- Email: [email protected]
Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells
Thin Solid Films 430 (2003) 202–207
Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells Stefan Kleina,d,*, Friedhelm Fingera, Reinhard Cariusa, Thorsten Dyllaa, Bernd Recha, Michael Grimmb, Lothar Houbenc, Martin Stutzmannd a
¨ Photovoltaik, Forschungszentrum Julich ¨ ¨ Institut fur GmbH, 52425 Julich, Germany ¨ Schichten und Grenzflachen, ¨ ¨ ¨ Institut fur Forschungszentrum Julich GmbH, 52425 Julich, Germany c ¨ Festkorperforschung, ¨ ¨ ¨ Institut fur Forschungszentrum Julich GmbH, 52425 Julich, Germany d ¨ Munchen, ¨ Walter Schottky Institut, Technische Universitat Am Coulombwall 3, 85748 Garching, Germany b
Abstract Microcrystalline silicon (mc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature TS and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at TS below 250 8C, mc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cmy1 IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages Voc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high Voc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high Voc . Raman spectra show a continuously increasing amorphous peak with increasing Voc. Crystalline fractions XC ranging from 50% for the highest Voc to 95% for the lowest Voc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest XC, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Hot-wire chemical vapour deposition; Microcrystalline silicon; Solar cells
1. Introduction The use of microcrystalline silicon (mc-Si:H) and amorphous silicon (a-Si:H) as absorber layers in tandem cells promises a considerable increase of the conversion efficiency of thin film silicon solar cells w1,2x. Much progress regarding the performance and preparation of mc-Si:H material using plasma enhanced chemical vapour deposition (PECVD) processes has been made in the past few years w2–4x. Nevertheless, further improvements concerning the deposition rate are necessary since the indirect band gap of silicon requires relatively thick absorber layers (1–2 mm even with good light trapping). Here, hot wire chemical vapour deposition (HWCVD) has attracted attention because of *Corresponding author. Tel.: q49-2461-612851; fax: q49-2461613735. E-mail address: [email protected] (S. Klein).
the high deposition rates reported for mc-Si:H w5x and the simplicity of the technology w6,7x, e.g. in terms of scaling up. In spite of the obtained large grain sizes and crystalline fractions, the electronic quality of the HWCVD-material was poor due to a porous structure and high defect densities w8x. The high filament temperatures Tf needed to achieve an effective process gas decomposition and high deposition rates, generally lead to a significant additional substrate heating w8,9x. By reducing the substrate temperatures TS to values below 300 8C (achieved by reducing the filament number and temperature), the material quality improves significantly w10x and high solar cell efficiencies could be achieved. The reduced spin densities NS and sub-gap absorption are interpreted as an improved grain boundary passivation due to a reduced hydrogen desorption during the growth process at lower TS. Unfortunately, the deposition ˚ sy1 for the optimised solar rates are low too, e.g. 0.9 A
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00111-1
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
cell material at TSs185 8C which yield cell efficiencies up to 9.4% w11x. Higher deposition rates for mc-Si:H can be achieved, e.g. by higher Tf, TS or deposition pressure Pd. The influence of these deposition parameters on the material quality, e.g. spin density and subgap absorption are presented and relations between structure and transport properties are discussed. Similar to PECVD material, the best HWCVD solar cell performance can be achieved with mc-Si:H absorber layers prepared near the transition to amorphous growth w3,10x. The solar cells prepared under these conditions exhibit very high open circuit voltages up to 600 mV at fill factors above 70% w11x. Raman spectroscopy revealed that these cells contain large fractions of amorphous material in the absorber layer. TEM imaging indicates that the amorphous material is homogeneously distributed across the i-layer, i.e. no nucleation zone is observed near the p-layer. X-ray diffraction (XRD) from the solar cells was measured to obtain absolute values for the crystallinity. Light-induced degradation of amorphous silicon solar cells, also known as the Staebler–Wronski Effect w12x, is a well-known phenomenon. Because of the high amorphous volume fractions in the solar cells with high Voc, these solar cells might exhibit light induced degradation. Results of light soaking experiments will be presented. 2. Experiment Intrinsic mc-Si:H and solar cells were prepared in a multichamber deposition system w8x using HWCVD for the i-layers. For the deposition of intrinsic mc-Si:H we used two or three tantalum wires at a temperature of Tfs1650–1800 8C, leading—through radiative heating—to substrate temperatures of TSs185–280 8C. As deposition feed gas we used a mixture of silane (SiH4) and hydrogen (H2) at different silane concentrations SC (wSiH4x y wSiH4 qH2 x) at a total pressure Pd of 3 Pa to 20 Pa. Intrinsic material was prepared on borosilicate glass, aluminium foil for electron spin resonance (ESR) measurements w13x and double-sided polished c-Si wafer for infrared absorption measurements. The solar cells were prepared at TSs185 8C in a p-i-n deposition sequence onto a textured ZnO substrate w14x, using a mc-Si:H p-layer, an a-Si:H n-layer, both prepared by PECVD and a sputtered ZnOyAg back reflector defining the area (1 cm2) of the individual cells. The solar cells were characterised by I–V measurements in the dark and under AM1.5 illumination (100 mW cmy2, class A simulator) and quantum efficiency measurements at 25 8C. The light soaking experiments were carried out at open circuit at a temperature of 50 8C using an AM1.5-like spectrum with an intensity of 100 mW cmy2. For structural analysis, Raman spectroscopy was carried out on the solar cells after
203
Fig. 1. Dark (closed symbols) and photoconductivity (open symbols) of films with different crystalline volume fractions prepared at the indicated substrate temperature.
