Effect of gas flow rates on PECVD-deposited nanocrystalline silicon thin film and solar cell properties

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Solar Energy Materials & Solar Cells 92 (2008) 385–392 www.elsevier.com/locate/solmat

Effect of gas flow rates on PECVD-deposited nanocrystalline silicon thin film and solar cell properties Amartya Chowdhury, Sumita Mukhopadhyay, Swati Ray Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Received 2 January 2007; received in revised form 17 September 2007; accepted 20 September 2007 Available online 5 November 2007

Abstract Nanocrystalline silicon films have been deposited at a plasma excitation frequency of 54.24 MHz by varying the flow rates of SiH4+H2 mixture in the reaction chamber. It has been found that with increase in gas flow rate from 100 to 300 sccm the defect density, microstructural defect fraction and the crystalline volume fraction in the film decrease. Films deposited at optimum total gas flow rate of 200 sccm with comparable crystalline volume fraction have shown better structural and optoelectronic properties compared to the films deposited at 100 sccm total gas flow rate for application in solar cell. Solar cells have been fabricated using these layers as absorber layers and the maximum cell efficiency obtained is 6.2% (AM1.5, 28 1C) at 200 sccm total gas flow rate. It has been found that material prepared using higher total gas flow rate of 200 sccm together with higher hydrogen dilution is better suited for solar cell application. r 2007 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline silicon; Gas flow rate; PECVD; TEM

1. Introduction Microcrystalline and nanocrystalline hydrogenated silicon thin films have attracted considerable attention for their promising applications in high efficiency and stable solar cells. As absorption coefficients of these silicon layers are low in the visible region of solar radiation, the thickness of the absorber layer should be more than 1 mm. So the deposition rate should be high to make solar cell commercially viable. For this purpose high power, pressure and very high plasma excitation frequency have been used. Plasma excitation frequency higher than conventional 13.56 MHz for deposition of nanocrystalline Si:H thin film reduces the maximum energy of ions impinging on the substrate while applying high excitation frequency and power to obtain an effective gas dissociation and a high deposition rate. Deposition rate also can be improved by

Corresponding author. Tel.: +91 33 24734971x601; fax: +91 33 24732805. E-mail address: [email protected] (S. Ray).

0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.09.013

supplying film-forming precursors at higher rate, i.e. by increasing the total gas flow rate. This also improves the film property by reducing the affect of powder formation on film growing surface [1,2]. Mai et al. [1,3] studied the effect of total gas (Tfl) flow and plasma power density mainly on the solar cell properties and found that higher Tfl have favourable effect on the solar cell performance. They also discussed the variation of crystalline volume fraction with Tfl and correlate them with abstraction reaction of atomic hydrogen. They suggested that hydrogen dilution should be increased at higher Tfl to reduce the effect of abstraction reaction. But they have not done a detailed analysis of the material prepared with different total gas flow rate. Niikura et al. [4] studied the effect of total gas flow rate using a special type of cathode and at a very high power pressure regime of 4 W/cm2 power density and 9.3 Torr chamber pressure. They found crystallinity decreases with increase in total gas flow rate and their sample become amorphous at very high total gas flow rate. So in the present article we have varied the total gas flow rate from 100 to 300 sccm and also varied other parameters to keep the film growth in microcrystalline or

