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Photovoltaic properties of reactively sputtered a-SiHx films
Journal of Non-Crystalline Solids 35 & 36 (1980) 719-724 ©North-Holland Publishing Company
PHOTOVOLTAIC PROPERTIES OF REACTIVELY SPUTTERED a-SiH x FILMS T.D. Moustakas, C.R. Wronski and D.L. Morel Corporate Research Laboratories Exxon Research and Engineering Company Linden, New Jersey U.S.A.
The e f f e c t of hydrogen incorporation on the photovoltaic properties of sputtered a-SiH films has been studied. By varying the hydrogen p a r t i a l pressure in the discharge at a substrate temperature of 275°C, films were f a b r i c a t e d having between I I and 27 at.% of bonded hydrogen. The corresponding o p t i c a l gaps were between 1.6 and 1.9 eV. The changes in open c i r c u i t voltages of Pd Schottky b a r r i e r devices made from these films indicated that the corresponding changes in b a r r i e r height followed that of the o p t i c a l gaps. The AM1 short c i r c u i t currents were found to depend non-monotonically on the p a r t i a l pressure of hydrogen, the highest current found in t h i s study is cons i s t e n t with the c o l l e c t i o n of photogenerated c a r r i e r s w i t h i n ~0•2 ~m of the metal/a-SiH x i n t e r f a c e . The observed v a r i a t i o n in the short c i r c u i t currents is i n t e r p r e t e d in terms of the modifications in both the defect and i n t r i n s i c e l e c t r o n i c s t r u c t u r e r e s u l t i n g from the hydrogen incorporation• I.
INTRODUCTION
Hydrogenated a-Si (a-SiH x) produced by glow discharge decomposition of silane has been used in the f a b r i c a t i o n of thin f i l m s o l a r cell structures with e f f i c i e n c i e s up to 5•5% under AM1 sunlight ( I ) . The technique of producing the material by sputtering from a c r y s t a l l i n e Si t a r g e t in an Ar+H plasma, f i r s t suggested by Paul et al (2), is now the subject of an intense i n v e s t i g a t i o n in many l a b o r a t o r i e s . Incorporation of hydrogen into the a-Si network results in s i g n i f i c a n t modifications of the density of states in the gap of the semiconductor as well as in the actual bands• Consequently the properties of a-SiH x alloys depend strongly on the amount and possibly the bonding configuration of incorporated hydrogen (3). Control of these parameters is essential in order to produce material with s u i t a b l e optical and e l e c t r o n i c properties f o r photovoltaic a p p l i c a t i o n s . In t h i s paper we report on a study of the photovoltaic properties of r e a c t i v e l y sputtered a-SiH, f i l m s . We f i n d that the s o l a r cell performance depends strongly on deposltlon condltlons and in p a r t i c u l a r on the amount of hydrogen incorporated into the network. This parameter was c o n t r o l l e d by varying the H2 to Ar r a t i o in the discharge at a given temperature and i t s effects on photovoltaic properties was evaluated using metal/a-SiH x Schottky b a r r i e r structures (4). Since the defect s t r u c t u r e due to i m p u r i t i e s or deposition temperature should conceptually be the same in a l l f i l m s , the study probes the effects of hydrogen in e l i m i n a t i n g or creating defects and in modifying the i n t r i n s i c e l e c t r o n i c s t r u c t u r e . The results on the c h a r a c t e r i s t i c s of the short c i r c u i t current, Jsc, are compared with those t h e o r e t i c a l l y predicted from o p t i c a l absorption data. •
II.
,
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.
