High-pressure behavior of disodered kesterite-type Cu2ZnSnS4 [original]

Vol.:(0123456789)

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Applied Physics A (2021) 127:616

https://doi.org/10.1007/s00339-021-04745-w

T.C. : MATERIALS BYDESIGN UNDER PRESSURE: EXPERIMENTS ANDTHEORY

High‑pressure behavior ofdisodered kesterite‑type Cu2ZnSnS4

IliasEfthimiopoulos1,2 · TimKüllmey3· SergioSpeziale1· AnnaS.Pakhomova4· MarcelQuennet3,5· BeatePaulus3·

AnnaRitscher5,6· MartinLerch6

Received: 24 January 2021 / Accepted: 7 July 2021 / Published online: 22 July 2021

Abstract

We have investigated the high-pressure structural and vibrational behavior of the disordered kesterite-type Cu2ZnSnS4 com-

pound at ambient temperature. Our experimental and theoretical investigations have revealed a clear structural transition

to a GeSb-type phase close to 15GPa, a tetragonally distorted variant of the NaCl-type phase. The latter transformation is

accompanied by a cationic coordination increase from fourfold to sixfold with respect to the sulfur anions. In addition, a

change in the compressibility rate was detected at about 8GPa within the pressure stability range of the disordered kesterite-

type phase. Upon decompression, a disordered zinc blende/sphalerite structure is recovered. We discuss our findings in close

conjunction with our recent high-pressure work on the ordered Cu2ZnSnS4 modification.

Keywords Kesterite· Pressure· Structure· XRD· Raman

1 Introduction

The quaternary semiconductor Cu2ZnSnS4 has attracted

considerable attention in recent years due to its potential

use as a solar absorber [1, 2]. The suitability of this mate-

rial in photovoltaic applications originates from its almost

optimal optical band gap (Eg ~ 1.5eV), its high absorption

coefficient in the visible range (~ 104 cm−1), and its earth-

abundant, low-cost, and non-toxic elemental constituents

[3–5]. The current power conversion efficiency for the

pristine Cu2ZnSnS4 thin films, however, has not reached

the theoretical limit of ~ 30% [6]. Hence, it is essential to

explore and understand the physical properties and the phase

diagram of this material further, so as todevelop strategies

to enhance in turn its photovoltaic performance.

From a structural aspect, Cu2ZnSnS4 adopts the ordered

kesterite-type (KS) structure at ambient conditions (SG I

, Z = 2, Fig.1) [7, 8]. This phase is derived from the well-

known zinc blende (ZB)/sphalerite-type structure (SG F

2m, Z = 4) by doubling the respective c-unit-cell parameter,

resulting from the alternating cationic arrangement of Cu/

Sn and Cu/Zn layers separated by sulfur anions [9]. Conse-

quently, all of the cations are tetrahedrally coordinated with

respect to the anions. Even though the KS phase represents

an ordered cationic arrangement, with each cation occupying

a unique Wyckoff site, cationic disorder is quite common in

this material [7, 10–13]. The most common type of disorder

is associated with the mutual anti-site exchange of the Cu+

and Zn2+ cations within the z = 1/4 and z = 3/4 layers, which

results in a disordered kesterite-type structural configura-

tion (DKS) with slightly higher symmetry compared to KS

(SG I

2m, Z = 2, Fig.1). This type of disorder leads also

to a reduction of the band gap Eg by up to ~ 0.2eV [14, 15].

Some interesting observations were made regarding the

compressibility behavior and structural transitions of the KS

and DKS phases under pressure. In particular, application

of compressive stress was predicted to increase the Eg of

Cu2ZnSnS4, whereas a KS → stannite structural transition

was found at about 32 GPa from theoretical studies [16–18].

