Nanoscale Measurements of the 

Surface Photovoltage in 

Cu(In,Ga)Se2, Cu2ZnSn4, and 

Cu2ZnSnSe4 Thin Films: The 

Role of the Surface Electronics 

on the Efficiency of Solar Cells 

Preprint 

H. Du, M. J. Romero, I. Repins, G. Teeter, 

R. Noufi, and M. M. Al-Jassim 

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37) 

Seattle, Washington 

June 19-24, 2011 

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. 

Conference Paper 

NREL/CP-5200-50711 

July 2011 

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NANOSCALE MEASUREMENTS OF THE SURFACE PHOTOVOLTAGE IN 

Cu(In,Ga)Se2, Cu2ZnSnS4, AND Cu2ZnSnSe4 THIN FILMS: THE ROLE OF THE 

SURFACE ELECTRONICS ON THE EFFICIENCY OF SOLAR CELLS 

H. Du, M. J. Romero, I. Repins, G. Teeter, R. Noufi, and M. M. Al-Jassim
 

National Renewable Energy Laboratory, Golden, Colorado, U.S.A. 

ABSTRACT 

We report on recent advances in the development of 

nanoscale measurements of the surface photovoltage 

(SPV) based on scanning tunneling microscopy (STM) 

and its application to the kesterites Cu2ZnSnS4 (CZTS) 

and Cu2ZnSnSe4 (CZTSe). One critical aspect of the 

electronic structure of Cu(In,Ga)Se2 (CIGS) that has yet to 

be determined in their related kesterite compounds is the 

character of the surface electronics. In CIGS, spontaneous 

deviations in the stoichiometry of the surface cause a 

depletion (or even a type inversion) region that reinforces 

the CIGS homojunction. First-principle calculations predict 

that this inversion region will be more difficult to form in 

CZTS. In this contribution, the characteristics of the 

surface space charge region for both CIGS and CZTS(e) 

are investigated by STM. The implications of the results of 

these measurements on the future development of CZTS 

solar cells will be discussed. 

INTRODUCTION 

Cu(In,Ga)Se2 (CIGS) has recently demonstrated a record 

efficiency of 20.3%, the absolute record for all thin-film 

photovoltaics (PV) [1]. The success of CIGS is based 

primarily on its electronic structure which, in the regime of 

Cu deficiency, is dominated by the formation of Cu 

vacancies and the stabilization of defect complexes with 

low formation energies [2,3]. First-principle calculations 

predict that our understanding of the electronic structure 

and defect formation in CIGS can be partly extrapolated to 

the kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 

(CZTSe) [4,5]. 

One critical aspect of the electronic structure of CIGS that 

has yet to be determined in CZTS (for both sulfides and 

selenides) is the character of the surface electronics. In 

CIGS, spontaneous deviations in the stoichiometry of the 

surface cause a depletion (or even a type inversion) 

region that reinforces the CIGS homojunction [6]. Firstprinciple calculations predict that this inversion region will 

be more difficult to form in CZTS [5]. This may be one 

negative factor limiting the future development of CZTS 

solar cells that needs to be addressed. 

In this contribution, the characteristics of the surface 

space charge region for both CIGS and CZTS and CZTSe 

are investigated by scanning tunneling microscopy (STM). 

The surface photovoltage (SPV) is determined by 

measurements of the contribution of the local photocurrent 

to the tunneling current while sweeping the voltage applied 

to the conductive tip. Another benefit of the STM is that 

nanoscale fluctuations of the SPV and multiple phases 

with different surface electronics can be resolved. 

EXPERIMENTAL 

Figure 1 shows the schematics of the STM equipped with 

the optics for the SPV measurements. A modulated optical 

beam (at a frequency above the response time for the zfeedback control of the STM of f > 2 kHz) is used to 

generate an excess of minority electrons in the p-type 

semiconductor. The contribution of the transient 

photocurrent (Iph) to the tunneling current (It) is obtained by 

lock-in detection at the modulation frequency. The optical 

chopper easily runs at frequencies above the tens of kHz, 

avoiding thermal expansion of the vacuum gap and 

allowing optimum settings (integral and proportional gains) 

for imaging in constant current mode with the STM. 

optical 

chopper 

parabol

I

t

tip V

x

y

z preamplifier 

Z feedback 

controller 

light 

source 

lock-in

amplifier 

ic 

mirror 

I

ph

Figure 1 Schematics of the setup used in the STM 

measurements of the SPV. 

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 470oC. 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 [7]. The stoichiometry 

of the films used in this study is confined to the region of 

stability for the CZTS kesterite and is 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 [8,9]. 

