HYDROGENATED AMORPHOUS SILICON AND SILICON GERMANIUM TRIPLE­JUNCTION SOLAR CELLS AT HIGH RATE USING RF AND VHF GLOW DISCHARGES

Guozhen Yue, Baojie Yan, Jeffrey Yang, and Subhendu Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI48084

ABSTRACT

We present our recent results on a-Si:H/a-SiGe:H/a­SiGe:H triple-junction solar cells made at high deposition rates using both RF and MVHF glow discharges. The growth parameters studied are under the constraints of manufacturing feasibility so that any improvement can be tr~nsferred to manufacturing lines. Using RF glow discharge, we achieved initial active-area efficiencies of 11.4% and 12.5% on AllZnO and AglZnO back reflectors, respectively, at deposition rates of -4-6 Als. Similar initial cell efficiency were achieved at even higher deposition rates (-6-8 Als) with MVHF glow discharge. Comparison studies of stability on these cells showed that the light­induced degradations are -15% and -22% for a-Si:H/a­SiGe:H/a-SiGe:H triple-junction solar cells deposited with RF at -1 Als and -4-6 Als, respectively, while it is only 6-11 % for the MVHF deposited triple-junction cells. The results demonstrate that VHF technology is desirable at high deposition rate in terms of increasing manufacturing throughput.

INTRODUCTION

A stable cell efficiency of 13.0% was achieved in 1997 incorporating hydrogenated amorphous silicon (a-Si:H) and silicon germanium (a-SiGe:H) alloys in a triple-junction structure [1], where the three junctions have different bandgaps to absorb photons of different energies in the solar spectrum. This technology has been successfully transferred to mass production of solar panels. We use a roll-to-roll continuous RF glow discharge deposition technique to make a-Si:H/a-SiGe:H/a-SiGe:H triple­junction solar cells on flexible Al/ZnO back reflector coated stainless steel (SS). It is generally known that the quality of a-Si:H and a-SiGe:H alloys decreases with the increase of deposition rate [2]. Therefore, a compromised deposition rate of -3 Als is used in the current manufacturing lines. In order to further increase the throughput and reduce the cost, it is desirable to explore new deposition regimes to improve cell efficiency and increase deposition rate. Several new deposition methods have been investigated, including microwave glow d~scharge [3], modified very high frequency (MVHF) glow dls~harge [4-6], and hot-wire CVD [7]. Some interesting fi~dlngs have been reported. In particular, MVHF glow discharge has shown a weak deposition rate dependence on the performance and stability of a-Si:H solar cells [6]. In this paper, we present our recent progress on the optimization of the triple-junction cells made with RF and MVHF glow discharges at deposition rates of -4-8 Als.

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The growth parameters of the n, i, and p layers are under the constraints of manufacturing feasibility so that they are transferable to manufacturing.

EXPERIMENTAL

. U~ing ~he RF and MVHF glow discharge techniques, Single-Junction n-i-p a-Si:H top and a-SiGe:H middle cells were deposited on bare SS substrates, while a-SiGe:H bottom and a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells were on both AllZnO and AglZnO back reflector coated SS substrates. Indium-Tin-Oxide dots were deposited on the p layer as top transparent contacts. Current-density versus voltage (J-V) characteristics were measured under an AM1.5 solar simulator at 25°C. The middle and bottom cells were also measured using long-pass filters for evaluating the long wavelength performance. Quantum efficiency (QE) was measured in a wavelength range from 300 to 1000 nm at room temperature under short circuit condition. The short circuit current density (J sc) was obtained from the integral of QE curves and AM1.5 spectrum in the corresponding wavelength range. Light­soaking of the triple-junction cells was conducted under the open circuit condition with 100 mW/cm 2 white light at 50°C for -1000 hrs.

RESULTS AND DISCUSSION

We first present the a-Si:H top, a-SiGe:H middle, and a-SiGe:H bottom component cells made by RF at a low rate of -1 Als to establish a baseline for comparison with the high rate cells. The J sc of the single-junction cells was designed to have a desired current mismatch when the component cells are incorporated into the a-Si:H/a­SiGe:H/a-SiGe:H triple-junction structure. Table I lists the J-V characteristics of the optimized component cells. One notes that the cell performance is of high quality for each type of cell. For example, the a-Si:H top cell shows an open-circuit voltage (Voc) of 1.016 V and a fill factor (FF) of 0.775. The maximum powers (P max ) are >4 mW/cm 2 and >3 mW/cm 2 for the middle and bottom cells measured under the AM1.5 solar simulator with 530-nm and 630-nm long pass filters, respectively. These cell characteristics will be used as references for high rate cells.