•
DEVICE FABRICATION AND EXPERIMENTAL METHODS
All the films in t h i s study were prepared at 275°C by r f sputtering from a polyc r y s t a l l i n e t a r g e t at an argon pressure of 15 mTorr, a v a r i a b l e hydrogen p a r t i a l 719
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T.Do Moustakas et al. / Reactively Sputtered a-SiR x Films
pressure and a constant power density of 1.6 watts/cm 2. The i n t r i n s i c f i l m s , approximately 1 ~m t h i c k , were deposited onto substrates having metal/n + doped a-SiH . This insured ohmic contacts to the Schottky b a r r i e r s o l a r c e l l structures whichXwere subsequently f a b r i c a t e d by evaporating a semitransparent Pd f i l m on the free surface. The p h o t o v o l t a i c properties of the c e l l structures were evaluated from l i g h t I-V curves, diode behavior in the dark and under i l l u m i n a t i o n and by c h a r a c t e r i z i n g t h e i r AM1 short c i r c u i t currents. The AM1Jsc'S were evaluated using a combination of s u n l i g h t , xenon s o l a r s i m u l a t o r and spectral response measurements. The spectral response of the c o l l e c t i o n e f f i c i e n c i e s , c a r r i e d out at d i f f e r e n t l e v e l s of i l l u m i n a t i o n and external biases, was used to obtain an independent measure of the J and to evaluate the e f f e c t i v e o p t i c a l transmission of the d i f f e r e n t metal e l ~ t r o d e s . This o p t i c a l transmission could be obtained from c o l l e c t i o n e f f i c i e n c i e s showing c l e a r - c u t s a t u r a t i o n under reverse bias, which could be associated with gains of u n i t y . In the case where such s a t u r a t i o n could not be observed, advantage was taken of the corresponding r e s u l t s obtained on discharge produced a-SiH x Schottky b a r r i e r structures f a b r i c a t e d during the same metal evaporation. The o p t i c a l absorption edge and i n f r a r e d v i b r a t i o n a l spectra were studied on films deposited at the same time on SiO 2 glass substrates and high p u r i t y s i l i c o n s i n g l e c r y s t a l wafers. The o p t i c a l d e n s i t i e s of the films on SiO 2 substrates were measured in a Carry 17 double beam spectrometer from 2.0 um to a wavelength at which the measured o p t i c a l density was about 3. From the spectral region below the o p t i c a l absorption edge the i n t e r f e r e n c e fringes were used to determine the r e f r a c t i v e index and i t s energy dispersion as well as the thickness of the sample as explained by DeNeufville et al (5). The o p t i c a l absorption constant was computed by allowing f o r m u l t i p l e incoherent r e f l e c t i o n s with c a l c u l a t e d reflectances between i n t e r f a c e s of the d i f f e r e n t media (6). I n f r a r e d v i b r a t i o n ~ l spectra were measured using a Digilab FT Spectrometer ( r e s o l u t i o n 4 cm-l). The s t r e t c h i n g v i b r a t i o n at 2000-2100 cm-I is dominated by the 2000 cm- I mode at low p a r t i a l pressure of hydrogen and there is a reversal at high p a r t i a l pressure of hydrogen. The concentration of hydrogen in the films was measured by the nuclear reaction 15N+H + 1ZC+ 4He+Y, and found to vary between I I and 27 at. % (5). III.
EXPERIMENTAL RESULTS
Solar c e l l structures f a b r i c a t e d by r e a c t i v e s p u t t e r i n g have shown values of Jsc and VO, comparable to those of e f f i c i e n t discharge produced a-SiH x c e l l s . Open circu1~ voltages have been obtained with MIS structures in excess of 0.8 v o l t s and i n t e r n a l short c i r c u i t currents of about I I mA/cm2 have been achieved. An example o f such a current is presented in Fig. l,which shows the spectral dependence of the c o l l e c t i o n e f f i c i e n c y , under white l i g h t i l l u m i n a t i o n comparable to AM1. (The transmission of the metal electrode is approximately 50%). However, the J- 's and the c e l l e f f i c i e n c i e s depend s t r o n g l y on the deposition conditions ,~c and in p a r t i c u l a r the amount of hydrogen bonded i n t o the a-Si network. The effects of t h i s on the o p t i c a l and p h o t o v o l t a i c properties f o r a p a r t i c u l a r set of preparation conditions is presented below. Fig. 2 shows the v a r i a t i o n of the o p t i c a l gaps of these f i l m s . The o p t i c a l gap has been a r b i t r a r i l y defined in two d i f f e r e n t ways. The s o l i d dots are obtained from the s t r a i g h t l i n e i n t e r c e p t of (~hv) i / ~ vs h~ curves with the h~ axis ( 7 ) , and the open c i r c l e s are the photon energies at which the o p t i c a l absorption has a value o f 104 cm- I , Thus, the o p t i c a l gaps of the i n v e s t i g a t e d f i l m s , according to the f i r s t d e f i n i t i o n , vary between 1.6 and 1.9 eV. The f i l m produced at p a r t i a l pressure of 7xlO -4 Torr was found to contain 22 at. % hydrogen and i t s spectral dependence of the absorption constant, compiled from films of d i f f e r e n t thicknesses, is shown in Fig. 3.
ToD. Moustakas et alo / Reactively Sputtered a-SiH
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Figure 1 The Spectral Dependence of the Collection Efficiency of a Sputtered a-SiHy/Pd Schottky Barrier Solar Cell Structure Exhibiting Internal AM1 Jsc ~ II mA/cm2, (The Optical Transmission of the Pd was Measured to be 50%) 10 6
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Figure 2 The Optical Gaps of the Investigated Films, Plotted Against the Partial Pressure of Hydrogen, (See Text for Details)
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Figure 3 The Optical Absorption vs hv for films Prepared at 7xlO-~ Torr of Hydrogen (The Hydrogen Content of this Film is 22 at,%)
722
ToDo Moustakas et al. / Reactively Sputtered a-SiH
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Fig. 4 shows the open c i r c u i t voltages obtained with the Pd Schottky b a r r i e r s formed on the films whose o p t i c a l gaps are shown in Fig. 2. The changes in the values of Vo. obtained with films deposited at the d i f f e r e n t p a r t i a l pressure of hydrogen are s l m l l a r to the corresponding changes o f the o p t i c a l gap. i
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Fig. 5 shows the internal short c i r c u i t currents of the same Pd Schottky barriers. These currents, whose accuracy is estimated to be within I0%, were calculated by taking into account the effective transmission of the Pd thin film as described earlier. 14
Figure 5 The Internal AMI Short Circuit Currents vs. Partial Pressure of Hydrogen For the Same Pd Schottky Barriers As in Fig. 4. (These Currents Were Calculated by Taking Into Account the Effective Transmission of the Pd Thin Films as Described in the Text).
