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Chemical bath deposition of Zn(O,S) and CdS buffers: Influence ofCu(In,Ga)Se2 grain orientationWolfram Witte, Daniel Abou-Ras, and Dimitrios HariskosCitation:Appl. Phys. Lett. 102, 051607 (2013); doi: 10.1063/1.4788717View online: http://dx.doi.org/10.1063/1.4788717View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i5Published by theAmerican Institute of Physics.Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/Journal Information: http://apl.aip.org/about/about_the_journalTop downloads: http://apl.aip.org/features/most_downloaded

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Chemical bath deposition of Zn(O,S) and CdS buffers: Influenceof Cu(In,Ga)Se2 grain orientation

Wolfram Witte,1 Daniel Abou-Ras,2 and Dimitrios Hariskos11Zentrum fur Sonnenenergie- und Wasserstoff-Forschung Baden-Wurttemberg (ZSW),Industriestrae 6, 70565 Stuttgart, Germany2Helmholtz-Zentrum Berlin fur Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

(Received 22 August 2012; accepted 3 January 2013; published online 6 February 2013)

The present contribution discusses buffer growth by chemical bath deposition (CBD) on

polycrystalline Cu(In,Ga)Se2 (CIGS) films deposited by in-line co-evaporation with an integral

[Ga]/([Ga][In]) ratio of 0.3. We report a correlation of the coverage of CBD Zn(O,S) and CdS filmswith the CIGS grain orientation as determined by electron backscatter diffraction. h221i-orientedCIGS grains are sparsely covered with the CBD films, whereas on h100i/h001i- and h110i/h201i-oriented CIGS grains, we found very dense coverage of the CIGS surfaces. This result may be

explained by lower energies of CIGS {112} surfaces compared with those of {100}/{001} and

{110}/{102}.VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4788717]

The most commonly used buffers layers for Cu(In,Ga)Se2(CIGS) thin-film solar cells are grown by the so-called chemi-

cal bath deposition (CBD) technique. All CIGS record devices

on laboratory or even on industrial scale use CBD CdS13 or

CBD Zn(O,S)46

as buffer layers. The CIGS/buffer interface

plays a key role in terms of recombination, interdiffusion, and

formation of the p-n junction for record solar cell parameters

in CIGS devices. Nevertheless, the relationships between

polycrystalline CIGS surface, CIGS grain orientation, and

growth mechanisms of buffer layers from the CBD process

are not well understood yet.

In the present contribution, we discuss the growth of

Zn(O,S) and very thin CdS layers from solution growth on

industrially relevant polycrystalline CIGS thin films, depos-

ited with an in-line co-evaporation process, in dependenceon the CIGS grain orientation as determined by means of

electron backscatter diffraction (EBSD).

CIGS films were deposited by an in-line multi-stage co-

evaporation process7 on Mo-coated soda-lime glass substrates.

The CIGS layers feature a tetragonal, chalcopyrite-type crystal

structure and are oriented nearly randomly, as confirmed by

X-ray diffraction (XRD). These films exhibit an integral

[Ga]/([Ga][In]) ratio of 0.3, an integral [Cu]/([Ga][In]) ra-tio of 0.8, and a thickness of around 2.4 lm, as determined by

X-ray fluorescence measurements. Directly after the CIGS

process, within a maximum exposition time of the CIGS

layers in air of less than 30 min, CdS8 or Zn(O,S)9 buffer

layers were grown on the glass/Mo/CIGS stacks by CBD. Forthe preparation of corresponding reference solar cells, the dep-

osition durations were 8 min for CdS at 65 C and 15 min for

Zn(O,S) at 80 C, resulting in different thicknesses of about

50 nm and 20-30 nm, owing to the different growth kinetics of

CdS and Zn(O,S).5 The denoted thicknesses of both buffer

layers are optimized for highest device performance. For CdS

and Zn(O,S), we used CdSO4 or ZnSO4 as well as NH4OH

and thiourea as educts. Thinner films were grown with shorter

deposition durations at the same temperatures. The reference

solar cells were completed by sputtered iZnO on top of CdSand sputtered Zn0.75Mg0.25O

10 on top of Zn(O,S). The front

contact for both device structures consists of sputtered ZnO:Al

with Ni/Al grids on top.

