HYDROGEN PASSIVATION OF POLYCRYSTALLINE SILICON PHOTOVOLTAIC CELLS

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HYDROGEN PASSIVATION OF POLYCRYSTALLINE SILICON PHOTOVOLTAIC CELLS

C. Seager, D. Sharp, J. Panitz, J. Hanoka

To cite this version:

C. Seager, D. Sharp, J. Panitz, J. Hanoka. HYDROGEN PASSIVATION OF POLYCRYSTALLINE

SILICON PHOTOVOLTAIC CELLS. Journal de Physique Colloques, 1982, 43 (C1), pp.C1-103-C1-

116. �10.1051/jphyscol:1982115�. �jpa-00221771�

JOURNAL DE PHYSIQUE

CoZZoque

C I ,

suppldrnent au

n o 10,

Tome

4 3 ,

octobre 1982 page

C1-103

HYDROGEN P A S S I V A T I O N OF P O L Y C R Y S T A L L I N E S I L I C O N P H O T O V O L T A I C C E L L S +

C.H. Seager, D.J. Sharp, J.K.G. Panitz and J.I. Hanoka*

Sandia Nationa Z Laboratories,

A

Zbuquerque,

NM

87185,

(I.

S.

A .

*Mobi

2

Tyco Solar Energy Corporation, WaZtharn,

MA, U . S . A .

Resume - L'effet de l'hydrogsne atomique sur les joints de grains dans le silicium est passe en revue ainsi que les recentes methodes qui per- mettent, par balayage d'un faisceau, de mesurer la vitesse de recombi- naison s, des porteurs minoritaires aux joints de grains. Les reshltats obtenus sur un grand nombre de joints de grains dans du silicium sur graphite dlHoneywell sont prgsentgs et montrent des variations de

s

de 2 x 105cm/s

2 3

x 103cm/s, ou moins, aprss exposition de quelques mi- nutes

?I

une source intense d8hydrogPne atomique. Le mode induit de la microscopie electronique 5 balayage a St6 applique de nombreux joints de grains d86chantillons dont la surface a St& exposee

5

la source ex- terne dlhydrogPne. Les resultats montrent que les diffgrences de reduc- tion des hauteurs de barrisre suivant les joints observes sont dues

2

des differences dans les proprietes des joints et non

2

des fluctuations dans les conditions de traitement. Les r6sultats obtenus en appliquant

la methode de passivation par source ionique Kaufman

.?I

des cellules solaires EFG indiquent que des aucynentations substantielles de rende- ment peuvent Gtre obtenues dans ces dispositifs avec une exposition de seulement quelques minutes

5

un faisceau d'ions hydrogene.

Abstract - The effect of atomic hydrogen on silicon grain boundaries is reviewed along with recent scanned spot methods for measuring the minority carrier recombination velocity, s, at grain boundaries.

Measurements on a large number of grain boundaries in Honeywell silicon-on-ceramic are presented which show that s typically changes from - ² x 10~cm/sec to 3 x 10~cm/sec, or less, after a few minutes exposure to an intense atomic hydrogen source. Electron-Beam-Induced- Current data o n numerous grain boundaries are discussed; these show that the differences in barrier height reduction from boundary to boundary in response to an external hydrogen source present at the sample surface are due to differences in boundary properties and not to fluctuations in the treatment conditions. The results of applying the Kaufman ion source passivation method to Edge Fed Growth ribbon solar cells indicate that substantial efficiency increases can be achieved in these devices with only a few minutes exposure to the hydrogen ion beam.

Introduction - The purpose of this paper is twofold: we shall briefly review our current understanding of the effects of atomic hydrogen on grain boundaries in silicon, and new data in this area will also be presented including the results of passivating a statistically signif- icant number of solar cells made from EFG silicon ribbon.

I. Grain Boundary Modelinq - In the last five years a great deal of understanding of the electrical and structural properties of grain boundaries in silicon has been achievedl. In particular, lattice imaging studies2-6 of both Si and Ge have indicated that the micro-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982115

JOURNAL DE PHYSIQUE

structure of these defects is generally quite complicated; dissociation or "facetting' of some boundaries into secondary structures is often observed, and as a consequence boundary microstructures may be inhomo- geneous on a scale of tens to hundreds of angstroms. The observation of arrays of individual dislocations at the boundary planes has been made4r6, and in some cases correlated with EBIC (electron-beam-induced- current) activity7. These observations, along with data discussed in the next section, give some plausibility to the suggestion that it is unpaired dangling bonds at dislocation cores which give rise to mid- gap acceptor/donor states at grain boundaries. These in turn cause the electrical activity of many of these boundaries. However, several investigators have emphasized the possible importance of im- purity segregation influencing this activity8-lo, and this issue is far from settled.

