficient resource because it is
inexhaustible and relatively pollution free. Photovoltaic (PV) systems have
become popular as they are capable of directly converting sunlight into
electrical energy. PV systems have long life and very low maintenance costs
owing to lack of any mechanical motion. Silicon based PV technology dominates
the solar market today. The best crystalline silicon solar cells have 27%
efficiency while the best thin film solar cells have efficiencies in the 20%
range. Thin film technologies such as CdTe and CuInGaSe2 (CIGS) use
direct band gap compound semiconductors5. Another thin film technology is Cu2SnZnS4
(CZTS), it is more environmentally friendly and abundantly available than
CuInGaSe2. In CZTS indium is substituted by tin, gallium by zinc,
and selenium by sulfur. CZTS based thin films are considered to be an excellent
PV material since it has a band gap in the range of (1.45-1.6) eV and a high
absorption coefficient (> 104 cm-1) 9-10. The cost
of raw material for CZTS based solar technology is significantly lower than
that of the other existing thin film PV technologies 17.

Sputtering

Sputtering is used for depositing materials on the
substrate, by emitting atoms on the substrate and then condensing them in high
vacuum environment. It is processed by high acceleration exchange between the
ions and atoms in the target materials, due to high speed collisions 1. The
target is bombarded with very high energy inert gas ions such as Argon. Vacuum
chamber is filled with argon atoms at a pressure of 1 to 10 m Torr. In between
the target and substrate a voltage source is introduced, which generates
plasma, hot gas?like phase consisting of ions and electrons, in the
chamber.  The Argon ions are charged and are moved at the high speed
towards the target.  Due to this the target atoms travel to the
substrate and finally settle down. . During the process of Argon ionization
electrons which are emitted  are moved at
high speed toward the substrate, leading to collision with other Argon atoms,
generating more ions and free electrons and this process continues. As
large numbers of atoms settle down on the substrate, they form a bond with each
other at the level of molecule, leading to formation of tightly bound atomic
layer which further lead to the formation of thin-film structures.

Sputtering process is mainly of 4 types: (i) DC, (ii) RF,
(iii) DC / RF magnetron and, (iv) Reactive. The system for DC sputtering
contains the planar electrodes, cold cathode and anode. The cathode is covered
with target material to be deposited and the anode is covered with the
substrate. Reactive sputtering is carried out by feeding the chamber with
oxygen or nitrogen along with the argon leading to production of oxidic or
nitridic films. In ac sputtering, for frequencies below 50 kHz, the target
voltage is periodically reversed, and highly mobile ions form a dc diode-like
discharge alternatively on each electrode, where the total potential drop is
near the cathode. The substrate chamber walls can be used as the counter
electrode. At frequencies above 50 kHz, the ions are not mobile leading to the
feeling of applied potential throughout the space between electrodes, due to
which they gain enough energy to cause ionizing collisions. The alternating
positive-negative potential is generated on the surface, by capacatively
coupling the RF voltage to electrode. In first half-cycle, the ions cause sputtering,
by moving at high speed towards the surface and gaining sufficient energy,
while in next half-cycle, electrons prevent building up of charge on surface. RF
sputtering has the disadvantage that it produces the insulating materials
mostly having worst thermal conductivity, large coefficients of thermal
expansion, and are brittle in nature. The magnetron sputtering, can be used
with DC or RF improves the efficiency of sputtering by confining sputtering
source through a magnetron source which causes the electrons to spiral, leading
to ionizing collision thus enabling the plasma to be operated at a higher
density. It causes low heating of substrate and low radiation damage. Fig. 1
shows basic set up of magnetron sputtering.

Fig.1 Magnetron sputtering

In
1988 Ito and Nakazawa used atom beam sputtering to deposit CZTS films on glass
substrates heated upto 240°C 3. The compound target was sputtered by a beam
consisting of mostly neutral particles. Pure Argon was used as the sputtering
gas and was maintained at 0.2 Pa in the sputtering chamber during the film
deposition. The discharge voltage and the current for the atom beam gun were 7
kV and 5mA, respectively. The (112) oriented polycrystalline films were
obtained on  glass substrates heated at
90°C and above while the diffraction curve of the film deposited at a substrate
temperature lower than 50°C contained (220) peak of the CZTS crystal Fig.2..

Figure 2. X-ray diffraction curves
of the films deposited from the CZTS target 3.