1 
 

SPV

 

 

 

 

CZTSe thin films are obtained by a two-stage 

coevaporation process as described in Ref. 10 . The 

molybdenum-coated glass substrates are actively heated 

to 470oC to 500oC. Under an overpressure of Se and Sn, 

Cu is delivered at a high rate during the first stage to form 

Cu-rich CZTSe, and then Zn continues to be delivered in 

the second stage to adjust the Cu/Zn ratio and form Cupoor CZTSe. Similarly to the sulfides, the selenide films 

used in this study are Cu-poor (Cu/(Zn+Sn) ~ 0.9) and Znrich (Zn/Sn ~ 1.1). 

Finally, CIGS thin films are obtained by the three-stage 

coevaporation process. The Cu/(In+Ga) ratio for our 

reference CIGS is ~0.9 and the In/(In+Ga) ~0.3, as 

required for record performance [11]. 

All of these thin films are stored in inert (nitrogen) 

atmosphere before being loaded in the vacuum chamber 

of the STM. The native oxide must be present as no 

surface treatments are applied prior or during the 

Figure 3(a) shows a constant current mode STM image of 

a CIGS thin film, acquired at V = –1 V and It = 500 pA. For 

CIGS, the STM operation is stable when applying a 

negative voltage to the tip (electrons tunneling from the 

tip) and becomes unstable when applying a positive 

voltage (holes tunneling from the tip). This is consistent 

with a surface depletion region (Δp <) when considering 

the rectifying characteristics of the tunneling junction. The 

existence of such a depletion region is confirmed by 

measuring the contribution of the transient photocurrent 

(Iph) to the tunneling current (It) while sweeping the tip 

voltage [see Fig. 3(c)]. The maximum photocurrent is 

measured at V ~ +2 V and, consequently, the intrinsic 

level is well below the Fermi level (Ei < Ef), causing an 

inversion at the surface (from p-type to n-type). The 

formation of this homojunction is critical to improving the 

efficiency of CIGS solar cells. 

(a) CIGS (V = -1 V, It = 500 pA) (c) 

2 μm (b) CZTS (V = 1.5 V, It = 500 pA) 

measurements. 

EF Ev

Ec

CIGS tip

Beam Off

EF Ev

Ec

CIGS tip 

Beam Off 

EF

Ev

Ec

CIGS tip

V

Beam On

SPV 

EF

Ev

Ec

CIGS tip 

V

Beam On 

Photocurrent [a.u.]

CZTS 

CIGS 

× 50 

-2 -1 0 1 2 3 

V
 

Figure 2 Generation of a photovoltage at the surface 

of CIGS. 

RESULTS AND DISCUSSION 

The origin of the surface photovoltage is illustrated in 

Figure 2 [12,13] for CIGS. In the absence of illumination, 

we assume a depletion region (Δp <) near the CIGS 

surface, which causes a downward band bending [5]. 

Photoexcited minority electrons diffuse and accumulate at 

the surface, hence reducing the band bending. 

Photoexcited majority holes diffuse in the opposite 

direction. As a result, a transient in the tunneling current is 

observed and measured as Iph using the lock-in detection 

under the modulated optical beam. With the beam on, the 

semiconductor surface is driven away from equilibrium, 

and the surface potential shifts; a SPV arises. The 

modulation frequency of the optical chopper is fast enough 

that the steady state is not achieved during the cycle. The 

magnitude of the SPV depends on the energy of the 

pinning of the Fermi level, the width of the surface space 

charge region, and the recombination. Therefore, multiple 

properties from both the surface and subsurface of the film 

contribute to the SPV. For a more detailed description, 

including pinning of the Fermi level and surface states, 

see Ref. 13. In our case, SPV measurements can detect 

the presence of a homojunction in the CIGS or CZTS film 

before the solar cell is fully processed (CdS and ZnO 

depositions). 

2 μm 

tip voltage (V) 

Figure 3 (a) STM image of a CIGS film. (b) STM image 

of a CZTS film. (c) Photocurrent measured in CIGS 

and CZTS as a function of the voltage applied to the 

tip. 

Because of the similar electronic structure of chalcopyrites 

and kesterites, we can anticipate that CZTS will benefit 

from a surface electronics comparable to that of CIGS. 

Figure 3(b) corresponds to a STM image of a Cu2ZnSnS4 

thin film, acquired at V = +1.5 V and It = 500 pA. 

Interestingly, the STM reproduces the microstructure of 

the CZTS at positive voltage, which is opposite to our 

observations for CIGS. Figure 3(c) shows the effect of the 

tip voltage on Iph for the kesterite film. The interpretation of 

these results is more difficult than in CIGS. The magnitude 

of the SPV is much reduced in the CZTS film, and the 

photocurrent seems to be dominated by surface electronic 

states. At low tip voltage (for both polarities), we observe a 

current of holes diffusing towards the surface associated 

with distinct surface states within the band gap. The 

2 
 

 

 

 

 

 

 

polarity of the photocurrent reverses at higher values of 

the tip voltage (near and above the band gap), although 

the level of the photocurrent is at least one order of 

magnitude below that of CIGS. These results suggest that 

(i) either the depletion region is very weak in the CZTS film 

or (ii) the recombination within the space charge region is 

very high and its contribution to the SPV signal is largely 

reduced. 