A FEI XL-30 Sirion SFEG scanning electron micro-

scope (SEM) served for imaging of the buffer layers of

CIGS and for estimation of buffer thicknesses. EBSD was

performed using a LEO 1530 GEMINI SEM equipped with

an Oxford Instruments HKL Nordlys II EBSD camera (ac-

quisition and evaluation software FastAcquistion/Channel5).

At a beam energy of 20 keV, the exit depth of those back-

scattered electrons which eventually reach the EBSD camera

is only a few tens of nanometers for CIGS.

The solar cells in this study with a Zn(O,S)/(Zn,Mg)O

buffer system reach conversion efficiencies in the range of

g 15-16% after a 30 min light soaking procedure at room

temperature and without further post-annealing. Our Zn(O,S)buffer layers with a thickness d between 20 and 30 nm cover

most of the CIGS grains contiguously, but there are also

grains with very poor coverage, as depicted in Fig. 1(a). If

we omit the Zn(O,S) buffer, the cells with the layer sequence

CIGS/(Zn,Mg)O/ZnO:Al exhibit efficiencies in the range of

g 8.5-9.5%. These results indicate that CIGS grains orareas with direct contact to (Zn,Mg)O may reduce the overall

efficiency of a cell with a CBD Zn(O,S) buffer, very prob-

ably due to sputter damage.

For the 40-60 nm thick CdS films, we find a complete

coverage of all CIGS grains [Fig. 1(b)]. The corresponding

reference cells with CdS buffers exhibit efficiencies in the

range of g 16-17%. Only for very thin CdS films withthicknesses of d< 10 nm, the coverage of the CIGS grains is

sparse, as shown in Fig. 1(c), and thus similar to the Zn(O,S)

films.

We analyzed the relationships between CIGS grain ori-

entation and Zn(O,S) buffer growth by use of EBSD

orientation-distribution maps in combination with SEM

images, as shown in Fig. 2 on CIGS grains with dense [Fig.

2(a)] and non-contiguous coverage [Fig. 2(b)] by a 25nm

thick Zn(O,S) layer. The two CIGS grains indicated in Fig. 2

with different buffer coverage also feature different crystal

orientations. The CIGS grain with dense Zn(O,S) [Fig. 2(a)]

0003-6951/2013/102(5)/051607/4/$30.00 VC 2013 American Institute of Physics102, 051607-1

APPLIED PHYSICS LETTERS 102, 051607 (2013)

http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://crossmark.crossref.org/dialog/?doi=10.1063/1.4788717&domain=pdf&date_stamp=2013-02-06http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717http://dx.doi.org/10.1063/1.4788717


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on top exhibits a h100i/h001i orientation, whereas thesparsely covered grain [Fig. 2(b)] shows a h221i orientation.For the EBSD analyses, we chose only CIGS grains which

exhibit surfaces parallel to the glass/Mo substrate. As an

overview for the reader, Fig. 3 illustrates typical crystal

planes for CIGS in the tetragonal system with indications of

their surface atoms and polarities: metal-termination with

Cu, In, and Ga atoms on polar (100)/(001) as well as on

(112) planes, metals, and Se on the non-polar (110)/(102)

and the polar Se-terminated (112) surfaces.

By means of EBSD, it is not possible to distinguish

between h100i- and h001i-oriented CIGS grains since CIGShas a pseudo-cubic crystal structure with a lattice constant

ratio c/a very close to 2 for [Ga]/([Ga][In]) ratios of 0.3.11

The EBSD patterns of CIGS grains with buffer layers on top

can be evaluated without any problems due to the small

thickness of Zn(O,S) or thin CdS films. The quality of the

SEM images shown in Figs. 2 and 4 is lower than those pre-

sented in Figs. 1 and 5, since these SEM images were

acquired as overviews for the EBSD measurements, for

which a high beam current of 10 nA and also a considerable

sample tilt of 70 were applied. The EBSD results presented

in this contribution are representative of our main findings

and are supported by numerous EBSD analyses on different

CIGS grains and also on samples from different deposition

processes.