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IV.
DISCUSSION
In order to account f o r the observed v a r i a t i o n s in the p h o t o v o l t a i c properties of the d i f f e r e n t films one needs t o consider both the change in the o p t i c a l as well as the e l e c t r o n i c properties of these f i l m s . For example, the open c i r c u i t voltage increases with the p a r t i a l pressure of hydrogen by about the same amount as the o p t i c a l gap of the m a t e r i a l . The changes in the Vn~ r e f l e c t the differences in the b a r r i e r height @ since i n t i m a t e metal/a-SiH x cSfltacts were i n d i c a t e d by the ideal diode c h a r a c t e r i s t i c s obtained under i l l u m i n a t i o n (4). Although the changes in @ depend on surface states they are also determined by the i n t r i n s i c e l e c t r o n i c s t r u c t u r e of the a-SiH . Therefore in considering the o p t i m i z a t i o n of s o l a r c e l l performance i t is necessary t o take i n t o account t h a t a-SiH x a l l o y s with smaller band gaps can r e s u l t in s i g n i f i c a n t l y lower values of Voc. Although in p r i n c i p l e the narrower the band gap of a material the higher is the short c i r c u i t current t h a t i t can generate, t h i s is not found to be the case in
ToD~ Moustakas et al. / Reactively Sputtered a-SiH
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723
our i n v e s t i g a t i o n . The results in Fig. 5 c l e a r l y indicate that the films produced at PH < 7xi0-4 Torr (CH ~ 22 at %) have short c i r c u i t currents which decrease as the band gap is lowered. This means that even though the lowering of the band gap results in enhanced absorption near the surface of the cell s t r u c t u r e , there is a reduction in Jsc due to a poorer c o l l e c t i o n of the photogenerated c a r r i e r s . This c o l l e c t i o n is determined by a v a r i e t y of e l e c t r o n i c properties which depend on the defect structure of the m a t e r i a l . The large decreas~ of Jsc f o r the films produced at p a r t i a l pressures of hydrogen higher than 7xlO -~ Torr cannot be due to only the changes in the o p t i c a l absorption indicated in Figs.2 and 3. Consequently, changes in e l e c t r o n i c properties determining the c a r r i e r c o l l e c t i o n also need to be included in accounting f o r these lower values of Jsc. In p r i n c i p l e the defect s t r u c t u r e c h a r a c t e r i s t i c of the deposition temperature and impurities should be the same in a l l investigated f i l m s . Therefore, the only defects changing are those which can be eliminated or created by the hydrogen incorporation. For example, the i n i t i a l increase of Jsc with the p a r t i a l pressure of hydrogen (CH ~ I I to 22 at.%) could be a t t r i b u t e d to the e l i m i n a t i o n of dangling bonds or reconstructed dangling bonds. The l a t t e r are weak bonds, formed by p a i r ing of two broken bonds over a distance considerably l a r g e r than the s m a l l e s ~ interatomic distance. The density of these weak bonds is estimatsd to be IOLL cm-3 while that of the single dangling bonds in pure a-Si is about 102 cm-3 (8,9). The fact2~hat ~he best Jsc was obtained f o r the films with hydrogen concentration of I0 cm" implies that the c o l l e c t i o n of the photogenerated c a r r i e r s is influenced by the defect states introduced by the reconstructed weak bonds. Elimination of these states by the hydrogen is also the cause of the i n i t i a l abrupt increase of the o p t i c a l gap shown in Fig. 2. The defect s t r u c t u r e of films produced at PH > 7xlO- Torr is f a r more d i f f i c u l t to unravel. However, there is evidence from work of Fritzsche e t . a l . ( l O ) that as the hydrogen concentration is increased in certain ways,there is a rapid increase in the spin density. Also, Knights e t . a l . ( I I ) have found that changes in the d i s t r i b u t i o n of hydrogen between d i f f e r e n t environments are accompanied by large changes in defect state density. Although we do not have spin density data on our films the findings in references I0 and I I are l i k e l y applicable to our experimental r e s u l t s . The f u l l evaluation of changes of the o p t i c a l gap as well as defect structure on the photovoltaic c h a r a c t e r i s t i c s requires d e t a i l e d c h a r a c t e r i z a t i o n of both j u n c t i o