A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Grain-Boundary Physics in 

Polycrystalline Photovoltaic 

Materials 

Preprint 

Y. Yan, C.S. Jiang, X.Z. Wu, R. Noufi, S.H. Wei, 

and M.M. Al-Jassim 

National Renewable Energy Laboratory

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42510 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
NOTICE

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
GRAIN-BOUNDARY PHYSICS IN POLYCRYSTALLINE 

PHOTOVOLTAIC MATERIALS 

Yanfa Yan, C.S. Jiang, X.Z. Wu, R. Noufi, S.-H. Wei, and M.M. Al-Jassim 

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA 

ABSTRACT 

We use the combination of high-resolution electron 

microscopy and density-functional theory to study the 

atomic structure and electronic effects of grain boundaries 

in polycrystalline photovoltaic materials such as Si, CdTe, 

CuInSe2, and CuGaSe2. We find that grain boundaries 

containing dislocation cores create deep levels in Si, 

CdTe, and CuGaSe2. Surprisingly, however, they do not 

create deep levels in CuInSe2. We further find that the 

presence of Ga in grain boundaries in CuInSe2 generates 

deep levels. These results may explain the fact that Si and 

CdTe solar cells usually require special passivation, 

whereas CuInSe2 solar cells do not. The passivation of 

grain boundaries in Si and CdTe is also studied. We find 

that grain boundaries in CdTe can be passivated very well 

by Cl, Br, and I. 

INTRODUCTION 

Polycrystalline semiconductor materials, such as Si, 

CdTe, and CuInSe2 (CIS), hold great promise for 

producing low-cost, high-efficiency, and large-area solar 

cells [1–3]. However, polycrystalline materials are typically 

expected to exhibit poor performance compared to their 

single-crystalline counterparts [4], because of grain 

boundaries, which are usually effective recombination 

centers for photon-generated electrons and holes, as well 

as scattering centers of free carriers. Therefore, 

passivation is usually required for making high-efficiency 

Si and CdTe solar cells [1,2]. However, CIS is an 

exception. Without special grain-boundary (GB) 

passivation, the efficiency of the polycrystalline 

Cu(In,Ga)Se2 (CIGS)-based device is now approaching 

20% [3], significantly surpassing the best corresponding 

single-crystal devices (~13%) [5]. CuGaSe2 (CGS) has the 

same structure as CIS. However, CGS solar cells have 

much lower performance than CIS cells. The classic 

“electronic” GB model concludes that GBs in 

polycrystalline semiconductors usually contain charged 

intrinsic defects that trap an excess number of majority 

carriers and form electrical potentials [6]. In the past, 

several scenarios were also presented to explain the 

behavior of GBs in CIS, but they are still under debate 

[7,8]. Therefore, there is still a need to understand the GB 

physics in photovoltaic materials. 

Here, we use the combination of high-resolution 

electron microscopy and first-principles calculations to 

determine the atomic structure of grain boundaries and 

calculate the electronic properties of the following 

photovoltaic materials: Si, CdTe, CIS, and CGS. We found 

that GBs in Si, CdTe, and CGS create deep levels and 

belong to the conventional GB model—the “electronic 

model.” However, GBs in CIS do not create deep levels 

and are electrically benign [10]. Our results explain why 

CIS solar cells do not require a special passivation 

process, and we propose that high Ga concentration of Ga 

in CIGS grain boundaries are not desired. Our results also 

indicate that GBs in Si can only be partially passivated. 

However, GBs in polycrystalline CdTe are well passivated 

by Cl and I impurities. 

METHOD 

Transmission electron microscopy (TEM) specimens 

were prepared by mechanical polishing to a thickness of 

~100 μm, then dimpling the central portion of the 

specimens down to ~5 μm. The samples were 

subsequently thinned to electron transparency using a 4-

kV Ar ion beam at 14° inclination, then cleaned at a lower 

voltage (1.5 kV). A liquid N2 cooling stage was used to 

minimize milling damage. High-resolution TEM (HRTEM) 

images were taken on a Philips CM30 microscope with a 

Cs=2.0 mm, operating at 300 kV. The image simulations 

were carried out using the MacTempas program 

packages. 