removing the n-doped layer. The intensity ratio IRS C s (I520qI500)y(I520qI500 qI480 ) of the crystalline peaks (520 cmy1 and 500 cmy1) and the amorphous peak (480 cmy1) was used as a semi-quantitative measure for the crystallinity. X-ray diffraction measurements were performed on the solar cells on textured ZnO glass substrates without removing the a-Si:H n-layer on the top, using the Cu-Ka X-ray radiation source and an asymmetric Bragg diffraction set-up in grazing incidence. Incident angles between 0.258 and 0.48 were chosen to achieve different penetration depths up to the solar cell thickness. However, the roughness of the ZnO causes variations of the penetration depth. 3. Results and discussion 3.1. Material properties The dark conductivity sd reflects the structural transition from amorphous to microcrystalline material by a characteristic increase of sd by several orders of magnitude, from sd-10y11 S cmy1 for a-Si:H to sds 10y8 –10y6 S cmy1 for mc-Si:H, as shown in Fig. 1. The photoconductivity sph shows only a weak dependence on IRS C , increasing gradually from amorphous to highly crystalline material. Surprisingly, sph and sd show only a weak dependence on TS, although the spin density and the sub-gap absorption in this temperature range differ by several orders of magnitude. Only the films prepared at the highest TS show significantly lower sph and sd. Therefore, sph and sd cannot be regarded as sensitive indicators of the material quality. The transition from a-Si:H to mc-Si:H can be controlled by varying the pressure, filament or substrate temperature and silane concentration. Higher Pd or Tf result in higher deposition rates, while higher Tf and
204
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
Fig. 2. Optical absorption measured by PDS for films near the transition to amorphous growth prepared at TSs220 8C and different deposition pressures.
Fig. 3. Spin density measured by ESR for samples prepared at various substrate temperatures TS and a filament temperature of 1650 8C and a pressure of 5 Pa.
3.2. Structural properties of solar cells TS (and lower Pd) shift the a-Si:Hymc-Si:H transition to higher SC. Unfortunately, deposition parameters like high Pd, Tf and TS severely affect the material properties, i.e. some of them might be not suitable to increase the deposition rates. Fig. 2 shows the optical absorption measured by photothermal deflection spectroscopy (PDS) of mc-Si:H films prepared with different Pd and SC selected in a way that the obtained films are all near the transition to amorphous growth, i.e. the growth regime where usually the best solar cell material is obtained. With increasing Pd, we observe a systematic increase of the absorption in the region below the bandgap. By increasing TS above 250 8C, a similar increase of the sub-gap absorption is observed w15x. A systematic reduction of the hydrogen content w16x and an increase of the dangling bond density NS is observed for increasing TS, too (see Fig. 3). The spin densities NS are shown in Fig. 3 for various TS as a function of SC. At TSs185 8C we find NSs4=1015 cmy3 and a (0.8 eV)s0.9 cmy1 for mc-Si:H material prepared near the transition to amorphous growth. For all TS we find a decrease of NS with increasing SC. A higher Pd results in higher spin densities w8x. The filament temperature (Tfs1650–1800 8C) has only a minor influence on the material quality. Additional indications for the decreasing material quality at high TS or Pd are found in infrared absorption spectra. At high TS (above 300 8C), a significant increase of the 2100 cmy1 SiH absorption mode and strong SiO absorption modes are observed w8,15x, both indications for a porous material structure. A similar behaviour was found for material prepared at Pd)10 Pa. Summarizing these results, we propose that low TS and Pd are prerequisites for good material quality and increasing Tf is the best way to achieve higher deposition rates.