The simple double depletion layer model, first proposed by Taylor et a1 in 195211, has been rather successfully used to describe majority carrier transport properties of grain boundaries in both n- and p-type polycrystalline siliconl28 13. These properties include the temperature dependence of the ac14 and dc15 dark resistance and capacitance14 as well as the thermal release of non-equilibrium charge injected into grain boundary trapping states16. Most of the reported data has been measured on rather weakly doped bicrystals where the extent of the grain boundary depletion layers is substan- tially greater than the scale of the structural inhomogeneities mentioned earlier. This is an important consideration since the present version of this theory assumes a uniform array of trapped majority carriers at the grain boundary plane. It is possible that charge inhomogeneities in real grain boundary structures could lead to deviations from the behavior predicted by theory when the grain doping level is sufficient to reduce the depletion region dimensions to values comparable with the scale of these inhomogeneities. Careful data on bicrystals doped in the 1017 - 1 0 ~ ~ c m - 3 range are needed to to resolve this issue.

Despite these concerns, the success of the double depletion layer picture in describing majority carrier effects has led investi- gators to apply it to the prediction of grain boundary-minority carrier interactions. Several calculations have appeared recently which describe the change of grain boundary barrier heights under illumination and predict the specific magnitude of minority carrier recombination velocities at boundaries1-/-lg.

A

number of measure- ments of the barrier hei ht @B, as a function of light intensity have also been published1a, lg, and these are in good 2vneral agree- ment with at least one of these calculation^^^. Seager has recently shown that the data from scanned spot excitation measurements on poly- crystalline silicon p/n diodes (using both light and electron beam excitation) can be analyzed using Zook's result21 to yield values of the grain boundary recombination velocity s. Furthermore, he showed that the observed temperature variation of s for a number of boundaries is similar to that of the dark zero bias resistance to across-boundary majority carrier flow22. This result is consistent with double de- pletion layer model calculations19 of s.

These recent efforts at measuring and modeling minority carrier/

grain boundary interactions have provided reasonable methods for

quantifying the effects of grain boundary passivation agents. In

section

I 1 1

of this paper we will use the Zook theory21 to estimate

values of minority recombination velocities from scanning ERIC measure-

ments on virgin and passivated grain boundaries.

11.

Hydrogen Passivation of Grain Boundaries in Silicon - While at- tempts at describing the electrical properties of grain boundaries have proceeded in a rather quantitative fashion, the characterization of the effects of passivating agents on silicon grain boundaries has had a more qualitative and empirical flavor. This has been due, not only to the difficulty in controlling and measuring the diffusion of the passivating species, but also to the variety of responses that different grain boundaries display in response to these treatment attempts.

It is well established that diffusion of monatomic hydrogen into silicon rain boundaries removes potential barriers at many of these

defect^^^*^^. Because this effect is observed for both n and p t y p e materia123~24, it is clear that majority carrier trapping sites are actually being removed from the forbidden energy gap. Recent work by Johnson et alZ9 has shown that progressive deuteration of thin film polysilicon removes the ESR resonance at g

=

2.0055 associated with the unpaired electron at a non-tetrahedrally coordinated silicon atom - the so-called "dangling bond" defect. Good correlation was observed between the amount of deuterium found in the polycrystalline films by SIMS analysis and the disappearance of the spin signal. The total number of unpaired electrons per unit area of grain boundary estimated from their experiment

( -

1 0 ~ ~ c r n - ~ ) is consistent with the estimates of the trapped majority carrier density obtained from transport measurements on bicrystals15 and bulk polysilicon30. Ginley and ~aaland3l have used infra-red spectroscopy to identify the exis- tence of Si-H bonding modes at grain boundaries after exposure of polycrystalline silicon samples to a hydrogen plasma at elevated temperatures.

Some characterization of the process of hydrogen diffusion into and subsequent bondin at silicon grain boundaries has been attempted in our l a b ~ r a t o r i e s ~ ~ e 28. 32. Seager and ~ i n l e ~ ~ ~ showed that the depth (below the plasma exposed Si surface) to which grain boundary barrier heights were reduced increased as the pressure in the hydrogen plasma was increased. More recent studiesz8 using the hydrogen ion beam of a Kaufman source have shown that passivation is more complete at higher ion fluxes and energies. Both these results have been in- terpreted as suggesting that, in the temperature range of interest, hydrogen in-diffusion is rapid enough to deplete the exposed surface during treatment. Higher ion energies (because of greater penetration) and fluxes appear to raise the near surface H concentration and hence affect the amount of hydrogen which reaches grain boundary defects located at substantial distances (tens to hundreds of

p )

from the exposed surfacez8. Johnson et a1 ' sZ9 recent SIMS determination that polycrystalline silicon films contain much more deuterium than simi- larly treated single crystals suggests that H diffusion down grain boundaries may be the important mechanism for getting the passivating species into the vicinity of grain boundary defects. Although we have previously noted26 that high temperature measurements of hydrogen permeation in single crystal silicon extrapolate to diffusion co- efficients large enough to attribute our passivation results to bulk permeation, Johnson et a129 point out that this extrapolation yields a value which is much higher (nine orders of magnitudel) than is indi- cated by their single crystal diffusion data. It is clear, then, that their work29 strongly favors the dominance of a grain boundary dif- fusion mechanism.