* Ilias Efthimiopoulos

[email protected]

1 GFZ German Research Center forGeosciences,

Telegrafenberg, 14473Potsdam, Germany

2 Institut Für Physik, Universität Greifswald,

Felix-Hausdorff-Str. 6, 17489Greifswald, Germany

3 Institut Für Chemie Und Biochemie, Freie Universität Berlin,

Takustr. 3, 14195Berlin, Germany

4 Deutsches Elektronen-Synchrotron, PETRA III,

22607Hamburg, Germany

5 Helmholtz-Zentrum Berlin Für Materialien Und Energie,

Hahn-Meitner-Platz 1, 14109Berlin, Germany

6 Institut Für Chemie, Technische Universität Berlin, Straße

des 17. Juni 135, 10623Berlin, Germany

I.Efthimiopoulos et al.

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616 Page 2 of 11

Our recent combined experimental and abinitio computa-

tional investigations on the KS Cu2ZnSnS4, however, dis-

missed the aforementioned KS → stannite transition; on the

contrary, a concomitant structural and electronic transfor-

mation toward a metallic GeSb-type phase (SG I4/mmm,

Z = 2), a distorted variant of the NaCl-type structure, was

discovered close to 15 GPa [19, 20]. A closer inspection of

the KS compressibility behavior, however, revealed clear

discontinuities in the compressibility of the tetragonal KS

a- and c-axis close to 6–8GPa, whereas no visible effect

was detected in the KS volume [20]. Such behavior is known

to reflect (subtle) structural-related transitions in materi-

als [21–23]. Even though the reason for this effect in KS

Cu2ZnSnS4 is not fully understood, a plausible scenario put

forward earlier involved a pressure-activated Cu/Zn cationic

disorder [19, 20].

Partly motivated by our aforementioned high-pressure

studies on the KS modification, and given that in the major-

ity of synthetic Cu2ZnSnS4 samples the Cu/Zn disorder is

always present to some extent [24–26], we have investigated

the pressure-induced structural and vibrational behavior of

the DKS Cu2ZnSnS4 compound by means of X-ray diffrac-

tion (XRD), Raman spectroscopy, and density functional

theory (DFT) calculations. Similar to KS, DKS Cu2ZnSnS4

undergoes a pressure-induced DKS → GeSb-type structural

transition close to 15GPa, with a disordered ZB-type struc-

ture being recovered upon full decompression [20]. The most

striking result, however, came from the close inspection of

the DKS compressibility behavior; the latter revealed certain

similarities with its KS counterpart, with a clear compress-

ibility change taking place close to 8GPa. This compress-

ibility change is reflected mainly in the pressure-induced

behavior of the a/c axial ratio (i.e., the a- and c-axis exhibit

similar compressibilities) above 8GPa, stemming most

likely from the expansion of the Sn–S bond length beyond

that pressure. The latter effects are common in both the KS

and DKS Cu2ZnSnS4 configurations. On the contrary, the

clear discontinuities observed previously in the F-f plots of

the KS Cu2ZnSnS4 modification were not detected in the

DKS case, implying that a potential pressure-induced Cu/

Zn disorder might be indeed at play for KS Cu2ZnSnS4 [20].

2 Experimental andcomputational details

The investigated disordered Cu2ZnSnS4 sample was avail-

able in polycrystalline powder form. Synthesis and charac-

terization details can be found elsewhere [27–29].

Pressure was generated with symmetric diamond anvil

cells (DACs) equipped with diamonds of 400μm culet diam-

eters. Drilled pre-indented rhenium gaskets with hole diam-

eters of 150–200μm served as sample chambers in separate

runs. The ruby luminescence method was used for measuring

pressure [30]. The error in pressure determination was ΔP

≈ 0.1GPa for the low pressure range (0 ≤ P ≤ 15GPa) and

ΔP ≈ 0.3GPa for the high pressure range (15 ≤ P ≤ 27GPa).

Argon served as a pressure transmitting medium (PTM) in

all experiments, in order to facilitate direct comparison

with our earlier high-pressure investigations on the ordered

Cu2ZnSnS4 modification [19, 20].

