Device grade hydrogenated polymorphous silicon deposited at high rates

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Journal of Non-Crystalline Solids 354 (2008) 2092–2095 www.elsevier.com/locate/jnoncrysol

Device grade hydrogenated polymorphous silicon deposited at high rates Y.M. Soro a,*, A. Abramov b,c, M.E. Gueunier-Farret a, E.V. Johnson b, C. Longeaud a, P. Roca i Cabarrocas b, J.P. Kleider a Laboratoire de Ge´nie E´lectrique de Paris, CNRS UMR8507; SUPELEC; Univ Paris-Sud; UPMC Univ Paris 6; 11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette cedex, France b Laboratoire de Physique des Interfaces et des Couches Minces, UMR 7647, CNRS, Ecole Polytechnique, F-91128 Palaiseau cedex, France c A.F.Ioffe Phys.-Technical Institute, St.-Petersburg 194021, Russia a

Available online 29 January 2008

Abstract Hydrogenated polymorphous silicon (pm-Si:H) thin films have been deposited by plasma-enhanced chemical vapor deposition at high ˚ /s), and a set of complementary techniques have been used to study transport, localized state distribution, and optical proprate (8–10 A erties of these films, as well as the stability of these properties during light-soaking. We demonstrate that these high deposition rate pmSi:H films have outstanding electronic properties, with, for example, ambipolar diffusion length (Ld) values up to 290 nm, and density of states at the Fermi level well below 1015 cm 3 eV 1. Consistent with these material studies, results on pm-Si:H PIN modules show no ˚ /s. Although there is some degradation after dependence of their initial efficiency on the increase of the deposition rate from 1 to 10 A light-soaking, the electronic quality of the films is better than for degraded standard hydrogenated amorphous silicon (values of Ld up to 200 nm). This result is reflected in the light-soaked device characteristics. Ó 2007 Elsevier B.V. All rights reserved. PACS: 71.23.Cq; 71.55.Jv; 73.50.Gr Keywords: Silicon; Solar cells; Photovoltaics; Chemical vapor deposition

1. Introduction The obtaining of device quality a-Si:H films has been and still is one of the major challenges of this field of research. Various approaches have been introduced for the deposition of a-Si:H at high rates combined with good quality materials or good conversion efficiency solar cells: very high-frequency plasmas (VHF) [1], microwave excitation [2], expanding thermal plasmas [3], hot wire CVD [4]. For instance stabilized conversion efficiency of 8.2% has been obtained on a-Si:H based single-junction at 2 nm/s using the VHF-PECVD. However, industrial systems gen-

*

Corresponding author. Tel.: +33 1 69 85 16 80; fax: +33 1 69 41 83 18. E-mail address: [email protected] (Y.M. Soro).

0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.10.047

erally operate at the standard RF frequency of 13.56 MHz and it would be interesting to increase the deposition rate without sacrificing material quality [5]. On the other hand, the increase of the efficiency of industrial modules requires the use of tandem devices incorporation with either microcrystalline or silicon germanium alloys as the bottom cell. In this tandem approach, the stability of the top cell, which roughly produces 2/3 of the power, is a crucial issue. Hydrogenated polymorphous silicon (pm-Si:H) is a good candidate material for thin film solar cells because it possesses a defect density and stability superior to that of conventional hydrogenated amorphous silicon (a-Si:H) [6–9]. However, detailed studies of the transport properties have been so far limited to pm-Si:H films deposited at low rates ˚ /s) by means of plasma-enhanced chemical vapor (61 A deposition (PECVD).The potential of such material for