The first-principles total-energy calculations are based 

on the density-functional theory, using the Vienna ab-initio 

Simulation Package (VASP) [11]. We used the local 

density approximation (LDA) and ultrasoft Vanderbilt-type 

pseudopotentials [12] as supplied by Kresse and Hafner 

[13]. The cutoff energy for the plane-wave basis was 380 

eV. The GBs were modeled using a 144-host-atom 

supercell. In all calculations, all atoms were allowed to 

relax to reach the minimum energies until the HellmannFeynman forces acting on them became less than 0.02 

eV/Å. 

RESULTS AND DISCUSSION 

We first determined the atomic structure of GBs in Si, 

CdTe, CIS, and CGS. It is usually very difficult to 

determine atomic structure of GBs in polycrystalline thin 

films. However, Si, CdTe, CIS, and CIGS have similar 

cubic crystal structures that can be described from the 

basic diamond structure. Si has a diamond structure that 

can be described by stacking Si layers along the [111] 

direction. CdTe can be described in the same manner, but 

with alternating Cd and Te layers. CIS can be described by 

alternately stacking a Se layer and a mixed Cu and In 

layer. Thus, it is reasonable to assume that GBs with the 

same characteristics should have the same atomic 

structure. We were only able to determine experimentally 

1
the atomic structure GBs in polycrystalline CdTe thin films. 

We will adopt the structure for the defects in Si, CIS, and 

CGS for density-functional theory calculations. 

Fig. 1. HRTEM image of a GB in CdTe. 

Figure 1 shows a HRTEM image of a GB obtained in a 

polycrystalline CdTe thin film. The GB consists of 

dislocation cores, as indicated by the white trapezoids. 

The GB is on the (11-2) plane. Based on the HRTEM 

image, eight possible structural models for the DP twin 

boundary are constructed with the guidance of the 

HRTEM image. These structure models are then relaxed 

using first-principles density-functional total-energy 

calculations. We find that there are only two structures that 

remain stable after the relaxation. Their optimized 

structures are shown in Figs. 2(a) and 2(b). The blue (dark 

color) balls indicate Cd atoms, whereas the gray (light 

color) balls indicate Te atoms. These two structures, 

called GB1 and GB2, respectively, look similar, but have 

different details. For example, in GB1, in Fig. 2(a), the 

atom numbered by 3 has four bonds and Te numbered by 

2 has a dangling bond. But in GB2, the atom numbered by 

3 has four bonds and Te numbered by 2 has a dangling 

bond. 

Our first-principles density-functional total-energy 

calculations indicate that both structures could exist. It is 

not clear which of the two structures should fit the HRTEM 

image of the DP twin shown in Fig. 1. Thus, the two 

structurally optimized models are used for HRTEM image 

simulations. The experimental parameters are obtained by 

through-focus and through-thickness simulations of the 

HRTEM image of the perfect regions near the boundary. 

We find that the defocus value is about -65 nm and 

thickness is about 55 nm. Figures 2(c) and 2(d) are the 

simulated images using the structural models of Figs. 2(a) 

and 2(b), respectively. For these simulations, the defocus 

value of -65 nm and thickness of 55 nm are used. It is 

seen that Fig. 2(c) fits with Fig. 1 much better than Fig. 

2(d). Thus, the combination of first-principles densityfunctional total-energy calculations and HRTEM image 

indicates that the GB1 structure shown in Fig. 2(a) is a 

convincing model for the GB that we have observed 

experimentally. However, it is noted that the GB2 structure 

may also be observed in CdTe. Thus, theoretically, we will 

consider both structures. 

Fig. 2. (a) and (b) Determined atomic structure of GB1 and 

GB2. (c) and (d) Simulated HRTEM images of GB1 and 

GB2. 

Fig. 3. Calculated PDOS of Cd and Te atoms around GB1. 

To study the possible effects of the DP twin structures 

on the electronic property of CdTe, we calculate site2
projected density of states (PDOS) of individual atoms 

around the boundaries and compare them with the PDOS 

of a Cd and Te pair in the perfect regions. Figure 3(a) 

shows the comparison of PDOS of the three Cd atoms on 

the GB1 shown in Fig. 2(a). The black solid line marked by 

Cd+Te is the PDOS of the Cd and Te pair in the perfect 

region. The PDOS of the Cd atoms marked by number 1, 

2, and 3 in Fig. 2(a) are indicated by Cd1, Cd2, and Cd3, 

respectively. We see that the Cd3 atom produces a peak 

in the gap of CdTe. This is likely due to the fact that the 

Cd3 atom has a dangling bond. The rest of the Cd atoms 

are all four-fold and have no dangling bonds; thus, they do 

not produce deep levels in the gap. 

