IMPROVEMENT OF a-Si:H AND nc-Si:H MULTI-JUNCTION SOLAR CELLS BY OPTIMIZATION OF TEXTURED BACK REFLECTORS

Guozhen Yue, Baojie Yan, Laura Sivec, Jessica M. Owens, Sherry Hu, Xixiang Xu, Jeffrey Yang, and Subhendu Guha

United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI 48084

ABSTRACT

The effect of the texture of Ag/ZnO back reflector (BR) on nc-Si:H single-junction solar cell performance has been investigated systematically. Using a high textured BR, a 74% gain in short circuit current density (Jsc) was obtained over a cell made using the same recipe on specular stainless steel. However, the texture reduced the fill factor (FF) from 0.73 to 0.54. Dark current versus voltage measurements showed a significant increase in reverse saturate current when the texture is increased, indicating a poor nc-Si:H material quality in the nc-Si:H cells deposited on highly textured Ag/ZnO BR. In order to maintain both high Jsc and FF, we have optimized the BR texture. With the improved BR, we have achieved initial efficiencies of 9.5% in a nc-Si:H single-junction and 13.4% in an a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells made at 10 Å/s.

INTRODUCTION

Light trapping techniques using textured back reflectors has been widely used to increase the short circuit current density (Jsc) in thin film solar cells, especially for hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) based multi-junction structures. We have developed advanced Al/ZnO and Ag/ZnO back reflectors for high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells [1] and achieved high efficiencies in small-area solar cells and large-area solar products [2]. nc-Si:H solar cells have received a great deal of attention because of the potential of forming high stablility solar modules[3]. Since nc-Si:H cells are usually much thicker than a-Si:H and a-SiGe:H solar cells, and the growth of nc-Si:H depends on the surface morphology, the back reflector developed for conventional a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures may not necessarily be the best choice for nc-Si:H solar cells. Therefore, we have carried out optimizations of Ag/ZnO back reflectors for nc-Si:H cells. Our previous studies mainly focused on the current enhancement [4,5]. In this paper, we systematically studied the effect of surface texture on the nc-Si:H material quality, which is closely related to the FF of the solar cell.

EXPERIMENTAL

We deposited nc-Si:H n-i-p single-junction and a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells using a modified very high frequency glow discharge system. Bare

stainless steel (SS) and Ag/ZnO coated SS were used as substrates. The surface textures of the Ag and ZnO layers were controlled by deposition parameters. The surface morphology was characterized by atomic force microscopy (AFM). Angular distributions of scattered light from various back reflectors were measured with a He-Ne laser. Light current-voltage (J-V) characteristics of solar cells were measured under an AM1.5 solar simulator at 25°C. Dark J-V characteristics of cells were measured in a vacuum chamber using a programmable multi-meter at controlled temperatures. Quantum efficiency (QE) curves of solar cells were measured in the wavelength range from 300 to 1200 nm. The integrals of QE curves with AM1.5 spectrum were used to calculate Jsc.

RESULTS AND DISCUSSION

As previously reported [4,5], the light scattering from textured substrates is the main source of light trapping in solar cells. AFM images provide a direct measurement of the surface texture. Figure 1 shows a comparison of two

Figure 1. AMF images of two Ag/ZnO back reflectors. The top one (R8839) has a root-mean-square (RMS) of 17 nm and the bottom one (R2283) has a RMS of 74 nm.

978-1-4244-2950-9/09/$25.00 ©2009 IEEE 000327

Figure 2. (a) Angular distribution of scattered light from the two back refelectors shown in Fig.1, and (b) integrated scattered light intensity and short circuit current desnity (Jsc) as a function of the roughness (RMS) of back reflectors. Ag/ZnO back reflectors deposited under different conditions, where R8839 is with a lower texture and R2283 with a high texture as defined by root-mean-square (RMS) values.

Figure 2(a) shows the angular distributions of the two

back reflectors, where the He-Ne laser illuminated normal to the substrate (90 degree). It clearly shows that the higher surface texture results in more scattered light as expected. Figure 2(b) shows the Jsc and the integrated scattered light intensity as a function of the RMS value of the substrate, where integration has taken into consideration the total internal reflection and the contribution from solid angles. The integrated scattered light intensity increases with RMS value up to around 40 nm and then levels off. The Jsc shows a similar trend as the scattered light intensity. Compared to the nc-Si:H solar

cells deposited on flat SS, the Jsc increased by ~74% when a highly textured back reflector was used. These results show that there is reasonable correlation between back reflector texture, light scattering, and photocurrent of nc-Si:H solar cells.

