The effect of Na in polycrystalline and epitaxial single ...

Thin Solid Films 480–481 (2005) 2 – 7

www.elsevier.com/locate/tsf

The effect of Na in polycrystalline and epitaxial
single-crystal CuIn1ÀxGaxSe2

A. Rockett*

University of Illinois, 1-107 ESB, MC-233 1101 W. Springfield Ave., Urbana, IL 61801, United States
Available online 15 December 2004

Abstract

Na is found to improve the performance of Cu(In,Ga)Se2 (CIGS) solar cells although the mechanism is not clear. This paper briefly
reviews some of the observations on Na in CIGS polycrystalline and epitaxial films. Experiments suggest weak electrical effects of Na within
grains, primarily by reducing compensation and in some cases by enhancing acceptor concentrations. As it segregates to surfaces, it has been
suggested that Na acts through passivation of grain-boundary defects. However, its main effect is on device open-circuit voltage (and
somewhat on fill factor), which does not correlate with grain size but rather with bulk grain defects. The Na concentration scales somewhat
with grain boundary density averaged over large areas of film, suggesting that it may be active there. Modest Na concentrations often increase
grain size in polycrystals, although not when the grain size is already large, and often changes preferred orientation. Na segregates to the
surfaces of CIGS grains. These results suggest that it may act at the surface, modifying growth mechanisms or defect organization during
growth. TEM evidence shows that strong concentration of Na in the grain boundaries, sufficient to passivate surface defects by itself, is
unlikely to occur. Finally, Na is removed from the surface of CIGS by aqueous solutions, such as those used to form the heterojunction. It is
concluded that Na acts at the surface during growth to organize point defects, probably including by reduction of vacancy populations, within
the bulk grains but that it has no residual effect once growth is complete.
D 2004 Elsevier B.V. All rights reserved.

Keywords: Open-circuit voltage; Cu(In,Ga)Se2; Na concentration

1. Introduction experimental results on the effect of Na appear either directly
contradictory or to disagree in broad terms.
Recent environmental and energy resource concerns have
increased interest in renewable energy sources, such as This paper argues that the literature is not in fact
photovoltaic devices. CuIn1ÀxGaxSe2 (CIGS)/CdS hetero- contradictory but rather that the results differ because of
junction devices are promising candidates having the highest the details of the experiments performed. Based on the
efficiency, exceeding 19%, of any thin film polycrystalline existing literature and recent results, it appears that Na must
solar cell [1]. The diodes work well when fabricated from act during film growth to improve crystal quality, rather than
polycrystalline materials and no evidence of a correlation having a specific ongoing effect in devices. Consistent with
between grain size and device performance is observed until this, there is no clear evidence of a persistent effect of Na
the grains are much less than 1 Am in diameter [2]. Typically, itself on bulk grains, grain boundaries, or the heterojunction
one finds that the efficiency of the devices is improved in the film, nor is there an obvious site at which Na acts
significantly by the presence of Na during growth [3], as within the films.
discussed below. The mechanism for this improvement is not
agreed to this point. To complicate the picture, many 2. Electronic effects of NA

* Tel.: +1 217 333 0417; fax: +1 217 244 2278. The effect of Na on devices has been studied by
E-mail address: [email protected]. numerous groups, primarily on materials produced by
multisource evaporation [3–10]. Typical results show as
0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2004.11.038

