CdS solar cells

Solar Energy Materials 20 (1990) 15-28 North-Holland

15

EVAPORATED COPPER SULPHIDE LAYERS F O R A L L - V A C U U M EVAPORATED C u x S / C d S SOLAR CELLS E. APERATHITIS, F.J. BRYANT and C.G. SCOTT Department of Physics, University, of Hull, Hull HU6 7RX, UK Received 14 June 1989 Copper sulphide layers have been prepared by vacuum evaporation from a single CuxS source, as an alternative to the chemiplating technique for fabricating the upper Cu x S layer in Cu x S / C d S solar cells. Deposition rates of less that 150 A / r a i n have been shown to produce CuxS layers with chalcocite being the major phase. Higher deposition rates increase the copper content of the layer which dominates its optoelectrical properties. Layers free from excess copper have a chalcocite-related phase transition between 75 and 8 0 ° C , room temperature resistivity between 10 .2 and 10- 3 ~2 cm and evidence of direct and indirect band gaps of 2.25 and 1.25 eV, respectively. With well controlled evaporation conditions the layers deposited on hot CdS thin film substrates are found to have highly reproducible characteristics, and are well suited for use as the absorber for the C u x S / C d S solar cell. Open-circuit voltages up to 0.58 V have been produced in cells with efficiencies in excess of 7%.

1. Introduction

Despite the long period of development for C u x S / C d S thin film solar cells and the significant advances that have been made in understanding the fundamental processes governing their behaviour, there is still a general lack of confidence concerning their reproducibility and long-term operational stability. These problems are partly due to the highly sensitive dependence of the CuxS material properties on composition and partly to the nature of the chemiplating ion-exchange or "Clevite" technique which is widely used for the CuxS formation in high efficiency C u x S / C d S cells. Cu~S is a p-type degenerate semiconductor and its optoelectrical properties are controlled by the stoichiometry of the material. It is widely believed that the material required for the absorber layer in high efficiency photovoltaic devices is CuzS (chalcocite) but CuxS can exist at room temperature in several different phases [1]. Under ideal conditions, Cu 2S is formed by the chemiplating technique as a result of Cu + ions displacing cadmium ions from a CdS film placed in a hot ( > 90 o C) CuC1 bath. Unfortunately, departures from ideal stoichiometry arise from the presence of Cu 2+ ions in the reaction bath and from subsequent oxidation of the newly prepared film upon exposure to air [2]. A further practical problem associated with layers formed in this way concerns the preferential formation of the CuxS at the CdS grain boundaries and other structural defects. This can easily lead not only to a non-uniform CUxS stoichiometry and thickness but also to an unintentional increase in the junction area of the device which has a deleterious effect on the cell 0165-1633/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

16

E. Aperathitis et al. • Evaporated copper ~'ulphlde lo~vers j?~r Uu, S / ('dS solar ~el:~

characteristics [3]. These inhomogeneities in the structure of the Cu,S layers have caused problems in analyzing their properties and, accordingly, have hampered further optimization of these properties. The above difficulties can be overcome by preparing and characterizing Cu~S thin film layers formed by vacuum evaporation onto different substrates. In this paper the woperties of Cu,S layers produced using a single source evaporation process will be presented. It will be shown that this technique is capable of producing layers with advantageous characteristics for use in Cu ,S/CdS solar cells.

2. Experimental methods

The Cu,S layers were vacuum-evaporated onto glass, ITO-coated glass or thin film polycrystalline CdS substrates. The polycrystalline CdS films (20-25 ~m thick) had been previously deposited by vacuum evaporation onto Zn-plated copper substrates kept at 220°C and with a deposition rate of 0.8 /~m/min in another evaporator. Any of the above substrates could be accommodated in one CuxS deposition run so that CuxS layers grown under identical conditions but on different substrate surfaces could be examined. The source material was copper(I) sulphide supplied by Ventron GmbH, which was used either in the as-received state or after sintering in hydrogen at 400 °C for 3 h. The powder was evaporated from a conventional integral tungsten-alumina crucible of the cone type (volume 0.25 ml, cavity diameter and depth 10 mm). The substrate holder, which could be heated to temperatures up to 300 ° C, was placed 20 cm vertically above the crucible. The characterization of the resultant CuxS thin films after preparation and after subsequent annealing in an ambient atmosphere (flowing hydrogen, vacuum or air) was made using the following experimental techniques: electrochemical analysis (ECA) for determination of thickness [4] using samples deposited on CdS substrates; X-ray diffraction measurements (using a Philips X-ray diffractometer PW1010) for structural studies; optical transmission measurements (using a SP700 spectrophotometer) and, resistivity measurements as a function of temperature in flowing argon for determination of the phase transitions of the layers with temperature. The sheet resistance measurements were performed using the two-probe technique with evaporated gold contacts in the form of two parallel lines of width 0.43 and 2.1 mm apart. For some room temperature resistance measurements of layers deposited on glass substrates the Van der Pauw technique [5] was used.