n and bulk properties beyond the scope of t h i s paper. However i t has been possible, in a number of these s o l a r cell s t r u c t u r e s , to characterize the J~c by a c o l l e c t i o n width, Xc. This can be carried out on cell structures which e x h i b i t e d internal c o l l e c t i o n e f f i c i e n c i e s close to 100% at the short wavelengths. In such cases, Xc, can be c l e a r l y i d e n t i f i e d with the region in which the e l e c t r i c f i e l d and the free c a r r i e r properties allow such e f f i c i e n t c a r r i e r c o l l e c t i o n to occur. As a r e s u l t the i n t e r n a l c o l l e c t i o n e f f i c i e n c i e s , n ( 4 ) , can, to a good approximation, be expressed by: q (4) = l-exp [-~ (~)Xc] (I) where m (k) is the o p t i c a l absorption constant. Using the o p t i c a l absorption data of Fig. 3 we generated with Eq. 1 a series of c o l l e c t i o n e f f i c i e n c i e s corresponding to d i f f e r e n t values of Xc. The changes in the spectral d i s t r i b u t i o n of the c o l l e c t ion e f f i c i e n c i e s , shown in Fig. 6, i n d i c a t e the e f f e c t that d i f f e r e n t defect structures can have on the Js ~ f o r a f i l m having the same o p t i c a l properties. The AM1Jsc can be r e a d i l y obtalned from these curves by t h e i r normalization with respect to AM1 s o l a r f l u x . For the s t r u c t u r e prepared at 7xlO -4 Torr both the spectral response and the AM1 Jsc were in good agreement with a value of Xc=O.2 ~m. Although we have not f u l l y characterized a l l the cell structures prepared at d i f ferent values of PH clear indications were found that in a-SiH x i t is d i f f i c u l t to t a y l o r the optical gap without also a f f e c t i n g the defect structure. V CONCLUSIONS We have demonstrated that hydrogen incorporation influences the o p t i c a l and photov o l t a i c properties of r e a c t i v e l y sputtered a-SiH x f i l m s . A consistent i n t e r p r e t a t i o n of the data can be made in terms of defects introduced by weak reconstructed
724
T.Do Moustakas et al. / Reactively Sputtered a-SiH
x
Films
dangling bonds. Elimination of these bonds through hydrogenation introduces the additional complication of increasing the optical gap of the material. In addition, as the amount of hydrogen increases above about 20 at. % new defect structures, related most probably to the bonding of hydrogen, begin to emerge. Thus, the interrelated roles of hydrogen in modifying the defect and i n t r i n s i c electronic structure of the material need to be considered carefully in optimizing the photovoltaic properties of the material. The optimization studies discussRd in this paper have yielded internal AM1 short c i r c u i t currents of about I0 mA/cm~. These currents have also been found to be consistent with e f f i c i e n t collection of the carriers photogenerated within ~ 0.2 ~m from the metal/a-SiHx interface. COLLECTION WIDTHSXclN ~ Figure 6 ' ' ' ' Theoretical Collection Efficiency Com~ i o o ~ puted From Eq. I, Using The Optical > ~ ~ ~ ~ Absorption Data of Fig. 3. 8o 60 40
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ACKNOWLEDGMENTS We gratefully acknowledge B. Abeles, G. Cody, J. DeNeufville and T. Tiedje for helpful discussions. The assistance of R. Friedman and B. Myers in sample preparation and characterization, and of K. Rogers for the infrared measurements is appreciated. REFERENCES I.
8.
D.E. Carlson, C.Ro Wronski, A.R. Triano and R.E. Daniel, Proceedings of the 12th Photevoltaic Specialists Conference, Baton Rouge, LA., (IEEE, N.Y., 1976), p. 893. W. Paul, A.J. Lewis, G.A.N. Connell and T.Do Moustakas, Solid State Commun., 20, 969 (1976). T__D. Moustakas, J. of Electronic Materials, Vol. 8, 391 (1979). C.R. Wronski, D.E. Carlson and R.E. Daniel, Appl.-Phys. Lett. Vol 29, 602 (1976). J.P. DeNeufville, T.D. Moustakas, A.F. Ruppert and W.A. Lanford, Proceedings of this Conference, p. G.A.N. Connell and A. Lewis, Phys. Stat. Sol.(b) 6(], 291 (1973). N.F. Mott and E.A. Davis, "Electronic Processes in Non-Crystalline Materials" Clarendon Press, Oxford (1971). G.A.N. Connell and J.R. Paulik,Phys. Rev. B L3, 787 (1976).
9.
M.H. Brodsky and D. Kaplan, J. of Non-Cryst. Solids 32, 431 (1979).
2. 3. 4. 5. 6. 7.
lO. H. Fritzsche, C.C. Tsai and P. Persans, Solid State Technology (Jan. 1978) p. 55. I I . J.C. Knights, G. Lucovsky and R.J. Nemanich, J. of Non-Cryst. Solids 32, 393 (1979).