Besides the different growth kinetics of Zn(O,S) and

CdS films,5 there is also a difference in the buffer coverage

on CIGS. 5 nm thick Zn(O,S) layers as well as films with a

thickness of 25 nm are not completely dense. In contrast, we

find coarse film coverage for CdS layers of 5 nm thickness or

slightly less, whereas thicker layers with deposition times

tdep> 3 min and d> 10 nm are completely closed. One main

difference between CdS and Zn(O,S) growth from solution,

apart from different kinetics, are the different solubility

products between the corresponding hydroxides and sulfides,resulting in a high amount of Zn(OH)2 in the Zn-containing

buffer, compared with Cd(OH)2 in CdS.12 Also, a different

behavior in an early growth state for Zn(O,S) was reported

due to Zn(OH)2 formation.13

FIG. 1. SEM images of different buffer

layers from solution growth on top of a

CIGS absorber. (a) 25 nm thick Zn(O,S)

buffer (deposition time tdep 15 min)with dense coverage (green rectangle)

and poor coverage (red circle) on differ-

ent CIGS grains. (b) Completely dense

standard CdS layer (tdep 8 min). (c)5 nm thin CdS layer (tdep 2 min) withpoor coverage on some CIGS grains.

FIG. 2. SEM and corresponding EBSD orientation-distribution maps with

local orientations given by colors of a 25nm thick CBD Zn(O,S)

(tdep 15 min) on CIGS. The grey unit cells indicate the exact crystallo-graphic orientation of each grain in the tetragonal crystal system. (a) Dense

Zn(O,S) coverage on a h100i/h001i-oriented CIGS grain. (b) Poor buffercoverage on a CIGS grain with h221i orientation.

FIG. 3. Scheme of important crystal planes for the tetragonal chalcopyrite-

like structure. The surfaces of the (100), (001), and (112) planes are metal-

terminated, whereas (112) is Se-terminated (blue color, bold). All these

planes are polar in contrast to the (110) and (102) planes, which are non-

polar and the surfaces are metal- and Se-terminated (red color, italic).

051607-2 Witte, Abou-Ras, and Hariskos Appl. Phys. Lett. 102, 051607 (2013)



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Fig. 4 depicts EBSD orientation-distribution maps onCIGS with initial growth of CBD CdS. Poor CBD CdS

coverage on a CIGS grain with h221i orientation and densecoverage on a neighboring h100i/h001i-oriented CIGS grainis observed, similar to the results of CBD Zn(O,S). In addi-

tion, we also found dense CBD buffer growth on h110i/h201i-oriented CIGS grains (not shown here) for CBD Zn(O,S) and

thin CdS.

In the SEM images shown in Figs. 5(a) and 5(b), we

were able to detect the triangular patterns of coarse growth

for Zn(O,S) and thin CdS on {112} CIGS planes, respec-

tively. This result agrees well with the poor coverage of

CBD buffers on h221i-oriented CIGS grains from EBSD

measurements. Fig. 5(c) shows the accumulation of CdS in avery early growth state alongside the {112} facets of a CIGS

grain. Such triangular islands were also found on {112} epi-

taxial CIGS grown on a {111} GaAs substrate.14

It should be

noted that by means of EBSD, it is not possible to distinguish

between CIGS grains with the metal-terminated (Cu/In/Ga)

(112) surfaces and the Se-terminated (112) surfaces.

The CIGS faces which form on the CIGS surface and

were analyzed by means of EBSD may have the same orien-

tation as the integral texture of the bulk CIGS layer as deter-

mined by XRD, but this is not very often the case. From ourexperience, it is more likely that the grain orientations on the

surface will differ from the preferred orientation of the bulk

CIGS layer, which is in our case randomly oriented with a

slight tendency to h221i. Also, our experiments indicate thatonly a small part of the CIGS grains on surfaces exhibits

{112} planes for high-efficiency solar cells.

Possible origins for the different growth behavior of buf-

fers grown by CBD on CIGS with various orientations are

lattice mismatch between CIGS and buffer layer, different

polarity of CIGS surface planes, the type of atoms at the

CIGS surface which are available as bonding partners for the

buffer films, as well as different surface energies of CIGS

grains with different orientations.

The crystal structure of CdS thin films grown by CBD

near the CIGS/buffer interface and lattice mismatch are still

an ongoing subject of discussion in the literature. Wada

observed a mixture of hexagonal and cubic CdS with a large

number of stacking faults from CBD on CIGS thin films but

also a dependence of the CIGS grain orientation.15 An epitax-

ial growth of cubic CdS from CBD on the {112} planes of

polycrystalline CIGS with the relationship {111}cubicCdS//

{112}CIGS due to very similar lattice spacings was reported by

Nakada.16 On the other hand, hexagonal CdS film growth

from aqueous solution on CIGS was observed by means of

electron diffraction.