The negative effects of textured back reflectors on nc-Si:H solar cells are reductions in FF and open circuit voltage (Voc). Table I lists the J-V characteristics of nc-Si:H solar cells deposited on various substrates. One can see that the Jsc increases with the substrate texture, but the FF and Voc drop significantly. A FF as high as 0.73 is achieved for the cell made on the bare SS with an RMS value of 6.0 nm, whereas it decreases to 0.54 when the RMS value increases to 73 nm. In the meantime, Voc decreases from 0.56 to 0.48 V, and Jsc increases from 15 to 26 mA/cm2.

Table I. J-V characteristics of nc-Si:H solar cells on different Ag/ZnO back reflectors. The RMS represents the roughness of the substrates. Sample 17108 was deposited on bare SS, and others on Ag/ZnO with different textures.

Run No.

Voc (V)

FF Jsc

(mA/cm2) Eff (%)

RMS (nm)

17108 0.561 0.729 14.85 6.07 6.0 17154 0.543 0.653 21.76 7.72 19.5 17180 0.544 0.640 21.06 7.33 17.0 17157 0.559 0.627 23.82 8.35 33.2 17156 0.537 0.646 23.82 8.26 42.6 17153 0.484 0.544 25.86 6.81 73.8

Two major factors have been indentified as possible reasons for the reduced FF and Voc of the cell on Ag/ZnO substrate. First, the light-intensity dependence could be a potential cause. Due to the light trapping effect, the light will undergo many paths, increasing the light absorption in nc-Si:H cells. This leads to a large quasi-Fermi level splitting, and increases the recombination probability of photo-excited carriers. In addition, the scattered light from the textured ZnO goes through the n layer first, which may potentially reduce FF due to the difference of electron and hole mobilities. Second, nc-Si:H materials deposited on textured surfaces may have a high defect density due to crystallite collisions. To clarify the first issue, we made a solar cell on a Cr coated Ag/ZnO substrate. Table II lists the light J-V characteristics of the three solar cells made with the same recipe but on different substrates of SS, ZnO/Ag/SS, and Cr/ZnO/Ag/SS. The Cr is very thin, ~20 nm, and has a nearly conformal surface texture with ZnO. Because of the low light reflectivity of Cr, most of the light goes through only one path, similar to the cell on SS. As can be seen in the table, the Jsc of the cell on Cr/ZnO/Ag/SS is similar to the cell on SS, however, the Voc and FF are still much smaller than the cell on SS. Therefore, the first possibility can be ruled out.

978-1-4244-2950-9/09/$25.00 ©2009 IEEE 000328

Table II. J-V characteristics of nc-Si:H single-junction cells made on SS, ZnO/Ag/SS, and Cr/ZnO/Ag/SS substrates.

Dark J-V measurement is a very useful technique to

characterize carrier recombination in a-Si:H and nc-Si:H solar cells. The carriers recombine through defects, and the forward dark current is directly associated with the properties of the defect density [6, 7]. To study the effect of the substrate texture on the material properties, we carried out dark J-V measurement on these three cells. As shown in Fig. 3, the dark J-V curves show typical diode characteristics as described by J=J0[exp(qV/nkT)-1], where J0 is the reverse saturation current density, q the unit charge, T the measurement temperature, k the Boltzmann constant, and n the diode quality factor. The dark J-V curves for the cells on Ag/ZnO and Cr coated Ag/ZnO substrates completely overlap, indicating a similar material property, both having a larger dark current than the cell on SS. By fitting the data, we found that n is around 1.25 for the cell on SS, and increases to 1.52 for cells deposited on the textured substrates. The reverse saturation current density J0 increases 30 times from 3.58 ×10-8 to 9.39 × 10-7 A/cm2, implying an enhanced recombination in the material on the textured substrates.

One may notice that the Voc of the two cells

deposited on ZnO/Ag/SS and Cr/ZnO/Ag/SS substrates are also different, although they have similar dark J-V characteristics. To clarify this, we measured the light intensity dependence of the solar cell performance. The light intensity was varied by neutral density filters. The result shows that Jsc has a linear relationship with the light intensity in the range of 3 to 100 mW/cm2. In Figure 3 (b), we plotted Voc as a function of Jsc. One can see that the data fit very well with typical logarithmic relationship of Voc

~ kT ln(Jsc/J0)/q. The lines of Voc versus Jsc from cells made on ZnO/Ag/SS and Cr/ZnO/Ag/SS are almost identical. Their Voc values are 50-70 mV smaller than the cell on SS for the same Jsc values. This result is in agreement with the dark J-V measurement. The large difference in Voc values between the cells on ZnO/Ag/SS and Cr/ZnO/Ag/SS measured under AM1.5 illumination is mainly from the difference of photocurrent density, while the difference in Voc values between the cells on SS and textured ZnO/Ag/SS mainly arises from the different nc-Si:H properties. Light intensity dependent measurement also shows that the FF of the cell on the ZnO/Ag/SS substrate is smaller than that on the Cr/ZnO/Ag/SS substrate for the same Jsc value. Two factors could contribute to the observation. First, the series resistance is different, and the second, the recombination centers

Figure 3. (a) Dark J-V characteristics of three nc-Si:H solar cells made on SS, Ag/ZnO, and Ag/ZnO/Cr coated SS, respectively. (b) Voc of three solar cells as a function of their Jsc under different illuminated light intensities. could be different when the cells are subjected to different illuminations.