PHOTOVOLTAIC PROPERTIES OF REACTIVELY SPUTTERED a-SiH x FILMS T.D. Moustakas, C.R. Wronski and D.L. Morel Corporate Research Laboratories Exxon Research and Engineering Company Linden, New Jersey U.S.A.
The e f f e c t of hydrogen incorporation on the photovoltaic properties of sputtered a-SiH films has been studied. By varying the hydrogen p a r t i a l pressure in the discharge at a substrate temperature of 275°C, films were f a b r i c a t e d having between I I and 27 at.% of bonded hydrogen. The corresponding o p t i c a l gaps were between 1.6 and 1.9 eV. The changes in open c i r c u i t voltages of Pd Schottky b a r r i e r devices made from these films indicated that the corresponding changes in b a r r i e r height followed that of the o p t i c a l gaps. The AM1 short c i r c u i t currents were found to depend non-monotonically on the p a r t i a l pressure of hydrogen, the highest current found in t h i s study is cons i s t e n t with the c o l l e c t i o n of photogenerated c a r r i e r s w i t h i n ~0•2 ~m of the metal/a-SiH x i n t e r f a c e . The observed v a r i a t i o n in the short c i r c u i t currents is i n t e r p r e t e d in terms of the modifications in both the defect and i n t r i n s i c e l e c t r o n i c s t r u c t u r e r e s u l t i n g from the hydrogen incorporation• I.
INTRODUCTION
Hydrogenated a-Si (a-SiH x) produced by glow discharge decomposition of silane has been used in the f a b r i c a t i o n of thin f i l m s o l a r cell structures with e f f i c i e n c i e s up to 5•5% under AM1 sunlight ( I ) . The technique of producing the material by sputtering from a c r y s t a l l i n e Si t a r g e t in an Ar+H plasma, f i r s t suggested by Paul et al (2), is now the subject of an intense i n v e s t i g a t i o n in many l a b o r a t o r i e s . Incorporation of hydrogen into the a-Si network results in s i g n i f i c a n t modifications of the density of states in the gap of the semiconductor as well as in the actual bands• Consequently the properties of a-SiH x alloys depend strongly on the amount and possibly the bonding configuration of incorporated hydrogen (3). Control of these parameters is essential in order to produce material with s u i t a b l e optical and e l e c t r o n i c properties f o r photovoltaic a p p l i c a t i o n s . In t h i s paper we report on a study of the photovoltaic properties of r e a c t i v e l y sputtered a-SiH, f i l m s . We f i n d that the s o l a r cell performance depends strongly on deposltlon condltlons and in p a r t i c u l a r on the amount of hydrogen incorporated into the network. This parameter was c o n t r o l l e d by varying the H2 to Ar r a t i o in the discharge at a given temperature and i t s effects on photovoltaic properties was evaluated using metal/a-SiH x Schottky b a r r i e r structures (4). Since the defect s t r u c t u r e due to i m p u r i t i e s or deposition temperature should conceptually be the same in a l l f i l m s , the study probes the effects of hydrogen in e l i m i n a t i n g or creating defects and in modifying the i n t r i n s i c e l e c t r o n i c s t r u c t u r e . The results on the c h a r a c t e r i s t i c s of the short c i r c u i t current, Jsc, are compared with those t h e o r e t i c a l l y predicted from o p t i c a l absorption data. •
II.
,
x
.