A high efficiency multi-junction solar cell not only needs optimized component cells, but also optimized tunnel-junctions. Because the tunnel-junction is a reverse junction and the current continuity is established by tunneling assisted recombination, a non-optimized tunnel­junction causes electrical loss in Voc and FF due to high

Table I. J-V characteristics of RF low rate component cells for incorporation into a-Si:H/a-SiGe:H/a-SiGe:H triple­junction solar cells The a-SiGe'H bottom cell was on an Al/ZnO coated SS substrate

Run Type Light Voe FF J sc Pmax No. M (mAlcm 2) (mW/cm2)

9082 Top AM1.5 1.016 0.775 7.24 5.70 9073 Middle >530 nm 0.764 0.663 8.30 4.20 9145 Bottom >630 nm 0.615 0.628 8.15 3.15

Table II. J-V characteristics of a-Si:H/a-SiGe:H/a-SiGe:H triple junction solar cells with the intrinsic layers made by RF on AllZnO back reflectors at a low rate of -1 Als The bolded numbers are used as J se

Run Voc FF QE (mAlcmL) Eff Rs Tunnel No. M Total Top

9066 2.354 0.694 21.09 6.89 9096 2.349 0.734 21.29 6.83 9112 2.359 0.745 21.24 7.05

series resistance (Rs). In addition, electron-hole pairs generated in the tunnel junctions do not contribute to the photo-current, therefore, an absorptive tunnel-junction

21.0

__ 20.5 NE ~ 1 20.0

w a 19.5

19.0

25

20 -N

5 15 E .c ~ 10

r2 5

0

C

C

T-C o T-O Tunnel junction

T-C o T-O Tunnel junction

Figure 1. (top) total J sc from QE; (bottom) Rs of a­Si:H/a-SiGe:H double-junction cells with different tunnel-junctions, where C, T-C, 0, and T-O represent conventional, thick conventional, optimized, and thick optimized tunnel-junction layers, respectively.

causes an optical loss in J se .

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Mid. Bott. (%) (Q·cm 2) junction

7.00 7.21 11.26 34.3 Conventional 6.92 7.54 11.78 27.3 Optimized 6.95 7.21 12.21 26.2 Optimized

Using the optimized component cells presented in Table I, we proceeded to optimize the tunnel-junction in an a-Si:H/a-SiGe:H double-junction structure on Al/ZnO coated SS substrates. Figure 1 shows the comparison of the total J sc calibrated from QE measurement and Rs of four cells with different tunnel-junctions. One can see that for the cells with the conventional tunnel-junction, Rs is high, -22 Q·cm 2. Rs can be lowered to 17 Q·cm 2 by increasing the thickness of the junction, but this causes a large J se reduction as illustrated in Fig. 1 (top). With the optimized tunnel-junction, Rs can be reduced to -16 Q·cm 2, and the J se remains its highest value. In order to check the optical loss on tunnel-junctions, we plot the quantum efficiency curves of two samples in Fig. 2: one with the conventional tunnel-junction and one with the optimized tunnel-junction. It appears that the optimized tunnel-junction reduces the optical loss in the wavelength

0.9 -+-C(top)

0.8 --C(Bottom)

0.7 - - • C (total)

-0 (Top)

0.6

W 0.5

a 0.4

0.3

0.2

0.1

0.0 300 400 500 600 700 800 900 1000

Wavelength (nm)

Figure 2. Quantum efficiency of two double-junction cells with conventional (C) and optimized (0) tunnel­junctions.

region of 450-700 nm. We attribute the enhanced QE

Table III. J-V characteristics of RF high rate component cells for a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells. The a-SiGe:H bottom cell was on an AllZnO coated SS substrate

Run Type Light Voe FF J sc Pmax No. M (mAlcm 2) (mW/cm 2)

9289 Top AM1.5 0.981 0.742 7.02 5.11 9248 Middle >530 nm 0.747 0.640 7.11 3.40 9277 Bottom >630 nm 0.579 0.625 8.21 2.97

Table IV. J-V characteristics of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells with the intrinsic layers made by RF at a rate of 4-6 Als on AllZnO back reflector The bolded numbers are used as J sc

Run Voe FF No. (V) Total

9335 2.319 0.744 19.87 9366 2.313 0.745 20.19 9412 2.324 0.742 20.18

values to the lower absorption in the optimized tunnel­junction than in the conventional ones.