3. Results and discussion 3.1. Evaporation conditions

The complicated nature of the Cu/S phase system, seen in fig. 1, and the relative higher evaporation rate of S compared to that of Cu, were found to produce CuxS thin films with properties which depended not only on the CuxS powder source

E. Aperathitis et aL / Evaporated copper sulphide layers for Cu x S / C d S solar ceils

17

250 Cv

(Hexagonal)

200

I 150 i.,i.I r.,."

Cv+Og

100

(a)~UJ

Dg (Eubic)

/

+Ch 93oc

,/

,~exagonaU Eh+Cu ~03.5 °C

73 °C

An (Cubic)

50

I---

[3j (Orfhorhombic)--~,- I o ~-?horhombic)

Cv+An

0

1.0

1.8

I.'/

1.9

2.0

CulS RATIO

150 f

Chd}s+Cu--~

O +O"

(b) ~. 12~

S0

-An*DjJ

OJ

"

•

l hi+Oh

/ChJCh+C~

,.=, F--

1.92

1.%

1,96

1.98

2.00

Cu/S RATIO Fig. ]. (a) Condensed and (b) detailed view of the phase diagram for the Cu/S system. Cv = covelite, A n = anilite, D g = digenite, Dj = djurleite, C h = low chalcocite, Chdi s ~ h e x a g o n a l c h a l c o c i t e [1].

used but also, as expected, on the evaporation conditions. The different vapour pressures of S and Cu caused the source to be depleted of S during the evaporation so that a Cu-rich powder resulted. This, in turn, caused an increased C u / S ratio of the vapour stream which consequently promoted the precipitation of elemental copper in the resultant CuxS film. The scanning electron microscope micrographs of fig. 2 show how the surface topography of a CuxS film deposited on a glass substrate at 220 °C is influenced by the evaporation time and by the use of sintered and unsintered copper sulphide powders. All these micrographs show surfaces which are covered with nodules which have been identified as copper. It is apparent that the longer the deposition time the bigger the Cu nodules on the surface. The effect of the sintering process, which produces a more stoichiometric and denser powder (due to the removal of oxygen, moisture and S in the form of H2S ), is the production of less copper precipitation on the CuxS film surface. Similar surface patterns have been seen for Cu~S layers deposited on ITO-coated glass and CdS substrates.

1~

f.. Aperatkitts ~'l ai.

l:z'apomlted ( opper ~u[phuh, /aver~ Cot ( u ,

d.~ ~*dat ~c/;~

F r o m these a n d further observations c o n c e r n i n g the vapour deposited growth o! the C u t s layers it became clear that the formation of the copper nodules was associated not only with the stoichiometry of the powder which becomes Cu-rich d u r i n g the evaporation, but also with the surface mobility of the atoms on the substrate surface a n d the smaller sticking coefficient of sulphur as compared with copper. Indeed, when Cu~S layers were deposited on substrates kept at room

Fig. 2. Cu,S layers evaporated on substrates kept at 220 ° C. (a) Evaporation using unsintered Cu,S powder: (al) ta~ = 1.5 min, d = 740 A; (a2) tdep = 20 min, d = 4100 A. (b) Evaporation using sintered ( ' u ~ S p o w d e r ; ( b l ) tde p = 1.5 m i n , d = 1 3 0 0 ,~; (b2) tdep = 20 m i n , d - 2400 A.