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nanocrystalline region. We have studied the properties of films with similar crystallinity but deposited with different total gas flow rate, have examined their role as an absorber layer of solar cell and have correlated these studies. Higher total gas flow rate reduces gas phase polymerisations. But for a microcrystalline growth at Tfl more than 300 sccm, hydrogen dilution greater than 98% is required which in turn reduces the deposition rate drastically. 2. Experimental Undoped hydrogenated nanocrystalline silicon thin films have been prepared decomposing mixture of silane (SiH4) and hydrogen gas (H2) in a multichamber PECVD system. The multichamber has a load lock and separate chambers for the deposition of different layers of pin solar cell. The plasma excitation frequency (fex) and substrate temperature (Ts) were kept at 54.24 MHz and 180 1C, respectively. Total gas flow rate (Tfl) was varied from 100 to 300 sccm. The hydrogen dilution [Y ¼ (H2/(SiH4+H2))  100] was varied from 97% to 98%. The chamber pressure (Pr) was varied from 2 to 3 Torr. For dark conductivity (sd), photoconductivity (sph), X-ray diffraction, Raman scattering and steady state photocarrier grating (SSPG) measurements about 700–800 nm thick films were deposited on Corning glass. The photosensitivity (G) of the films is defined as the ratio sph/sd. The thicknesses of the samples were measured using a stylus type profilometer. Crystalline volume fraction (Xc) of the films was estimated from Raman spectra where 514 nm Ar Laser source was used for Raman scattering studies. The transverse optical (TO) mode peak in Raman spectra was deconvoluted into three Gaussian peaks for crystalline, grain boundary and amorphous component around 520, 510 and 480 cm1, respectively. Crystalline volume fraction have been calculated from the simplified empirical relation Xc ¼ (Ic+Ib)/ (Ic+Ib+Ia), where Ic, Ib and Ia represent the integrated area of peak of crystalline, grain boundary and amorphous component, respectively. Pc is the peak centre of the crystalline silicon component. X-ray diffraction studies were done by CuKa radiation. The grain sizes of the films were calculated from ‘full width half maximum’ of /2 2 0S peaks using Scherrer’s formula. For transmission electron microscopy very thin films (80 nm) were deposited on carbon-coated copper grid. About 3 mm thick samples were deposited on aluminium foil for electron spin resonance (ESR) measurements. For Fourier transformed infra-red (FTIR) studies samples were deposited on crystalline silicon wafer. Finally the nanocrystalline silicon films of about 1.5 mm thickness have been applied as absorber layer in a single junction solar cell with p-i-n configuration. The solar cells were deposited on TCO coated glass substrate and aluminium was used as back electrode. The J–V characteristics of the solar cells were measured under 100 mW/cm2 of AM1.5 illumination at 28 1C. Light induced degradation studies of solar cells were done using 100 mW/cm2 tungsten lamp for 500 h of light soaking.

3. Results and discussion 3.1. Deposition rate Fig. 1 shows variation of deposition rate with total gas flow rate for samples deposited under different deposition conditions. Deposition rate increases as Tfl increases from 100 to 300 sccm under all conditions but the rates of increase are different. This increase in deposition rate is due to higher supply of film forming precursors at higher Tfl. Maximum deposition rate of 7.8 A˚/s obtained at Tfl of 300 sccm. But the increase in deposition rate is not proportional to Tfl due to lower gas utilisation at higher Tfl [4]. The rate of increase of deposition rate with increase in Tfl almost saturates for Tfl above 200 sccm at Pw of 0.5 W/cm2. At very high Tfl of 300 sccm the stay time of the film forming precursors in the plasma is low as Pr is kept fixed and a high fraction of SiH4+H2 gas mixture comes out from the plasma region in undissociated condition. At Pw of 0.7 W/cm2 saturation of deposition rate is not observed as Tfl is increased from 200 to 300 sccm. This may be due to more efficient dissociation of SiH4+H2 gas mixture at higher power density. Deposition rate of samples increases with increase in chamber pressure and power density and it also increases with decrease in hydrogen dilution. It is also observed from Fig. 1 that for films deposited at higher Pw of 0.7 W/cm2 or at higher Y of 98%, the rate of increase in deposition rate with increase in Tfl from 100 to 200 sccm is higher. We shall see later from Raman scattering study that crystallinity of the films is high at higher Pw or at higher Y. So effect of Tfl on deposition rate is more prominent when the films are more crystalline in nature.

Deposition rate (Å /sec)

386

7.5 c' 7.0 6.5 6.0

b' c b a'

5.5 a 5.0 100 150 200 250 300 Total gas flow rate (SCCM)

Fig. 1. Variation of deposition rate with total gas flow rate (Tfl) for samples deposited under different Pr, Pw and Y: (a) Pr ¼ 2 Torr, Pw ¼ 0.5 W/cm2, Y ¼ 98%, (a0 ) Pr ¼ 3 Torr, Pw ¼ 0.5 W/cm2, Y ¼ 98%; (b) Pr ¼ 2 Torr, Pw ¼ 0.5 W/cm2, Y ¼ 97%, (b0 ) Pr ¼ 3 Torr, Pw ¼ 0.5 W/cm2, Y ¼ 97%; (c) Pr ¼ 2 Torr, Pw ¼ 0.7 W/cm2, Y ¼ 97%, and (c0 ) Pr ¼ 3 Torr, Pw ¼ 0.7 W/cm2, Y ¼ 97%.

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3.2. Structural properties

Intensity (a.u.)