Y.M. Soro et al. / Journal of Non-Crystalline Solids 354 (2008) 2092–2095

mass production is therefore limited. Efforts are thus underway to achieve good quality silicon thin films combined with high deposition rates. Here, we present the main optical and electronic properties of pm-Si:H films depos˚ /s. Our aim is to compare these ited at rates up to 10 A properties with those of a-Si:H or pm-Si:H films deposited at low deposition rates, to address the stability properties and to demonstrate that the material is suitable for photovoltaic applications by looking at the solar cell properties. 2. Experimental The pm-Si:H films were deposited on Corning 1737 glass substrates by conventional RF plasma-enhanced chemical vapor deposition (RF PECVD) at 200 °C from 30 sccm of silane diluted in 200 sccm of hydrogen and under high pressure (4 Torr) and high RF power (25 W). In view of our previous results, we assume that the increase of SiH4 flow rate in the gas mixture as well as of the RF power can increase the concentration of reactive species and therefore can stimulate secondary reactions in the gas phase which lead to the growth of silicon nanocrystals in the plasma. On the other hand, the decrease of the interelectrode distance to 12 mm can facilitate the diffusion of neutral nanocrystals out of plasma and should prevent excess nanoparticle growth and agglomeration (formation of powders). Application of these conditions let us achieve ˚ /s. high deposition rates (HDR) of 8–10 A The optical properties of the films were deduced from transmission measurements, while a set of complementary characterization techniques was used to evaluate the defects and electronic properties. The density of states (DOS) above the Fermi level was calculated from modulated photocurrent (MPC) measurements performed in the high-frequency regime [10]. Dark current (DC) and steady-state photoconductivity (SSPC) measurements were used to derive the dark conductivity (rd) and its activation energy (Ea), and to deduce the mobility-lifetime product of electrons (lnsn), respectively. Steady-state photocarrier grating technique (SSPG) was used to derive the ambipolar diffusion length (Ld), which is assumed to be determined by holes in these disordered thin film silicon materials. All these measurements were performed on samples deposited onto Corning 1737 substrates with parallel top electrodes after annealing at 453 K during 1 h (initial state) and after light-soaking (LS state).

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In addition, two types of devices were fabricated using our HDR pm-Si:H films. Schottky diodes with top platinum rectifying contacts were used to determine the DOS at the Fermi level from capacitance versus temperature and frequency measurements [11]. As well, PIN photodiode modules with an active area of 63.2 cm2 were fabricated ˚ /s. The modules consisted using material grown at 9 A of eight subcells connected in series. The layer stack was ˚ )/ as follows: glass/textured SnO2:F/p+ a-SiC:H (200 A + ˚ ˚ pm-Si:H (3800 A)/n a-Si:H (180 A)/Al. The p-aSiC:H layer was grown at 175 °C and the rest of the layers at 210 °C. 3. Results and discussion Material properties are summarized for a series of four samples with varying thickness in Table 1. In addition, the dark conductivity rd and its activation energy Ea were found in the range 1–5  10 11 S cm 1 and 0.7–0.9 eV, respectively, which are both characteristic of undoped samples. Values of lnsn and Ld obtained in the initial state are comparable to those measured on pm-Si:H having equiva˚ /s [8,12]. Indeed, lent rd and Ea and deposited around 1 A in the initial state, Ld takes values higher than 200 nm, reaching a maximum of 290 nm. In the LS state values as high as 200 nm can still be reached. Values of lnsn at 300 K are between 4  10 7 and 1.6  10 5 cm 2 V 1 in the initial state and remain well above 10 7 cm 2 V 1 in the LS state (for a dc flux of 1014 cm 2 s 1). Fig. 1 shows the reconstructed DOS from MPC measurements using the high-frequency formulas [10] with values of the electron mobility ln and capture coefficient cn equal to 10 cm2 V 1 s 1 and 4  10 8 cm3 s 1. It is known that in the MPC experiment, the upper envelope of the reconstructed DOS points obtained at different temperatures and different frequencies give the shape of the actual DOS provided there is a good overlap of the DOS points calculated from the high-frequency data measured at two consecutive temperatures (for temperature steps set at 30 K, as in our set-up) [13]. In Fig. 1 we observe that the required overlap could not be obtained in the initial state for the deep defects (EC E > 0.4 eV). This is a characteristic of our pm-Si:H samples. From a detailed analysis of the MPC technique, it can be deduced that such behavior can only occur for samples with a low deep defect density. To confirm the low deep defect DOS, we performed

Table 1 Deposition rate r, thickness d, optical bandgap Eg, deduced from optical transmission measurements, diffusion length Ld and electron mobility-lifetime product lnsn (in initial and LS states) ˚ /s) Samples r (A d (lm) Eg (eV) Ld (nm) lnsn (cm2 V 1) 610 102 610 105 610 113 610 117