We used the optimized component cells and tunnel­junctions to make a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells. The efficiency was significantly improved as listed in Table II, where the bolded numbers were used as J se for the efficiency calculation. The best efficiency for an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cell with conventional tunnel-junction was 11.3%. Using the optimized tunnel-junction, the Rs was reduced, and the FF was increased from 0.694 to 0.734. As a result, the efficiency was increased to 11.8%. By optimizing other parameters, we have achieved an initial active-area efficiency of 12.2%, also listed in Table II.

Second, we present the a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells made by RF at high rates on AllZnO coated SS substrates. As reported previously [2], the quality of high rate deposited a-Si:H and a-SiGe:H is generally poorer than the low rate materials, presumably resulting from high ion bombardments and a short relaxation time for the impinging species before being incorporated into the films. Therefore, the optimization of high rate deposition is focused on substrate temperature and hydrogen dilution for enhancing the surface mobility of the impinging species, and the RF power and pressure for reducing the ion bombardments. The deposition time of the high rate intrinsic layer was 6 times shorter than the low rate intrinsic layer. Table III listed the typical J-V characteristics of single-junction cells made at a high rate. A 530-nm and a 630-nm long-pass filters were used to measure middle and bottom cells, respectively. Although the component cells do not perform as well as the low rate cells listed in Table I, they show reasonable characteristics for high rate solar cells and are close to the high rate solar cells made using MVHF glow discharge [6).

By incorporating the high rate RF component cells into a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures, we have achieved an initial active-area efficiency of -11.4% as listed in Table IV. At the same rate, using a Ag/ZnO coated SS substrate, we have achieved an initial

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QE (mAlcm"') Top Mid. 6.63 6.45 6.51 6.60 6.62 6.71

2

1 (a)

0

-1 .-. .... S -2

Col < -3

S-4 .., -5

-6

-7

-8

-0.5 0.0

1.0

(b)

0.8

0.6 r.ol 01

0.4

0.2

300 400

Bott. 6.79 7.08 6.85

Eft (%)

11.13 11.22 11.42

Eff.=12.5 %

1sc=7.17 mAlcm2

Voc=2.343 V FF=O.744

0.5 1.0 1.5

Voltage (V)

2.0

21.65 mAlcm2

500 600 700 800 900

Wavelength (nm)

2.5

1000

Figure 3. (a) J-V characteristics and (b) quantum efficiency of a triple junction cell with the intrinsic layers made by RF at 4-6 Als on a Ag/ZnO back reflector coated SS.

Table V. J-V h II d b MVHF 6 8 AI Th b c aractenstlcs 0 component ce s ma e )y at - s. e ottom ce IS on f II' AllZnO BR. Run Type Light Voe FF J se Pmax No. M (mA/cm2) (mW/cm 2)

9585 Top AM1.5 1.000 0.712 7.37 5.25 9579 Middle >530nm 0.720 0.646 7.69 3.58 9593 Bottom >630nm 0.623 0.623 7.34 2.85

Table VI. J-V characteristics of a-Si:H/a-SiGe:H double-junction solar cells made by MVHF at 6-8 Als on Al/ZnO back reflector The bolded numbers are used as J sc

Run Voe FF QE (mA/cm«) Efficiency No. M Total Top Bottom (%)

16071-32 1.672 0.716 19.48 9.40 10.08 11.25 16072-32 1.679 0.719 19.24 9.42 9.82 11.37 16073-33 1.602 0.693 20.54 9.84 10.70 10.92

Table VII. J-V characteristics of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells with the intrinsic layers made by MVHF at 6-8 Als on Al/ZnO back reflector The bolded numbers are used as J sc

Run Voe FF No. M Total

9606 2.235 0.713 20.97 9610 2.295 0.720 20.07 9614 2.252 0.728 21.12

active-area efficiency of 12.5%. Figure 3 shows its J-V characteristics and quantum efficiency.