3.2.1. Raman scattering studies Fig. 2 shows the Raman spectra of films deposited at different total gas flow rates along with different plasma power. The crystalline volume fraction estimated from Raman spectra are shown in Table 1. It is observed that Xc in general decreases with increase in total gas flow rate. For samples prepared at Pw of 0.7 W/cm2, Xc decreases from 85% to 48% as Tfl increases from 100 to 300 sccm. At higher Tfl supply of silane precursors increases but atomic hydrogen density decreases on the film-growing surface due to abstraction reaction (SiH4+H ¼ SiH3+H2). So relative density of atomic hydrogen decrease on the film-growing surface which reduces the crystallinity of the films. Generally Xc increases with increase in power density. As Pw increases the ion bombardment on the film growing surface increases. Such increase in surface temperature helps in crystalline growth in disordered silicon matrix and maximum Xc of 85% is obtained for Pw of 0.7 W/cm2 and Tfl of 100 sccm.

c' c b' b a' a 440

480 Raman shift

520

560

(cm-1)

Fig. 2. Raman spectra of samples deposited under different conditions with Y ¼ 97%: (a) Tfl ¼ 300 sccm, Pw ¼ 0.5 W/cm2, (a0 ) Tfl ¼ 300 sccm, Pw ¼ 0.7 W/cm2; (b) Tfl ¼ 200 sccm, Pw ¼ 0.5 W/cm2, (b0 ) Tfl ¼ 200 sccm, Pw ¼ 0.7 W/cm2; (c) Tfl ¼ 100 sccm, Pw ¼ 0.5 W/cm2, and (c0 ) Tfl ¼ 100 sccm, Pw ¼ 0.7 W/cm2.

387

Variation of Xc at different hydrogen dilution is shown in Table 2. Independent of Tfl, Xc increases as hydrogen dilution of silane is increased. Higher atomic density of hydrogen helps to etch out disordered part of silicon matrices, as a result Xc increases. It is observed that the ratio of grain boundary component at 510 cm1 to the total TO mode peak in the Raman spectra decreases as gas flow rate increases, i.e. the grain boundary region decreases as Tfl is increased. It is also observed that with the increase in plasma power density from 0.5 to 0.7 W/cm2 the crystalline peak centre shifts from 517.6 to 520 cm1 for samples deposited at Y of 97% and Tfl of 100 sccm. This indicates with increase in power density stress [5] in the film increases. The crystalline peak shifts towards lower wave number as total gas flow rate increases. 3.2.2. X-ray diffraction studies Fig. 3 shows the X-ray diffraction spectra of samples prepared at different total gas flow rate, plasma power density and hydrogen dilution. The intensity of the peaks in XRD spectra of samples decreases as Tfl is increased. It indicates that crystallinity in the films decreases with increase in Tfl. When samples are deposited at lower power density of 0.5 W/cm2 and Y of 97% only single /2 2 0S orientation is observed, whereas for the samples deposited at higher power density of 0.7 W/cm2 multiple orientations are observed. At higher power density the ion bombardment on the film-growing surface is high which creates multiple orientations. /2 2 0S is the most dominant peak for the sample deposited at Pw of 0.7 W/cm2 and Tfl of 100 sccm total flow. For samples deposited at Pw of 0.7 W/cm2 and Tfl of 200 sccm both /1 1 1S and /2 2 0S are prominent peaks. It is observed from Fig. 3 that at 0.5 W/cm2 plasma power density with the increase in hydrogen dilution to 98%, /1 1 1S and /3 1 1S; peak both appear. Shah et al. [6] found similar results when they increase Y beyond 98.75%. It reveals from the works of different groups that the flux density of hydrogen, electron bombardment at the surface and the nature of precursors can influence over the growth of crystallites and their orientations. At the beginning of the growth, nuclei are formed in various directions, such as /1 1 1S, /2 2 0S and

Table 1 Structural properties, defect densities and transport properties of films prepared at different deposition conditions with Y ¼ 97% Tfl (sccm)

Pw (W/cm2)

Xc (%)

100

0.5 0.7

60.7 84.9

200

0.5 0.7

300

0.5 0.7

CH (at%)

R

Defect density (cm3)

Ld (nm)

mt product (minority carriers) (cm2/V)

9.04 5.04

0.33 0.37

3.1  1017 6.5  1017

610 340

1.5  107 4.6  108

45.2 71.1

11.32 7.89

0.22 0.25

4.3  1016 7.2  1016

276 235

3.1  108 2.2  108

27.9 48.2

15.4 10.8

0.19 0.23

2.3  1016 4.6  1016

– –

– –

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Table 2 Effect of total gas flow rate, hydrogen dilution and power density on solar cells Xc of i-layer (%)

i-Layer parameters Total flow (sccm)