7.8 7.9 9.0 8.0

0.30 0.54 1.30 4.00

1.85 1.84 1.83 1.78

I

LS

I

LS

180 290 210 230

110 200 150 170

5  10 6 4  10 7 1.6  10 5 7.  10 6

1.7  10 2.6  10 2.4  10 5  10 7

7 7 7

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Y.M. Soro et al. / Journal of Non-Crystalline Solids 354 (2008) 2092–2095 1019 610117 (d=4 µm)

1018

1017

-3

-1

MPC-DOS (cm eV )

Initial state After light-soaking

1016

1015

1014

1013

0.2

0.4

0.6

0.8

1

T with a slope equal to the inverse of the Debye length, LDebye [11]. Our data yield LDebye = 1.15 ± 0.10 lm. Since the Debye length is directly related to the DOS at the Fermi level, N(EF), by: (LDebye)2 = e/q2N(EF), we obtain N(EF) = (4.6 ± 0.8)  1014 cm 3 eV 1. This is the lowest value we ever measured on silicon thin films. It is even lower than the values previously obtained on Schottky diodes fabricated from pm-Si:H deposited at low rate ˚ /s) [14]. (<1 A To characterize how such high-quality material properties translate into device efficiency properties, material ˚ /s was used as the i-layer in PIN photodeposited at 9 A diode modules. After deposition, patterning, and metallization, the modules were annealed for 2 h at 50 °C. The modules were subsequently light-soaked at 0.6 suns under a sodium lamp at 30 °C. Measurements were taken immediately after annealing, after 1 h of light-soaking, and after 65 h of light-soaking. The current–voltage curves for the

E -E (eV) c

capacitance measurements of Schottky diodes. In Fig. 2 we present the temperature dependence of the quantity Y = {d/dT( eA/C)} 1, where e is the dielectric permittivity, A the diode area, T the temperature and C the measured capacitance. It has been shown that above the so-called turn-on temperature T0, Y increases linearly with

a

0.12 Annealed 1hr

0.10

65hr

0.08 pm-Si:H 1.5A/s Initial State

I(A)

Fig. 1. Density of states (MPC-DOS) reconstructed from MPC data for sample 610 117 (d = 4.0 lm) in the initial state and after light-soaking. The MPC experiment has been performed under red (k = 660 nm) illumination with a dc flux equal to 1014 cm 2 s 1.

0.06

0.04

0.02 1×106 0.00

1 kHz 2.3 kHz 5.2 kHz

L

-1

[d/dT(-εA/C)] (Kcm )

8×10

0

1

2

3

N(E )=(4.6 ± 0.8)×10 F

-3

cm eV

5

6

7

8

600

650

700

750

1.1

-1

Annealed

1.0

EQE(0,5V)/EQE(0V)

-1

6×105

b

=1.15 ± 0.10 µm

Debye

14

4

V(V)

5

4×105

2×105

0.9

0.8 Light soaked

0.7

0.6 0 320

340

360

380

400

420

440

460

Temperature (K) Fig. 2. Temperature dependence of the quantity {d/dT( eA/C)} 1 for three measurement frequencies. The straight lines are linear fits to these curves above turn-on. The inverse slope of these fits is equal to the Debye length of the material which is proportional to the square of the DOS at the Fermi level.

0.5 350

400

450

500

550

Wavelength (nm) Fig. 3. (a) IV-characteristics of photovoltaic module with pm-Si:H i-layer ˚ /s, after annealing and after two periods of light-soaking. grown at 9 A ˚ /s. (b) Ratio of EQE at forward bias Full line is for a module made at 1.5 A (0.5 V) and at short circuit (0 V).

Y.M. Soro et al. / Journal of Non-Crystalline Solids 354 (2008) 2092–2095 Table 2 Solar module parameters for HDR pm-Si:H at different durations of lightsoaking, as well as percentage decrease in each parameter Condition

Jsc (mA cm 2)

Voc/cell (V)

FF

g (%)

Annealed 1h 65 h Total decrease (%)