The high-pressure Raman measurements were conducted

with a Horiba Jobin Yvon LabRam HR800 VIS single-stage

Raman spectrometer, equipped with a blue (λ = 473nm)

diode-pumped solid-state laser, a 20 × objective lens, an

1800l/mm diffraction grating, and a Peltier-cooled charge-

coupled device (CCD) detector. The measured frequency

range was 100–800 cm−1, with the collection time set to 3

accumulations of 10min each. The incident laser power was

kept below 1mW, in order to avoid any potential damage

on the sample [31]. The Raman-relevant parameters were

obtained from the fitting of the Raman spectra with Lorentz-

ian functions, accompanied by linear background correction/

subtraction.

Angle-resolved high-pressure powder XRD measure-

ments were performed at the Extreme Conditions Beam-

line P02.2 of PETRA III (Hamburg, Germany) [32] with

an incident X-ray wavelength λ = 0.289Å and a beam size

of 2μm × 2μm. Two-dimensional XRD patterns were

collected with a fast flat panel detector XRD1621 from

PerkinElmer (2048 pixels × 2048 pixels, 200 × 200μm2

pixel size) and processed with the FIT2D software [33].

Refinements were performed using the GSAS + EXPGUI

Fig. 1 Unit cell of the kesterite (KS, SG I

, Z = 2, left) and disor-

dered kesterite (DKS, SG I

2m, Z = 2, right) Cu2ZnSnS4 modifi-

cations. The brown, green, gray, yellow, and black spheres represent

Cu+, Zn2+, Sn4+, S2−, and mixed Cu+/Zn2+ ions, respectively. Notice

that in the KS structure, the Cu+ and Zn2+ cations lying in the same

plane occupy the distinct Wyckoff sites 2c and 2d, respectively. In the

DKS structure on the other hand, the respective Cu+ and Zn2+ cations

are randomly distributed in these two positions, leading to a change in

the space group from I

to I

2m, whereas the Wyckoff positions 2c

and 2d transform to position 4d

High-pressure behavior ofdisodered kesterite-type Cu2ZnSnS4

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Page 3 of 11 616

software packages [34]. The Birch–Murnaghan equation

of state (B–M EoS) function [35, 36] was fitted to the

obtained pressure–volume (P–V) data of each Cu2ZnSnS4

phase. Given that the Ar PTM becomes solid at 1.4 GPa

[37], we used its EoS as an additional pressure calibrant

[38, 39].

The periodic density functional theory (DFT) calcula-

tions were performed with the Vienna Abinitio Simula-

tion Package (VASP) 5.3.5 in the athermal limit [40–43].

A plane wave basis set with an energy cutoff of 550eV

with the projector-augmented wave (PAW) potentials [44,

45] was used, whereby the 4s and 3d electrons of Cu and

Zn, the 5s, 5p, and 4d electrons of Sn, and the 3s and

3p electrons of S were explicitly considered. The elec-

tronic convergence criteria were set at least to 10–5eV,

whereby the block Davidson algorithm was applied as

implemented in VASP. The structural relaxation of inter-

nal and external lattice parameters was set to a force con-

vergence of 4 × 10–2eV/Å2, performed with the conjugate

gradient algorithm implemented in VASP. The freedom of

spin polarization was enabled, and a Gaussian smearing

approach with a smearing factor σ of 0.01eV was utilized

for all structural optimizations.