•
DEVICE FABRICATION AND EXPERIMENTAL METHODS
All the films in t h i s study were prepared at 275°C by r f sputtering from a polyc r y s t a l l i n e t a r g e t at an argon pressure of 15 mTorr, a v a r i a b l e hydrogen p a r t i a l 719
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T.Do Moustakas et al. / Reactively Sputtered a-SiR x Films
pressure and a constant power density of 1.6 watts/cm 2. The i n t r i n s i c f i l m s , approximately 1 ~m t h i c k , were deposited onto substrates having metal/n + doped a-SiH . This insured ohmic contacts to the Schottky b a r r i e r s o l a r c e l l structures whichXwere subsequently f a b r i c a t e d by evaporating a semitransparent Pd f i l m on the free surface. The p h o t o v o l t a i c properties of the c e l l structures were evaluated from l i g h t I-V curves, diode behavior in the dark and under i l l u m i n a t i o n and by c h a r a c t e r i z i n g t h e i r AM1 short c i r c u i t currents. The AM1Jsc'S were evaluated using a combination of s u n l i g h t , xenon s o l a r s i m u l a t o r and spectral response measurements. The spectral response of the c o l l e c t i o n e f f i c i e n c i e s , c a r r i e d out at d i f f e r e n t l e v e l s of i l l u m i n a t i o n and external biases, was used to obtain an independent measure of the J and to evaluate the e f f e c t i v e o p t i c a l transmission of the d i f f e r e n t metal e l ~ t r o d e s . This o p t i c a l transmission could be obtained from c o l l e c t i o n e f f i c i e n c i e s showing c l e a r - c u t s a t u r a t i o n under reverse bias, which could be associated with gains of u n i t y . In the case where such s a t u r a t i o n could not be observed, advantage was taken of the corresponding r e s u l t s obtained on discharge produced a-SiH x Schottky b a r r i e r structures f a b r i c a t e d during the same metal evaporation. The o p t i c a l absorption edge and i n f r a r e d v i b r a t i o n a l spectra were studied on films deposited at the same time on SiO 2 glass substrates and high p u r i t y s i l i c o n s i n g l e c r y s t a l wafers. The o p t i c a l d e n s i t i e s of the films on SiO 2 substrates were measured in a Carry 17 double beam spectrometer from 2.0 um to a wavelength at which the measured o p t i c a l density was about 3. From the spectral region below the o p t i c a l absorption edge the i n t e r f e r e n c e fringes were used to determine the r e f r a c t i v e index and i t s energy dispersion as well as the thickness of the sample as explained by DeNeufville et al (5). The o p t i c a l absorption constant was computed by allowing f o r m u l t i p l e incoherent r e f l e c t i o n s with c a l c u l a t e d reflectances between i n t e r f a c e s of the d i f f e r e n t media (6). I n f r a r e d v i b r a t i o n ~ l spectra were measured using a Digilab FT Spectrometer ( r e s o l u t i o n 4 cm-l). The s t r e t c h i n g v i b r a t i o n at 2000-2100 cm-I is dominated by the 2000 cm- I mode at low p a r t i a l pressure of hydrogen and there is a reversal at high p a r t i a l pressure of hydrogen. The concentration of hydrogen in the films was measured by the nuclear reaction 15N+H + 1ZC+ 4He+Y, and found to vary between I I and 27 at. % (5). III.
EXPERIMENTAL RESULTS
Solar c e l l structures f a b r i c a t e d by r e a c t i v e s p u t t e r i n g have shown values of Jsc and VO, comparable to those of e f f i c i e n t discharge produced a-SiH x c e l l s . Open circu1~ voltages have been obtained with MIS structures in excess of 0.8 v o l t s and i n t e r n a l short c i r c u i t currents of about I I mA/cm2 have been achieved. An example o f such a current is presented in Fig. l,which shows the spectral dependence of the c o l l e c t i o n e f f i c i e n c y , under white l i g h t i l l u m i n a t i o n comparable to AM1. (The transmission of the metal electrode is approximately 50%). However, the J- 's and the c e l l e f f i c i e n c i e s depend s t r o n g l y on the deposition conditions ,~c and in p a r t i c u l a r the amount of hydrogen bonded i n t o the a-Si network. The effects of t h i s on the o p t i c a l and p h o t o v o l t a i c properties f o r a p a r t i c u l a r set of preparation conditions is presented below. Fig. 2 shows the v a r i a t i o n of the o p t i c a l gaps of these f i l m s . The o p t i c a l gap has been a r b i t r a r i l y defined in two d i f f e r e n t ways. The s o l i d dots are obtained from the s t r a i g h t l i n e i n t e r c e p t of (~hv) i / ~ vs h~ curves with the h~ axis ( 7 ) , and the open c i r c l e s are the photon energies at which the o p t i c a l absorption has a value o f 104 cm- I , Thus, the o p t i c a l gaps of the i n v e s t i g a t e d f i l m s , according to the f i r s t d e f i n i t i o n , vary between 1.6 and 1.9 eV. The f i l m produced at p a r t i a l pressure of 7xlO -4 Torr was found to contain 22 at. % hydrogen and i t s spectral dependence of the absorption constant, compiled from films of d i f f e r e n t thicknesses, is shown in Fig. 3.
ToD. Moustakas et alo / Reactively Sputtered a-SiH
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Figure 1 The Spectral Dependence of the Collection Efficiency of a Sputtered a-SiHy/Pd Schottky Barrier Solar Cell Structure Exhibiting Internal AM1 Jsc ~ II mA/cm2, (The Optical Transmission of the Pd was Measured to be 50%) 10 6
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Figure 2 The Optical Gaps of the Investigated Films, Plotted Against the Partial Pressure of Hydrogen, (See Text for Details)
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Figure 3 The Optical Absorption vs hv for films Prepared at 7xlO-~ Torr of Hydrogen (The Hydrogen Content of this Film is 22 at,%)
722
ToDo Moustakas et al. / Reactively Sputtered a-SiH
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Films
Fig. 4 shows the open c i r c u i t voltages obtained with the Pd Schottky b a r r i e r s formed on the films whose o p t i c a l gaps are shown in Fig. 2. The changes in the values of Vo. obtained with films deposited at the d i f f e r e n t p a r t i a l pressure of hydrogen are s l m l l a r to the corresponding changes o f the o p t i c a l gap. i
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Fig. 5 shows the internal short c i r c u i t currents of the same Pd Schottky barriers. These currents, whose accuracy is estimated to be within I0%, were calculated by taking into account the effective transmission of the Pd thin film as described earlier. 14
Figure 5 The Internal AMI Short Circuit Currents vs. Partial Pressure of Hydrogen For the Same Pd Schottky Barriers As in Fig. 4. (These Currents Were Calculated by Taking Into Account the Effective Transmission of the Pd Thin Films as Described in the Text).