An increasing SC for the deposition of the i-layer increases the open circuit voltage Voc. The maximum of the fill factor is found near the transition to amorphous growth. In the following discussion Voc is used—instead of SC—as the general parameter for the solar cells, since the deposition pressure was varied too. We have investigated the structural, optical and electrical properties for a set of 1-mm-thick p-i-n solar cells showing Voc between 518 and 600 mV to gain deeper insight into the origin of the high Voc. Fig. 4 shows the results of the structural analysis of the solar cells by Raman and XRD. The Raman intensity ratio decreases from RS IRS C s0.65 for the cell with the lowest Voc to IC s0.25 for the cell with the highest Voc, independent of the excitation wavelength and therefore independent of the penetration depth and the Raman cross-sections. X-ray diffraction is the best way to determine the absolute crystalline fraction XC of the cells. As shown
Fig. 4. Raman intensity ratio IRS C (jh) calculated from Raman spectra and crystalline fraction XC (s q =) determined from X-ray diffractograms vs. Voc of the solar cells.
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
Fig. 5. Azimuthal scans over selected area (200 nm spot) electron diffraction patterns measured in the front, middle and the back of a cross-sectional TEM sample of the solar cell with Vocs600 mV.
in Fig. 4, all XC are systematically higher than the IRS C obtained by Raman scattering, what is well known from studies on mc-Si:H single layers w17x. For an incident beam angle of 0.358 (probing the full i-layer), the highest crystallinity with XCs0.95 is found in the cell with the lowest Voc, decreasing to XCs0.55 in the cell with the highest Voc. Compared to our previous studies on PECVD solar cells w3x, this is a very low value for a mc-Si:H solar cell with still a high fill factor. For smaller incident beam angles, a decrease of XC is observed in all cells, likely caused by the amorphous nlayer, which contributes most to the XRD spectra at small angles. The linewidths of the crystalline peaks are slightly narrower for the cells with high XC. The calculated domain sizes are all in the range of 5–10 nm with significant scatter for the individual solar cells. A preferred crystallographic orientation was not observed in any of these samples, all of them showed the typical intensity distribution of a random powder sample. As Raman spectroscopy and XRD are not sensitive to the spatial distribution of the amorphous fractions, TEM imaging in cross-section was performed on selected cells with high amorphous fractions (Vocs562 mV and 600 mV) w11,18x. These TEM images show a fibrelike arrangement of crystalline columns separated by incoherent boundaries with little variation in the typical column size from bottom to top. The radial intensity distribution in selected area electron diffraction (ED) patterns of regions with 1200 nm diameter (see insert for an example) are shown in Fig. 5 for the cell with Vocs600 mV. The three ED patterns from the bottom, middle and top of the cell show a similar intensity distribution, i.e. the amorphous volume fraction is the same across the i-layer. Thus, no nucleation layer is found despite the low crystallinity.
205
Fig. 6. (a) Short circuit current density jsc and fill factor FF under AM1.5 (b) dark I–V characteristics: dark saturation current density j0 and diode quality factor n as a function of open circuit voltage Voc.
3.3. Optical and electronic properties of solar cells Fig. 6 summarises the I–V characteristics in the dark and under AM1.5 illumination. The short circuit current density jsc decreases by 2 mA cmy2 upon an increase of Voc from 518 to 600 mV. The corresponding quantum efficiency (QE) spectra in Fig. 7 show that the reduced jsc results from a decreasing long wavelength response (l)700 nm) for the solar cells with higher Voc. This reduced QE is linked to an increase of the amorphous fraction in the cells with higher Voc. In this wavelength region, the amorphous fraction of the i-layer material does not contribute to the QE, because of the low absorption coefficient of amorphous silicon. With increasing Voc, the fill factor (FF) is increasing, exceeding 71% in the cell with the highest Voc. A small decrease of the diode quality factor n is
Fig. 7. Quantum efficiency spectra of mc-Si:H solar cells with open circuit voltages Voc between 518 mV and 600 mV.
206
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
8c) and a reduction up to 10% of the initial efficiency in the cells with XCf0.55 (Fig. 8a,b). No saturation is observed, even after more than 1000 h of light soaking. As one can see from Fig. 8a,b, there are distinct differences in the behaviour of the I–V parameters of different cells. In some cells, the decrease of h is mainly a result of the reduced Isc, while FF and Voc show only small reductions (Fig. 8b). Other cells show a completely different degradation behaviour (Fig. 8a). There, the decrease of h was dominated by a decrease of FF and Voc, while Isc exhibited only small changes and even small increases of approximately 1% in the cells with high amorphous fractions. After annealing the solar cells at 160 8C for 1 h, the solar cells recover and in some cases a small improvement of the efficiencies occurs. The dependence of the degradation magnitude on the amorphous volume fraction and the recovering after annealing suggest that Staebler–Wronski-like defect generation in the amorphous phase is responsible for the light induced degradation of these high Voc solar cells. Fig. 8. Normalised I–V parameters of mc-Si:H cells with different Voc as a function of light soaking time ts.