A. Rockett / Thin Solid Films 480–481 (2005) 2–7 3

much as a 50% increase in device efficiency with Na Reduction in compensation must be due to direct insertion
addition, decreasing at both higher and lower concentrations into the defect causing passivation (in the case of a vacancy)
(see, for example, Refs. [8,10]). The primary improvements or elimination of the defect (for either a vacancy or an
are in the open-circuit voltage (VOC) and often in fill factor antisite). The two most likely sources of the compensating
(FF) with little or no change in collected current. The lack of donor are Se vacancies (VSe) and In on Cu antisites (InCu).
a grain-size effect and analysis of both single-crystal and Inserting a group I alkali metal on a group VI lattice site
polycrystalline devices suggests that VOC is largely deter- seems unlikely and it is not obvious how the Na atom would
mined by recombination in the space-charge region; thus, an passivate a Se vacancy directly. Rather, it seems probable
increase in VOC indicates a reduction in recombination for either donor that the Na acts to remove the defect rather
centers or an increase in carrier concentration or both within than somehow deactivating it.
the grains near the surface of the film with the addition of
Na. In typical devices grown on soda-lime glass substrates Na has been found by several researchers [3,4] to reduce
coated with Mo, the Na is incorporated into the Mo back the resistivity of polycrystalline CIGS films. This could be
contact in the form of Na2O from the glass and diffuses into due to an increase in acceptor density as observed in some
the device [11]. However, there is a strong preference for Na films, or could be due to a reduction in grain boundary
to segregate to the surface of CIGS films, indicating that Na resistivity. The more likely explanation in this case is the
incorporation into CIGS is energetically unfavorable [12– latter as in-plane resistivity is probably dominated by grain
15]. Although it leaves the glass as Na2O, it is probably boundary terms [17]. Reduced boundary resistivity could be
converted at least mostly to a selenide compound in the due either to the presence of a conductive phase in the
CIGS layer or on the surface due to the high activity of Se boundaries or to reduction in carrier depletion in the bulk
and the low activity of O during deposition process. grains around the boundaries. No conductive phase has been
observed to be connected with the presence of Na nor has
Several groups have studied the electronic effect of Na any phase in the boundaries been directly observed by high-
within CIGS grains by Hall effect [16,17] and photo- resolution transmission electron microscopy (TEM) [22,23].
luminescence [17,18]. The results show no correlation of an A high-resolution TEM image of a typical grain boundary in
effect of Na with the presence of grain boundaries nor do a material produced at the University of Delaware Institute
they show a difference in the effect of Na within for single- for Energy Conversion as described in Ref. [2] is given in
crystal as opposed to polycrystalline materials [17]. Fig. 1 [23], showing a very abrupt interface with no
Although Na has been suspected of doping CIGS, none evidence of any interfacial phases or distortion of lattice
of these studies showed any evidence of new states in the planes near the boundary.
energy gap. Schroeder and Rockett [16] and Kimura et al.
[18] found a reduction in compensation in CuInSe2 (CIS) Fig. 1. A high-resolution transmission electron micrograph [21] showing
epitaxial and polycrystalline thin films, respectively, with the structure of a grain boundary in a multisource-evaporated polycrystal-
no increase in carrier concentration while Schuler et al. line CIGS film [9]. This is a high-angle grain boundary so lattice image
found both reduction in compensation and increase in contrast is not obtained in both grains. However, fine fringes are visible in
carrier concentration associated with Na addition in both the right-hand grain from which it can be seen that no distortion exists up
polycrystalline and epitaxial CuGaSe2 (CGS). In addition, the boundary. The boundary is sharp (roughly one monolayer wide) and
most device results have been interpreted as having there is no evidence of a chemical or structural change present.
improved voltages resulting strictly from an increase in
hole concentration in the absorber. While the effect on the
majority carrier states varies, it seems generally agreed that
Na reduces compensating defects and usually enhances
carrier concentrations. The absence of an identifiable new
state associated with Na also indicates that it is not a dopant
in CIGS of any sort. In addition to direct measurements of
the effect of Na on electrical properties of films, an
additional indirect indication may be found in comparison
of transient photocapacitance spectroscopy [19,20] and
cathodoluminescence [21] depth profiling for epitaxial
films without Na and polycrystals with Na, both of which
showed identical deep defects and band tails in films with
and without Na.

Reduction in donor compensation without creation of a
new acceptor state shows that rather than adding new point
defects, Na is removing or reducing the donor defect
responsible for compensation. This is consistent with the
observed increase in carrier concentration in many devices.