The absence of an inversion region near the surface of the 

kesterite structure can be a direct consequence of the 

much higher energetic preference for acceptor states in 

kesterites relative to chalcopyrites, leading to n-type 

doping difficulty [5]. The “doping pinning rule” proposed by 

Zhang et al. [ 14 ] can be applied to determining the 

dopability of these compounds. Thus, the energy of the 

Fermi-level pinning for n-type doping is above the 

conduction band minimum (CBM) of CuInSe2 and below 

the CBM of CZTS, as shown in the band diagram of 

Figure 4 [5,15]. Consequently, the n-type doping is more 

difficult in CZTS than it is in CuInSe2. That said, the higher 

recombination in the kesterites associated with 

substitutional defects leading to transition energy levels 

close to the midgap (CuSn, SnCu, and SnZn) can overrun the 

potential beneficial effects of such a depletion region. 

CuInSe2 CZTS CZTSe 

Ec (n) Epin 

Ev 

Figure 4 Band alignment and pinning energy of the 

Fermi level for n-type doping in CuInSe2, CZTS, and 

CZTSe. 

The prospect of finding the inverted surface in the 

selenides is much higher than in sulfides because of the 

band alignment between CuInSe2, CZTS, and CZTSe (see 

Fig. 4). The lower band gap of the selenide allows for the 

pinning energy of the Fermi level to move towards the 

CBM of CZTSe, improving the n-type dopability of the 

selenide. 

Similar STM measurements of the SPV are then 

performed to probe the surface electronics of the selenide. 

Fig. 5(a) corresponds to a STM image of a Cu2ZnSnSe4 

thin film acquired at V = +1.5 V and It = 500 pA. As is the 

case for the sulfide, the STM reproduces the 

microstructure of the selenide only at positive voltage. 

Figure 5(b) shows the effect of the tip voltage on Iph for the 

CZTSe film. A transient photocurrent is measured when 

applying a negative bias to the tip. No response is 

observed when reversing the polarity of the tip. 

(a) CZTSe (V = 1.5 V, It = 500 pA) Photocurrent [a.u.] 

2 μm 

(b)

CZTSe 

-2 -1 0 1 2 3 

Tip voltage [V] 

Figure 5 (a) STM image of a CZTSe film. (b) 

Photocurrent measured in CZTSe as a function of the 

voltage applied to the tip. 

In CZTS, the photocurrent Iph saturates rapidly when 

increasing the intensity of the external illumination, 

suggesting that a limited number of states is involved in 

the effect (i.e., surface electronic states). In CZTSe, on the 

other hand, the photocurrent increases linearly with the 

excitation, suggesting surface band bending, as is the 

case for CIGS. However, this band bending is of opposite 

sign to that seen in CIGS, which is consistent with a 

surface accumulation region (Δp >) when considering the 

rectifying characteristics of the tunneling junction. 

Unfortunately, an accumulation region (upward band 

bending) near the surface will work against the collection 

of the photocurrent in the solar cell. On the other hand, the 

band bending that leads to accumulation in CZTSe is not 

as pronounced and the impact over the cell characteristics 

is limited. As is the case for CZTS, the presence of a 

depletion region can be difficult to detect if the 

recombination in the film is high enough. The observed 

behavior can be simply explained by charged states in the 

native oxide. 

These results show that, at this stage of development, the 

surface photo response from the kesterites is very 

different from their related chalcopyrites, and the beneficial 

effects of the surface depletion observed for the 

chalcopyrites do not arise spontaneously in kesterites. If 

the presence of such a depletion region proves to be 

critical to the efficiency of CZTS(e) solar cells, we suggest 

to investigate different variations of the surface 

stoichiometry during the growth and/or the introduction of 

impurities that improve the n-type dopability of kesterites. 

3 
SUMMARY REFERENCES 

In summary, we have investigated the surface 

photovoltage of chalcopyrites and kesterites (both 

selenides and sulfides) by STM-based measurements. 

The photo response from the surface of the chalcopyrites 

is much more pronounced than from kesterites, most likely 

because of the high recombination in the CZTS(e) films. 

The main contribution to the SPV in the kesterites is then 

associated with surface/interfacial electronic states. These 

results are based on kesterite films in the early stages of 

development and, therefore, we can anticipate changes in 

the surface electronics as the quality of the films improves. 

What is certain is that advancing our understanding of the 

effects of stoichiometry, surface reconstruction, and 

dopability will be needed to improve the characteristics of 

the junction and the performance of kesterite solar cells. 

ACKNOWLEDGEMENT 

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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