Because of the variety of boundary microstructures that are ex-

pected to exist, we might also anticipate a substantial spread in

boundary diffusion rates for hydrogen. This could well explain the

C1-106 JOURNAL DE PHYSIQUE

differences seen in "treatabilit&" from boundary to boundary; these have been alluded to previously and are elaborated on in section 111.

Finally, it is worth noting that several experiments suggest that the Si-H bonds in passivated grain boundaries are reasonably stable - at least from the viewpoint of typical solar cell environ- ments. Two hour vacuum anneals have shown no appreciable loss o f passivation (as monitored by EBIC measurements) until the sample temperature exceeds 4 0 0 ' ~ ~ ~ . Associated mass spectrometry s t u d i e ~ 3 ~ have also confirmed that the majority of the hydrogen outgassed from grain boundaries during ramped temperature measurements leaves above 400°C.

111. Measurements of the Hydrogen Passivation Efficiency-on Single Grain Boundaries - As we have previously reportedLu, not all grain boundaries in silicon respond in the same fashion to hydrogen passivation. In this section we shall present some recent data which bear on this issue and suggest some possible explanations for this phenomenon.

The majority of the measurements discussed in this section were made by measuring the short circuit current of Honeywell

SOC

(silicon- on-ceramic) polycrystalline n+/p diodes33 as a 25 keV electron beam was scanned across the n+ surface. zook21 has suggested that the resulting current, when plotted versus the distance of the beam from a grain boundary plane, can be used to deduce both the bulk diffusion length and the minority carrier recombination velocity at the grain boundary. While Zook's calculation specifically assumed that excita- tion was due to an infinitesimally thin ray of light, it has recently been shown20 that his result also describes the shape of EBIC scans as long as the diffusion length, L, is substantially larger than the diameter of the electron-hole excitation volume. This latter dimension is about 7

pn

for 25 keV electrons in silicon34. Figure 1, reproduced from reference 20, shows a comparison between EBIC traces obtained on Honeywell silicon-on-ceramic polycrystalline diodes and Zook's calculated result. Scanned light spot data at various wavelengths were also made on this same grain boundary, and this allowed an independently obtained estimate of L and s to be made.

The point to be made here is that knowledge of the EBIC profile near any specific grain boundary can be used to reliably estimate the minority carrier recombination velocity. While only the maximum ERIC

I(')

using the notation of Figure 1) loss (which we define as 1 - 1 0

will be shown in the data plotted below, we used the scanned excita- tion results of reference 20 to provide a rough calibration of peak losses in terms of real recombination velocities. Because the actual minority carrier diffusion length varies somewhat in our samples, there is not an exact one-to-one correspondence between maximum EBIC loss

and

s ,

but this variation is far smaller (proportionately) than that

produced by hydrogen exposure.

We have designed a fairly straightforward experiment to help us

understand the variability of grain boundary response to hydro-

gen diffusion seen in our earlier work on Honeywell polycrystalline

silicon.

A

total of

6

Honeywell

SOC

samples, comprising about 50

separately observable grain boundaries were EBIC characterized, then

H passivated in the Kaufman ion source for 4 minutes at 325OC. The

peak ion energy and flux (1.4 keV and 1.4 m ~ / c m ~ ) were selected to

be in the region where our prior results28 have indicated good passi-

vation efficiency in this material. Typical results for several

samples are shown in Figure 2. The first point worth making about

- ^{ZOOK THEORY}

Figure

1

Room temperature electron-beam-induced-current scans of a grain bound- dary in a Honeywell silicon-on-ceramic n+/p diode at two different electron beam energies. The solid lines are theoretical fits to the data using the theory of Zook with L

=

40

p

and s = 1.25

x

lo5 cm/sec.

Previous light-beam-induced-current data on the same boundary were used to establish the values of s and L and determine the effective absorp tion coefficients used for fitting this data

( a =

3280 and 12,000 cm-l for 25 and 15 keV, respectively). Reference (20) contains full details o f these measurements.