All
the deposited films showed p-type conductivity. The resistivity of the film
decreased with the increase in substrate temperature up to 240°C (Fig. 3) while
the Hall mobility of the film was 1 cm2/V-s. The films exhibited an
absorption coefficient greater than 104 cm-1. The direct
optical band gap of the CZTS film was estimated at 1.45eV. film exhibited an
open circuit voltage of 165 mV under AM 1.5 illumination.

 

Figure. 3. Graph showing dependence
of CZTS films resistivity on substrate temperature 3

Seol et al., in 2003 fabricated CZTS thin films using RF
magnetron sputtering . The constituents of the target were fine mixture of Cu2S,
ZnS and SnS2 which were cold pressed at pressure of 250MPa.
After this the films were annealed at the temperature of 250-400°C. Mixture of
Ar and S2(g) was used as the inert gas atmosphere.  When,
RF power greater than 100 W was applied, rapid variation of Cu and Sn contents
with RF power was found which depended on the plasma density Fig. 4. 
However at the RF power from 50 to 100 W, atomic ratio of thin film was fine.
At the RF power of 75 W, S/(Cu+Zn+Sn) ratio of thin film was less than
stoichiometry, while Cu/(Zn+Sn) ratio was close to stoichiometry. The thin
films deposited by as were stochiometric and amorphous in nature and were annealed
in the environment of Ar and S2 (g). When the annealing was
done at the temperature greater than 250°C, thin films became crystallized. Most
of the diffraction peaks of the CZTS thin films were obtained at (1 1 2), (2 0
0), and (2 2 0), (3 1 2) planes Fig.5 and was stochiometric to all the
re?ection of a kesterite structure. The optical absorption coef?cient was found
to be more than 104 cm-1 and energy band gap
was found to be 1.51 eV. The sheet resistance was found to be indirectly
proportion to temperature, and the intensity of the orientation (112) increased
with annealing temperature Fig.6. 

                   Fig.4. Atomic percent vs RF
power curves for CZTS thin films

                                               

                                                           Fig.5. Atomic percent vs annealing
temperature

                                     

                                       Fig.6.
XRD patterns of thin films as a function of annealing temperature

In
2005, Tanaka et al. used a hybrid sputtering system with two sputter sources to
fabricate CZTS films 5.The films were fabricated by sequentially depositing Sn,
Zn, and Cu followed by annealing with S flux. The authors proposed that use of
binary compound ZnS in place of Zn or introduction of S vapor during the
deposition of Zn could help prevent loss of Zn. Increase in the substrate
temperature, led to decrease in the film thickness Fig.9. The adhesion of the
films improved with increase in the substrate temperature.  Absorption coefficient was larger than 104
cm-1, the direct optical band gap was found to be about 1.5 eV.

The
main problem of CZTS prescursor films is moisture adsorption on their surface,
when they are taken out of deposition chamber before sulfurization process. In
the year 2007 Jimbo et al.6 used the reaction of N2 and H2S
(20%) for processing sulfurization in the inline-type vaccum chamber to
fabricate CZTS thin films.  The substrate
was heated till the temperature of  800
°C. The annealing was carried out by transferring precursor to SiC heater. The
testing was done on different samples, and it was found that the constituents
with ratio of  0.87 for Cu/(Zn+Sn) and
1.15 for Zn/Sn were best. The author obtained open circuit voltage of 6.62 mV,
short circuit current of 15.7 mA/cm2, fill factor of 0.55 and
conversion efficiency of 5.74% for this best Cu-lean and Zn-rich sample. According
to the authors annealing in  inline-type
vaccum chamber in which moisture did not adhered to the precursor caused the
conversion efficiency to improve further

Ericson
et al.7 deposited CZTS thin films using the method of reactive sputtering and
determined the stability of the constituents of film. Reaction of CuSn alloy
along with Zn
alloy targets was carried out in H2S.
These alloy constituents removed the disadvantage of Sn having low melting
point. The composition of alloy constituents was varied as Cu- 67% Sn -33%
(99.99% purity) and Cu-65% Sn-35% (99.999% purity). Surface sulfurization is
accompanied by preferential removal of Sn which leads to long term changes in
the Sn concentration in the deposited films. It was proved that Cu2S
formation can be curtailed by two methods; one is by decreasing the partial
pressure of H2S and other by erasing the surface of target
consistently. Table 1 describes various CZTS thin films fabricated under
different conditions and Table 2 explains various solar cells fabricated and
the results of the parameters achieved by them.