Figure 3(b) shows the PDOS of the three Te atoms on 

the DP(1) boundary shown in Fig. 7(b) and a Cd and Te 

pair in a perfect region. The black solid line is the PDOS of 

the Cd and Te pair in the perfect region. The PDOS of the 

three Te atoms marked by numbers 1, 2, and 3 in Fig. 2(a) 

are indicated by Te1, Te2, and Te3, respectively. We see 

that the Te2 atom produces a very large peak in the gap 

region. This is because the Te2 atom has weak interaction 

with another Te atom. 

Fig. 4. (a) Atomic structure of GB in Si. (b) Calculated 

PDOS of Si atoms around the GB. 

Figure 4(a) shows the GB structure in Si relaxed by 

density-functional theory. Compared to Si in perfect 

regions, Si1 has one extra bond, Si2 has wrong bonding 

angles, and Si3 has a dangling bond. Typically, the 

dangling bonds create deep-level states. However, our 

calculations revealed different results. Figure 4(b) shows 

the PDOS of the three Si atoms on the DP boundary and a 

Si atom in a perfect region. The black solid line is the 

PDOS of the Si in the perfect region. The PDOS of the 

three Si atoms marked by numbers 1, 2, and 3 in Fig. 4(a) 

are indicated by 1, 2, and 3, respectively. Our results 

reveal that Si3 does not create deep levels. Instead, Si1 

and Si2 create deep levels, as indicated by the arrows. 

Fig. 5. Calculated PDOS of (a) Cu, (b) In, and (c) Se 

atoms around GB1 in CIS. 

We have also adopted the GB structure for CIS. 

Surprisingly, however, they do not create deep levels in 

CuInSe2. Figure 5 shows the calculated PDOS of (a) Cu, 

(b) In, and (c) Se atoms around GB1 in CIS. It is seen that 

unlike the GBs in Si and CdTe, the GBs in CIS exhibit no 

deep levels. We found that the reason for this unusual 

behavior in CIS is due to the huge expansion around the 

boundary. Our results thus explain why Si and CdTe solar 

cells usually require special passivation, whereas CIS 

solar cells do not. 

Fig. 6. Calculated PDOS of (a) Cu, (b) In, and (c) Se 

atoms around GB1 in CIS without expansion. 

3
To demonstrate our explanation, we have calculated the 

PDOS of (a) Cu, (b) In, and (c) Se of GB1 without 

expansion (Fig. 6). It is seen that without expansion, the 

very same GB showed deep levels. 

We further find that the presence of Ga can affect the 

behavior of GBs in CIS. Ga at some In sites are found to 

present deep levels. In addition, the Se atoms bonding to 

these Ga atoms are also affected. 

Fig. 7. Calculated PDOS of GB1 passivated by (a) Br, (b) 

Cl, (c) I, and (d) S. 

We now discuss how the GBs in Si and CdTe can be 

passivated. We find that hydrogen can passivate the 

dangling bond of Si3 in Fig. 4(a). However, this 

passivation does not affect Si2 and Si 3 atoms. It interacts 

strongly with Si3. 

It is believed that one of the effects of CdCl treatment is 

the passivation of defects by Cl. In our calculations, in 

addition to Cl, we also considered S and two other groupV elements, Br and I. Our total-energy calculations reveal 

that all these atoms are more favorable to bond with two 

Cd(1) atoms with dangling bonds, rather than to bond with 

Te atoms. To study the passivation effects of the GB1 by 

Br, Cl, I, and S, we calculate PDOS of individual atoms 

around the boundaries. Figure 7 shows the PDOS of a 

Cd(1) atom in the GB1 boundary after it is bonded with a 

Br, Cl, I, or S atom (dashed line) and the PDOS of the Br, 

Cl, I, and S atoms (dotted line). We see that the Cd(1) 