It is worthwhile to mention that the difference of the Voc and FF for nc-Si:H cells made on flat SS and textured BR substrates depends on the intrinsic layer thickness. Degradation of the solar cell performance caused by the use of a thick nc-Si:H intrinsic layer on textured BRs might be due to the following reason. Grains in nc-Si:H deposited on a textured BR have a tendency to grow perpendicular to the local substrate surface, and they collide with each other when the film exceeds a certain thickness [8]. The nanocrystallite collision results in poor grain boundaries, high defect density, and poor cell performance. Therefore, a thin nc-Si intrinsic layer before collisions can take place may have cell performance as good as cells made on flat SS. In order to confirm this conjecture, we made two sets of nc-Si:H cells with thicknesses varied from 0.2 to 1.5 µm, which were realized by changing the deposition time only. One set is on SS,

Run No.

Voc (V)

FF Jsc

(mA/cm2)

Eff (%)

Subst.

17108 0.561 0.729 14.85 6.07 SS 17110 0.541 0.600 23.18 7.52 ZnO/Ag/

SS

17109 0.514 0.664 14.78 5.04 Cr/ZnO/Ag/SS

978-1-4244-2950-9/09/$25.00 ©2009 IEEE 000329

Table III. nc-Si:H cells with different intrinsic layer thicknesses made on SS and Ag/ZnO substrates. FFb and FFr were measured using the blue and red lights with low light intensities to study the interface and bulk properties, respectively.

FF Run No.

Voc (V) AM1.5 FFb FFr

Q(>610nm) (mA/cm2)

Pmax (mW/cm2)

Thickness (µm)

Substrates

17797 0.523 0.713 0.677 0.664 7.47(0.86) 2.79 0.2 17779 0.523 0.717 0.681 0.681 9.48(1.70) 3.55 0.4 17777 0.527 0.720 0.696 0.698 13.06(3.39) 4.96 0.8 17780 0.518 0.691 0.675 0.683 15.75(5.18) 5.64 1.5

SS

17793 0.529 0.692 0.684 0.689 15.8(6.37) 5.22 0.2 17806 0.519 0.674 0.675 0.680 19.07(8.98) 6.67 0.4 17784 0.508 0.658 0.661 0.663 22.47(11.71) 7.51 0.8 17781 0.501 0.598 0.618 0.623 25.20(14.05) 7.81 1.5

BR

0.495

0.500

0.505

0.510

0.515

0.520

0.525

0.530

0.535

0.0 0.5 1.0 1.5 2.0

Voc(V)

Thickness (µµµµm)

On SSOn BR

(a)

0.55

0.57

0.59

0.61

0.63

0.65

0.67

0.69

0.71

0.73

0.0 0.5 1.0 1.5 2.0

FF

Thickness (µµµµm)

On SS

On BR

(b)

Figure 4. (a) Voc and (b) FF as a function of the intrinsic layer thickness for nc-Si:H solar cells deposited on specular SS and textured BR substrates.

and the other is on the textured BR. The growth recipe of the cells is identical for each particular thickness. Table III lists the J-V characteristics of the two sets of solar cells. One can see that the Voc and FF are almost the same for the 0.2 µm thick cell on SS and BR. With increasing the thickness, the Voc and FF of the cells on SS does not change too much, indicating a uniform material quality along the growth direction and low defect density material.

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100

QE

Wavelength (nm)

29.22mA/cm2

Figure 5. The quantum efficiency of a nc-Si:H cells with Jsc of 29.22 mA/cm2. However, the Voc and FF of the cells on BR decrease dramatically with the increase of the nc-Si:H thickness. The Voc decreases from 0.529 to 0.501 V, and the FF from 0.692 to 0.598, when the cell thickness increases from 0.2 to 1.5 µm. In Fig. 4, we plot the Voc and FF as a function of thickness for the two sets of solar cells. Clearly, the difference of both Voc and FF is widened with increased intrinsic layer thickness. This is in agreement with the conjecture mentioned above. From this experiment, we learned that a large opening angle of the ZnO valleys on the substrate surface may delay the nanocrystallite collisions, and is beneficial for the thick nc-Si:H cell performance.