17

There are hardly any reports aboutCBD Zn(O,S) in the literature concerning crystal structure

near the CIGS/buffer interface as a result of its more recent

development as a buffer material. It seems likely that the crys-

tal structure of the buffer itself and the resulting lattice mis-

match on different CIGS planes will determine whether

epitaxial, polycrystalline, amorphous, or nanocrystalline

growth occurs but does not conclusively explain the dense and

non-contiguous buffer coverage we found in the present work.

One should keep in mind that a few atomic layers of the CIGS

surface are etched by ammonia solution before the actual

buffer growth commences. Nevertheless, we expect that the

orientation of the CIGS grains on the absorber surface remains

the same. In addition, an intermixing of buffer and CIGS takesplace at the interface.18

FIG. 4. SEM and corresponding EBSD orientation-distribution maps with

local orientations given by colors (legend see Fig. 2) of a 5 nm thick CBD

CdS film with tdep 2 min on CIGS. The grey unit cells indicate the exactcrystallographic orientation of each grain in the tetragonal crystal system.

Top: Dense CdS coverage on a h100i/h001i-oriented CIGS grain. Bottom:Poor buffer coverage on a CIGS grain with h221i orientation.

FIG. 5. SEM images of CIGS grains

with sparse CBD buffer coverage form-

ing typical triangular patterns on the

{112} CIGS planes. (a) 25 nm thick

CBD Zn(O,S) with tdep 15min. (b)Approximately 5 nm thick CBD CdS

with tdep 2 min. (c) CBD CdS withtdep 1 min growing alongside the CIGSfacets and terraces.




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The issue of polarity (see Fig. 3) can be considered less

dominant owing to the fact that while we found poor buffer

growth on the polar {112} planes, there was also good cover-

age on the polar {100}/{001} planes. On the only non-polar

plane types in CIGS, {110}/{102}, the buffer layers exhib-

ited dense coverage.

Polar CIGS surfaces like {112} and {001} may undergo

reconstruction to achieve surface charge neutrality via defect

formation such as Cu vacancies for [Cu]/([Ga][In]) < 1(as used for high-efficiency CIGS solar cells), resulting in

lower surface energies compared with non-polar {110} and

{102} surfaces.1921 These different energies of CIGS surfa-

ces as well as the type of atoms (metals or Se) with different

orientations can also influence the initial growth of the CBD

buffer layers. The present work indicates that the wetting

during CBD is more impeded on CIGS {112} surfaces with

the lowest energy after reconstruction as determined by theo-

retical calculations,19,20 as compared with the {110}/{102}

surfaces exhibiting higher energy. In addition, Siebentritt

et al. suggested that the surface energy of {001} is higher

than for {112} in the Cu-poor regime.21 This result concurs

well with the different growth behaviors we found on {112}

and {100}/{001} surfaces. Furthermore, different growth

phenomena for different orientations were also reported for

CBD CdS grown on cubic InP single crystals.22 Lower sur-

face energies due to different orientations can also explain

the detected nucleation of buffer growth at the step edges of

{112} facets.

In conclusion, we report on the growth of Zn(O,S) and

CdS buffers from solution in dependence of CIGS grain orien-

tation as determined with EBSD. Dense CBD Zn(O,S) films

with a thickness of 20-30 nm grow on CIGS grains with

h100i/h001i and h110i/h201i orientations. Non-contiguous

layers were found on h221i-oriented grains of polycrystallineCIGS absorbers. This correlation is also valid for very thinCBD CdS films with thicknesses of around 5 nm in an early

growth stage. For both buffer materials, typical triangular pat-

terns of non-contiguous CBD films are visible on CIGS grains

with {112} surfaces. The low surface energy for the {112}

surface as well as the type of atoms (Cu/In/Ga or Se) serving

as bonding partners for Zn(O,S) and CdS in the nucleation

state at the CIGS surface after reconstruction are the most

likely candidates to influence the growth behavior and density

of the final CBD buffer layer itself.

The authors are grateful to the CIGS team at ZSW, espe-

cially Daniela Muller for SEM measurements. We thankDaniel Lincot of the Institut de Recherche et Developpement

sur lEnergie Photovoltaque (IRDEP) for fruitful discus-

sions. This work was funded by the Federal Ministry of Edu-

cation and Research (BMBF) within the GRACIS project

under contract number 03SF359.

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