NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

A Comparative Study of the 

Defect Point Physics and 

Luminescence of the Kesterites 

Cu2ZnSnS4 and Cu2ZnSnSe4 and

Chalcopyrite Cu(In,Ga)Se2

Preprint

Manuel J. Romero, Ingrid Repins, Glenn Teeter, 

Miguel A. Contreras, Mowafak Al-Jassim, 

and Rommel Noufi

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5200-54140

August 2012
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1

A Comparative Study of the Defect Point Physics and Luminescence of 

the Kesterites Cu2ZnSnS4 and Cu2ZnSnSe4 and

Chalcopyrite Cu(In,Ga)Se2

Manuel J. Romero, Ingrid Repins, Glenn Teeter, Miguel A. Contreras, Mowafak Al-Jassim, Rommel Noufi

National Renewable Energy Laboratory, Golden, CO 80401 USA

Abstract — In this contribution, we present a comparative 

study of the luminescence of the kesterites Cu2ZnSnS4 (CZTS) 

and Cu2ZnSnSe4 (CZTSe) and their related chalcopyrite

Cu(In,Ga)Se2 (CIGSe). Luminescence spectroscopy suggests that 

the electronic properties of Zn-rich, Cu-poor kesterites (both 

CZTS and CZTSe) and Cu-poor CIGSe are dictated by 

fluctuations of the electrostatic and chemical potentials. The 

large redshift in the luminescence of grain boundaries in CIGSe, 

associated with the formation of a neutral barrier is clearly 

observed in CZTSe, and, to some extent, in CZTS. Kesterites can 

therefore replicate the fundamental electronic properties of 

CIGSe.

Index Terms — chalcopyrite, kesterite, thin film, 

cathodoluminescence.

I. INTRODUCTION

The kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4

(CZTSe) are attracting considerable interest because first 

principles calculations predict that their electronic properties 

must be similar to their associated chalcopyrite Cu(In,Ga)Se2

(CIGSe) compounds [ 1, 2]. Kesterite-based solar cells can 

therefore potentially achieve the high conversion efficiencies 

demonstrated by chalcopyrites (> 20%) [3] while covering the 

demand in a projected TeraWatt-scale photovoltaic (PV) 

scenario in which the relative scarcity of indium and the 

competition for this resource with other manufacturing sectors

will limit the expansion of CIGSe-based PV [4,5].

The electronic structure of Cu-poor CIGSe is dominated by 

the formation of copper vacancies (VCu) and the stabilization 

of defect complexes with low formation energies (2VCu– + 

InCu2+) [6,7]. The high densities of shallow donor and acceptor 

levels lead to the delocalization of their associated electronic 

states and introduces local variations in the chemical and 

electrostatic potentials as revealed by luminescence 

spectroscopy [8, 9]. In chalcopyrites, these local variations of 

the chemical potential are accommodated by the spontaneous 

segregation of α-like and (copper-deficient) β-like domains 

(named after the α and β phases of the phase diagram) in the 

nanometer scale [ 10, 11]. This nanoscale morphology is 

regarded as beneficial to the efficiency of the solar cell 

because electrons and holes will dissociate respectively into 

electron-donor (α) and electron-acceptor (β) domains and then 

diffuse to the appropriate electrode within the α or β phase 

(hence reducing recombination).

Another critical aspect to be considered is the electronic 

properties of grain boundaries. In Cu-poor CIGSe, grain 

boundaries develop a neutral barrier for holes that do not 

otherwise impede the electron transport and reduce 

recombination [12,13]. The formation of this neutral barrier 

is largely responsible for the benign behavior of grain 

boundaries [ 14 ] and critical to achieving high conversion 

efficiencies.

In this contribution, luminescence spectroscopy and 

spectrum imaging [15] measurements are applied to Zn-rich, 

Cu-poor kesterite thin films and the results are compared to 

Cu-poor CIGSe. The electronic structure of grain boundaries 

in the kesterite is investigated by luminescence microscopy

and the results compared to those from Cu-poor CIGSe 

obtained by NREL's three-stage process [16].

II. EXPERIMENTAL

CZTS thin films are obtained by coevaporation of Cu, Zn, 

and Sn from three independent effusion cells under a 

molecular beam of S2 supplied by a valved-cracking source. 