The DKS and the high-pressure GeSb-type phases of

Cu2ZnSnS4 were treated in the same manner as stated before

[15, 20, 46]. In a nutshell, for the DKSstructure, we use the

most stable Cu ↔ Zn disorder pattern which can be found

in the KS unit cell, with SG P

2c [47]. A total of 16 atoms

were simulated, which corresponds to the number of atoms

in the DKS unit cell (Z = 2). On the other hand, for the GeSb-

type phase, we omit the disorder in our model and calculate

only one distribution of the cations (Fig.2), rather than

averaging over all possible distributions. This approach is

sufficient to predict the transition pressure, since the energy

differences between the different GeSb-type cationic pat-

terns are smaller compared to the difference associated with

the DKS → GeSb-type structural change. For the GeSb-type

phase, we created 1 × 1 × 2 supercells to match the number

of atoms of the DKS phase. The cells were fully optimized

with a 8 × 8 × 4k-grid constructed via the Monkhorst–Pack

scheme [48] and centered at the Γ-point with the Perdew,

Burke, and Ernzerhof (PBE) functional [49].

The EoS for each phase was determined by selecting vol-

ume points in a range of about ± 100Å3 around the minima.

This corresponds to a pressure range of 0–100GPa. We used

a step size of 8Å3, which led to 23 (DKS) and 24 (GeSb-

type) volume points. At each volume point, we optimized

the cell shape and the atomic positions. We fitted the B–M

EoS function [35, 36] to the total energy E as a function of

volume V for each Cu2ZnSnS4 phase. Then, the pressure of

each volume was obtained from the P(V) formulation of the

same B–M EoS. Consequently, the enthalpy (H = E + PV)

can be obtained for each phase as a function of pressure.

3 Results anddiscussion

3.1 Effects ofdisorder intheRaman spectra

of Cu2ZnSnS4

Before proceeding to our high-pressure results for the DKS

phase, we find it useful first to briefly address the effects of

the Cu/Zn anti-site disorder in the structural and vibrational

properties of Cu2ZnSnS4. As we mentioned earlier, this type

of disorder is mainly restricted in the z = ¼ and z = ¾ cati-

onic planes of the I

KS phase due to the ZnCu and CuZn

anti-site exchange (Fig.1) [11, 25, 28]. Consequently, this

random distribution of the Zn2+ and Cu+ cations leads to

the adoption of the ‘more symmetric’ I

2m DKS phase.

Except from the change in space group, the Cu/Zn disorder

results also in an expansion of the c-axis due to the inequiva-

lence of the Zn–S and Cu–S bond length values; the a-axis,

on the other hand, does not appear to be affected [15, 46].

Regarding now the Raman activity, a total set of 14

Raman-active modes is expected for the DKS phase [24, 50]:

whereas 15 Raman-active modes are predicted for the

ordered Cu2ZnSnS4 KS phase [51]:

In Fig.3, we compare the Raman spectra of the DKS and

KS Cu2ZnSnS4 modifications measured at ambient condi-

tions with the same experimental parameters (λ = 473nm).

Both of these KS and DKS samples were readily available

(1)

Γ=2A1

2B1

4B2

(2)

Γ=3A +6B + 6E

Fig. 2 The GeSb-type unit cell (SG P4/mmm, Z=) used in our DFT

calculations. In order to match the number of atoms of the DKS unit

cell, we use 2 GeSb-type unit cells stacked along the c-axis. The

brown, magenta, gray, and yellow spheres represent Cu+, Zn2+, Sn4+,

and S2− ions, respectively. Notice that in the GeSb-type structure, all

of the Cu+, Zn2+, Sn4+ cations are randomly distributed in the same

Wyckoff site 2a, whereas the S2− anions occupy the Wyckoff position

I.Efthimiopoulos et al.

1 3

616 Page 4 of 11

in polycrystalline powder form and originate from the

same source [27, 28]. Generally, the ambient-pressure KS

Cu2ZnSnS4 Raman response is consistent with the reported

Raman spectra collected either with green or blue laser

light excitation, with the most intense Raman features lying

at ~ 291 cm−1 and ~ 338 cm−1 [13, 51–53]. These Raman

peaks correspond to vibrations of A symmetry and reflect

sulfur motions mainly along the ab plane (291 cm−1) and the

c-axis (338 cm−1), respectively [54].