[ ~ a l a ~ a ~ e EBIC

3

temperat~re ambiante d'un joint de grains d'une diode silicium sur ceramique n /p Honeywell pour 2 energies differentes du faisceau dV6lectrons. Les lignes solides correspondent

.3

un lissage theorique des donn6es obtenu en appliquant la theorie de Zook pour L

=

4 0 ~ x 1 et s

=

1,25 x 105 cm/s. Des donnees ant6c6dentes LBIC sur le mEme joint ont permis de determiner les valeurs de L et de s ainsi que

les coefficients d'absorption effective utilises pour lisser ces donnees

(a

=

3280 et 12000 cm-l respectivement pour 25 et 15 keV) la reference

(20)contient les details de ces mesuresd

C1-108 JOURNAL DE PHYSIQUE

this data is the variety of EBIC losses seen for the untreated bar- riers. Using the Zook formula (with

L =

40

pm)

we find that the more active barriers having a peak current loss of - ^{0.40 have s}

⁼

^{2.0 x}

10~cm/sec, while the smallest shown in Figure 2

( -

0.15 loss) have s values about one decade smaller. The peak EBIC loss values in this figure, in fact scale roughly20 as the logarithim of s. The variety of s values seen for untreated barriers can be compared with past observations of an equivalently large variation in the zero-bias barrier resistan in various types of p o l y ~ i l i c o n ~ ' ~ ~ . As has been pointed o , ' 5 t u Rg&th Ro and s are expected to be exponential functions of the band bending,

@B,

and hence both are equally sen- sitive to fluctuations in this parameter. These fluctuations could result from differences in boundary microstructure.

Aside from these initial differences, variations in the response to nominally the same treatment with hydrogen are even more noticeable.

The decrease of the peak loss after passivation varies wildly and appears to be almost totally uncorrelated with the virgin loss value.

This implies that each boundary either sees quite a large difference in local H concentration, or that the ability of a defect to bond to a locally available H atom varies strongly from boundary to boundary.

If we naively assume that the same "dangling bond" defect dominates the microstructure of all boundaries, the former supposition becomes more likely - the risks involved in drawing this conclusion are clearly substantial, however.

Variations in the way that our samples are processed in the Kaufman ion source could, in principle, result in some differences in local hydrogen concentrations. These could arise from different local flux densities in various locations of the ion beam or tem- perature gradients across our specimens. To check for these effects we outgassed all of our treated samples for one half hour at 600°C in vacuum. This has previously been shown to drive out all the bonded hydrogen and return the EBIC losses to values comparable to their

original values32. Having accomplished this, we retreated all samples (at random positions in the ion beam) with the original passivation conditions. As can be seen in Figure 2 passivation efficiency after this second treatment was markedly improved, a result which we have observed before35. The average EBIC loss of - ^0.05 after this second treatment corresponds to s values of -

^{3 x}

10~cm/sec.

Another general result observed in this retreatment experiment

was a general correlation between how well a boundary passivated the

first time and the second time. While this correlation was not per-

fect it was quite noticeable. There were only two boundaries

(#9

in

Figure

2 A r

was one) which showed more EBIC loss after the second

passivation; a statistical analysis of the data showed that far more

of these events should have been observed if the degree of passivation

was not a function of the boundary structure or its immediate environ-

ment in the polycrystalline sample. Our conclusion is, that to first

order, it is not variations in our externally controlled passivation

environment which cause most of the variations seen, but rather certain

properties of the grain boundaries themselves. The more efficient

passivation of already treated and outgassed boundaries complicates

the interpretation of these experiments somewhat and it is by itself

a rather surprising and interesting result. It is possible that the

first passivation/outgassing cycle purges impurities from the grain

boundaries which tend to block hydrogen diffusion. More experiments

of this type are presently underway in our laboratories to probe this

effect.

Figure 2

The effect of atomic hydrogen treatment and retreatment for several grain boundaries located on two samples of Honeywell silicon-on- ceramic. Maximum

EBIC

loss (defined as 1 - m , ^I(0) using the notation of Figure 1) is shown after Kaufman ion source treatment for

4

minutes at

325OC.

The peak ion energy was 1.4 keV and the ion current den- sity was 1.4 m ~ / c m ~ . Also shown is the EBIC loss after vacuum out- gassing at

600°C

for

30

minutes and retreatment with hydrogen under the same Kaufman source conditions.

[~ffet de traitements successifs par hydrogene atomique sur plusieurs joints de grains appartenant

2

2 echantillons silicium sur cgramique

I (O) en utilisant Honeywell. La perte EBIC maximale (d6finie comme

1

- ^{I ( . . )}

les notations de la figure

1)

aprPs traitement la sourcd ionique

Kaufman de 4mn

2 3 2 5 ° C

est prGsent6e. L16nergie ionique de crete etait

1,4

keV et la densit6 du courant d'ions

1,4

m~/cm2. Est egalement

montree la perte EBIC obtenue aprPs d6gazage dans le vide

2

600°C pen-

dant 30 mn et le retraitement par hydrogene dans des conditions iden-

tiques pour la souce ~aufrnan.]