E. Aperathitis et al. / Evaporated copper sulphide layers for CuxS/ CdS solar cells

19

Fig. 2. Continued.

temperature the Cu nodules were greater in number but smaller in size. This was due to the reduced surface atom mobility and the reduced re-evaporation of sulphur from the cold substrate [6]. Similar observations concerning the Cu enrichment of the CuxS powder [7-9], and the formation of Cu-rich CuxS layers resulting from vacuum deposition onto hot substrates [10-13] or by other deposition techniques such as RF sputtering [14], sulphurization of Cu [15,16], etc., have been reported. Although the presence of excess Cu enhances the formation of a highly stoichiometric Cu 2S layer, the presence of the Cu nodules is not expected to be beneficial if

20

E. 4perathitis et at.

l',~,<6ooratedcopper sulphide Ic~ve~ ]or ('u~ X / ( dS ~'olar c('l~s

Fig. 3. Cu:,S layers deposited at a slow rate on (a) glass and (b) textured CdS substrates kept at 220 o C.

this layer is to be employed as an absorber for a C u x S / C d S solar cell. However, copper precipitation at the surface of the growing CuxS layer was prevented by performing the evaporation at a very slow rate. Fig. 3 shows the surface topography for layers produced by such an evaporation. N o free copper can be seen for the 160 ,~ thick Cux S layer, and similar results were obtained for thicknesses up to 1000 A. The electrical resistivity of such Cu-free layers on CdS substrates was around 10-3 cm. Typical variations in the properties of evaporated CuxS layers with evaporation rate and time are shown in table 1. The evaporation rate, thickness and resistivity p were determined for layers grown on CdS substrates, whereas for the

E. Aperathitis et aL / Evaporated copper sulphide layers for CuxS/ CdS solar cells

21

Table 1 Variations of the properties of the evaporated CuxS layers with the evaporation rate on substrates held at 220 o C (resistivity values p and pl are for layers on CdS and glass substrates, respectively) Evaporation time

Evaporation rate

Layer thickness

(min)

(A/min)

(A.)

4.5 2.0 7.0 20.0 40.0

538 489 124 12 3

2422 978 865 240 130

O ( £ cm)

01 ( £ cm)

1.8 × 101 5.3×10 2 4.7 x 10 3 1.5 x 10 -3 1.4x10 3

2.3 x 102 1.8×10 o 2.3×10-1 8.8×10 o

resistivity of the layers grown on glass, pl, the Van der Pauw method was used. It is apparent that to increases as the evaporation rate increases. The higher resistance of the Cu~S layers on glass compared with those grown on CdS is attributed to the diffusion of copper into the CdS substrate during the evaporation. Taking into account the previously discussed growth process for the CuxS layers, the observed increase in resistivity for Cu~S layers thicker than - 1000 ]~ is clearly associated with the excess copper in their structure. The pl values for the thinnest sample of table 1 is higher than expected and it is believed that this indicates the onset of excess copper formation (resulting from Cu enrichment of the source during the 40 min evaporation period), despite the fact that these samples are those seen in fig. 3. For the layers deposited on CdS it must be assumed that the effect of the Cu-enriched source is offset by the loss of copper from the CuxS layer by diffusion into the CdS layer. For samples grown on cold substrates resistivity values were generally higher than those on hot substrates. However, in addition to some adhesivity problems which were encountered with these layers, prolonged deposition times were accompanied by an increase in the temperature of the substrate due to radiation from the source crucible. These problems hampered the production of reliable, reproducible results for CuxS layers on cold substrates. 3.2. Post-fabrication heat treatments

Changes in resistance as a result of post-fabrication treatments were found to be dependent on the substrate on which the layers were deposited. CuxS layers deposited on CdS substrates with post-preparation resistivities between 10 -3 and 10 -2 £ cm showed an increase in resistivity by 1-1.5 orders of magnitude after subsequent annealing in flowing H 2 for more than two hours at 200°C. It is well-known that annealing in hydrogen and other reducing atmospheres causes an increase in the stoichiometry of Cu~S layers whereas exposure to or annealing in air always causes oxidation, with a deterioration in the stoichiometry and a decrease in the resistivity [17]. However, long periods of hydrogen annealing for samples deposited on glass were always accompanied by a decrease in resistivity of up to

22

E. .4perathitls el aL

i::'ap:>rated < cJl>lJcr ~'ulph~d~: lai'
Fig. 4. The effect of annealing the ( u

t

,1,<7,,d,. ~ <'/i,

S layer ~f figure 2bl in flowing hydrogen for 3 h at 300 o C.

three orders of magnitude. Similar changes in sample resistance have been reported for samples annealed in argon for 24 h at 250 ° [13] and in nitrogen at 200°C for 20 min [11]. SEM examination of a hydrogen annealed sample, prepared in this study, revealed changes in the structure of the layer and evidence for the formation of a thin conductive copper layer. A typical SEM micrograph (fig. 4) shows the surface topography of sample bl of fig. 2 after annealing in hydrogen at 300°C for 3 h. Even though loss of sulphur in the form of HzS always takes place during hydrogen annealing, the observed changes in the surface structure must also be associated