atom is no longer producing significant energy levels in the 

gap when Cd(1) atoms are bonded by the Br, Cl, I, and S 

atoms. The Br, Cl, and I atoms themselves also do not 

produce significant peaks in the gap. These indicate that 

Br, Cl, I, and S atoms can passivate well the Cd dangling 

bonds in the DP(1) twin boundaries. However, unlike Br, 

Cl, and I atoms, the S atom does not effectively passivate 

the Cd dangling bonds, as seen in Fig. 7(d), though the S 

atom bonds with the Cd atoms with dangling bonds. The S 

atom itself, as well as the Cd atom bonded by the S atom, 

produces high densities of energy levels in the bandgap. 

CONCLUSIONS 

We studied the atomic structure and electronic properties 

of GBs in polycrystalline photovoltaic materials such as Si, 

CdTe, CIS, and CGS, using a combination of highresolution electron microscopy and first-principles 

calculations. We found that GBs in Si, CdTe, and CGS 

create deep levels, and belong to the conventional GB 

model (the “electronic model”). However, GBs in CIS do 

not create deep levels and therefore are electrically benign 

and do not exactly belong to the conventional GB model. 

Our study of GB passivation indicates that GBs in Si can 

only be partially passivated by H. GBs in polycrystalline 

CdTe can be passivated well by Cl and I impurities. 

ACKNOWLEDGEMENTS 

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

Energy under Contract No. DE-AC36-99GO10337. This 

research used resources of the National Energy Research 

Scientific Computing Center, which is supported by the 

Office of Science of the U.S. Department of Energy under 

Contract No. DE-AC02-05CH11231. 

REFERENCES 

[1] A. Rohatgi et al., Appl. Phys. Lett. 84, 145 (2004). 

[2] X. Wu et al., Proc. 17th European PVSEC, 2001, 

p.995. 

[3] K. Ramanathan et al., Prog. Photovoltaics 11, 225 

(2003). 

[4] Proceedings of the 31st IEEE Photovoltaic Specialists 

Conference (IEEE, New York, 2005). 

[5] C.H. Champness, Proceedings 29th IEEE Conference

(IEEE, Piscataway, NY, 2002), p.732. 

[6] J.Y.W. Seto, J. Appl. Phys. 46, 5247 (1975). 

[7] C. Persson and A. Zunger, Phys. Rev. Lett. 91, 

266401 (2003). 

[8] C. Leendertz et al., Appl. Phys. Lett. 89, 113120 

(2006). 

[9] Y. Yan et al., Thin Solid Films 389, 75 (2001). 

[10] Y. Yan et al., Phy. Rev. Lett. 99, 235504 (2007). 

[11] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 

(1993). 

[12] D. Vanderbilt, Phys. Rev. B 41, 7892 (1990). 

[13] G. Kresse and J. Hafner, J. Phys.: Condens. Matter 6, 

8245 (1994). 

4
F1147-E(09/2007) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

May 2008 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

11-16 May 2008 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Grain-Boundary Physics in Polycrystalline Photovoltaic Materials: 

Preprint 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-520-42510 

5e. TASK NUMBER

PVA73201 

6. AUTHOR(S)

Y. Yan, C.S. Jiang, X.Z. Wu, R. Noufi, S.H. Wei, and 

M.M. Al-Jassim 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-520-42510 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

We use the combination of high-resolution electron microscopy and density-functional theory to study the atomic 

structure and electronic effects of grain boundaries in polycrystalline photovoltaic materials such as Si, CdTe, 

CuInSe2, and CuGaSe2. We find that grain boundaries containing dislocation cores create deep levels in Si, CdTe, 

and CuGaSe2. Surprisingly, however, they do not create deep levels in CuInSe2. We further find that the presence of 

Ga in grain boundaries in CuInSe2 generates deep levels. These results may explain the fact that Si and CdTe solar 

cells usually require special passivation, whereas CuInSe2 solar cells do not. The passivation of grain boundaries in 

Si and CdTe is also studied. We find that grain boundaries in CdTe can be passivated very well by Cl, Br, and I. 

15. SUBJECT TERMS

PV; grain boundary; large-area; transmission electron microscopy; polycrystalline; atomic structure; electronic 

properites; impurity; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18