found in the dark I–V curves for the cells with high Voc. The dark saturation current density j0 continuously decreases with increasing Voc. Considering the high solar cell efficiencies w11x and the very low spin densities of these layers, we propose that in microcrystalline material with homogeneously distributed amorphous material fraction, we have an excellent passivation of the grain boundaries. Further, the high QE at short wavelengths clearly shows that the amorphous phase is of high quality as it fully contributes to the solar cell performance. 3.4. Light soaking It is well known that a-Si:H shows an increase of the defect density under illumination w12x, thus the performance of those mc-Si:H solar cells with different amorphous fraction was monitored during light exposure. Fig. 8 shows some typical results of the light soaking experiments conducted on the same set of cells presented above. The values are averaged over approximately 10 cells on a single substrate. Although the results of these experiments are inconsistent in terms of the solar cell parameters, some general trends are found. During the first 100 h of light soaking, only small degradation effects are observed and in some cases even a small increase of h was found. After more than 100 h, all cells show a decrease of the solar cell efficiency h. The decrease of h depends on the crystallinity of the i-layer material, with almost no degradation in the cell with the most crystalline i-layer and lowest Voc of 518 mV (Fig.
4. Conclusions mc-Si:H of very high quality can be prepared using HWCVD. For this purpose, low substrate temperatures below 250 8C are essential. Under these conditions, compact materials with low spin densities, high photosensitivities and low optical absorption below the band gap can be obtained. Applied as the i-layer in p-i-n solar cells efficiencies, up to 9.4% and Voc up to 600 mV were obtained for HWCVD mc-Si:H prepared close to the transition to amorphous growth. The solar cells contain large amorphous fractions up to 50% in the cells with the highest Voc of 600 mV. XRD and TEM indicate a homogeneous distribution of the amorphous material across the i-layer, resulting in an excellent passivation of the grain boundaries. Solar cells containing amorphous material also show reversible light induced degradation, leading to a reduction of the efficiency by 10% in the solar cells with the highest amorphous volume fractions. Acknowledgments ¨ ¨ The authors thank J. Wolff, M. Hulsbeck, G. Schope, F. Birmans and W. Reetz for their contributions to this study. This work was supported by the BMBF (contract 412-ET-9612A) and BMWi (contract 0329854A). References w1x J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, Ch. Hof, D. Fischer, H. ` J. Koehler, Keppner, A. Shah, K.-D. Ufert, P. Giannoules, Mater. Res. Soc. Symp. Proc. 420 (1996) 3.
S. Klein et al. / Thin Solid Films 430 (2003) 202–207 w2x K. Yamamoto, M. Yoshimi, Y. Tawada, Y. Okamoto, A. Nakajima, J. Non-Cryst. Solids 266–269 (2000) 1082. w3x O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, ¨ A. Lambertz, A. Muck, B. Rech, H. Wagner, Solar Energy Mater. & Solar Cells 62 (2000) 97. w4x T. Roscheck, T. Repmann, J. Muller, ¨ B. Rech, H. Wagner, J. Vac. Sci. Technol. A 20 (2) (2002) 492. w5x R.E.I. Schropp, Y. Xu, E. Iwaniczko, G.A. Zaharias, A.H. Mahan, Mater. Res. Soc. Symp. Proc. 715 (2002) A26.3. w6x H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175. w7x A. Ledermann, U. Weber, C. Mukherjee, B. Schroder, ¨ Thin Solid Films 395 (2001) 61. w8x S. Klein, F. Finger, R. Carius, H. Wagner, M. Stutzmann, Thin Solid Films 395 (2001) 305. w9x M. Karasawa, A. Masuda, K. Ishibashi, H. Matsumura, Thin Solid Films 395 (2001) 71. w10x S. Klein, J. Wolff, F. Finger, R. Carius, H. Wagner, M. Stutzmann, Jpn. J. Appl. Phys 41 (2002) L10.
207
w11x S. Klein, F. Finger, R. Carius, B. Rech, L. Houben, M. Luysberg, M. Stutzmann, Mater. Res. Soc. Symp. Proc. 715 (2002) A26.2. w12x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. w13x F. Finger, J. Muller, ¨ C. Malten, H. Wagner, Phil. Mag. B 77 (1998) 805. w14x O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schope, ¨ C. ¨ Beneking, H. Wagner, A. Loffl, H.W. Schock, Thin Solid Films 351 (1999) 247. w15x S. Klein, F. Finger, R. Carius, L. Baia Neto, H. Wagner, M. Stutzmann, in: B. McNelis, W. Palz, H.A. Ossenbrink, P. Helm (Eds.), Proceedings of the 17th EPVSEC, Munich, Germany, 2001, p. 2965. w16x F. Finger, S. Klein, T. Dylla, A.L. Baia Neto, O. Vetterl, R. Carius, Mater. Res. Soc. Symp. Proc. 715 (2002) A16.2. w17x L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner, Phil. Mag. A 77 (6) (1998) 1447. w18x M. Luysberg, C. Scholten, L. Houben, R. Carius, F. Finger, O. Vetterl, Mater. Res. Soc. Proc. 664 (2001) A15.2.1.
Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells Stefan Kleina,d,*, Friedhelm Fingera, Reinhard Cariusa, Thorsten Dyllaa, Bernd Recha, Michael Grimmb, Lothar Houbenc, Martin Stutzmannd a
¨ Photovoltaik, Forschungszentrum Julich ¨ ¨ Institut fur GmbH, 52425 Julich, Germany ¨ Schichten und Grenzflachen, ¨ ¨ ¨ Institut fur Forschungszentrum Julich GmbH, 52425 Julich, Germany c ¨ Festkorperforschung, ¨ ¨ ¨ Institut fur Forschungszentrum Julich GmbH, 52425 Julich, Germany d ¨ Munchen, ¨ Walter Schottky Institut, Technische Universitat Am Coulombwall 3, 85748 Garching, Germany b
Abstract Microcrystalline silicon (mc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature TS and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at TS below 250 8C, mc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cmy1 IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages Voc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high Voc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high Voc . Raman spectra show a continuously increasing amorphous peak with increasing Voc. Crystalline fractions XC ranging from 50% for the highest Voc to 95% for the lowest Voc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest XC, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Hot-wire chemical vapour deposition; Microcrystalline silicon; Solar cells
1. Introduction The use of microcrystalline silicon (mc-Si:H) and amorphous silicon (a-Si:H) as absorber layers in tandem cells promises a considerable increase of the conversion efficiency of thin film silicon solar cells w1,2x. Much progress regarding the performance and preparation of mc-Si:H material using plasma enhanced chemical vapour deposition (PECVD) processes has been made in the past few years w2–4x. Nevertheless, further improvements concerning the deposition rate are necessary since the indirect band gap of silicon requires relatively thick absorber layers (1–2 mm even with good light trapping). Here, hot wire chemical vapour deposition (HWCVD) has attracted attention because of *Corresponding author. Tel.: q49-2461-612851; fax: q49-2461613735. E-mail address: [email protected] (S. Klein).
the high deposition rates reported for mc-Si:H w5x and the simplicity of the technology w6,7x, e.g. in terms of scaling up. In spite of the obtained large grain sizes and crystalline fractions, the electronic quality of the HWCVD-material was poor due to a porous structure and high defect densities w8x. The high filament temperatures Tf needed to achieve an effective process gas decomposition and high deposition rates, generally lead to a significant additional substrate heating w8,9x. By reducing the substrate temperatures TS to values below 300 8C (achieved by reducing the filament number and temperature), the material quality improves significantly w10x and high solar cell efficiencies could be achieved. The reduced spin densities NS and sub-gap absorption are interpreted as an improved grain boundary passivation due to a reduced hydrogen desorption during the growth process at lower TS. Unfortunately, the deposition ˚ sy1 for the optimised solar rates are low too, e.g. 0.9 A
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00111-1
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
cell material at TSs185 8C which yield cell efficiencies up to 9.4% w11x. Higher deposition rates for mc-Si:H can be achieved, e.g. by higher Tf, TS or deposition pressure Pd. The influence of these deposition parameters on the material quality, e.g. spin density and subgap absorption are presented and relations between structure and transport properties are discussed. Similar to PECVD material, the best HWCVD solar cell performance can be achieved with mc-Si:H absorber layers prepared near the transition to amorphous growth w3,10x. The solar cells prepared under these conditions exhibit very high open circuit voltages up to 600 mV at fill factors above 70% w11x. Raman spectroscopy revealed that these cells contain large fractions of amorphous material in the absorber layer. TEM imaging indicates that the amorphous material is homogeneously distributed across the i-layer, i.e. no nucleation zone is observed near the p-layer. X-ray diffraction (XRD) from the solar cells was measured to obtain absolute values for the crystallinity. Light-induced degradation of amorphous silicon solar cells, also known as the Staebler–Wronski Effect w12x, is a well-known phenomenon. Because of the high amorphous volume fractions in the solar cells with high Voc, these solar cells might exhibit light induced degradation. Results of light soaking experiments will be presented. 2. Experiment Intrinsic mc-Si:H and solar cells were prepared in a multichamber deposition system w8x using HWCVD for the i-layers. For the deposition of intrinsic mc-Si:H we used two or three tantalum wires at a temperature of Tfs1650–1800 8C, leading—through radiative heating—to substrate temperatures of TSs185–280 8C. As deposition feed gas we used a mixture of silane (SiH4) and hydrogen (H2) at different silane concentrations SC (wSiH4x y wSiH4 qH2 x) at a total pressure Pd of 3 Pa to 20 Pa. Intrinsic material was prepared on borosilicate glass, aluminium foil for electron spin resonance (ESR) measurements w13x and double-sided polished c-Si wafer for infrared absorption measurements. The solar cells were prepared at TSs185 8C in a p-i-n deposition sequence onto a textured ZnO substrate w14x, using a mc-Si:H p-layer, an a-Si:H n-layer, both prepared by PECVD and a sputtered ZnOyAg back reflector defining the area (1 cm2) of the individual cells. The solar cells were characterised by I–V measurements in the dark and under AM1.5 illumination (100 mW cmy2, class A simulator) and quantum efficiency measurements at 25 8C. The light soaking experiments were carried out at open circuit at a temperature of 50 8C using an AM1.5-like spectrum with an intensity of 100 mW cmy2. For structural analysis, Raman spectroscopy was carried out on the solar cells after
203
Fig. 1. Dark (closed symbols) and photoconductivity (open symbols) of films with different crystalline volume fractions prepared at the indicated substrate temperature.