E. Aperathitis et al. / Evaporatedcopper sulphide layersfor CuxS/CdS solar cells

23

with the mobility of copper which would be expected to be high on smooth substrates, such as glass, and to increase with temperature. Such changes have not been observed for layers deposited on CdS substrates which were annealed in hydrogen for at least 8 h at 200 ° C. These annealing conditions are similar to the post-fabrication heat treatments needed for the C u x / C d S solar cells to obtain their best performance. 3.3. X-ray diffraction data

Table 2 shows the X-ray diffraction lines of two layers deposited on hot glass substrates. Their resistivity values just after preparation were 2.1 x 100 ~2 cm for the 1040 A thick sample and 1.6 x 102 ~2 cm for the 1800 A thick sample. Samples deposited on CdS substrates in the same deposition runs showed resistivities of 7.0 × 10 -2 f] cm and 9.8 x 10 -2 ~ cm, respectively. CuxS layers with thickness less than 1000 ,~ yielded no diffraction peaks whereas the diffraction peaks for layers deposited on CdS or ITO-coated glass substrates were overlapped by the peaks arising from their substrates. The diffraction lines that could immediately be identified beyond any doubt are indicated in table 2. The 1040 A layer is not sufficiently thick to provide enough lines from which a firm diagnosis can be made. However, as expected, free copper clearly exists in the freshly prepared thick sample, for which the "good diagnostic" line for low chalcocite (20 = 40.95 ° ) is also present. A similar association between the diffraction peaks for Cu and Cu2S can be seen in the data presented by Arjona [12]. The loss of sulphur during the annealing cycles of the samples is apparent since elemental copper appeared after the first annealing cycle for the thin sample and the corresponding peaks was significantly enhanced after the second annealing cycle for both samples. It is clear that thick CuxS layers on smooth glass substrates have a superior and more stable structure than thin samples when they are subjected to such treatments. The similarities of the diffraction peaks between the two samples just after preparation and after their post-fabrication treatments and the presence of elemental copper suggests that both samples consist of the same CuxS phase. Although some of the unidentified peaks in table 2 could be attributed to the presence of djurleite and anilite as well as chalcocite, it is expected that they are all associated with chalcocite for the following reasons: (i) the films are copper rich; (ii) the best diagnostic peak for chalcocite exists; (iii) the three characteristic peaks of djurleite, i.e. at 26.3 °, 37.6 ° and 46.3 °, do not exist; (iv) the unidentified lines are maintained rather than diminished by hydrogen annealing at 200 ° C and, (v) resistivities of more than 100 f~ cm are characteristic of chalcocite. 3.4. Phase transitions with temperature

As an alternative means of identifying the dominant phases present in the evaporated CuxS layers the film resistance was monitored as a function of temperature in order to determine the temperature at which the various phase transitions occur. The result for a CuxS layer deposited on a cold substrate is seen in curve a of

24

E. Aperathins el al. ., Et~aporated copper sulphide layers j~r ("u, S / ('dS solar, ells

"r-

o~ o

d8

b~ "G

o~3

8 tt3

~

q'~

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"N 0

d o tt~

r~

~

¢q

~4D

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0

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~o

#o

=X ".~

P-

t'~

A A

~5~5 o

o

..~o ¢q ¢)

.d.d X.=

E. Aperathitis et aL / Evaporated copper sulphide layers for CuxS/CdS solar cells 105

'I

I

I

'

lW

11

I

25

'

°o o

10 ~

•

o °~

•

•

o o

C

l

103

rw

•

•

qto=O=oe

(b) 0 O0 000 QOQO000000

W%-O O~ • ~ ' OOOo o°Q~Q ° ° ° .

10 2 • -

..

lal°°Oooooooo _ " 101

i I , I 0

20

40

, I , I , I , 60

TEMPERATURE

80

100 120 = °C

Fig. 5. Temperature dependence of resistance of freshly prepared CuxS layers deposited on CdS substrates held at (a) room temperature, (b) 220 o C with slow rate and (c) 220 o C at a high rate.