C1-110

JOURNAL DE PHYSIQUE

A

final point about these Honeywell samples is that sample to sample variations in the average boundary treatability were also evident, implying one rather small area of this material (the samples were - 1 x 5 m m ) can possess grain boundaries with properties which are distinctly different than those in another area. In fact, one particular sample had all of its six observable boundaries passivate completely (no observable EBIC loss) after both Kaufman source ex- posuresl A second hint that the source of the polycrystalline silicon can be crucial in this regard comes from some rather preliminary ERIC studies of n+/p diodes fabricated on Mobil-Tyco EFG (Edge Fed Growth) ribbon silicon. Several areas of these diodes were monitored before and after passivation at 275OC (with the Kaufman source) with t3e results shown in Figure 3. Essentially all the EBIC losses seen before passivation disappear after treatment. It is possible that the particular boundary structures (higher order twin boundaries) common to this material are very permeable to hydrogen. As shown in the following section the changes in device parameters observed after passivating solar cells made from this material are also impressive.

BEFORE AFTER

Figure

3

Qualitative EBIC scans of an n+/p EFG silicon ribbon solar cell before and after hydrogen passivation at 1.4 keV, 1.4 m ~ / c m 2 , for 4 minutes at a sample temperature of 275°C. The wide dark structures are due to beam attenuation by the front grid structure. Virtually all the boundary-related contrast is absent after treatment.

[~mage qualitative EBIC d'une cellule solaire EFG n +

/p

en couche mince de

silicium avant et apr6s passivation

5

llhydrog&ne 3 1,4 keV, 1,4 m ~ / c m 2 ,

4mn pour une temp6rature d16chantillon de 275OC. Les larges bandes sombre

sont dues

5

l'attcnuation du faisceau par la grille frontale. De fait,

tous les contrastes dus aux joints sont absents aprPs traitement.]

IV. Passivation of Solar Cells - While there is considerable evi- dence that hydrogen has a large (and beneficial) effect on the prop- erties of individual grain boundaries, there have been only a few reports of its effects on the performance ~f~polycrystalline silicon solar 27r28. Robinson and DIAiello and weZ8 have shown that a few epitaxial silicon on metallurgical grade silicon cells show 10-20% improvements in Air Mass 1 efficiencies after exposure to atomic hydrogen. A few preliminary s t ~ d i e s 3 ~ # 3 ~ have shown that cells made from fine grain polysilicon

( <

10

pm

grain size) show large increases in cell performance after hydrogen exposure, but there has been little recent effort to grow and fabricate these types of struc- tures. Thus, while work on improving passivation procedures has pro- gressed considerably, no comparable effort has proceeded at the same time to fabricate clean, fine grain polysilicon solar cells to test the ultimate utility of this process. In lieu of demonstrating passi- vation on fine-grained materi'al we have concentrated our recent device passivation efforts on large grained Edge Fed Growth (EFG) ribbon silicon cells. This die-pulled method of growin solar cell silicon has been under development for a number of years38, and consequently these devices have undergone considerable refinement in processing.

Two groups of cells were passivated in this study. The first group had considerably lower average efficiencies than the second and was representative of "non-production" ribbon cells. All devices were n+/p configurations with base resistivities on the order of 5 Q-cm. No anti-reflection coatings were present for the measure- ments quoted here, although as we shall emphasize later, the usual

- 40% increase in AM1 efficiency appears to be observed when passi- vated cells are subsequently AR coated. Results of treating the first group of four cells are shown in Table 1. The average efficiency improves from -

⁷

to 8.48, a 21% increase after

4

minutes of treat- ment. The Kaufman ion source parameters were in the high ion energy and flux parameter range dictated by prior optimization studies 28. As mentioned in section 111, EBIC losses appear to be almost totally

eliminated after treatment.

The second group of EFG cells studied had pretreatment effi- ciencies more characteristic of actual production devices. The his- togram shown in Figure 4 shows that in addition to an - 13% increase in the average efficiency (8.4

+

9.5%), hydrogen exposure also markedly narrows the distribution of cell efficiencies. The average gains in short circuit current, open circuit voltage, and fill factors were

- 6%, 3%, and 38, respectively for this group of cells.

It is clear from the increases seen in short circuit current that hydrogen exposure is accomplishing more than just removing current losses at grain boundaries. The density of these defects is actually quite low (a few per mrn) in these cells, and hence they are probably affecting the device currents by less than 3%. While we have previously reported28 that the normal incidence reflectivity of H bombarded surfaces is 10-15% lower than its virgin value

( -

.40-.45), this effect does not seem to be responsible for all

JOURNAL DE PHYSIQUE

the current gains seen in Table 1. In fact we have observed that when several cells from both groups have been AR coated, the usual

( -

40%) gains in AM1 efficiency are observed - this suggests that

our observed reflectivity changes could be due to scattering effects, and that these prior data d o not imply real gains in transmittance.