               Table 1. Bandgap achieved for CZTS
thin films fabricated by different method.

Year

Author

Temperature

Precursor

Bandgap

2003

Seol et.al.

250–400°C.

Cu2S, ZnS and SnS2

1.51eV

2005

Tanaka et al

300 – 500°C

Cu, Sn, Zn, S

1.5 eV

2013

Wang et. al

750 °C

Cu2S-ZnS-SnS2-S

1.7 eV

2014

Tanaka et al.

530°C

Cu, Zn, Sn

1.23,1.35, and 1.48 eV

2014

Singh et al.

80-450° K

Cu, Zn, and Sn

1.49 eV

 

              Table 2. Various parameters achieved for CZTS
solar cells fabricated by different method.

Year

Author

Open
circuit voltage (mV)

Short circuit current (mA/cm2)

Fill factor

Efficiency

2007

Jimbo et
al.

646

13.7

0.60

5.33%

2013

He et al.

484

14.56

50.1%

3.52%,

2014

Pawar et.
al

561

18.4

48.2

5%

 

Tanaka et al. deposited the CZTS films using the method of
sputtering-sulfurization in the year 2014. The Cu to (Zn+Sn) ratio was varied
between 0.60 and 4.21 and Zn to Sn ratio was varied between 0.29 and 1.68. The
precursors were pre heated at 530 during sulfurization process in an atmosphere
of H2S with N2. Precursor in which Cu quantity was least
and Zn quantity was maximum was found to have the best efficiency. The analysis
of Power spectrum was performed by varying the dependencies of power on the PL
spectrum. The spectrum was found to have 3 emission bands in B1, BT and BB
bands, as shown in figure 7. The BI bands were having maximum intensity in the
PL spectra it was observed that BI band dominated specifically for samples with
Cu to Sn ratio less than equal to 2.0. Authors concluded that with this
composition a deep acceptor state can be formed easily which is required to
achieve high efficiency in solar cells.

Figure 7. PL spectra showing temperature dependence  (a)  PL
group A and (b)  PL group B measured with
24mW excitation power. 8

In
2014 Pawar et. al 9 investigated the various properties of CZTS absorbers
because of the effects of sulfurization temperature.  Mo-coated glass substrates were used on which
CZTS absorbers were grown. Rapid thermal processing (RTP) sulfurization technique
is used in which  metallic precursor of
CZT in stacked forms was deposited using targets Zn , Cu and Sn  in their purest forms ( 99.999%).  The temperature of annealing varying from 500
to 580°C was preferred for the sulfurization. Raman spectra and XRD of the
precursor films Fig. 9 shows that films mainly constituted of  metal elements such as alloys of Zn, Sn and
Cu. This indicates that Cu atoms diffuse into the Zn and Sn metallic ?lms
results in binary inter-metallic phases 11. During the process of
sulfurization authors noted that, the CZTS absorber was formed from stacked
metal precursor due to the significant increase of volume which was around
twice the previous thickness of CZT film. The crystallinity of the absorber
was  improved by increasing the
temperature of sulfurization. The sulfurized CZTS absorber ?lms are polycrystalline
in nature and has dense morphology. The solar cell which are fabricated using
CZTS absorber films are observed under a temperature range of 500°C to 580°C .
They showed highest conversion efficiency of 5% for a 0.44 cm2 area
with Voc to be 561 mV, Jsc=18.4 mA/cm2, and
FF=48.2, at an optimized sulfurized temperature of 580°C.

In
2014, Singh et al. 10 carried out Raman studies on CZTS thin film grown using
a two-step method; co-sputtering Cu, Zn, and Sn metal targets on cleaned lime
glass substrate and sulfurizing it in H2S ambient in the temperature
range of 80-450° K. SEM micrograph study revealed that the film had dense and
compact morphology and the average grain size was in the range of 500-600 nm.
Raman spectrum of CZTS thin film recorded at different ranges of  temperatures with  wave number in range of 200-450 cm-1 (Fig.9
(a)). It was observed that the peak shifts from 337 cm-1- 329 cm-1
when the measurements were carried out at 80 K and 450 K, respectively (Fig.8
(b)). The shift in the Raman peak position with temperature due to the thermal
expansion as well as an harmonic coupling with other phonons which become
active at higher temperature. The experimental values for temperature dependent
Raman frequency matched well with the values plotted using Equation (1):