The molybdenum-coated glass substrates are actively heated 

to 470 oC. The base pressure of the chamber is 8.0 × 10-6 Torr, 

rising to 1.7 × 10-4 Torr during the deposition. More details, 

including information on the elemental sources, can be found 

in reference 17. The stoichiometry of the films used in this 

study is confined to the region of stability for the kesterite 

phase and slightly displaced towards the ZnS-SnS2 boundary 

of the phase diagram. These CZTS films are therefore Cu-poor 

(Cu/(Zn+Sn) ~ 0.74) and Zn-rich (Zn/Sn ~ 1.36), which is 

consistent with the stoichiometry of the best CZTS solar cells 

reported in the literature [18,19].

CZTSe thin films are obtained by coevaporation from 

elemental sources. The molybdenum-coated glass substrates 

are actively heated to 470-500 oC. In a two-stage process, Cu 

and Zn are first supplied under an overpressure of Sn and Se 

to form the Cu-rich (Cu/(Zn+Sn) > 1) kesterite with copper 

selenide as a secondary phase. During the second stage, the Cu 

source is shut off and the deposition proceeds to form Zn-rich 

((Zn/Sn = 1.15) and Cu-poor (Cu/(Zn+Sn) = 0.86) CZTSe. 

Similarly to the three-stage process for CIGSe, the two-stage 

process for CZTSe revolves around the formation of excess 

copper selenide and the accompanying emissivity signature 

that can be monitored during deposition. More details can be 

found in reference 20.
2

These films are compared to NREL’s high efficiency 

CIGSe, with the standard Cu/(In+Ga) ratio ~ 0.9 and 

Ga/(In+Ga) ~ 0.3, as required for record performance [21].

III. LUMINESCENCE SPECTROSCOPY

The fundamental aspects of the recombination in these 

coevaporated Zn-rich, Cu-poor kesterites are first investigated 

by luminescence spectroscopy and the results compared to 

those previously obtained in Cu-poor chalcopyrites. The 

influence of the excitation density (measured by the electronbeam current Ib[22]) on the emission spectrum at cryogenic 

temperatures (T = 15 K) for CIGSe is shown in Fig. 1a. These 

measurements are complemented by the influence of the 

temperature (∆T = 20-300 K) –Fig. 1b. The spectrum consists 

of a broadband luminescence centered at few hundreds of 

milielectronvolts (meV) below the bandgap Eg. The emission 

spectrum shows a pronounced blueshift with excitation density

(~40 meV over three decades of excitation density) and a 

blueshift with temperature (~50 meV from T = 300 K down to 

T = 20 K).

Fig. 1. Influence of the excitation density (measured by the electronbeam current Ib) on the emission spectrum of Cu-poor CIGSe at 

cryogenic temperatures (T = 15 K). Influence of the temperature (T= 

20-300 K) at Ib = 500 pA.

The luminescence of Cu-poor CIGSe has been extensively 

investigated by other authors [8, 9] and here we summarize 

the aspects most relevant to our comparative study of 

chalcopyrites and kesterites. This is better described by first 

considering the point defect structure of chalcopyrites under 

Cu deficiency. From first principles calculations [ 23 ], the 

dominant donors and acceptors are, respectively, the indiumon-copper-antisite (InCu) (stabilized through the formation of 

(2VCu– + InCu2+) defect complexes) and the copper vacancy 

(VCu). The prevalence of these intrinsic point defects in the 

chalcopyrite structure leads to the delocalization of the donor 

and acceptor states and the formation of bands. In this 

scenario of high density of intrinsic point defects and 

compensation, local deviations in the distribution of donors 

(+) and acceptors (–) will introduce potential fluctuations in 

the band structure [24]. The schematics of the band diagram 

(including these potential fluctuations) are shown in Fig. 2. 

Using this representation, the luminescence includes two 

emission bands: one associated to the band tails of the donor 

and acceptor bands (BT)—which follow the potential 

fluctuations—and the other to transitions between the donor 

and acceptor bands (BB) [9], as shown schematically in Fig. 2.

Fig. 2. Schematics of the band diagram for Cu-poor CIGSe. The 

luminescence is dominated by band-tail (BT) and band-to-band (BB) 

transitions between delocalized donor and acceptor states. Local 

deviations in the distribution of donors and acceptors lead to potential 

fluctuations.