It is possible that the observed increases are due to substantial passivation of point or line defects. Since the mechanism which

limits the minority carrier lifetime in EFG ribbon has not been completely established, this suggestion remains a rather speculative one.

The gains seen in the open circuit voltage and fill factor after

passivation are likely due to grain boundary effects. Several inves- t i g a t o r ~ ~ ~ ~ ~ ~ have pointed out that a fairly low density of these defects can seriously degrade p/n diode characteristics. To firmly establish this connection it would be desirable to fabricate and passivate small area devices having various numbers of boundaries intersecting the metallurgical junction as was done by Sopori 39.

V. Conclusions - W e have reviewed the present status of grain boundary passivation in silicon as well as some of the methods for quantifying this phenomena. We noted that evidence was recently presented by Johnson et that the ESR resonance associated with "dangling bond"

defects in polycrystalline silicon is removed by deuteration. In addition their data showed that grain boundary diffusion of deuterium (and hence of hydrogen, by implication) is exceedingly rapid and is therefore probably the dominant mechanism during passivation. Our prior studies2* of the ion energy and flux dependence of passivation have also indicated that rapid diffusion of hydrogen into the bulk occurs during H ion beam bombardment. In fact, the importance of maintaining a high concentration of atomic species near the surface was a major conclusion of this work28.

In the present paper we have presented data which explicitly shows the differences in response to hydrogen from one boundary to another. From the results of these experiments we have ascribed these differences to specific differences among grain boundaries rather than possible passivation process variations. Likely candidates for pro- ducing such effects could include differences in hydrogen permeation coefficients as a result of specific microstructural variations or perhaps, the effect of boundary segregated impurities. The fact that passivation efficiency is greatly diminished when hydrogen is forced to pass through a heavily phosphorous doped (junction) region has already been noted26; this observation also tends to draw atten- tion to the possible influence of impurities.

Finally, we have shown that significant improvements are seen in large grained EFG ribbon solar cells after only a short exposure to atomic hydrogen. These changes are largest in the poorer cells measured, but are still significant

( -

13%) in production quality cells. While some of the observed improvement can be related to grain boundary effects, it is likely that increases in the minority carrier lifetime in the bulk of the material are also taking place as a result of the passivation process.

Acknowledgements - The authors are grateful for useful discussions concerning grain boundary passivation with N. M. Johnson and S. J.

Fonash. L. N. Rapp provided invaluable assistance in carrying out

many of the experiments reported here.

AM1 EFFICIENCY

Figure 4

A histogram describing the results of passivating eleven EFG silicon ribbon solar cells with the Kaufman ion source for

4

minutes. Sample temperature was 275°C during treatment, and the peak ion energy and current density were 1.4 keV and 1.4 mZi/cm2, respectively. The

e f f i -

ciences quoted were measured with no cell AR coatings at a light in- tensity of 100 mw/cm2 using a tungsten-halogen light source as AM1 simulation.

[Elistogramme decrivant les rGsultats de passivation sur onze cellules

solaires en silicium EFG par exposition de

4

mn

Z

la source ionique

Kaufman. La temp6rature de 116chantillon etait de

2 7 5 O C

pendant le

traitement, l'energie de crete et la densit6 de courant des ions de

1,4

keV et

1,4

mA/cm2 respectivement.

L e s

rendements port6s ont Bt6

rnesures avec des couches sans revetement antireflet

2

une intensite

de lumisre de

100

m ~ / c m 2 en utilisant pour la simulation AM1 une

source de lumisre h a l o g h e au tungstGne.1

JOURNAL DE

PHYSIQUE

T a b l e I

CELL # TREATMENT ~,,(mA/cm 2

)

f . f . e f f .

I I

1 i

VIRGIN

I 1 9 . 6 I .541 I .77 I 8.2

" I He, 275'C, 4 min. I 22.3 1 .570 1 - 7 5 1 9 . 5

I I I I I

2 1

V I R G I N

I 1 6 . 4 1 .508 1 .73 1 6 . 1

"

I

H+,

275'C, 4 min. I ^21.5 1 .566 1 .71 1 8 . 6

I I 1 I I

3 1

V I R G I N

I 1 5 . 9 1 - 5 2 9 1 .78 1 6.6

"

I

H+, 275'C,

4 min. 1 18.2 1 .545

]

.78 1 7.7

I I I 1 1

4 1

V I R G I N

I 17.4 1 .543 1 .74 1 7.0

" I + , 2 7 5 ' C , 4 m i n . I 1 8 . 8 1 .553 1 .74 1 7.7

I I I I I

The r e s u l t s o f p a s s i v a t i n g s e v e r a l 4 c m 2 Mobil Tyco EFG r i b b o n s o l a r c e l l s a t 275'

C

f o r 4 m i n u t e s a t a peak i o n e n e r g y o f 1 . 4 keV.