?p(T)=?0+??expansion
(T)+ ??d(T)                                                                                                                         (1)

where
?0 is the harmonic frequency , ?? expansion (T) is the frequency due
to thermal expansion and ??d(T) is the frequency due to an  harmonic coupling of different phonons.
Therefore, the peak intensity decreases with the increase in measurement
temperature and it also shifts to the lower frequency values. The line width
changed from 17 cm-1 to 26 cm-1 when measurements were
taken at 80 K and 450 K, respectively (Fig.9 (b)), leading to the conclusion
that there is a decrease in the intensity of the peaks. Similarly, at higher
temperature, the activated phonon interacts with the “A” mode phonon resulting
in increase in the FWHM of the peaks. Therefore, for a multi component system
it is necessary to carry out Raman measurements at lower temperature.

                                     

(a)                                                                                                 
                         (b)

Figure. 8.
(a) Raman Spectrum analysis of CZTS thin films observed at different
temperatures and (b) Mode “A” of Raman spectra at 80 and 450K 8

 

  

 

 

 

 

 

                             

Fig.
9. (a) Raman mode A Spectra analysis at room temperature with different counts
(b) Raman “A” mode of CZTS thin film using damped harmonic oscillator
model, (c) temperature dependence of Raman peak (A mode)  with line width.

Recently, in 2015, Cormier et al.
successfully synthesized CZTS, which was crystallized in nature by the method
of reactive magnetron sputtering, in which author used two targets- Zinc and
Copper (67%)–Sn alloy (37%) 13. During deposition, authors used soda lime
glass substrates and varied the substrate temperature in the range of 25°C to
600°C. Raman data indicates that there is strong affect on spectra mainly due
to the substrate temperature particularly especially in the range of 240 to 500
cm-1. For temperatures less than 400°C, broad peaks are shown by
Raman spectra in the range of 160 to 500 cm-1. The disadvantage of
annealing is that it creates the voids because the secondary phases gets
sublimed and as such is not used in the industrial processes 14. Emrani et
al. 15 studied the structural and optical properties of CZTS thin films
fabricated at different sulfurization time duration. Sputtering of precursors
was followed by a short duration sulfurization in dilute H2S EDS
analysis revealed that the annealed films were zinc rich and copper poor which
is ideal for the CZTS solar cells 20. It is observed that higher annealing
temperature result in larger grains but more voids Fig.10 (P1) and (P2). With
the increase in sulfurization time, the grains start to coalesce into larger
sizes Fig. 10 (bottom).

 

 

 

   

 

                                 

 

                                    

Fig.10. Top pictures shows surface morphology of the films
and bottom pictures shows the cross section of the films.

    

                              Table 3. Average
and RMS values of surface roughness measured for different samples

Samples

Sulfurization Conditions

Average(nm)

Root mean square (nm)

P1

590oC, 10 min

           76

                 113

P2

590oC, 60 min

          112

                 152

P3

525oC, 10 min

           86

                 198

 

 

            Table 4
Current-Voltage data of the (P1:P5) samples processed through the sulfurization
processes

 
Sample

Process temperature and time taken
for sulfurization

 
Efficiency (%)

 
VOC (mV)

 
ISC (mA/cm2)

 
RS (? cm2)

 
RSH (? cm2)

 
Fill factor(%)

P1

590oC, 10 min

2.56

441

17.8

13.5

56

36

P2

590oC, 60 min

1.68

434

8.47

18.9

200

42

P3

525oC, 10 min

1.56

446

12.36

34.2

68

29

P4

580oC, 30 min

3.8

492

22.5

14.9

65

34

P5

550oC, 180 min

5.75

593

20.5

8.55

279

48

 

Conclusion
This work has focused on the basic
aspects of sputtering grown CZTS film. The paper provides a brief overview of
the various experiments that have been carried out to prepare these films.
Several factors such as elemental composition of the targets, deposition
temperatures, choice of sputtering gas, substrate temperatures etc. have an
effect on the properties of the prepared films and eventually on the solar
cells fabricated using them. 
Improvements in CZTS based solar cells have helped in achieving a high
efficiency of 6.77% with the sputtering process. This efficiency is higher as
compared with other techniques such as PLD, sol–gel, electro-deposition etc. A
better understanding of the sputtering process and the effect of various
parameters on the film properties is required for the preparation of CZTS based
solar cells with better performance.

 

 

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