Growth of a-Si:H on transparent conductive oxides for solar cell applications

Solar Ener~ Materials and Solar Cells

ELSEVIER

Solar Energy Materials and Solar Cells 41/42 (1996) 519-527

Growth of a-Si:H on transparent conductive oxides for solar cell applications H.N. Wanka, M.B. Schubert, E. Lotter Unioersiti~t Stuttgart, lnstitut fiJr Physikalische Elektronik, PfaffenwaMring 47, D-70569 Stuttgart, Germany

Abstract Different concepts for optimizing the transparent conductive oxide (TCO)/p-interface in hydrogenated amorphous silicon (a-Si:H) based solar cells have been studied in order to avoid the segregation of metal layers, and hence considerable reductions in short circuit current. We analysed structural and chemical changes which occur at the surface of transparent conductive oxides - - TCOs (SnO2, ZnO, and Indium Tin Oxide - - ITO) in silane, hydrogen and CO 2 plasmas. We also used a-SiO:H instead of a-SiC:H in the p-doped layer. In-situ ellipsometry and SIMS/XPS depth profiling show that room temperature as well as fast deposition easily overcome all detrimental effects. TCO deterioration by ion and radical bombardement at high deposition rates is more than compensated if the surface is protected by a rapidly growing a-Si:H film. Using ZnO as a TCO, or as a 20 nm buffer layer only, completely suppresses metal formation. In-situ ellipsometry in conjunction with atomic force microscopy reveals significant changes in surface morphology, namely filling of the TCO-texture during deposition, which is of crucial importance for light trapping in solar cells.

1. Introduction Although subject of numerous investigations [1-4] the optimum choice of materials and cell design for the crucial T C O / p region in a-Si:H based solar cells is still a matter of debate. Successful concepts include the use of a higher bandgap intrinsic buffer layer for improved electrical field profiling, lowering of the substrate temperature during p-layer deposition, replacement of the standard a-SiC:H by a-SiO:H [5], CO 2 plasma treatment of tin oxide front contacts [6], or the use of ZnO for enhanced chemical stability of the TCO (transparent conductive oxide) [7]. Especially in-situ ellipsometry in conjunction with SIMS and XPS depth profiling revealed that optical losses at the T C O / p interface are mainly due to the formation of metallic phases originating from a chemical reduction of the TCOs upon exposure to 0927-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-0248(95)00141-7

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silane or hydrogen plasmas. Whereas the standard layer sequence glass/tin oxide/p-aSiC:H is well characterized, no systematic study of the above listed options for improvement is available as yet. Benefits for solar cell performance due to the implementation of these options can only be assessed from the data of different laboratories and from rather different overall cell structures. Therefore, we present a study of in-situ ellipsometry and SIMS/XPS analyses for addressing the problem of optical loss by the investigation of chemical and structural changes at the interface. We consider this to be a prerequisite for studying interface states, electrical field profiling, and finally optimizing solar cell efficiency on safe grounds.

2. Experimental We have characterized different types of commonly used substrates: (i) ZnO, sputtered from an Al-doped ZnO target at room temperature (rf-power 1.1 W / c m 2, thickness (10-230) nm, room temperature conductivity 2000 S/cm); (ii) commercially available tin oxide (delivered by Nippon Sheet as well as Asahi Glass, thickness 450 nm), referred to as SnO 2. Amorphous silicon films (a-Si:H) were deposited from a dc glow discharge (substrate temperature 300K or 520K, respectively, Sill 4 flow 6 sccm at 20 Pa); growth rate has been controlled by adjusting Re plasma power (3 ,~,/s at 30 m W / c m 3 - - referred to as f a s t deposition, 0.16 A / s at 2 m W / c m 3 - - slow deposition). Plasma treatment of the TCOs has been performed by using H 2 or CO 2 (at 300K as well as 520K, l0 sccm, 20 Pa and power levels of 27 and 14 m W / c m 3, respectively), a-SiO:H has been deposited from a mixture of Sill 4 (6 sccm) and CO 2 (9 sccm); in order to qualify TCO interaction, the deposition rate has been changed as mentioned above (2.4 A / s at 20 m W / c m 3 and 0.12 A / s at 1.25 mW/cm3). In-situ ellipsometry has been performed by using a phase-modulated instrument (Jobin-Yvon, UVISEL [8]). Kinetic measurements (10 data points per sec) were usually carded out at a photon energy of 4 eV, for optimum surface sensitivity. A proximity shutter system has been employed in front of the substrate for providing stable plasma conditions at the very beginning of the depositions. Ellipsometric data were evaluated by applying multilayer analyses [9] and the Bruggeman effective medium approximation [10]. XPS (MgK,~ X-ray source, Ar + sputtering, Ekin=5 keV) and SIMS (Ar + s p u t t e r i n g , Eki n = 2 keV) depth profiles have been recorded from a multi-purpose surface analysis system (Leybold-Heraeus, LHS 10). For atomic force microscopy (AFM) we used the contact mode of a TopoMetrix instrument with a Si3N 4 probe tip (spring constant 0.3 N/m). 3. Results and discussion 3.1. TCO / p-interface