Coming now to the DKS sample, the main Raman-related

differences between the KS and DKS modifications are: (1)

the slight broadening (by ~ 4 cm−1) and frequency downshift

of the strongest 338 cm−1 KS peak (down to 335 cm−1) and

(2) the notable Raman intensity reduction of the 291 cm−1

KS mode. Both of these observations are consistent with the

KS → DKS Raman-related changes reported in the litera-

ture [13, 31]. We should point out that the frequency drop

of the strong 338 cm−1 KS peak (S motions along c-axis)

upon passing to the DKS modification is connected with

the expansion of the c-axis in the latter due to the Cu/Zn

exchange. Given that the Cu/Zn cationic disorder does not

seemingly affect the a-axis as mentioned earlier; however, no

apparent frequency shift would be expected for the 291 cm−1

KS peak present also in the DKS phase (S motions mainly

along the ab-plane). Contrastingly, the Cu/Zn disorder has

a pronounced influence on the 291 cm−1 Raman peak inten-

sity, leading to a substantial reduction (Fig.3); the latter

effect has been frequently used in the literature as a means

of monitoring the Cu/Zn disorder in Cu2ZnSnS4 thin films

via resonant Raman scattering [55–57].

We finally mention that the presence of Cu/Zn disorder

diminishes the electronic/optical band gap of Cu2ZnSnS4 by

up to 200meV, thus effectively affecting the photovoltaic

properties of the material [5, 16, 56].

3.2 High‑pressure Raman investigation ofDKS

Cu2ZnSnS4

In Fig.4, we present collectively our high-pressure Raman

spectroscopic results on DKS Cu2ZnSnS4. At ambient condi-

tions, we could resolve 10 Raman-active modes for the DKS

modification (Table1) out of the 14 expected (Eq.1). We

note that only 3–4 Raman-active modes are clearly visual-

ized in our Raman spectra (Fig.4a); the remaining Raman

features arise from the careful inspection and deconvolution

of the low-intensity part of the measured spectra.

As already shown (Fig.3), the strongest Raman features

of the DKS phase at ambient conditions are the two Raman

peaks at 291 cm−1 and 335 cm−1. Both of these modes, as

well as the rest of the resolved DKS Raman peaks, exhibit

a rather regular behavior under compression, with their

frequencies shifting to higher values upon increasing pres-

sure (Fig.4b). Contrary to its KS counterpart, no additional

changes are observed in the DKS Cu2ZnSnS4 Raman spectra

upon increasing pressure, e.g., the gradual emergence of a

sideband in the strongest 335 cm−1 Raman mode as in the

KS case [19].

A clear change of the DKS Cu2ZnSnS4 Raman spectra

can be evidenced close to 15GPa, with the vanishing of the

DKS-related Raman features and the appearance of a broad

band centered at ~ 320 cm−1 (Fig.4a). In conjunction with

our high-pressure XRD results presented in the next Section,

this Raman-related change is the signature of a pressure-

induced structural transition of the DKS Cu2ZnSnS4 com-

pound toward a disordered GeSb-type phase, similar to KS

Cu2ZnSnS4 [19]. We should note here that no first-order

Raman activity is expected for the GeSb structure according

to group theory [61]; the size inequivalence of the Cu, Zn,

and Sn cations occupying the unique Wyckoff site available

in the GeSb-type structure, however, may result in disorder-

induced Raman activity [62].

Upon full decompression, we recover a Raman signal

reminiscent of the original DKS phase, yet with substantial

disorder as indicated by the broad Raman features (Fig.4a).

As we discuss below, this recovered phase corresponds to

a disordered zinc blende/sphalerite-type structure, a differ-

ently disordered phase compared to the DKS Cu2ZnSnS4

modification. We note finally that both the GeSb-type and

ZB-type Raman spectra of the DKS and KS Cu2ZnSnS4

modifications appear essentially identical {see Fig.4a above

& Fig.2b from Ref. [19]}.