fig. 5, whilst curves b and c show the corresponding results for layers prepared on hot CdS substrates with a very low and a high deposition rate, respectively. The sample with the higher resistance, curve c, being Cu-rich, exhibited a fast phase transition at around 9 0 ° C during heating and a hysteresis of 1 5 - 2 0 ° C during cooling from this temperature region. This feature is associated with the transformation of monoclinic chalcocite (7-Cu2S) to hexagonal chalcocite (fl-Cu 2S) [9]. The transformation of fl-Cu2S to the cubic a-Cu2S phase is expected to occur at higher temperatures [18]. The sluggish transition at around the same temperature for a sample deposited with a lower deposition rate on a hot substrate (figures 5b) gives additional proof of the existence of chalcocite in these layers. These are the samples that have yielded high efficiency all-vacuum evaporated CUxS/CdS photovoltaic cells, as will be seen later. Layers deposited in the same deposition run on glass substrates showed a decrease in resistance at the same transition temperature by 2.5 orders of magnitude but the resistance did not return to the initial room temperature value after the cooling cycle. This effect was seen to be associated with the migration of copper under the action of the applied electric field and the accumulation at the negative electrode. Such effects have not been observed for CuxS layers deposited on hot

26

t:'. Aperathitis et al

"

A)'aporated copper s'u[phide hoer~' Ira" ( ' u , N / ( dN ~oh,'r cc//~

CdS substrates, suggesting again that surface migration o f copper is made easier (m smooth surfaces like glass. For Cu.,S layers formed on cold substrates no clear transition could be observed. but as seen in fig. 5a there was an increase in resistance following the temperature cycle due to an irreversible annealing effect. After further annealing of this sample in vacuum at 200 ° C for 2 h an increase in the resistance was observed by almost two orders of magnitude. During a subsequent heating cycle a fast transition appeared at 56-60 ° C with a reduction in resistance by a factor of 5, and with small fluctuations in resistance within 20 ° C of the transition temperature. Such changes are attributed to multiple phases present in the structure, with the major phase change being associated with the presence of digenite [9]. Non-reversibility of the resistance at room temperature was observed after the cooling cycle, and accumulation of copper was seen again on the negative electrode.

3.5. Optical properties Fig. 6 shows the spectral dependence of the absorption coefficient for a typical layer deposited on a glass substrate at 220 ° C. The high absorption coefficient for samples deposited on hot substrates is a consequence of the good structure and

18 16

8

14

7 --

0-

"o o -

6

12

o O •

10 , ~

O o

o

o O ~

?

,

X

o

°

•

o

6

o

-

(b)

o

(a)

-

z,

o

2

%

•

o o

1

o

2

•

o

I

0./+

0.8

I

I

1.2

I .I,"~

1.6

I

2.0 hv

J

I

'

2./+

I

2,8

0

,

3.2

= eV

Fig. 6. Spectral dependence of the optical absorption coefficient of a 400 A thick evaporated Cu.~S layer deposited on a hot substrate.

E. Aperathitis et al. / Evaporated copper sulphide layers for CuxS/CdS solar cells

27

stoichiometry of these layers and the expected beneficial effects of the use of such layers as the absorber in C u ~ S / C d S solar cells will be seen in the next section. The direct and indirect energy band gaps for the above sample, as deduced from the square power and square root of the absorption coefficient given by curves a and b are 2.25 and 1.25 eV, respectively. The presence of copper deficient phases in the CuxS layer deposited on a cold substrate results in a larger indirect band gap (1.45 eV). Comparable values for the direct and indirect band-gap energies have been deduced from optical data obtained by other researchers [19] but, in view of the sensitivity of these properties to the exact structure and composition of the layers, it is not surprising to find a wide variation in the reported results concerning both the nature and the values of the bandgaps in evaporated CuxS [12,19,20]. 3. 6. A l l - v a c u u m evaporated solar cells

Using the slow evaporation conditions described in section 3.1, Cu~S films with well controlled and reproducible properties were achieved. The slow deposition onto heated CdS substrates promoted diffusion of copper into the CdS substrate generating a near-surface semi-insulating region in the CdS, as required for good photovoltaic behaviour [3]. Thus the Cu x S / C d S devices just after preparation were found to generate a short-circuit current Isc of between 7 and 10 m A / c m 2 and open-circuit voltage Voc between 0.52 and 0.58 V, depending on whether the CdS substrate used was etched or unetched. Such a variation in Vo~ is in accordance with theoretical predictions for the dependence of Vo~ on the junction area [21,22]. Additional post-fabrication annealing in hydrogen at 200 ° C improved further the stoichiometry of the CuxS layer and resulted in an increase in Is~ to around 20 m A / c m 2 and a corresponding efficiency in excess of 7%. A full account of the properties of these cells and their long-term stability will be presented in a future paper.