The i o n c u r r e n t d e n s i t y was 1 . 4 m ~ / c m ~ d u r i n g t r e a t m e n t .

A l l

s o l a r c e l l d e v i c e p a r a m e t e r s w e r e measured w i t h a n A i r Mass 1 s i m u l a t o r w i t h n o a n t i - r e f l e c t i o n c o a t i n g s p r e s e n t on t h e c e l l s .

R e f e r e n c e s

+ T h i s work p e r f o r m e d a t S a n d i a N a t i o n a l L a b o r a t o r i e s s u p p o r t e d by t h e U.S. Department o f Energy u n d e r c o n t r a c t XDE-AC04-76DP00789.

1. S e e f o r i n s t a n c e , G r a i n B o u n d a r i e s i n S e m i c o n d u c t o r s , e d . b y

G.

E. P i k e ,

C . H.

S e a g e r , and

H .

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2. A. B o u r r e t and

J.

Desseaux, P h i l . Mag. e, ⁴⁰⁵ ( 1 9 7 9 ) . 3.

C .

B . C a r t e r ,

J .

Rose, and D.

G .

A s t , P r o c . 3 9 t h EMSA ( 1 9 8 1 ) . 4 . B. Cunningham and D . A s t i n r e f e r e n c e 1, p . 21.

5. Anne-Marie Papon, M a u r i c e P e t i t , Georges S i l v e s t r e , a n d

J . J.

Bacmann, r e f e r e n c e 1, p . 27.

6.

C .

B . C a r t e r , r e f e r e n c e 1, p. 33.

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G .

A s t , i n r e f e r e n c e 1, p . 51.

8 . D. R e d f i e l d , Appl. Phys. L e t t . 38, 1 7 4 ( 1 9 8 1 ) .

9. P. E. R u s s e l l ,

C.

R. H e r r i n g t o n , D.

E.

Rurke, and P. H . Holloway i n r e f e r e n c e 1, p. 185.

10. L. L. Kazmerski,

J .

Vac. S c i . T e c h n o l . 20, 423 ( 1 9 8 2 ) .

11.

W.

E. T a y l o r ,

N .

H . O d e l l , a n d H . Y. Fan, Phys. Rev. 88, 8 6 7 ( 1 9 5 2 ) .

12. S e e f o r i n s t a n c e ,

G .

E. P i k e and

C .

H . S e a g e r , Adv. i n C e r a m i c s 1, 53 ( 1 9 8 1 ) .

13.

J.

Werner,

W .

J a n t s c h , K .

H .

F r o e h n e r , a n d

H . J .

Q u e i s s e r i n r e f e r e n c e 1, p . 99.

14. C. H. S e a g e r and

G .

E . P i k e , Appl. Phys. L e t t . 37, 747 ( 1 9 8 0 ) . 15.

C .

H. S e a g e r a n d

G .

E . P i k e , Appl. Phys. L e t t . 35, 709 ( 1 9 7 9 ) . 16.

C .

H . S e a g e r ,

G .

E . P i k e , a n d D . S . G i n l e y , ~ h y s . Rev. L e t t .

4 3 , 532 ( 1 9 7 9 ) .

-

17. H. C. Card a n d E. S. Yang, IEEE T r a n s . E l e c t . D e v i c e s ED-24, 397 ( 1 9 7 7 ) .

18. J.

G .

Fossum and F.

A .

Lindholm, IEEE T r a n s . E l e c t . D e v i c e s ED-27, 692 ( 1 9 8 0 ) .

19. C. H. S e a g e r , J. Appl. Phys. 2, 3960 ( 1 9 8 1 ) .

20.

C .

H. S e a g e r ,

J .

Appl. Phys. 53, 5968 ( 1 9 8 2 ) .

21. J. D.

ZOO^,

Appl. Phys. L e t t . 37, 223 ( 1 9 8 0 ) .

22. To b e p u b l i s h e d i n A p p l i e d ~ h y x c s L e t t e r s .

C. H. Seager and D. S. Ginley, Appl. Phys. Lett. - 34, 537 (1979).

D. R. Campbell, M. H. Brodsky, J. C. M. Hway, R. E. Robinson, and M. Albert, Bull. Am. Phys. Soc. 24, Abstract

JK3

(1979), and D. R. Campbell, Appl. Phys. Lett. - 3 c 604 (1980).

C. H. Seager, D. S. Ginley, and J. D. Zook, Appl. Phys. Lett.

36, 831 (1980).

- C. H. Seager and D. S. Ginley, J. Appl. Phys. 52 ^{(21, 1050}

(

1981

)

.

P. H. Robinson and R. V. DIAiello, Appl. Phys. Lett. 39, 63 (1981).

C. H. Seager, D. J. Sharp,

J.