For MVHF deposition, we started with an a-Si:H/a­SiGe:H double-junction structure. The deposition rate of intrinsic layers is higher than RF high rate ones. The deposition time for both the top and bottom intrinsic layers is 4 minutes. For the a-Si:H/a-SiGe:H/a-SiGe:H triple­junction structure, the deposition times of top, middle, and bottom intrinsic layers are 2.5, 4, and 4 minutes, which are 50% shorter than those made by the RF at a high rate. The doped layers and tunnel-junctions are the same as RF high rate cells. For the intrinsic layers, the substrate temperatures and deposition pressures of two types of cells are the same, but the hydrogen dilution ratio of active gas is much higher in MVHF cells than in RF high rate cells. This is because VHF plasma has higher disassociation rates of reaction gas and lower ion bombardment on the surface of the growing film [4]. Therefore, even if high hydrogen dilution is used, a high deposition rate can still be maintained. Tables V, VI, and VII list the typical J-V characteristics of single-junction, double-junction, and triple-junction cells made by MVHF. At this moment, the performance of cells is similar to that of high rate RF cells. Figure 4 shows the J-V characteristics and quantum efficiency of the best triple­junction solar cell made by MVHF at 6-8 Als on Al/ZnO coated SS substrates. Further optimization of component cells and current mismatches is on going.

QE (mA/cm«) Top Mid. 7.21 7.23 6.72 6.73 6.87 7.02

2

(a) 0

-1 -. .... a -2

Col :;;: -3 a ;:;'-4

-5

-6

-7

-8

-0.5 0.0

0.9

0.8 (b)

0.7

0.6

rOil 0.5

0' 0.4

0.3

0.2

0.1

0.0 300 400

Eff Bott. (%) 6.53 10.4 6.62 10.9 7.23 11.3

EiI=11.26%

Voc=2.252 V FF=O.728

J8c=6.87 rnA/cm 2

0.5 1.0 1.5 2.0

Voltage (V)

21.12 mA/cm2

500 600 700 800 900

Wavelength (nm)

2.5

1000

Next, we present the stability of solar cells made by different techniques and at different rates. As previously mentioned, RF high rate a-Si:H and a-SiGe:H materials show more light-induced defects than the corresponding low rate materials, hence the high rate solar cells degrade more than the low rate solar cells after prolonged light Figure 4. (a) J-V characteristics and (b) quantum

efficiency of a triple junction cell with the intrinsic layers made by VHF at 6-8 Als on an AllZnO back reflector.

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soaking [5]. On the other hand, MVHF high rate cells showed a weak dependence of the cell performance and

25.--------------------------, 23

_ 21

'#. 19 -= 17 .~ 1; 15 ~ E 13 ifll Q 9 •

7

5+---~~--~----~----~--~

f VHF

o 2 4 6 8 10

Deposition rate (A/s)

Figure 5. Degradation of the efficiency as a function of deposition rate for a-Si:H/a-SiGe:H/a-SiGe:H triple­junction cells. Connection lines are used only as guides.

the light-induced degradation on the deposition rate in the range of 5-14 Als [6]. To compare the stability of triple­junction cells, we carried out light soaking experiments on triple-junction solar cells made by RF and MVHF at both low and high rates on Al/ZnO coated SS substrates.

Again, the stable status was reached by light-soaking samples under 100 mW/cm 2 white light at 50°C for - 1000 under 100 mW/cm 2 white light at 50°C for -1000 hrs. Figure 5 shows the efficiency degradation as a function of the deposition rate for triple-junction solar cells. One can see that, for RF cells, the degradation increases with increasing deposition rate, which is consistent with the previous results [5]. For VHF cells, the deposition rate is 1.5 times deposition rate of RF high rate cells. However, the degradation is much smaller than that of RF high rate ones as shown in Fig. 5.

Table VIII lists the cell performance of triple-junction cells at their initial and stable statuses. The results show that the degradation is -15-17% for the triple-junction cells made by RF at the low rate, and - 22% for the cells made by RF at the high rate. The best stabilized efficiency we have achieved is 10.1 % for the triple-junction solar cells made at the low rate, and 8.6% for the cells made at the high rate. For MVHF triple cells, the degradation is 6-11%,

Table VIII. Stability results of a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells made by RF and MVHF on AllZnO coate d SS b b hid h· h Th b Id d b d ~ J I I ffi . su strates at ot ow an Igi rates. e 0 e num ers were use or sc to ca cu ate e IClency.

