Fabrication of Large Area Amorphous Silicon/Nanocrystalline Silicon Double Junction Solar Cells

Gautam Ganguly, Guozhen Yue, Baojie Yan, Jeff Yang, and Subhendu GuhaUnited Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI 48084

ABSTRACT

We have developed n-i-p type nc-Si:H solar cells using a large area RF plasma deposition at a rate of 3Å/s. We have used the knowledge that ion bombardment and increasing temperatures cause defects in the nanocrystalline material. We therefore used higher pressures to reduce ion energy and lower temperatures to reduce defects. In order to maintain deposition uniformity over large areas at the higher pressures, we reduced the inter-electrode spacing (Paschen’s Law). We have thus been able to obtain initial, aperture area (420cm2) efficiency of 11.8% with Ag/ZnO back reflectors. After laminating and light soaking these large cells, we obtained a stable, aperture area efficiency of 9.5%.

INTRODUCTION

The study of the growth mechanism and characteristics of hydrogenated nanocrystalline silicon (nc-Si:H) materials revealed the importance of the ratio of hydrogen to silicon hydride species arriving at the growth surface, as well as the effect of ion bombardment, to the formation of nanocrystalline material [1]. Later, the effect of growth conditions on the defect density was investigated [2]. The interest in making nanocrystalline silicon cells however did not take off till a reasonable efficiency of 7% was obtained [3]. Since then, several groups have been active in developing a-Si:H/ nc-Si:H double junction and a-S:Hi/ nc-Si:H / nc-Si:H triple cell structures as an alternative to the use of amorphous silicon germanium as a bottom/middle cell material. It soon became clear that the highest efficiency could be obtained by maximizing Voc while staying on the nc-Si side of the ‘nc-Si:H to mixed phase transition’ region. We have thus optimized nip type nc-Si:Hcells with a deposition rate of 3Å/s for the intrinsic layer to obtain an efficiency of 7.5%. We have then used this as a bottom cell to make a-Si:H/nc-Si:H double junction solar cells and obtained an efficiency of 13.6% for 0.25 cm2

active area cells, which stabilized at 12.4%. Using larger substrates with Ag/ZnO back reflectors we were able to obtain initial aperture area (420cm2) efficiency of 11.8%. After laminating and light soaking these large cells, we obtained a stable, aperture area efficiency of 9.5%.

EXPERIMENTAL

The nc-Si single junction and a-Si/nc-Si double junction cells were fabricated in a multi-chamber system (2B) using RF glow discharge decomposition of the relevant gases. The smaller cells (0.25 cm2) were defined using indium tin oxide (ITO) and a gold grid was evaporated on top. The IV characteristics were obtained under a Xenon lamp solar simulator with the cells set on a temperature controlled

stage set at 25°C, and the short circuit current density (Jsc)was obtained from quantum efficiency measurements. The larger (45 and 420cm2) cells were defined by etching after ITO deposition, the cells were wired and bus bars applied prior to lamination. The larger cells were measured at United Solar on a Spire 240A pulsed lamp simulator and sent to the National Renewable Energy Laboratory (NREL) for certified measurements. Light soaking was done using ~100mW/cm2 of white light with the temperature held at 50°C.

RESULTS AND DISCUSSION

The use of higher pressure has been shown to reduce the defect density and increase the grain size of nanocrystalline material [2]. We have successfully adapted this idea to the fabrication of nc-Si:H at higher rates as illustrated in Table I. The data illustrate that all cell parameters improve with increasing pressure and lead to higher efficiency.

Table I: Cell parameters for 0.25 cm2 nc-Si:H cells deposited at different pressures.

Cell # Pressure Voc(V)

FF Jsc(mA/cm2)

Eff.(%)

10138 Low 0.336 0.407 18.0 2.510153 Medium 0.460 0.644 20.9 6.210997 High 0.503 0.673 22.1 7.5

In order to improve the current further, we optimized the back reflector using the nc-Si:H bottom cell as the test device. Table II shows the performance of nc-Si:H cells on

Table II. 0.25 cm2 a-Si:H/nc-Si:H cells deposited on 6 different Ag/ZnO BR.

Cell# Cell BRVoc(V) FF

Jsc (mA/cm2)

Eff (%)

10897 23 1 0.443 0.661 23.6 6.910909 32 1 0.455 0.643 24.1 7.110898 32 2 0.443 0.65 23.2 6.710912 32 2 0.451 0.669 23.2 7.010899 32 3 0.445 0.654 23.2 6.710914 22 3 0.437 0.622 22.9 6.210759 32 4 0.422 0.621 22.4 5.910761 32 4 0.425 0.621 22.2 5.910902 32 5 0.444 0.622 23.4 6.510910 32 5 0.453 0.666 23.3 7.010903 33 6 0.448 0.662 23.1 6.910908 32 6 0.448 0.66 23.2 6.9

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V (V)

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Voc (V) 1.429 1.423 FF 0.783 0.726Jsc (mA/cm2) 12.15 12.01η (%) 13.6 12.4

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2B 10949 LC1 (#43)Initial/Light Soaked

Jsc(QE)=12.61/12.01mA/cm2

Jsc(QE)=12.15/12.01mA/cm2

Jsc(QE)=24.76/24.02mA/cm2

Fig. 1 The initial and post light soaking IV-characteristics (top) and QE spectra for one a-Si:H/nc-Si:H double junction cell of area 0.25 cm2.

different Ag/ZnO back reflectors. We see a significant improvement in the value of Jsc by ~2mA/cm2. Using this improved nc-Si:H cell as a bottom cell, we fabricated a-Si:H/nc-Si:H double junction devices. The initial and stable performance of the best 0.25 cm2

device is shown in Fig. 1. We see that the initial performance reaches 13.6% and the stable performance is over 12%. The cell parameters all degrade, resulting in a 10% overall loss of performance after light soaking. From the QE curves it is apparent that Jsc losses occur primarily in the top cell.