Y (%)

Pw (W/cm2)

100

97

0.5 0.7 0.5 0.7

98 200

97 98

300

97 98

Voc (V)

Jsc (mA/cm2)

FF

Z (%)

60.7 84.9 72.1 82.5

0.48 0.45 0.47 0.43

20.6 18.9 18.6 16.7

0.60 0.58 0.58 0.57

5.93 4.93 5.07 4.09

0.5 0.7 0.5 0.7

45.2 71.1 58.3 75.2

0.88 0.47 0.48 0.42

12.2 20.1 21.5 19.2

0.50 0.59 0.60 0.59

5.40 5.57 6.19 4.76

0.5 0.7 0.5 0.7

27.9 48.2 39.2 59.1

0.89 0.64 0.72 0.48

11.3 15.4 13.2 20.1

0.51 0.56 0.55 0.59

5.13 5.51 5.22 5.69

<111> <220> <311>

g Intensity (a.u.)

f e d c b a 20

30

Cell properties

40

50

60

2θ θ (degree) Fig. 3. X-ray spectra of samples deposited under different conditions at Pr ¼ 2 Torr: (a) Tfl ¼ 300 sccm, Pw ¼ 0.7 W/cm2, Y ¼ 97%; (b) Tfl ¼ 200 sccm, Pw ¼ 0.7 W/cm2, Y ¼ 97%; (c) Tfl ¼ 100 sccm, Pw ¼ 0.7 W/cm2, Y ¼ 97%; (d) Tfl ¼ 200 sccm, Pw ¼ 0.5 W/cm2, Y ¼ 97%; (e) Tfl ¼ 100 sccm, Pw ¼ 0.5 W/cm2, Y ¼ 97%; (f) Tfl ¼ 200 sccm, Pw ¼ 0.5 W/cm2, Y ¼ 98%; and (g) Tfl ¼ 100 sccm, Pw ¼ 0.5 W/cm2, Y ¼ 98%.

/3 1 1S, etc. But, in the subsequent growth of layers, with the coalescence of nuclei, the preferential growth and the grain size evolve in the film. Kakinuma et al. [7] pointed out that the /1 0 0S orientation is the easiest direction both for deposition and etching. In our films, no /1 0 0S growth was observed, as it was etched out with the atomic hydrogen during deposition. The planes (1 1 1) and (2 2 0) are similar in stability against etching. Because, for both the orientations /1 1 1S and /2 2 0S, the minimum numbers of bonds to desorb one Si atom from the top surface layer are same, i.e. 2 or 3. But, for the nucleation and the lateral growth, the minimum numbers of atoms required for the former are 3 and 2, and for the later are 2 and 1, respectively. Hence, the nucleation rate and lateral

growth will be much larger for the later. The net result of the competing processes of nucleation, lateral growth and etching, which depend on orientation, will determine the preferential growth in the film. So in this work, it is found that in most of the films /2 2 0S is the prominent peak. Grain sizes do not depend on Tfl. The grain size for the samples deposited at Tfl of 100 sccm, Pr of 2 Torr and Y of 97% increases from 9.7 to 14.4 nm as calculated from /2 2 0S orientation with the increase in Pw from 0.5 to 0.7 W/cm2. 3.2.3. Transmission electron microscope studies The microstructures of the layers have been investigated with transmission electron microscope (TEM). Dark spots have been observed in the TEM image (Fig. 4a) for the sample prepared at Pw of 0.5 W/cm2 and Tfl of 100 sccm which are clusters of crystallites. As Pw increases to 0.7 W/cm2 larger numbers of dark spots are observed (Fig. 4b), but the sizes of the grains are smaller. Fig. 4c and 4d is the TEM images of samples prepared at Tfl of 200 sccm at power density of 0.5 and 0.7 W/cm2, respectively. Completely different types of grains with different shapes are observed in the transmission electron micrograph, some of the grains being elongated. Fig. 4e shows a typical long grain inside a long dark region while the films are deposited under high flow rates and high power. For both 100 and 200 sccm total gas flow rate, the size of the dark regions reduces when Pw increases from 0.5 to 0.7 W/cm2. At higher gas flow rate the number of dark spots decreases which indicates decrease in crystallinity of the samples. It has been observed that the size of the clusters decreases as the total gas flow rate increases. Density of crystallites is low for films deposited at 300 sccm and is not shown here. 3.2.4. Microstructural defect in the film The FTIR spectra of Si:H films have major infra-red transmission bands near 640 and 2000 cm1 associated