Sample Status Voc FF QE (mA/cm") Efficiency Comments No. M Total Top Middle Bottom (%)

9096 Initial 2.353 0.739 21.69 6.83 6.92 7.54 11.9 Stable 2.258 0.677 20.75 6.61 6.72 7.42 10.1 RF low rate DeQ. 4.0% 8.4% 4.3% 3.2% 2.9% 1.6% 15.1% (1 Als)

9112 Initial 2.359 0.745 21.24 7.05 6.95 7.21 12.2 Stable 2.256 0.674 20.61 6.64 6.79 7.18 10.1 DeQ. 4.4% 9.5% 2.9% 5.8% 2.3% 0.4% 17.2%

9188 Initial 2.204 0.713 20.01 6.62 6.61 6.78 10.4 Stable 2.096 0.631 19.39 6.52 6.27 6.60 8.3 RF high rate DeQ. 4.9% 11.5% 3.1% 1.5% 5.1% 2.7% 20.3% (4-6 Als)

9391 Initial 2.293 0.734 20.13 6.57 6.50 7.06 10.9 Stable 2.196 0.635 19.27 6.28 6.15 6.85 8.57 DeQ. 4.2% 13.5% 4.3% 4.4% 5.4% 3.0% 21.7%

9610 Initial 2.295 0.720 20.07 6.72 6.73 6.62 10.9 Stable 2.256 0.691 19.77 6.58 6.63 6.57 10.2 VHF high rate DeQ. 1.7% 4.0% 1.5% 2.1% 1.5% 0.8% 6.4% (6-8 Als)

9614 Initial 2.252 0.728 21.12 6.87 7.02 7.23 11.3 Stable 2.211 0.680 20.79 6.71 6.90 7.18 10.1 DeQ. 1.8% 6.6% 1.6% 2.3% 1.7% 0.7% 10.6%

Table IX. Stable total-area cell efficiencies of a-Si:H/a-SiGe:H double-junction cells made by MVHF at 6-8 Als as mea sured at United Solar Ovonic (USO) and National Renewable EnerQV Laboraton (NREL).

~ 16071-32 16072-32 16073-22 Measurements

USO 9.26 9.27 9.09

NREL 9.22 9.67 9.60

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and the highest stabilized efficiency is -10.2%. Although the initial efficiency of MVHF high rate cells is similar to that of RF high rate one, the stabilized efficiency is much higher, indicating that the MVHF high rate cells are superior to the RF high rate cells in terms of long term stability.

The MVHF a-Si:H/a-SiGe:H double-junction cells were also light-soaked. The degradation is -16%, which is higher than its corresponding triple-junction cells. We believe that this is mainly due to the thicker top cell intrinsic layer in double-junction cells than in triple cells. The stabilized cells were sent to the National Renewable Energy Laboratory (NREL) for efficiency confirmation. Table IX lists total-area cell efficiencies measured at United Solar Ovonic (USO) and NREL. The highest stabilized total-area efficiency is 9.7% for an a-Si:H/a­SiGe:H double-junction cell measured at NREL.

SUMMARY

We have optimized a-Si:H/a-SiGe:H/a-SiGe:H triple­junction cells at a high deposition rate of 4-8 Als using both RF and MVHF glow discharges. Preliminary results show initial active-area efficiencies of 11.4% on AllZnO and 12.5% on AglZnO coated SS substrates, respectively for RF high rate cells. For MVHF cells, we have achieved an initial active-area efficiency of 11.3% at an even higher rate. The light-induced degradation of the RF high rate deposited triple-junction cells is around 22%. On the other hand, the MVHF high rate cells show much less degradation (6-11%) than the RF high rate cells. As a result, an active-area stabilized efficiency of -10.2% has been achieved in a cell made at -8 Als on an AllZnO coated SS substrate. This result is promising for moving manufacturing towards a higher throughput.

ACKNOLEDGEMENTS

The authors thank J. Noch, E. Chen, and L. Sivec for sample preparation and measurements. This work was supported by Solar America Initiative (SAl) program with contract No. DE-FC36-07 GO 17053.

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[3] S. Guha, X. Xu, J. Yang, and A Banerjee, "High Deposition Rate Amorphous Silicon-Based

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[4] B. Yan, J. Yang, S. Guha, and A. Gallagher, "Analysis of Plasma Properties and Deposition of Amorphous Silicon Alloy Solar Cells Using Very High Frequency Glow Discharge", Mater. Res. Soc. Symp. Proc. 557, 1999, pp. 115-120.

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