removing the n-doped layer. The intensity ratio IRS C s (I520qI500)y(I520qI500 qI480 ) of the crystalline peaks (520 cmy1 and 500 cmy1) and the amorphous peak (480 cmy1) was used as a semi-quantitative measure for the crystallinity. X-ray diffraction measurements were performed on the solar cells on textured ZnO glass substrates without removing the a-Si:H n-layer on the top, using the Cu-Ka X-ray radiation source and an asymmetric Bragg diffraction set-up in grazing incidence. Incident angles between 0.258 and 0.48 were chosen to achieve different penetration depths up to the solar cell thickness. However, the roughness of the ZnO causes variations of the penetration depth. 3. Results and discussion 3.1. Material properties The dark conductivity sd reflects the structural transition from amorphous to microcrystalline material by a characteristic increase of sd by several orders of magnitude, from sd-10y11 S cmy1 for a-Si:H to sds 10y8 –10y6 S cmy1 for mc-Si:H, as shown in Fig. 1. The photoconductivity sph shows only a weak dependence on IRS C , increasing gradually from amorphous to highly crystalline material. Surprisingly, sph and sd show only a weak dependence on TS, although the spin density and the sub-gap absorption in this temperature range differ by several orders of magnitude. Only the films prepared at the highest TS show significantly lower sph and sd. Therefore, sph and sd cannot be regarded as sensitive indicators of the material quality. The transition from a-Si:H to mc-Si:H can be controlled by varying the pressure, filament or substrate temperature and silane concentration. Higher Pd or Tf result in higher deposition rates, while higher Tf and
204
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
Fig. 2. Optical absorption measured by PDS for films near the transition to amorphous growth prepared at TSs220 8C and different deposition pressures.
Fig. 3. Spin density measured by ESR for samples prepared at various substrate temperatures TS and a filament temperature of 1650 8C and a pressure of 5 Pa.
3.2. Structural properties of solar cells TS (and lower Pd) shift the a-Si:Hymc-Si:H transition to higher SC. Unfortunately, deposition parameters like high Pd, Tf and TS severely affect the material properties, i.e. some of them might be not suitable to increase the deposition rates. Fig. 2 shows the optical absorption measured by photothermal deflection spectroscopy (PDS) of mc-Si:H films prepared with different Pd and SC selected in a way that the obtained films are all near the transition to amorphous growth, i.e. the growth regime where usually the best solar cell material is obtained. With increasing Pd, we observe a systematic increase of the absorption in the region below the bandgap. By increasing TS above 250 8C, a similar increase of the sub-gap absorption is observed w15x. A systematic reduction of the hydrogen content w16x and an increase of the dangling bond density NS is observed for increasing TS, too (see Fig. 3). The spin densities NS are shown in Fig. 3 for various TS as a function of SC. At TSs185 8C we find NSs4=1015 cmy3 and a (0.8 eV)s0.9 cmy1 for mc-Si:H material prepared near the transition to amorphous growth. For all TS we find a decrease of NS with increasing SC. A higher Pd results in higher spin densities w8x. The filament temperature (Tfs1650–1800 8C) has only a minor influence on the material quality. Additional indications for the decreasing material quality at high TS or Pd are found in infrared absorption spectra. At high TS (above 300 8C), a significant increase of the 2100 cmy1 SiH absorption mode and strong SiO absorption modes are observed w8,15x, both indications for a porous material structure. A similar behaviour was found for material prepared at Pd)10 Pa. Summarizing these results, we propose that low TS and Pd are prerequisites for good material quality and increasing Tf is the best way to achieve higher deposition rates.
An increasing SC for the deposition of the i-layer increases the open circuit voltage Voc. The maximum of the fill factor is found near the transition to amorphous growth. In the following discussion Voc is used—instead of SC—as the general parameter for the solar cells, since the deposition pressure was varied too. We have investigated the structural, optical and electrical properties for a set of 1-mm-thick p-i-n solar cells showing Voc between 518 and 600 mV to gain deeper insight into the origin of the high Voc. Fig. 4 shows the results of the structural analysis of the solar cells by Raman and XRD. The Raman intensity ratio decreases from RS IRS C s0.65 for the cell with the lowest Voc to IC s0.25 for the cell with the highest Voc, independent of the excitation wavelength and therefore independent of the penetration depth and the Raman cross-sections. X-ray diffraction is the best way to determine the absolute crystalline fraction XC of the cells. As shown
Fig. 4. Raman intensity ratio IRS C (jh) calculated from Raman spectra and crystalline fraction XC (s q =) determined from X-ray diffractograms vs. Voc of the solar cells.