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Fig. 4. TEM images of samples deposited at Y ¼ 97% and Pr ¼ 2 Torr: (a) Tfl ¼ 100 sccm, Pw ¼ 0.5 W/cm2; (b) Tfl ¼ 100 sccm, Pw ¼ 0.7 W/cm2; (c) Tfl ¼ 200 sccm, Pw ¼ 0.5 W/cm2; (d) Tfl ¼ 200 sccm, Pw ¼ 0.7 W/cm2; and (e) Tfl ¼ 200 sccm, Pw ¼ 0.7 W/cm2.

with the wagging and stretching modes of vibration of Si:H films. The stretching mode peak is deconvaluated into two Gaussian peaks around 2000 and 2100 cm1 which evolved due to the monohydride bonding and dihydride or polyhydride bonding, respectively. The microstructural defect fraction may be defined as R ¼ I2100/(I2000+I2100), where I2000 and I2100 are the integrated intensities of peaks

due to Si–H and Si–Hn (n ¼ 2, 3, y) species, respectively. Table 1 shows the variation of R with total gas flow rate and power density. R decreases from 0.37 to 0.23 as Tfl increases from 100 to 300 sccm for the films deposited at Pw ¼ 0.7 W/cm2. Generally, dihydride or polyhydride bond dominates microcrystalline and nanocrystalline silicon films, i.e. microstructural defect fraction is high

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3.2.5. Defect density study Defect density of Si:H films deposited at different Pw and Tfl are shown in Table 1. Defect density decreases as Tfl increases. It is found that for samples prepared at Pw ¼ 0.5 W/cm2, the defect density improves from 3.1  1017 cm3 to 2.3  1016 cm3 as Tfl is increased from 100 to 300 sccm. Similar trend is observed for sample prepared at Pw of 0.7 W/cm2. With the increase in Pw, the defect density increases. This indicates that for samples prepared under same gas flow rate film with higher crystalline volume fraction has higher defect density. It is observed that for films deposited at a fixed Tfl, as Pw increases crystallinity of the films increases and defect density also increases. But from Table 1 it is found that the film deposited at Pw of 0.7 W/cm2 and Tfl of 200 sccm have higher crystalline volume fraction of 71% and lower defect density of 7.2  1016 than the film deposited at Pw of 0.5 W/cm2 and Tfl of 100 sccm. This indicates that for films with similar Xc, defect density decreases as Tfl increases. This is very important criteria for application of the material as the active layer of solar cell.

3.3. Transport properties 3.3.1. Conductivity measurements Fig. 5 shows the variations of dark conductivity and photosensitivity of samples with total gas flow rate under different deposition conditions. It is observed that sd decreases and photosensitivity increases as Tfl is increased from 100 to 300 sccm. The variation of sd can be correlated to the crystalline volume fraction of the materials, i.e. sd decreases as crystallinity of the films decreases at higher Tfl. sd increases as plasma power density increases or chamber pressure decreases. Maximum dark conductivity of 3.8  105 S/cm is obtained for sample deposited at Tfl ¼ 100 sccm, Pw ¼ 0.7 W/cm2 and Y ¼ 97%. sd decreases to 1.6  108 S/cm as Tfl increases to 300 sccm keeping all other conditions same. From Fig. 5 and Table 2 it is found that for the film deposited at Y of 98%, Pw of 0.5 W/cm2 and Tfl of 200 sccm has Xc of 58.3% and sd of 3.69  107 S/cm. It is also found that the film deposited at Y of 97% and Tfl of 100 sccm has slightly higher Xc of 60.7 and sd of 5.56  107 S/cm. This increase in sd may be due to small increase in Xc. But the first sample has much higher