200 300 400

DKS

).u.a(ytisnetninamaR

Raman shift (cm

-1

)

Cu2ZnSnS4

P = 1 bar

291

338

335

Fig. 3 Raman spectra of the DKS (black, top) and the KS (blue, bot-

tom) Cu2ZnSnS4 modifications at ambient conditions (λ = 473nm)

High-pressure behavior ofdisodered kesterite-type Cu2ZnSnS4

1 3

Page 5 of 11 616

3.3 Structure ofDKS Cu2ZnSnS4 underpressure

Following our Raman results, we have performed insitu

high-pressure XRD investigations on DKS Cu2ZnSnS4 in

order to identify the structural changes under compres-

sion. We should mention that minor traces of Cu2S were

detected at ambient conditions, a well-known secondary

phase formed during the synthesis of Cu2ZnSnS4 [12].

The results are presented in Fig.5. We can observe that

the DKS phase persists up to ~ 15GPa; at this pressure, the

vanishing and merging of individual DKS Bragg peaks, as

well as the appearance of novel Bragg features in the XRD

patterns, denote a clear structural transition. The structural

Fig. 4 a Raman spectra of

Cu2ZnSnS4 at selected pressures

(λ = 473nm, T = 300K). The

black, red, and green spectra

correspond to the DKS, GeSb-

type, and ZB-type phases,

respectively. b Raman mode

frequency evolution as a func-

tion of pressure for the DKS

(black) and the GeSb-type (red)

Cu2ZnSnS4 phases. The solid

lines passing through the data

points representpolynomial

least square fittings. The vertical

dashed line indicates the DKS-

to-GeSb-type transition

200 300 400 500

).u.a(ytisnetninamaR

Raman shift (cm-1)

0.7

2.7

6.5

9.5

13.2

18.8

1 bar (R)

P (GPa)

DKS

(a)

0510 15

200

300

400

500

mc(tfihsnamaR

-1

)

Pressure (GPa)

DKS

(b)

Table 1 Assignment [51, 58,

59], zero-pressure frequencies

ω0, pressure coefficients, and

mode Grüneisen parameters

γi of the Raman-active modes

of DKS Cu2ZnSnS4 and its

high-pressure GeSb-type

modification.

The Raman mode pressure dependence is given by the relation: ωi(P) = ωi0 + aiP + biP2. The mode Grü-

neisen parameters are estimated from the equation: γi = (B0/ωi0) × (∂ωi/∂P), where B0 = 43GPa is the bulk

modulus at ambient conditions for the DKS phase, as estimated here (Table2). The data in parentheses

correspond to earlier results for the KS phase [19]

a We have additionally detected a low-intensity Raman feature at 411 cm−1; since no Raman activity

is expected in this frequency region for either the KS or DKS-Cu2ZnSnS4 phases [51, 59, 60], and the

411 cm−1 band does not correspond to any known Cu2ZnSnS4-related impurity [53], we tentatively assign

it as a second-order Raman feature

Mode symmetry ωi0 (cm−1) ∂ωi/∂P (cm−1/GPa) ∂2ωi/∂P2 (cm−1/GPa2)γi

B2 (E) 150 (146) 2.9 (2.7) − 0.02 0.83

E (B) 163 (168) 3.5 (2.2) − 0.07 0.92

E (E) 251 (254) 5.6 (5.1) - 0.96

E (A) 273 (270) 4.8 (5.1) - 0.76

A1 (A) 290 (291) 5 (4.2) - 0.74

B1 (E) 307 (299) 6.7 (8.7) − 0.13 (− 0.24) 0.94

A1 (A) 335 (338) 4.5 (4.7) − (− 0.09) 0.58

E (B) 355 (356) 6 (6) − 0.09 (− 0.09) 0.73

B2 (B) 374 (369) 4.9 (4.7) - 0.56

Second-order 411 (411)a7.2 (5.8) − 0.21 (− 0.11) 0.75

GeSb-type 280 (280) 0.1 (0.4)

330 (284) 0.2 (2.8)

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