We found that for uniformity over larger areas, the conditions that yield the best performance at the center yield mixed phase material at the edges and are therefore not suitable for making larger area cells. We have found that improving the temperature uniformity over the cell deposition area did not significantly change the performance uniformity. We therefore attribute the uniformity limitation to that of the RF intensity distribution.In order to obtain uniform performance over larger areas one approach is to move away from the ‘nanocrystalline

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Fig. 2. The uniformity of performance on a-Si:H/nc-Si:H double junction cells over an area of 645 cm2.

Table III: Initial and light soaked performance of 45 cm2 a-SiH/nc-Si:H double junction cells.Sample # State Temp

(°C)Voc(V)

FF Jsc(mA/cm2)

Pmax(W)

η (%)

ηT-corr(%)

10490F2 Initial 26.6 1.425 0.741 11.40 0.542 12.03 12.07600h 25.4 1.403 0.688 11.25 0.489 10.86 10.87

10490G3 Initial 26.9 1.437 0.735 11.30 0.536 11.92 11.97600h 25.9 1.418 0.671 11.39 0.488 10.84 10.85

10491F1 Initial 26.4 1.436 0.742 10.94 0.524 11.65 11.69600h 25.4 1.413 0.692 10.91 0.480 10.67 10.68

10500F1 Initial 26.6 1.430 0.738 11.46 0.544 12.08 12.12600h 25.6 1.409 0.665 11.43 0.482 10.71 10.72

10500F2 Initial 26.9 1.432 0.736 11.64 0.552 12.26 12.31600h 25.6 1.406 0.682 11.44 0.494 10.97 10.98

10500G3 Initial 26.6 1.441 0.743 11.37 0.548 12.18 12.22600h 25.9 1.419 0.686 11.12 0.487 10.82 10.83

to mixed phase’ transition region [4]. This allows one to obtain similar Jsc over larger areas. Before proceeding to make larger area devices, we ascertained the uniformity by measuring 0.25 cm2 cells covering about 645cm2. The results are shown in Fig. 2. The coefficient of variation (COV = standard deviation/mean) is 5.2%.

We then fabricated several medium area (45 cm2) devices covering the area that would be covered by the larger area devices (420 cm2).The results are shown in Table III. We see that for these devices, the Voc and Jscare somewhat larger while the FF is lower so that overall, the aperture area efficiency is similar to the

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Fig. 3. The stabilized IV characteristics of a 420cm2 laminated a-Si:H/nc-Si:H cell.

Table IV: The performance of two large area cells measured on Spire 240A solar simulators at NREL and USOC.

Serial #/ LocationArea (cm2)

Temp (°C)

Voc(V)

Isc(A)

Jsc(mA/cm2) FF

Pmax(W)

Vmp(V)

η (%)

ηTcorr(%)

2B-2/ USOC 420.0 27.6 1.400 4.362 10.39 0.641 3.917 1.067 9.33 9.382B-2 /NREL 411.8 25.0 1.461 4.009 9.74 0.648 3.797 1.137 9.222B-3/USOC 420.0 27.6 1.382 4.419 10.52 0.624 3.807 1.063 9.06 9.122B-3 /NREL 418.0 24.9 1.455 4.048 9.68 0.648 3.815 1.142 9.13

small area (0.25 cm2) devices. The differences in the cell parameters may be attributed in part to the different spectra for the solar simulators used to measure them. The performance is very uniform (COV=1.9%).We then used these conditions to make larger area (420cm2) devices. The performance of the best device was 11.8% initially. After lamination and light soaking for 500h, the IV-characteristics of the cell with the best stable performance (9.5%) is shown in Fig. 3. We sent several large area cells to NREL and a comparison of measurements at the two locations on the Spire 240A solar simulator is shown in Table IV. The differences are within 2%.

SUMMARY

We have fabricated a-Si:H/nc-Si:H cells on large area substrates using conditions identified to reduce defects in nanocrystalline material. The best initial efficiencies obtained are 13.6% (0.25 cm2) 12.3% (45 cm2), and 11.8% (420 cm2). The best stable performance of a 0.25cm2 cell is 12.4%, of a 45 cm2 cell is 11% and of a laminated, 420 cm2 cell is 9.5%.We expect further improvement in the performance of large area cells from better RF field intensity distribution.

ACKNOWLEDGEMENTS

This work was supported in part by NREL Subcontract #ZXL-6-44205-14.

REFERENCES

[1] A. Matsuda, J. Non-Cryst Solids, 59/60, 1983, 767.

[2] G. Ganguly, M. Fukawa, T. Ikeda and A. Matsuda, Mater. Res. Soc. Symp. Proc.467, 681 (1997); S. Klein, F. Finger, R. Carius, B. Rech, L. Houben, M. Luysberg, and M. Stutzmann, Mater. Res. Soc. Symp. Proc. 715, 2002, A26.2.1.

[3] J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, Ch. Hof, D. Fischer, H. Keppner, and A. Shah, Mater. Res. Soc. Symp. Proc.420, 3 (1996); K. Yamamoto, T. Suzuki, M. Yoshimi, and A. Nakajima, Proc. 25th IEEE PVSC, 1996, 661.

[4] J. Yang, K. Lord, B. Yan, A. Banerjee, S. Guha, D. Han and K. Wang, Mater. Res. Soc. Symp. Proc. 715, 2002, 601.

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