Based on the above results, we have further optimized our Ag/ZnO back reflectors by considering the surface texture of ZnO in terms of total and diffusive reflections, and Ag plasmon absorptions. The nc-Si:H material quality was carefully optimized to obtain high efficiency nc-Si:H single-junction solar cells. Using the optimized BR for a thick nc-Si:H cell, a Jsc of 29.2 mA/cm2 has been obtained with Voc=0.452 V, FF=0.583, and Eff.=7.69%. To our best knowledge, this is one of the highest Jsc values reported for nc-Si:H single-junction cells. Figure 5 shows its quantum efficiency curve. Using

978-1-4244-2950-9/09/$25.00 ©2009 IEEE 000330

Figure 6. (a) J-V characteristics and (b) quantum efficiency of a nc-Si:H single-junction solar cell on Ag/ZnO back reflectors.

the same back reflector, we further optimized the growth recipe to obtain high efficiency. An initial efficiency of 9.5% has been achieved for a nc-Si:H single-junction cells made on Ag/ZnO back reflectors. Figure 6 shows its J-V characteristics and quantum efficiency.

By combining the optimized back reflector and the

improved component cells, we proceeded to make a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells. An initial 13.4% and a stable 12.1% active-area efficiencies have been achieved, where the nc-Si:H intrinsic layers were made at a high deposition rate of 10 Å/s. Figure 7 shows its initial J-V characteristics and quantum efficiency of the triple-junction solar cell.

SUMMARY

We have systematically studied the effect of

substrate texture on the nc-Si:H solar cell performance.

Figure 7. (a) J-V characteristics and (b) quantum efficiency of an a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cell on Ag/ZnO back reflectors.

The Ag/ZnO back reflectors with high textures increases the Jsc by up to 74%, but it reduces the FF and Voc. We found that the reduction in FF and Voc mainly arises from the deterioration of the nc-Si:H material quality. Based on this finding, we have optimized our Ag/ZnO back reflectors by considering the surface texture of ZnO in terms of total and diffusive reflections, and Ag plasmon absorptions. As a result, we have achieved initial efficiencies of 9.5% in a nc-Si:H single-junction and 13.4% in an a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells. The stabilized efficiency of the triple-junction cell is 12.1%.

ACKNOLEDGEMENTS

The authors thank T. Palmer, D. Wolf, K. Younan, and

E. Chen for sample preparation and measurements. The work was supported by US DOE under the Solar America Initiative Program Contract No. DE-FC36-07 GO 17053.

978-1-4244-2950-9/09/$25.00 ©2009 IEEE 000331

REFERENCES

[1] A. Banerjee and S. Guha, “Study of back reflectors for amorphous silicon alloy solar cell application”, J. Appl. Phys. 69, 1991, pp.1030-1035. [2] J. Yang, A. Banerjee, and S. Guha, “Triple-junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies”, Appl. Phys. Lett. 70, 1997, pp. 2975-2977. [3] J. Meier, R. Fluckiger, H. Keppner, and A. Shah, “Complete microcrystalline p-i-n solar cell—Crystalline or amorphous cell behavior?”, Appl. Phys. Lett. 65, 1994, 860-862. [4] B. Yan, J. M. Owens, C.-S. Jiang, J. Yang, and S. Guha, “Improved Back Reflector for High Efficiency Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar Cells”, Mater. Res. Soc. Symp. Proc. 862,

2005, 603-608. [5] B. Yan, G. Yue, C.-S. Jiang, Y. Yan, J. M. Owens, J. Yang, and S. Guha, “Optical Enhancement by Textured Back Reflector in Amorphous and Nanocrystalline Silicon Based Solar Cells”, Mater. Res. Soc. Symp. Proc. E1066 2008, KK13-2.

[6] J. Deng and C.R. Wronski, “Carrier recombination and differential diode quality factors in the dark forward bias current-voltage characteristics of a-Si:H solar cells”, J. Appl. Phys. 98, 2005, pp. 24509-24518.

[7] B. Yan, J. Yang, and S. Guha, “Temperature Dependence of Dark Current-Voltage Characteristics of Hydrogenated Amorphous and Nanocrystalline Silicon Based Solar Cells”, Mater. Res. Soc. Symp. Proc. 910, 2006, pp. 713-718. [8] Y. Nasuno, M. Kondo, and A. Matsuda, “Microcrystalline silicon thin-film solar cells prepared at low temperature using RF-PECVD”, Proceedings of the 28

th

IEEE Photovoltaic Specialists Conference, 2000, pp.142-145.

978-1-4244-2950-9/09/$25.00 ©2009 IEEE 000332