At low temperature and low excitation density (Ib = 50 pA 

in Fig. 1a), the spectrum is dominated by band-tail transitions 

corresponding to the lowest energy states in the landscape of 

potential fluctuations. The blueshift with the excitation density 

at low temperature results from the reduction of the amplitude 

of the potential fluctuations leading to an increase in the 

transition energy (towards BB, Ib = 1 nA in Fig. 1a). In our 

case, we observe a saturation of the transition energy at high 

excitation densities (Ib > 1 nA) below the measured bandgap 

[25]. This suggests that (quasi)-free electron-to-acceptor band 

transitions (associated with the VCu) are dominant at high 

excitation densities (∆n,p > 1017 cm-3

) with no contribution of 

the excitonic transitions observed in near stoichiometric 

chalcopyrites [ 26 ]. The overall blueshift with temperature 

(Fig. 1b) is a direct consequence of the thermal excitation of 

electrons (holes) from the band-tail states to the donor 

(acceptor) band—resulting in the quenching of the BT 

transition observed at moderate excitation densities Ib = 500 

pA, Fig. 1b. It also results in the increase in the energy of free 

electrons (holes) in the donor (acceptor) bands with further 

increasing temperature—resulting in the blueshift of the BB 

transition.

Similar spectroscopy measurements in Zn-rich, Cu-poor 

kesterites reveal a near one-to-one correspondence with Cupoor CIGSe (Fig. 3). The spectrum also consists of BT and 

BB emission bands with similar excitation density and 

temperature dependencies (not shown) to those observed in
3

chalcopyrites. There is a dramatic loss of quantum efficiency 

of the luminescence of the CZTS when compared to CIGSe 

(0.02 to 1), which improves significantly for CZTSe but 

remains well below than that of CIGSe (0.2 to 1).

Fig. 3. Influence of the excitation density (measured by the electronbeam current Ib) on the emission spectrum of Zn-rich, Cu-poor CZTS 

and CZTSe at cryogenic temperatures (T = 15 K).

In the coevaporated CZTS, the high recombination led by 

antisite substitutions between the cations Cu(I), Zn(II), and 

Sn(IV), sulfur vacancies (VS) and their associated electronic 

levels deep inside the bandgap [2] overruns the prospective 

beneficial effects of the potential fluctuations. Because of the 

high contribution of nonradiative recombination, it is very 

difficult to extract information on the amplitude and length 

scale of these fluctuations.

In the coevaporated CZTSe, on the other hand, the blueshift 

with the excitation density at cryogenic temperatures is 

estimated in ~40 meV over three decades of excitation density 

(to transition from BT to BB), and saturates at Ib ~ 2 nA with 

BB at ~100 meV below the estimated bandgap. These are very 

similar values to those obtained from CIGSe, suggesting that 

the amplitude and length scale of the fluctuations of the 

electrostatic potential are comparable in both kesterite and 

chalcopyrite selenides (see Fig. 4). Indeed, similar transition 

energy at saturation (relative to the bandgap) points toward a 

large number of Cu vacancies for both CIGSe and CZTSe: the 

VCu-acceptor state has similar shallow levels at ε(–/0) = 0.02 

eV in CZTSe and ε(–/0) = 0.03 eV in CIGSe. Although these 

measurements suggest that the copper vacancies are primarily 

involved in the band-to-band transitions in CZTSe, it is likely 

that the deeper acceptor level at ε(–/0) = 0.12 eV associated 

with copper-on-zinc antisites (CuZn)—the most energetically 

favorable point defect in CZTS(e)—also participates in the 

formation of the acceptor band, even under Cu-poor, Zn-rich 

conditions.

To reproduce the fluctuations, the formation of neutral 

(2VCu– + ZnCu2+

) defect complexes to accommodate the large 

number of Cu vacancies in the kesterite structure (under Znrich conditions) might parallel that of the (2VCu– + CuIn

2+) in 

chalcopyrites [2].

Fig. 4. Schematics of the band diagram for Cu-poor CIGSe and Cupoor, Zn-rich CZTSe. Based on the results of the luminescence 

spectroscopy measurements, the amplitude and length scale of the 

potential fluctuations should be similar in CIGSe and CZTSe.

All these results suggest that the electronic properties of Znrich, Cu-poor kesterites (both CZTS and CZTSe) and Cu-poor 

CIGSe are dictated by fluctuations of the electrostatic and 

chemical potentials. From the increase in the efficiency of the 

luminescence (CZTSe vs. CZTS), the density of midgap states

is largely mitigated in the selenide and, consequently, the 

point defect structure of CZTSe becomes more closely related 

to that of CIGSe.