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
Fig. 5. Azimuthal scans over selected area (200 nm spot) electron diffraction patterns measured in the front, middle and the back of a cross-sectional TEM sample of the solar cell with Vocs600 mV.
in Fig. 4, all XC are systematically higher than the IRS C obtained by Raman scattering, what is well known from studies on mc-Si:H single layers w17x. For an incident beam angle of 0.358 (probing the full i-layer), the highest crystallinity with XCs0.95 is found in the cell with the lowest Voc, decreasing to XCs0.55 in the cell with the highest Voc. Compared to our previous studies on PECVD solar cells w3x, this is a very low value for a mc-Si:H solar cell with still a high fill factor. For smaller incident beam angles, a decrease of XC is observed in all cells, likely caused by the amorphous nlayer, which contributes most to the XRD spectra at small angles. The linewidths of the crystalline peaks are slightly narrower for the cells with high XC. The calculated domain sizes are all in the range of 5–10 nm with significant scatter for the individual solar cells. A preferred crystallographic orientation was not observed in any of these samples, all of them showed the typical intensity distribution of a random powder sample. As Raman spectroscopy and XRD are not sensitive to the spatial distribution of the amorphous fractions, TEM imaging in cross-section was performed on selected cells with high amorphous fractions (Vocs562 mV and 600 mV) w11,18x. These TEM images show a fibrelike arrangement of crystalline columns separated by incoherent boundaries with little variation in the typical column size from bottom to top. The radial intensity distribution in selected area electron diffraction (ED) patterns of regions with 1200 nm diameter (see insert for an example) are shown in Fig. 5 for the cell with Vocs600 mV. The three ED patterns from the bottom, middle and top of the cell show a similar intensity distribution, i.e. the amorphous volume fraction is the same across the i-layer. Thus, no nucleation layer is found despite the low crystallinity.
205
Fig. 6. (a) Short circuit current density jsc and fill factor FF under AM1.5 (b) dark I–V characteristics: dark saturation current density j0 and diode quality factor n as a function of open circuit voltage Voc.
3.3. Optical and electronic properties of solar cells Fig. 6 summarises the I–V characteristics in the dark and under AM1.5 illumination. The short circuit current density jsc decreases by 2 mA cmy2 upon an increase of Voc from 518 to 600 mV. The corresponding quantum efficiency (QE) spectra in Fig. 7 show that the reduced jsc results from a decreasing long wavelength response (l)700 nm) for the solar cells with higher Voc. This reduced QE is linked to an increase of the amorphous fraction in the cells with higher Voc. In this wavelength region, the amorphous fraction of the i-layer material does not contribute to the QE, because of the low absorption coefficient of amorphous silicon. With increasing Voc, the fill factor (FF) is increasing, exceeding 71% in the cell with the highest Voc. A small decrease of the diode quality factor n is
Fig. 7. Quantum efficiency spectra of mc-Si:H solar cells with open circuit voltages Voc between 518 mV and 600 mV.
206
S. Klein et al. / Thin Solid Films 430 (2003) 202–207
8c) and a reduction up to 10% of the initial efficiency in the cells with XCf0.55 (Fig. 8a,b). No saturation is observed, even after more than 1000 h of light soaking. As one can see from Fig. 8a,b, there are distinct differences in the behaviour of the I–V parameters of different cells. In some cells, the decrease of h is mainly a result of the reduced Isc, while FF and Voc show only small reductions (Fig. 8b). Other cells show a completely different degradation behaviour (Fig. 8a). There, the decrease of h was dominated by a decrease of FF and Voc, while Isc exhibited only small changes and even small increases of approximately 1% in the cells with high amorphous fractions. After annealing the solar cells at 160 8C for 1 h, the solar cells recover and in some cases a small improvement of the efficiencies occurs. The dependence of the degradation magnitude on the amorphous volume fraction and the recovering after annealing suggest that Staebler–Wronski-like defect generation in the amorphous phase is responsible for the light induced degradation of these high Voc solar cells. Fig. 8. Normalised I–V parameters of mc-Si:H cells with different Voc as a function of light soaking time ts.