Photovoltaic applications suffer from remarkable transmission losses if there is metallic tin present at the interface which reduces light incidence into the active layer of

H.N. Wanka et a l . / solar Energy Materials and Solar Cells 4 1 / 4 2 (1996) 519-527

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~(deg.) Fig. 1. Real time ellipsometry trajectories at an incident energy of 4 eV, taken during deposition of a-Si:H on top of SnO~, and during H2-plasma treatment of SnO 2, respectively.

pin solar cells. Fig. 1 comprises real-time trajectories of the ellipsometric angles recorded during a-Si:H deposition on SnO 2 at different growth rates and temperatures, plus one obtained from a H 2 plasma treatment of the TCO. "Slow" deposition data exhibit a sharp kink after 60 s deposition time which is characteristic of the formation of metallic interlayers [2]. Initially this trajectory is governed by a strong increase in extinction coefficient and by a lowering of the real part of the refractive index. This results from a chemical reduction of SnO 2 and the corresponding formation of metallic tin close to the TCO surface. The same effect clearly occurs during H 2 plasma treatment at deposition temperature. The solid line in Fig. 1 represents the calculated time evolution of the effective optical parameters of a-Si:H growing on top of a 10~, thick film containing 50% metallic Sn and 50% SnO 2. The amorphous silicon deposition after 60 s can consistently be modelled by homogeneous growth in conjunction with a decrease in surface roughness (details see below). The "fast" deposition data in Fig. 1 do not give direct hints at a reduction of the SnO 2 surface. The "fast" trajectory is well reproduced if the change in surface roughness is properly accounted for. At room temperature in any case nucleation behavior prevails over homogenous growth (see Fig. 1 for fast deposition, slow deposition data not displayed). DUring slow deposition, and e v e n H E plasma treatment, chemical reduction could completely be avoided at 30OK. Although this kind of remedy is not feasible for application in real production, it points to a threshold activation of SnO 2 reduction, independent of the energy impact from the plasma. The effect of different growth rates on the optical transmission of a2Si:H on top of SnO 2 is shown in Fig. 2. Metallic tin at the interface of the slowly deposited films causes remarkable transmission losses as compared to layers grown at a high rate. It is worth noting that the energy dependence in Fig. 2 definitely proves that there was no significant difference in film thickness between the two samples plotted, and thus the difference is attributed to increased absorption and scattering by the metallic layer at the interface of the slowly deposited films. Consequently the incidence of light into the active layer of pin solar cells is reduced in this case. In order to trace the formation of metal layers by chemical analyses, XPS and SIMS depth profiling has been applied. An~SiOx barrier layer forming at the interface, which

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results from the reduction of SnO 2, is evident from Fig. 3. For better comparison the maxima positions of the SiO x spectra have been aligned, clearly showing a larger thickness of the oxide barrier in slowly deposited films. It should be mentioned that the depth profiles are broadened since the ion beam used for sputter etching hit the film at an angle of 30 degrees, and the surface roughness of our samples therefore leads to shadowing of ions and results in non-uniform etching. All spectra in Fig. 3, however, should have been affected in a similar way due to comparable roughness of the samples. The XPS data reveal that even in the rapidly deposited samples SnO 2 has been reduced to metal close to the interface, although no signature of such a layer is obvious from Fig. 1. Based on the knowledge of room temperature deposition this contradictory evidence can easily be explained: Nucleation processes (initial upward bending of the 300K data in Fig. 1) and the formation of metallic tin (downward kink) compensate each other in the optical parameters and the trajectory starts smooth. Nevertheless the rapidly grown interface is significantly sharper, and there is less Sn present owing to the quick coverage of the SnO 2 surface, which is thereby being protected against the impact of radicals from the discharge. It should be mentioned that in our case of a de-powered