4 A. Rockett / Thin Solid Films 480–481 (2005) 2–7

Fig. 2. This figure shows the composition of grains (filled points) and grain although another study [25] found a decrease in grain size
boundaries (open points) for two samples obtained from Shell Solar with added Na. In addition, Bodeg3rd et al.[5,10] found an
(triangles) and Global Solar Energy (squares) for baseline device materials increase in (112) preferred orientation in the crystallites
(after Ref. [20]). which has subsequently been observed by others. Although
the results differ, the experiments also changed, especially in
It seems most likely given the structure of the boundaries the form of delivery of the Na. Furthermore, the grains in
that the change in conductivity is the result of a change in Rudmann et al. [25] were large even in the absence of added
boundary defects pinning the Fermi energy, or bulk defects Na. Thus, although not a ubiquitous observation, the
within the grains. Certainly, an increase in conductivity in increase in grain size and change in preferred orientation
the grains might reduce the effective resistance of grain appears to be consistent throughout the preponderance of
boundaries in the same way that doping can effectively the experiments reported to date while differing results are
convert a Schottky barrier to an ohmic contact due to probably valid but were produced in noncomparable experi-
tunneling breakdown of the barrier. One would expect that a ments. Interpretation of the effects of Na on microstructure
direct and measurable effect within the grain boundaries are further complicated in multistep processes where
might be correlated with a measurable deviation of the grain massive changes in stoichiometry and recrystallization of
boundary composition relative to that of the bulk grains, the film in general are occurring during growth. Nonethe-
considering the number of dangling bonds that boundaries less, a strong implication of an increase in grain size with
normally include. Such a deviation has been inferred but not the addition of Na for an otherwise constant process is either
observed to date. Indeed, recent microchemical and micro- that the nucleation process is being altered or that the
structural characterizations show the contrary—no differ- diffusivity of atomic species is being enhanced, especially
ence in grain and grain boundary composition [22,23]. A on grain surfaces where growth occurs.
typical comparison of the grain and grain boundary
compositions for CIGS polycrystalline device layers meas- A change of nucleation could account for the differing
ured by nanoprobe EDS is shown in Fig. 2. (The intrinsic results and is also consistent with the change in grain
scatter in the data in this figure is roughly F3 at.%, much orientation in many studies. A close-packed-plane-up
less than the total scatter. A large amount of similar but orientation [(112) for chalcopyrite] is what one would
unpublished data also exists, indicating a similar lack of expect for growth of any material on a surface having no
difference in grain boundary composition [23]). This result preference for orientation—the close-packed plane brings
suggests that any change in conductivity due to the presence adatoms closest together and is widely observed in thin
of Na is the result of redistribution of atoms to reduce films of all sorts grown on amorphous substrates. Thus, a
defects in the boundary and/or within the grains rather than decreased interaction with the substrate could enhance such
because of a large change in boundary chemistry. preferred orientation. Alternately, a study of the surface
morphology and growth of epitaxial CIGS films has
3. Structural effects of NA strongly suggested that the (112) surfaces have the lowest
energy [26,27]. Thus, enhanced atomic diffusion would also
Several investigators have shown an increase in grain be expected to favor this surface. The observations available
size in CIGS polycrystals grown by multisource evaporation to date are therefore not sufficient to establish the change in
as a result of addition of Na prior to growth [5,6,10,18,24], growth mechanism resulting from Na addition. However, it
appears to be a surface-related phenomenon. Furthermore,
Na does not appear to enhance diffusivities in the bulk
material so the changed nucleation model seems more
probable.

The argument that Na changes growth modes by action
upon adatoms on the surfaces of grains rather than in the
bulk of the grains is supported by several studies showing a
strong tendency of Na to segregate to the surface of bulk
single crystals [15], epitaxial layers [16], and polycrystalline
films [14]. Likewise, there is some indication that the
concentration of Na is coupled to grain boundary density
[8,28], suggesting that segregation is not limited to surfaces
but may include rejection from grains to other interfaces,
such as boundaries [29].