found in the dark I–V curves for the cells with high Voc. The dark saturation current density j0 continuously decreases with increasing Voc. Considering the high solar cell efficiencies w11x and the very low spin densities of these layers, we propose that in microcrystalline material with homogeneously distributed amorphous material fraction, we have an excellent passivation of the grain boundaries. Further, the high QE at short wavelengths clearly shows that the amorphous phase is of high quality as it fully contributes to the solar cell performance. 3.4. Light soaking It is well known that a-Si:H shows an increase of the defect density under illumination w12x, thus the performance of those mc-Si:H solar cells with different amorphous fraction was monitored during light exposure. Fig. 8 shows some typical results of the light soaking experiments conducted on the same set of cells presented above. The values are averaged over approximately 10 cells on a single substrate. Although the results of these experiments are inconsistent in terms of the solar cell parameters, some general trends are found. During the first 100 h of light soaking, only small degradation effects are observed and in some cases even a small increase of h was found. After more than 100 h, all cells show a decrease of the solar cell efficiency h. The decrease of h depends on the crystallinity of the i-layer material, with almost no degradation in the cell with the most crystalline i-layer and lowest Voc of 518 mV (Fig.
4. Conclusions mc-Si:H of very high quality can be prepared using HWCVD. For this purpose, low substrate temperatures below 250 8C are essential. Under these conditions, compact materials with low spin densities, high photosensitivities and low optical absorption below the band gap can be obtained. Applied as the i-layer in p-i-n solar cells efficiencies, up to 9.4% and Voc up to 600 mV were obtained for HWCVD mc-Si:H prepared close to the transition to amorphous growth. The solar cells contain large amorphous fractions up to 50% in the cells with the highest Voc of 600 mV. XRD and TEM indicate a homogeneous distribution of the amorphous material across the i-layer, resulting in an excellent passivation of the grain boundaries. Solar cells containing amorphous material also show reversible light induced degradation, leading to a reduction of the efficiency by 10% in the solar cells with the highest amorphous volume fractions. Acknowledgments ¨ ¨ The authors thank J. Wolff, M. Hulsbeck, G. Schope, F. Birmans and W. Reetz for their contributions to this study. This work was supported by the BMBF (contract 412-ET-9612A) and BMWi (contract 0329854A). References w1x J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, Ch. Hof, D. Fischer, H. ` J. Koehler, Keppner, A. Shah, K.-D. Ufert, P. Giannoules, Mater. Res. Soc. Symp. Proc. 420 (1996) 3.
S. Klein et al. / Thin Solid Films 430 (2003) 202–207 w2x K. Yamamoto, M. Yoshimi, Y. Tawada, Y. Okamoto, A. Nakajima, J. Non-Cryst. Solids 266–269 (2000) 1082. w3x O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, ¨ A. Lambertz, A. Muck, B. Rech, H. Wagner, Solar Energy Mater. & Solar Cells 62 (2000) 97. w4x T. Roscheck, T. Repmann, J. Muller, ¨ B. Rech, H. Wagner, J. Vac. Sci. Technol. A 20 (2) (2002) 492. w5x R.E.I. Schropp, Y. Xu, E. Iwaniczko, G.A. Zaharias, A.H. Mahan, Mater. Res. Soc. Symp. Proc. 715 (2002) A26.3. w6x H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175. w7x A. Ledermann, U. Weber, C. Mukherjee, B. Schroder, ¨ Thin Solid Films 395 (2001) 61. w8x S. Klein, F. Finger, R. Carius, H. Wagner, M. Stutzmann, Thin Solid Films 395 (2001) 305. w9x M. Karasawa, A. Masuda, K. Ishibashi, H. Matsumura, Thin Solid Films 395 (2001) 71. w10x S. Klein, J. Wolff, F. Finger, R. Carius, H. Wagner, M. Stutzmann, Jpn. J. Appl. Phys 41 (2002) L10.
207
w11x S. Klein, F. Finger, R. Carius, B. Rech, L. Houben, M. Luysberg, M. Stutzmann, Mater. Res. Soc. Symp. Proc. 715 (2002) A26.2. w12x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. w13x F. Finger, J. Muller, ¨ C. Malten, H. Wagner, Phil. Mag. B 77 (1998) 805. w14x O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schope, ¨ C. ¨ Beneking, H. Wagner, A. Loffl, H.W. Schock, Thin Solid Films 351 (1999) 247. w15x S. Klein, F. Finger, R. Carius, L. Baia Neto, H. Wagner, M. Stutzmann, in: B. McNelis, W. Palz, H.A. Ossenbrink, P. Helm (Eds.), Proceedings of the 17th EPVSEC, Munich, Germany, 2001, p. 2965. w16x F. Finger, S. Klein, T. Dylla, A.L. Baia Neto, O. Vetterl, R. Carius, Mater. Res. Soc. Symp. Proc. 715 (2002) A16.2. w17x L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner, Phil. Mag. A 77 (6) (1998) 1447. w18x M. Luysberg, C. Scholten, L. Houben, R. Carius, F. Finger, O. Vetterl, Mater. Res. Soc. Proc. 664 (2001) A15.2.1.