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deposition higher power predominantly translates into an enhanced flux of precursors rather than an increase in translational energy of the individual precursors. It is well known that ITO and SnO 2 are substantially being reduced in H 2 discharges, and metallic layers are formed at the TCO surfaces [11,12]. In order to find out, whether TCO reduction in Sill 4 plasmas is mainly due to hydrogen present in the discharge, ITO as well as SnO 2 have been exposed to H 2 plasmas (at 520K) for 15 min, which deteriorated the optical transmission of both TCOs by about 50%. On the other hand no reduction effects could be observed upon exposure to an H 2 discharge at 300K. ZnO on the contrary has not been reduced at all in either H 2 plasma, its optical transmission as well as its spectroscopic ellipsometry data remaining unchanged. For gaining more detailed information SIMS depth profiling of 230 nm thick ZnO has been performed after 15 min exposure to a hydrogen discharge (Fig. 4). Untreated samples do not show any change in H and ZnH signals (i.e., in the background originating from the surface analysis system) throughout the sample, whereas H and ZnH concentrations clearly rise towards the surface (cf. Fig. 4). Peaking signals at the interface to the glass substrate are artificial, resulting from well known SIMS matrix effects. SIMS depth profiling in this case is impervious to shadowing effects because our rf-ZnO is very smooth. Atomic force microscopy reveals surface structures of about 5 nm height and 60 nm lateral extension on the average. Relying on ellipsometric data Fig. 5 nicely demonstrates that ZnO can serve as a protection layer for the more common TCOs like SnO 2 or ITO [7]. Layers as thin as 20 nm of room temperature if-sputtered ZnO efficiently suppress metal formation at the T C O / p interface. Similarly, the reduction of SnO 2 during slow deposition could be suppressed by ZnO shielding; the total optical transmission of 30 nm a-Si:H on SnO 2 has been improved by about 15% compared to the case of fast deposition (SnO 2 uncoated with ZnO). In order to analyze the actual effect of ZnO shielding on solar cell efficiencies SnO 2 substrates have been half-area coated with ZnO (20 nm), and pin-diodes have been completed on top of these substrates. Solar cells within the area protected by ZnO exhibited larger short circuit currents due to a higher incidence of light into the photovoltaicly active i-layer, thus improving cell efficiency by more than

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HJV. Wanka et al. / solar Energy Materials and Solar Cells 4 1 / 4 2 (1996) 519-527 65 0.16 A/s

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half a percent. ZnO protecting layers therefore seem to be feasible for shielding ITO as well as SnO 2 substrates against silane plasma impact. More recent approaches for improved T C O / p performance have been studied, too: real-time ellipsometry data taken during a-SiO:H deposition at high and low rates are plotted in Fig.° 6. Slow deposition trajectories are well reproduced by adding an interlayer of 6A with equal contents of Sn and SnO 2 to the numerical modelling. Compared with a-Si:H, or even with a-SiC:H, the use of a-SiO:H reduces metal formation at the interface, which however cannot completely be avoided. The lower hydrogen concentration in a S i H d C O 2 plasma compared with a pure silane or SiH4/CH 4 plasma accounts for that; and most promising, after CO 2 plasma treatment of SnO 2 the trajectory does not indicate any reduction at all, not even for the case of slow a-Si:H deposition [13]. On the contrary, the data exhibit a strong nucleation comparable to room temperature growth.

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H.N. Wanka et al./ Solar Energy Materials and Solar Cells 41/42 (1996)519-527

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The surface morphology of a-Si:H films, growing on top of smooth ZnO (average ZnO surface structures are about 5 nm high and 60 nm wide), has been examined with atomic force microscopy (AFM). It does not change with increasing film thickness (Fig. 7), and corresponding ellipsometry spectra also remain unchanged after the layers have become opaque. Combining both analysis tools we are able to re-calculate the spectroscopic ellipsometry bulk data, and hence to determine the surface roughness R (thickness of the surface roughness layer) of similarly deposited films merely from spectroscopic ellipsometry. When depositing a-Si:H on Coming glass (surface roughness approx. 8,~, Fig. 8), however, the surface roughness is enhanced. In Fig. 8 the final morphology of a-Si:H on Coming 7059 is shown, which is close to that of Fig. 7. The half size of the lateral dimensions of the structures (20-30 nm) can be correlated to the surface diffusion length [ 14]. Unlike a-Si:H on top of smooth ZnO or glass, the surface of amorphous silicon growing onto texturized SnO 2 substrates is partly planarized with increasing thickness.