106

Photosensitivity

[8]. But in our case, we found low microstructural defect fraction which implies that the amorphous part of the films are more ordered. R decreases as Pw decreases. These can be correlated with the crystalline volume fraction of the samples, i.e. samples with higher crystalline volume fraction have higher microstructural defect fraction. But from Table 1, it is observed that the film deposited at Pw of 0.7 W/cm2 and Tfl of 200 sccm have higher crystallinity (Xc ¼ 71%) and lower microstructural defect fraction (R ¼ 0.25) than the film deposited at Pw of 0.5 W/cm2 and Tfl of 100 sccm which have Xc ¼ 61% and R ¼ 0.33. It indicates for a similar value of Xc, microstructural defect fraction is lower at higher Tfl of 200 sccm which is good for solar cell application. As Tfl is further increased to 300 sccm, the crystallinity of the films become low and the microstructural defect fraction decreases to 0.19 for the film deposited at Pw of 0.5 W/cm2. For a fixed Pw of 0.7 W/cm2 and Y of 97%, R changes from 0.37 to 0.25 as Tfl increases from 100 to 200 sccm and R marginally decreases from 0.25 to 0.23 as Tfl increases from 200 to 300 sccm. It implies that rate of decrease of R with Tfl almost saturates at higher Tfl. In this work microstructural defect fraction cannot be reduced further by increasing total gas flow rate, but a optimum low value of R can be achieved for a material with certain value of crystallinity by controlling Tfl. The bonded hydrogen content has been calculated from the wagging mode of FTIR vibrational spectra around 640 cm1. CH increases as total gas flow rate increases and decreases with the increase in plasma power, i.e. CH is inversely proportional to the crystalline volume fraction of the films. Minimum value of CH of 5.04 at% is obtained at Pw of 0.7 W/cm2 and Tfl of 100 sccm where the crystalline volume fraction is 85%, i.e. Xc is maximum.

104

102

100 10-4 Dark Conductivity (S/cm)

390

10-6

10-8

10-10 100

150 200 250 Total gas flow rate (SCCM)

300

Fig. 5. Variation of dark conductivity and photosensitivity with Tfl for samples deposited at Pr ¼ 2 Torr (open symbol), Pr ¼ 3 Torr (solid symbol), Pw ¼ 0.5 W/cm2 (triangle), Pw ¼ 0.7 W/cm2 (circle), Y ¼ 97% (up triangle) and Y ¼ 98% (down triangle).

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3.3.2. Steady state photocarrier grating studies In order to study the effect of gas flow rate on the carrier transport properties of the samples, the ambipolar diffusion length and mt product of the minority carriers are calculated from steady state photocarrier grating study. The results of the study are shown in Table 1. It is found that with the increase in total flow from 100 to 200 sccm, the diffusion length of minority carriers drastically decreases from 610 to 276 nm and mt product decreases from 1.5  107 to 3.1  108 cm2/V for the samples deposited at the same power pressure condition of Pr ¼ 2 Torr, Pw ¼ 0.5 W/cm2. Variation of Ld with power density for different total gas flow rates are similar in nature. As Pw increases electron temperature and ion bombardment on the film surface increases. The increase in ion bombardment creates more defects in the film. Hence Ld and mt product of the minority carriers decreases. 3.4. Solar cell Results of solar cell measurement are shown in Table 2. The efficiencies of the solar cells are measured under 100 mW/cm2 of AM1.5 illumination at 28 1C. Maximum efficiency of 6.2% is obtained for solar cell with absorber layer deposited at Tfl of 200 sccm. It is observed that at Tfl of 100 and 200 sccm, higher cell efficiencies are obtained with absorber layer deposited at a lower power density of 0.5 W/cm2. At Pw ¼ 0.5 W/cm2, the absorber layer have lower microstructural defect fraction and defect density as compared to absorber layer deposited at Pw ¼ 0.7 W/cm2. The absorber layer deposited at Y ¼ 98%, Pw ¼ 0.5 W/cm2, Tfl ¼ 200 sccm and Y ¼ 97%, Pw ¼ 0.5 W/cm2, Tfl ¼ 100 sccm have similar value of Xc (60%) and the solar cell fabricated using them also have same Voc. The first cell has higher value of Jsc which may be due to lower defect density of the film prepared at 200 sccm total gas flow rate. But when the absorber layer with Xc ¼ 59%, deposited at Tfl ¼ 300 sccm is applied in solar cell, the efficiency and Jsc of the cell decreases. As Tfl increases from 200 to 300 sccm efficiency of solar cell decreases. This may be because at Tfl of 300 sccm high power together with high hydrogen dilution is needed to achieve moderate crystalline absorber layer. But such a high value Pw and Y may create defect in the film. So