IV. GRAIN BOUNDARY LUMINESCENCE

In Section III, we have shown evidence that (for the 

coevaporation route) the point defect physics of Zn-rich, Cupoor CZTSe and Cu-poor CIGSe are closely related, with Znrich, Cu-poor CZTS being somehow related. Along with the 

point defect physics, another critical aspect of the electronic 

properties in Cu-poor CIGSe is the benign behavior of grain 

boundaries [14]. The leading explanation is based on the 

formation of a large neutral barrier for holes that do not 

otherwise impede the electron transport and reduce 

recombination [12]. The existence of this neutral barrier has 

been confirmed experimentally by several methods [13,27]. 
4

Among these methods, luminescence reveals a redshift of the 

emission spectrum at grain boundaries in CIGSe consistent 

with the incorporation of VCu acceptors to the boundary, as 

predicted by the neutral barrier model.

Fig. 5a shows a map of the photon energy of the 

luminescence from CIGSe (color coded so that red and 

blueshifts in the emission spectrum are intuitive [29]). We 

observe a 10 to 15 meV redshift of the luminescence at grain 

boundaries when compared to the grain interiors. This is 

consistent with the copper depletion observed at grain 

boundaries by atom probe tomography [30]: the incorporation 

of VCu-acceptor states will extend the acceptor band into the 

gap, hence the observed redshift. We have firmly established 

that this redshift is absolutely critical in achieving high 

efficiency in CIGSe and must be related to the formation of 

the neutral barrier.

In the kesterites, the redshift at the grain boundaries is not 

clearly seen for CZTS (see Fig. 5b), and the spatial variation

in the emission spectrum is largely dominated by grain-tograin inhomogeneity. The redshift at grain boundaries, when 

present, is much less pronounced (only 2-4 meV). In the 

CZTSe, on the other hand, we can clearly observe this redshift 

at the grain boundaries (see Fig. 5c). We are finding grain 

boundaries in CZTSe where the magnitude of the redshift is 

par with that of the grain boundaries of CIGSe (10-15 meV). 

This is an extremely important result suggesting that the 

kesterite can duplicate the electronic structure of the grain 

boundaries found in chalcopyrites, possibly because of similar 

atomistic configuration and point defect structure. 

Unfortunately not all boundaries show this effect, in contrast 

to CIGSe where this redshift is commonplace. The higher 

degree of cation disorder of the kesterite structure—induced 

by substitutions among the cations Cu(I), Zn(II), and Sn(IV)

— can account for the variability in the defect point structure 

of grain boundaries and their luminescence.

V. SUMMARY AND CONCLUSIONS

We have found that the point defect luminescence of Znrich, Cu-poor CZTSe and Cu-poor CIGSe are closely related, 

with Zn-rich, Cu-poor CZTS being somehow related. The 

results of the spectroscopy measurements are consistent with 

high densities of intrinsic point defects with donor and 

acceptor character leading to fluctuations of the electrostatic 

and chemical potentials.

Not only does Zn-rich, Cu-poor CZTSe mimic very 

exclusive aspects of the point defect luminescence of Cu-poor 

CIGSe, but the grain boundaries of the kesterite can replicate 

the luminescence observed in chalcopyrites, in agreement with 

the neutral barrier model. This is an extremely important 

result, as the formation of this neutral barrier is largely 

responsible for the benign behavior of grain boundaries and 

obviously critical to achieving high conversion efficiencies.

In conclusion, for these films obtained by coevaporation, 

the recombination processes operating in Zn-rich, Cu-poor 

CZTSe are closely related to its relative Cu-poor CIGSe and 

the similarities between the kesterite and chalcopyrite 

selenides are more pronounced than in the case of the kesterite 

sulfide. As a result, current conversion efficiencies for CZTSe 

solar cells at NREL are higher than those of CZTS. Moving 

forward to higher efficiencies will require a reduction of the 

variability in the point defect structure within grain interiors 

and particularly among the grain boundaries of CZTSe.

Fig. 5. Spectrum imaging measurements of the luminescence. (a) A representative map of the photon energy of the luminescence from 

CIGSe thin films used in PV applications. A redshift of the emission spectrum (10 to 15 meV) is observed at grain boundaries. Similar 

maps of the transition energy for CZTS (b) and CZTSe (c). The spatial variation in the emission spectrum of CZTS is mostly a result of 

grain-to-grain inhomogeneity. The characteristic redshift at grain boundaries is more clearly seen in CZTSe.
5

ACKNOWLEDGEMENTS

This work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-08-GO28308 with the National 

Renewable Energy Laboratory.

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