0nm boo nm 0 nm Fig. 8. Surfacestructureof a-Si:Hdepositedon glass.

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H.N. Wanka et al./ Solar Energy Materials and Solar Cells 4 1 / 4 2 (1996) 519-527

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Fig. 10. Surfaceroughnessof a-Si:Has a functionof film thickness. The imaginary part of the dielectric function is enhanced during growth as a result of decreasing surface roughness (Fig. 9) [2,3]. The spectroscopic data from Fig. 9 yield an exponential drop (Fig. 10) in surface roughness like R ( d ) = ( R s - Rf)exp( - d / c ) + R f (film thickness d, SnO 2 roughness Rs = 45 nm) with c = 330 nm and a final roughness of Rf = 5 nm. This smoothing of the initial TCO texture during a-Si:H deposition deteriorates light trapping in solar cells, for which light scattering at the back electrode is of crucial importance [15,16], and thereby remarkably lowers cell efficiency. Further work has been started in order to quantify the loss of TCO texture under production-like growth conditions in real solar cells.

Acknowledgements

The authors wish to thank Dr. G. Bilger for XPS analyses and helpful discussions, and M. Wieber for technical assistance. Atomic force microscopy would not have been

H.N. Wanka et al./ Solar Energy Materials and Solar Cells 4 1 / 4 2 (1996) 519-527

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p o s s i b l e without the c o o p e r a t i o n and g e n e r o u s h e l p o f U. G r i e s i n g e r at the 4. Physics D e p a r t m e n t o f Stuttgart University. F i n a n c i a l support by the G e r m a n ' B M B F ' under contract No. 0 3 2 9 5 2 1 A is also gratefully a c k n o w l e d g e d .

References [1] G. Bilger, H.C. Weller, A. Eicke and G.H. Bauer, Int. Symp. on Trends and New Applications in Thin Films, Strasbourg, France (Le Vide Les Couches Minces) 235 (1987) 91. [2] S. Kumar and B. Drevillon, J. Appl. Phys. 65 (1989) 3023. [3] B. Drevillon, J. Non-Cryst. Sol. 114 (1989) 139. [4] R.W. Collins, J. Non-Cryst. Sol. 114 (1989) 160. [5] Y.-M. Li, F. Jackson, L. Yang, B.F. Fieselmann and L. Russell, Proc. Mat. Res. Soc. Symp. 336 (1994) 663. [6] J. Bauer, H. Calwer, P. Marklstorfer, P. Milla, F.W. Schulze and K.-D. Ufert, J. Non-Cryst. Solids 164-166 (1993) 685. [7] G. Bilger, A. Eicke and G.H. Bauer, Proc. 19th IEEE Photovoltaic Specialists Conf., 1987, p. 615. [8] S.N. Jasperson, D.K. Burge and R.C. O'Handley, Surface Science 37 (1973) 548. [9] R.M.A. Azzam and N.M. Bashara, Ellipsometry and Polarized Light (North-Holland, Amsterdam, 1977). [I0] D.A.G. Bruggeman, Ann. Phys. 24 (1935) 636. [11] H. Schade, Z.E. Smith, J.H. Thomas and A. Catalano, Thin Solid Films 117 (1984) 149. [12] R. Banerjee, S. Ray, N. Basu, A.K. Batabyal and A.K. Barua, J. Appl. Phys 62 (1987) 912. [13] H.N. Wanka, G. Bilger and M.B. Schubert, submitted to Appl. Surf. Sci. [14] H.N. Wanka, E. Lotter and M.B. Schubert, submitted to J. Appl. Phys. [15] X. Deng and K.L Narasimhan, Proc. lth WCPEC, 1994, in print. [16] A. Banerjee, K. Hoffman, X. Xu, J. Yang and S. Guha, Proc. lth WCPEC, 1994, in print.