It has been proposed that Na, being isovalent with Cu,
could form a chalcopyrite surface phase or related com-
pound [30]. Although this phase is not thermodynamically
stable as a bulk material, it could exist in thin layers on
chalcopyrite surfaces. Evidence for such a compound has

A. Rockett / Thin Solid Films 480–481 (2005) 2–7 5

been found under certain conditions [31]. However, in boundaries. In a recent series of tests, Li et al. [22] and
typical completed devices finished with a dip-coated CdS Lei et al. [23] examined individual grains and grain
layer forming the heterojunction, there is no evidence from boundaries in CIGS from complete polycrystalline device
secondary ion mass spectrometry (SIMS) analysis of a layers produced by a variety of techniques at both
significantly high Na concentration at the heterojunction or commercial and academic laboratories. The studies involved
any correlation of residual interfacial Na with device microenergy dispersive X-ray analysis (A-EDS) carried out
performance. For example, Fig. 3 shows a typical profile in a high-resolution analytical scanning transmission elec-
for Na in a CIGS/CdS heterojunction. The CIGS was tron microscope (STEM). The probed region diameter was
deposited by a multilayer multisource evaporation process. ~1 nm. The results showed no change in grain boundary
The profile is not surprising as experimental evidence shows composition relative to the bulk grains (Fig. 2) and no
that Na is effectively removed from the surface of CIGS correlation of Na or O signal with grain composition. Na
polycrystals by a water rinse [32]. The aqueous environment concentration was linearly related to O concentration,
of the dip process would therefore be expected to remove although the stoichiometry suggested by the slope of the
the majority of any surface Na prior to finishing the relationship would be roughly NaO2, indicating that some of
heterojunction. Therefore, although surface segregation is the O may have been bonding to the matrix. I note that
widely observed, the effect of the Na does not appear to be although this appears to be an odd stoichiometry for a Na
associated with its presence in the junction. compound, it is similar to that of Na selenides observed by
Braunger et al. [28]. These authors also observed a
We may then turn to the grain boundaries for evidence of correlation between Na and O concentration by SIMS in
an effect of Na. One of the longstanding questions with air-exposed films. This result supports the proposition that
regard to CIGS is how the devices can operate satisfactorily Na is simply bringing O into the structure and that O is
in the presence of grain boundaries to the point of showing replacing missing Se. However, this is ruled out by the fact
very little change in efficiency as a function of grain size that device performance is not correlated with O concen-
until the grains are very small [2,9]. The failure of epitaxial tration in the grain boundaries [8], that the grain and grain
layers and single crystals to produce good devices also boundary compositions are identical to within the error in
supports the idea that grain boundaries actually help the the measurements [22], and that the measured stoichiometry
device performance. The question is, how? As noted above, of the grains and grain boundaries does not suggest a S+Se
Na concentration is often found to be correlated with grain deficiency (Fig. 2). In short, there is no evidence from the
size, although clear evidence of Na in the grain boundaries STEM analysis of a connection of Na to the grain
themselves is limited [29]. It has been proposed [33] that Na boundaries or to compensation for any nonstoichiometry
acts to passivate grain boundaries, possibly by bringing O either by itself or through added O.
with it and that this eliminates Se vacancies in the
To summarize the arguments to this point: Na does not
Fig. 3. A typical SIMS profile for a CdS/CIGS heterojunction showing the act in the heterojunction to improve performance as no
absence of significant accumulation of Na at the heterojunction. Indeed, significant excess of Na is present there and it would be
there appears to be no more Na present at the junction than elsewhere in the expected to be removed during junction formation in any
film. For this chalcogen/chalcogen interface, no significant ion yield case. Na does not act in large quantities at the grain
changes for Na are expected. boundaries either directly or through incorporation of O as
what little is present of either element is not correlated with
stoichiometry—there is no detectable difference in stoichi-
ometry between the bulk grains and grain boundaries that
would suggest a need for another element to be present and
no separate phase is observable in the boundaries. Finally,
Na segregates strongly out of the bulk of grains and does not
dope the grains when it is present, suggesting that it is not
acting on its own within the grains to improve the devices.
Then the question arises, where is the Na that is certainly
observed in the films? Probably some of it is in the grains
and some in the grain boundaries. Certainly, too little is
present in the grain boundaries to detect in EDS measure-
ments (about 5% of the boundary sites); therefore, it is not
correcting a major stoichiometry problem itself. Even less is
present in the grains, less than would be needed to account
directly for the change in the density of holes by substitional
doping with Na (nor is it found to be a dopant). This leaves
only one possible explanation for the effect of Na that is
consistent with all of the experimental data—that Na acts