maximum solar cell efficiency with crystallinity of absorber layer around 60% can be obtained at an optimum Tfl of 200 sccm. From the structural analysis carried out on the intrinsic layer we find that the samples deposited at Y ¼ 97%, Pw ¼ 0.5 W/cm2 at 100 and 200 sccm total gas flow rate have only single /2 2 0S orientation. Single /2 2 0S orientation is beneficial for solar cell application as it contains a tilt orientation which is electrically least active w.r.t. other orientations of Si [6,9–11]. But in our case the best cell (6.2% efficiency) is fabricated using an absorber layer with three different orientations of crystalline silicon. It is found that irrespective of deposition condition solar cells with high efficiencies are fabricated using absorber layer with Xc around 60%. Mai et al. also found similar result [1]. 3.5. Light-induced degradation studies Fig. 6 shows the variation of solar cell efficiency for 200 h of light soaking. It is found that the light-induced degradation of all the solar cells with nanocrystalline absorber layer almost saturates after 20 h of light soaking. The cell prepared at Tfl ¼ 200 sccm, Y ¼ 98% and Pw ¼ 0.5 W/cm2 have highest initial efficiency of 6.2% and after light induced degradation of 3.4% the stabilised efficiency drops to 6.0%. The cell prepared at Tfl ¼ 100 sccm, Y ¼ 97% and Pw ¼ 0.5 W/cm2 degrades about 2.8% and have a stabilised efficiency of 5.77%. Absorber layer of both these cells have similar Xc of 58% and 61%, respectively. Minimum degradation of 2.06% is observed for the absorber layer deposited at Tfl ¼ 100 sccm, Y ¼ 97% and Pw ¼ 0.7 W/cm2. This very low degradation may be due to very high Xc of 85% of the i-layer. The cell prepared at Tfl ¼ 300 sccm, Y ¼ 98% and Pw ¼ 0.7 W/cm2 degrades about 3.4%. 6.4 6.2 6.0 Efficiency (%)

photosensitivity of 110 compared to the later (G ¼ 30). This indicates not only the higher Tfl makes the sample less defective, it also make the film better for application in solar cell as absorber layer. It is found that sd decreases and photosensitivity increases as Pr increases from 2 to 3 Torr. With the increase in Pr, the collision among the film forming precursors and atomic hydrogen increases. As a result the effective reactivity of atomic hydrogen on the surface of a fast-growing Si:H network happens to reduce drastically and that leads to decrease in crystallinity.

391

5.8 5.6

e

Xc=58%

d

Xc=61%

c

Xc=59%

b

Xc=85%

a

Xc=75%

5.4 5.0 4.8 4.6 0

100 Light soaking time (hour)

200

Fig. 6. Variation of solar cell efficiency with illumination time: (a) Tfl ¼ 100 sccm, Pw ¼ 0.7 W/cm2, Y ¼ 97%, (b) Tfl ¼ 200 sccm, Pw ¼ 0.7 W/cm2, Y ¼ 98%, (c) Tfl ¼ 300 sccm, Pw ¼ 0.7 W/cm2, Y ¼ 98%, (d) Tfl ¼ 100 sccm, Pw ¼ 0.5 W/cm2, Y ¼ 97%, and (e) Tfl ¼ 200 sccm, Pw ¼ 0.5 W/cm2, Y ¼ 98%.

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A. Chowdhury et al. / Solar Energy Materials & Solar Cells 92 (2008) 385–392