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IV.
DISCUSSION
In order to account f o r the observed v a r i a t i o n s in the p h o t o v o l t a i c properties of the d i f f e r e n t films one needs t o consider both the change in the o p t i c a l as well as the e l e c t r o n i c properties of these f i l m s . For example, the open c i r c u i t voltage increases with the p a r t i a l pressure of hydrogen by about the same amount as the o p t i c a l gap of the m a t e r i a l . The changes in the Vn~ r e f l e c t the differences in the b a r r i e r height @ since i n t i m a t e metal/a-SiH x cSfltacts were i n d i c a t e d by the ideal diode c h a r a c t e r i s t i c s obtained under i l l u m i n a t i o n (4). Although the changes in @ depend on surface states they are also determined by the i n t r i n s i c e l e c t r o n i c s t r u c t u r e of the a-SiH . Therefore in considering the o p t i m i z a t i o n of s o l a r c e l l performance i t is necessary t o take i n t o account t h a t a-SiH x a l l o y s with smaller band gaps can r e s u l t in s i g n i f i c a n t l y lower values of Voc. Although in p r i n c i p l e the narrower the band gap of a material the higher is the short c i r c u i t current t h a t i t can generate, t h i s is not found to be the case in
ToD~ Moustakas et al. / Reactively Sputtered a-SiH
x
Films
723
our i n v e s t i g a t i o n . The results in Fig. 5 c l e a r l y indicate that the films produced at PH < 7xi0-4 Torr (CH ~ 22 at %) have short c i r c u i t currents which decrease as the band gap is lowered. This means that even though the lowering of the band gap results in enhanced absorption near the surface of the cell s t r u c t u r e , there is a reduction in Jsc due to a poorer c o l l e c t i o n of the photogenerated c a r r i e r s . This c o l l e c t i o n is determined by a v a r i e t y of e l e c t r o n i c properties which depend on the defect structure of the m a t e r i a l . The large decreas~ of Jsc f o r the films produced at p a r t i a l pressures of hydrogen higher than 7xlO -~ Torr cannot be due to only the changes in the o p t i c a l absorption indicated in Figs.2 and 3. Consequently, changes in e l e c t r o n i c properties determining the c a r r i e r c o l l e c t i o n also need to be included in accounting f o r these lower values of Jsc. In p r i n c i p l e the defect s t r u c t u r e c h a r a c t e r i s t i c of the deposition temperature and impurities should be the same in a l l investigated f i l m s . Therefore, the only defects changing are those which can be eliminated or created by the hydrogen incorporation. For example, the i n i t i a l increase of Jsc with the p a r t i a l pressure of hydrogen (CH ~ I I to 22 at.%) could be a t t r i b u t e d to the e l i m i n a t i o n of dangling bonds or reconstructed dangling bonds. The l a t t e r are weak bonds, formed by p a i r ing of two broken bonds over a distance considerably l a r g e r than the s m a l l e s ~ interatomic distance. The density of these weak bonds is estimatsd to be IOLL cm-3 while that of the single dangling bonds in pure a-Si is about 102 cm-3 (8,9). The fact2~hat ~he best Jsc was obtained f o r the films with hydrogen concentration of I0 cm" implies that the c o l l e c t i o n of the photogenerated c a r r i e r s is influenced by the defect states introduced by the reconstructed weak bonds. Elimination of these states by the hydrogen is also the cause of the i n i t i a l abrupt increase of the o p t i c a l gap shown in Fig. 2. The defect s t r u c t u r e of films produced at PH > 7xlO- Torr is f a r more d i f f i c u l t to unravel. However, there is evidence from work of Fritzsche e t . a l . ( l O ) that as the hydrogen concentration is increased in certain ways,there is a rapid increase in the spin density. Also, Knights e t . a l . ( I I ) have found that changes in the d i s t r i b u t i o n of hydrogen between d i f f e r e n t environments are accompanied by large changes in defect state density. Although we do not have spin density data on our films the findings in references I0 and I I are l i k e l y applicable to our experimental r e s u l t s . The f u l l evaluation of changes of the o p t i c a l gap as well as defect structure on the photovoltaic c h a r a c t e r i s t i c s requires d e t a i l e d c h a r a c t e r i z a t i o n of both j u n c t i o n and bulk properties