13.1 13.0 12.3 6.1

0.94 0.90 0.88 6.4

0.65 0.62 0.55 15.4

7.7 7.2 5.9 23.4

As a comparison, the initial values of a module made of low deposition rate pm-Si:H are: Jsc = 11.4 mA/cm2, Voc/cell = 0.85 V, FF = 0.70, g = 7.2%

module are presented in Fig. 3(a), and the corresponding photovoltaic parameters are listed in Table 2 for each of the light-soaking times. As a comparison, the full line in ˚ /s. Fig. 3(a) shows a curve for a module grown at 1.5 A The performance of the annealed module reflects the high-quality of the pm-Si:H material used as the i-layer. ˚) Despite the i-layer thickness being quite high (3800 A compared to that typically used for stable a-Si:H devices ˚ ), the high value of the FFVoc product (FF (2500 A and Voc being the fill factor and open circuit voltage, respectively) indicates good collection over the whole ilayer, consistent with the high values of the diffusion length. Shown in Fig. 3(b) is the ratio of the external quantum efficiency of the devices at short circuit (0 V) and at forward bias (0.5 V), for both annealed and light-soaked conditions. It can be seen that the collection from the front of the cell (short wavelength light) is most affected by lightsoaking, indicating that the collection is not limited by hole-transport properties, as in this region, the distance holes must travel is shortest. This is consistent with the light-soaking results for the material itself.

4. Conclusion Increasing the deposition rate of thin film materials continues to be an important area of research. We have characterized pm-Si:H samples deposited at deposition rates ˚ /s with thickness between 0.3 and 4 lm using around 8 A a set of complementary experiments before and after

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light-soaking. It appears that the transport and the defect related properties are comparable or better than those of device grade a-Si:H samples, even after light-soaking. The degradation of the PIN devices under light-soaking was appreciable, possibly due to the thicker i-layer, but external quantum efficiency measurements indicate that holes transport is not the limiting factor. Device design to fully exploit this material behavior remains a subject of future research. Acknowledgements The authors thank SOLEMS for making the PIN layer stacks into modules. This work has been partly supported by ANR project ATOS. E.V Johnson acknowledges the generous support of NSERC. References [1] U. Kroll, J. Meier, P. Torres, J. Pohl, A. Shah, J. Non Cryst. Solids 227–230 (1998) 68. [2] M. Kitagawa, K. Setsune, Y. Manabe, T. Hirao, Jpn. J. Appl. Phys. 27 (1988) 2026. [3] W.M.M. Kessels, R.J. Severens, A.H.M. Smets, B.A. Korevaar, G.J. Adriaenssens, D.C. Schram, M.C.M. van de Sanden, J. Appl. Phys. 89 (2001) 2404. [4] A.H. Mahan, Y. Xu, D.L. Wiliamson, W. Beyer, J.D. Perkins, M. Vanecek, L.M. Gedvilas, B.P. Nelson, J. Appl. Phys. 90 (2001) 5038. [5] A. Matsuda, M. Takai, T. Nishimoto, M. Kondo, Sol. Energ. Mat. Sol. Cells 78 (2003) 3. [6] C. Longeaud, J.P. Kleider, P. Roca i Cabarrocas, S. Hamma, R. Meaudre, M. Meaudre, J. Non-Cryst. Solids 227–230 (1998) 96. [7] R. Butte´, R. Meaudre, M. Meaudre, S. Vignoli, C. Longeaud, J.P. Kleider, P. Roca i Cabarrocas, Philos. Mag. B 79 (1999) 1079. [8] M. Meaudre, R. Meaudre, R. Butte´, S. Vignoli, C. Longeaud, J.P. Kleider, P. Roca i Cabarrocas, J. Appl. Phys. 86 (1999) 946. [9] J.P. Kleider, C. Longeaud, M. Gauthier, M. Meaudre, R. Meaudre, R. Butte´, S. Vignoli, P. Roca i Cabarrocas, Appl. Phys. Lett. 75 (1999) 3351. [10] C. Longeaud, J.P. Kleider, Phys. Rev. B 45 (1992) 11672. [11] J.D. Cohen, D.V. Lang, Phys. Rev. B 25 (1982) 5321. [12] J.P. Kleider, M. Gauthier, C. Longeaud, D. Roy, O. Saadane, R. Bru¨ggemann, Thin Solid Films 403/404 (2002) 188. [13] J.P. Kleider, C. Longeaud, M.E. Gueunier, J. Non-Cryst. Solids 338– 340 (2004) 390. [14] J.P. Kleider, C. Longeaud, M. Gauthier, M. Meaudre, R. Meaudre, R. Butte´, S. Vignol, P. Roca i Cabarrocas, Appl. Phys. Lett. 75 (1999) 3351.