4. Discussions The crystalline volume fraction of the films deposited at higher total gas flow rate is lower while deposited under same power, pressure condition. The decrease in crystallinity at higher total gas flow rate is mainly due to abstraction reaction which reduces the atomic density of H atoms in the growth zone and crystalline volume fraction of the film. So with increase in Tfl the hydrogen dilution should be increased. From Table 1 it can be noted that for Pw ¼ 0.5 W/cm2, Tfl ¼ 100 sccm the value of Xc increases from 61% to 72%, i.e. an increase of about 19% as hydrogen dilution is increased from 97% to 98%. But this increase in Xc is about 29% for same increase in Y at Tfl ¼ 200 sccm. From Table 2 we found that at Tfl ¼ 200 sccm the value of Voc decreases and Jsc increases as hydrogen dilution is increased from 97% to 98%. At Tfl ¼ 100 sccm both Voc and Jsc decreases as Y is increased. It indicates that at lower total gas flow rate defect states may be developed in the material which traps charge carriers and reduces the short circuit current. By VHF PECVD method one cannot achieve very high crystallinity using high dilution as defect states and cracks appears in the sample [6], otherwise using Pw ¼ 0.5 W/cm2, Tfl ¼ 100 sccm and Y ¼ 98% a Xc higher than 72% can be achieved. It is observed that with the increase in power density, the microstructural factor increases. The film deposited at Pw of 0.7 W/cm2 and Tfl of 200 sccm have higher crystallinity (Xc ¼ 71%) and lower microstructural defect fraction (R ¼ 0.25) than the film deposited at Pw of 0.5 W/cm2 and Tfl of 100 sccm which have Xc ¼ 61% and R ¼ 0.33. From Raman study we found that grain boundary region is less for films deposited at higher Tfl. All these imply that higher gas flow rate produces films with less microstructural defect fraction. At this higher power and pressure regime powder formation may degrade the film structure and polyhydride bonds are formed. As Tfl is increased this affect reduces and films become more uniform. From ESR study it is observed that with the increase in total flow the defect density decreases. The samples prepared at Tfl ¼ 100 sccm, Y ¼ 97%, Pw ¼ 0.5 W/cm2 and Tfl ¼ 200 sccm, Y ¼ 98%, Pw ¼ 0.5 W/cm2 have similar value of crystalline volume fraction. The first sample have single /2 2 0S orientation and the second sample have all the three orientations of silicon, but both layers form solar cells of initial efficiencies around 6%. It is widely believed that a material with single /2 2 0S orientation is good for solar cell application. But in our study it is found that materials having all three orientations can form comparable solar cells. 5. Conclusions The effect of total gas flow rate on nanocrystalline silicon thin films have been investigated in this paper. Abstraction

reaction of atomic hydrogen is more prominent when total gas flow rate is higher. Deposition rate increases as Tfl increases, but rate of increase in deposition rate with Tfl saturates for Tfl above 200 sccm for lower power deposited films. Both microstructural defect fraction and crystalline volume fraction decrease as total gas flow rate increases. Higher total gas flow rate reduces the effect of powder formation on film growing surface and also makes the amorphous part of the film more homogeneous. Diffusion length of carriers in the film deposited at higher total gas flow rate reduces but still have reasonably high value. Some elongated grains and clusters are observed at higher total gas flow rate. Very low light induced degradation of solar cells are observed and it almost saturates after 20 h of light soaking. Films with comparable crystalline volume fraction shows better structural and optoelectronic properties when they are deposited at higher total gas flow rate. Defect density also decreases in the films prepared with higher gas flow rates. Best solar cells are fabricated using absorber layer with Xc around 60%. Optimum layer properties and solar cell performance are obtained at a total gas flow rate of 200 sccm. Acknowledgments The steady state photo carrier grating (SSPG) measurements have been done by Dr. C. Longeaud of LGEP, France. TEM studies were done under Nano Science and Technical Initiative Programme, DST, Government of India. The work has been carried out under a project funded by Ministry of Non-Conventional Energy Sources, Government of India. One of the authors (SM) gratefully acknowledges the financial support of the Council of Scientific and Industrial Research (CSIR), Government of India. References [1] Y. Mai, S. Klein, R. Carius, J. Wolff, A. Lambertz, F. Finger, X. Geng, J. Appl. Phys. 97 (2005) 114913. [2] P. Cabarrocas, A. Morral, S. Lebib, Y. Poissant, Pure Appl. Chem. 74 (2002) 359. [3] Y. Mai, S. Klein, R. Carius, T. Repmann, X. Geng, F. Finger, in: Proceedings of the 15th International Photovoltaic Science and Engineering Conference (PVSEC 15), Shanghai, China, 2005, pp. 941–942. [4] C. Niikura, M. Kondo, A. Matsuda, J. Non-Cryst. Solids 338–340 (2004) 42. [5] C. Das, T. Jana, S. Ray, Jpn. J. Appl. Phys. 43 (2004) 3269. [6] A. Shah, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, U. Graf, Sol. Energy Mater. Sol. Cells 78 (2003) 469. [7] H. Kakinuma, M. Mohri, M. Sakamoto, T. Tsuruoka, J. Appl. Phys. 70 (1991) 7374. [8] S. Mukhopadhyay, C. Das, S. Ray, J. Phys. D 37 (2004) 1736. [9] J.K. Rath, Sol. Energy Mater. Sol. Cells 76 (2003) 431. [10] T. Matsui, M. Tsukiji, H. Saika, T. Toyama, H. Okamoto, Jpn. J. Appl. Phys. 41 (2002) 20. [11] A.H. Mahan, Sol. Energy Mater. Sol. Cells 78 (2003) 299.