4. Summary and conclusions It has been shown that a single source vacuum evaporation process is capable of producing CuxS thin films with properties which make them suitable for use as the absorber layer in C u x S / C d S solar cells. Performing the evaporation with a rate of less than 150 A / m i n prevents the formation of copper-rich layers which can be formed due to the CuxS source becoming sulphur-deficient during the course of the evaporation. Studies of X-ray diffraction and phase transitions with temperature, in conjunction with electrical and optical measurements, have shown that the highly stoichiometric layers have a resistivity of 10 3-10 2 [2 cm, an indirect band gap of 1.25 eV and a direct band gap of 2.25 eV. For layers thicker than 1000 A these parameters are found to increase with the copper content of the layers. Layers deposited onto heated CdS substrates leave the CdS surface intact so that the resulting cells exhibit an increase in the Voc of up to 65 mV when compared with

28

E. Aperathitis et aL / E1~aporated copper sulphide layer; .[Dr Uu, S / UdS soMr ce/Lv

t h e Voc o f d e v i c e s f o r w h i c h t h e C u x S l a y e r s h a v e b e e n f o r m e d u s i n g t h e c o n v e n tional chemiplating or "Clevite" technique.

References [1] R.W. Potter, Econ. Geol. 72 (1977) 1524. [2] B. Baron, A.W. Catalano and E.A. Fagen, in: Proc. 13th IEEE Photovoltaic Specialists Conf., Washington, DC, 1978, p. 406. [3] A. Rothwarf, in: Proc. 2nd E.C. Photovoltaic Solar Energy Conf., Berlin, 1978, p. 370. [4] E. Castel and J. Vedel, Analysis 3 (1975) 487. [5] L.J. Van der Pauw, Philips Tech. Rev. 20 (1958/59) 220. [6] L.I. Maissel and R. Glang, Handbook of Thin Film Technology (McGraw-Hill, New York, 1970). [7] T. Vanderwel, R. Clarke, F. Scholz, D. Burk, N. Dalacu and J. Shewchun, in: Proc. 14th IEEE Photovoltaic Specialists Conf., San Diego, CA, 1980, p. 694. [8] L. Stolt and D. Sigurd, in: Proc. 5th E.C. Photovoltaic Solar Energy Conf., Athens, Greece, 1983, p. 866. [9] M. Leon, N. Terao and F. Rueda, J. Mater. Sci. 19 (1984) 113. [10] H. Nimura, A. Atoda and T. Natau, Jpn. J. Appl. Phys. 16 (1977) 403. [11] B. Rezig, S. Duchemin and F. Guastavino, Sol. Energy Mater. 2 (1979) 53. [12] F. Arjona, E. Elizalde, E. Garcia-Camarero. A. Feu, B, Lacal, M. Leon, J. Llabres and F. Rueda, Sol. Energy Mater. 1 (1979) 379. [13] P.N. Uppal and L.C. Burton, J. Vac. Sci. Technol. A 1 (1983) 479. [14] E. Vanhoecke, M. Burgelman and L. Anal, in: Proc. 17th IEEE Photovoltaic Specialists Conf., Orlando, FL, 1984, p. 890; Thin Solid Films 144 (1986) 223. [15] J. Shewchun, J.J. Loferski, A. Wold, R. Arnott, E.A. DeMeo, R. Beaulieu, C.C. Wu and H.L. Hwang, in: Proc. l l t h IEEE Photovoltaic Specialists Conf., Scottsdale, 42. 1975, p. 482. [16] A.W. Czanderna, E.T. Prince and H.F. Heibig, Proc. SPIE 240 (1980) 74. [17] A. Rothwarf and K.W. Boer, Progr. Solid State Chem. 10 (1975) 71. [18] K. Okamoto and S. Kawai, Jpn. J. Appl. Phys. 12 (1973) 1130. [19] M. Savelli and M. Bougnot, in: Solar Energy Conversion, Vol. 31 of Topics in Applied Physics, Ed. B.O. Seraphin (Springer, New York, 1979) p. 213. [20] S. Couve, L. Gouskov, L. Szepessy, J. Vedel and E. Castel, Thin Solid Films 15 (1973) 223. [21] J.A. Bragagnolo, in: Proc. 2nd E.C. Photovoltaic Solar Energy Conf., Berlin, 1979, p. 882, [22] U.K. Muckerjee, F. Pfisterer, G.H. Hewig, H.-W. Schock and W.H. Bloss, J. Appl. Phys. 48 (1977) 1538.