beyond the scope of t h i s paper. However i t has been possible, in a number of these s o l a r cell s t r u c t u r e s , to characterize the J~c by a c o l l e c t i o n width, Xc. This can be carried out on cell structures which e x h i b i t e d internal c o l l e c t i o n e f f i c i e n c i e s close to 100% at the short wavelengths. In such cases, Xc, can be c l e a r l y i d e n t i f i e d with the region in which the e l e c t r i c f i e l d and the free c a r r i e r properties allow such e f f i c i e n t c a r r i e r c o l l e c t i o n to occur. As a r e s u l t the i n t e r n a l c o l l e c t i o n e f f i c i e n c i e s , n ( 4 ) , can, to a good approximation, be expressed by: q (4) = l-exp [-~ (~)Xc] (I) where m (k) is the o p t i c a l absorption constant. Using the o p t i c a l absorption data of Fig. 3 we generated with Eq. 1 a series of c o l l e c t i o n e f f i c i e n c i e s corresponding to d i f f e r e n t values of Xc. The changes in the spectral d i s t r i b u t i o n of the c o l l e c t ion e f f i c i e n c i e s , shown in Fig. 6, i n d i c a t e the e f f e c t that d i f f e r e n t defect structures can have on the Js ~ f o r a f i l m having the same o p t i c a l properties. The AM1Jsc can be r e a d i l y obtalned from these curves by t h e i r normalization with respect to AM1 s o l a r f l u x . For the s t r u c t u r e prepared at 7xlO -4 Torr both the spectral response and the AM1 Jsc were in good agreement with a value of Xc=O.2 ~m. Although we have not f u l l y characterized a l l the cell structures prepared at d i f ferent values of PH clear indications were found that in a-SiH x i t is d i f f i c u l t to t a y l o r the optical gap without also a f f e c t i n g the defect structure. V CONCLUSIONS We have demonstrated that hydrogen incorporation influences the o p t i c a l and photov o l t a i c properties of r e a c t i v e l y sputtered a-SiH x f i l m s . A consistent i n t e r p r e t a t i o n of the data can be made in terms of defects introduced by weak reconstructed
724
T.Do Moustakas et al. / Reactively Sputtered a-SiH
x
Films
dangling bonds. Elimination of these bonds through hydrogenation introduces the additional complication of increasing the optical gap of the material. In addition, as the amount of hydrogen increases above about 20 at. % new defect structures, related most probably to the bonding of hydrogen, begin to emerge. Thus, the interrelated roles of hydrogen in modifying the defect and i n t r i n s i c electronic structure of the material need to be considered carefully in optimizing the photovoltaic properties of the material. The optimization studies discussRd in this paper have yielded internal AM1 short c i r c u i t currents of about I0 mA/cm~. These currents have also been found to be consistent with e f f i c i e n t collection of the carriers photogenerated within ~ 0.2 ~m from the metal/a-SiHx interface. COLLECTION WIDTHSXclN ~ Figure 6 ' ' ' ' Theoretical Collection Efficiency Com~ i o o ~ puted From Eq. I, Using The Optical > ~ ~ ~ ~ Absorption Data of Fig. 3. 8o 60 40
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ACKNOWLEDGMENTS We gratefully acknowledge B. Abeles, G. Cody, J. DeNeufville and T. Tiedje for helpful discussions. The assistance of R. Friedman and B. Myers in sample preparation and characterization, and of K. Rogers for the infrared measurements is appreciated. REFERENCES I.
8.
D.E. Carlson, C.Ro Wronski, A.R. Triano and R.E. Daniel, Proceedings of the 12th Photevoltaic Specialists Conference, Baton Rouge, LA., (IEEE, N.Y., 1976), p. 893. W. Paul, A.J. Lewis, G.A.N. Connell and T.Do Moustakas, Solid State Commun., 20, 969 (1976). T__D. Moustakas, J. of Electronic Materials, Vol. 8, 391 (1979). C.R. Wronski, D.E. Carlson and R.E. Daniel, Appl.-Phys. Lett. Vol 29, 602 (1976). J.P. DeNeufville, T.D. Moustakas, A.F. Ruppert and W.A. Lanford, Proceedings of this Conference, p. G.A.N. Connell and A. Lewis, Phys. Stat. Sol.(b) 6(], 291 (1973). N.F. Mott and E.A. Davis, "Electronic Processes in Non-Crystalline Materials" Clarendon Press, Oxford (1971). G.A.N. Connell and J.R. Paulik,Phys. Rev. B L3, 787 (1976).
9.
M.H. Brodsky and D. Kaplan, J. of Non-Cryst. Solids 32, 431 (1979).
2. 3. 4. 5. 6. 7.
lO. H. Fritzsche, C.C. Tsai and P. Persans, Solid State Technology (Jan. 1978) p. 55. I I . J.C. Knights, G. Lucovsky and R.J. Nemanich, J. of Non-Cryst. Solids 32, 393 (1979).