K. G. Panitz, and R. V. D'Aiello, J. Vac. Sci. Technol. 20, 430 (1982).

N. M. Johnson, D. K. Biegelsen, and M. D. Moyer, Appl. Phys.

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C. H. Seager and T. G. Castner, J. Appl. Phys. 49, 3879 (1978).

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C. H. Seager and D. S. Ginley, Fundamental Studies of Grain Boundary Passivation with Application to Improved Photovoltaic Devices, Sandia National Laboratories Report tSAND80-2461, August 1980.

For a description of this polycrystalline silicon, see for instance, J. D. Zook, S. B. Schuldt, R. B. Maciolek, and J. D.

Heaps in "Proceedings of the Thirteenth IEEE Photovoltaic

Conference", Washington, D.C., 1979, (IEEE, New York, 1979), p. 472.

T .

E. Everhart and P. H. Hoff, J. Appl. Phys. 42, 5837 (1971).

Unpublished data.

C. H. Seager at the Solar Energy Research Institute Poly- crystalline Silicon Subcontractors Meeting, Golden, CO, June 17-18, 1982, SERI CP-211-1648.

P. H. Fang, C. C. Shubert,

J.

H. Kinnier, and Dawen Pang, Appl.

Phys. Lett. 2, 256 (1981).

See for instance, K. V. Ravi, H. B. Serreze, H. E. Rates, A. D.

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DISCUSSION

J . C . C . FAN.- S i n c e you found b e t t e r p a s s i v a t i o n much h i g h

1) Dosage r a t e a n d h i g h i o n e n e r g i e s , any s u r f a c e t e m p e r a t u r e t h a t w i l l a f f e c t t h e r e s u l t s .

- 2 ) Any b u l k damages i n S i s e e n i n i o n i m p l a n t a t i o n 3) Any GaAs r e s u l t s .

C.H. SEAGER.- 1) I f t h e s a m p l e s a r e w e l l h e a t - s i n k e d n o s i g n i f i c a n t sample tempera- t u r e r i s e .

2 ) Some b u l k damage h a s been o b s e r v e d . The e v i d e n c e is a d e c r e a s e i n u l t r a - v i o l e t r e s p o n s e .

3) N o p a s s i v a t i o n i n G a A s s o f a r .

JOURNAL DE PHYSIQUE

D. HELMRE1CH.- Are there difficulties to integrate this passivation step by H- implantation in a production line (vis. the Mobil Tyco production line, mentioned in the talk) ?

-

additional cost ?

C.H. SEAGER.- I do not have a finer answer to this question but my feeling is that this additional passivation step should not involve significant additional processing costs.

G.O. MULLER.- You talked about the facts, would you say something on the mechanism ? The removal of localized state, by hydrogen seems to be likely if you think of them as dangling bonds, do you assume the grain boundaries.to be undecorated ?

C.H. SEAGER.- I think it is likely that the passivation effect is a result of a hydrogen atom bonding to a non-tetrahedrally coordinated silicon atom-the "dangling bond" defect. I'm not sure if any impurity segregation (other than of hydrogen, of course) at grain boundaries plays an important role in the electrical activity of these defects.

H.F. MATA&.- I think it has been shown that thermal treatment will relax grain boundary strain. With respect to the g.b. model I want to mention that the

thermionic approach has been shown to be equivalent to the Schottky barrier approach (Taylor et al.). See "Defect Electronics in Semiconductors" 1978 (Wiley Interscience).

C.H. SEAGER.- The model that I am using is, in fact, the Taylor, Odell, and Fan model.

K.L. CH0PR.A.- 1) Did you try passivation with neutralized ion beam ?

2) Is the large hydrogen implantation depth calculated for your specimens due to channeling effect ?

C.H. SEAGER.- 1) No. The hydrogen ion bean is actually a mixture of neutral, atomic, molecular and ionized hydrogen.

2) The calculations are probably for

[loo]

channeling direction.

Note added after the session.- This calculation (of penetration depths) does not directly incorporate any directional properties of the crystalline lattice, i.e. it is for a idealized uniform medium.

M. ROD0T.- If the penetration of H is smaller than 1/10 Dm, how can you explain the improvement of solar cells which seems to need penetration of some hundreds lun ? C.H. SEAGER.- The penetration of 1/10

urn

is the bulk penetration (no diffusion included) but there is a very fast diffusion of t1 along grain boundaries.

S. MARTINUZZ1.- Have you verified the stability of the improvement, for example after annealings or long exposures to sunlight ?

C.H. SEAGER.- Thermal anneals for two hours in vacuum show that the sample tempera- ture must be above % 400 O C for any noticeable loss of the passivation effect. Only limited data on the stability of passivated solar cells under illumination are available. No problems are seen after 5-8 hours.