6 A. Rockett / Thin Solid Films 480–481 (2005) 2–7

during growth of the material upon the CIGS matrix energy gap of CIGS and their electronic effects has been
material itself (as also proposed in Ref. [29]), and that after found [16–21]. These defects are known to be affected by
growth is complete, Na has no further direct effect. To test Na as summarized briefly above, although Na itself does
this, my group has begun to study the effects of Na on the not create new defects. At the same time, Na is widely
structure growth mode of epitaxial CIGS layers and to thought based on device analyses to enhance carrier
determine the resulting change in device performance [34]. concentrations although it is not a dopant. Therefore, it
Details will be presented elsewhere but some of the initial must be increasing hole concentration by modifying the
results are summarized here. native defect concentration. This, in turn can affect the in-
plane conductivity of the films. The most likely target of
We observe, as have others, that Na reduces the Na is the Se vacancy or the In on Cu antisite defect.
diffusivity of Ga in CIGS. In my group’s results, the effect However, it may be sufficient that Na changes the
is reflected in apparently reduced diffusion of Ga from the tendency to organize these point defects into clusters,
GaAs substrate into the CIS epitaxial layers and is quite rendering them electrically inoffensive. Reduction in
pronounced. In our results, the reduced diffusion is vacancy populations in particular would be consistent with
accompanied by an increase in dislocation density, a observed reduced Ga diffusivity. Having provided an
decrease in the formation of Kirkendall voids at the CIS/ improved crystal structure during growth, Na is not needed
GaAs heterojunction, and a roughening of the growth in or on the grains in the final device, which is why a clear
surface. All of these are consistent with a decreased atomic effect and sensitivity to residual Na levels is not observed.
diffusion rate and probably a decreased vacancy concen- The conclusions are also consistent with the absence of an
tration in the epitaxial layer as a result of the addition of Na. obvious physical location for Na activity or any systematic
Further details will have to await the completion of the composition variation correlated with device performances.
study. However, a reduced diffusivity would be consistent
with the reduction or organization of native point defects in Acknowledgements
the material.
I gratefully acknowledge the support of the U.S.
4. Conclusion Department of Energy through the National Renewable
Energy Laboratory and the Basic Energy Sciences Program
Certainly, the jury is still out on the effect of Na. subcontract DEFG02-91ER45439. Microanalysis was car-
However, in spite of apparent contradictions in almost all ried out in the Center for Microanalysis of Materials
of the experimental results, a general picture has emerged (CMM), supported by the Department of Energy. The
and the contradictions can be seen to be primarily the result author thanks all of the CMM staff for their invaluable help
of the experiments performed rather than necessarily any without which this research could not have been accom-
being specifically invalid. It is the conclusion of this author plished. Finally, I thank my graduate students, postdoctoral
that the primary effect of Na is to organize the point defects researchers, and the many colleagues around the world who
during thin film growth of CIGS acting primarily as a have offered samples, comments, and collaborations over
surfactant without needing to enter the bulk of the material. the course of this research.
As the improved surface structure is overgrown, the result
is an improved bulk crystal quality. Furthermore, I References
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