Broadband Enhancement in Thin-Film Amorphous Silicon Solar Cells

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Broadband Enhancement in Thin-Film Amorphous Silicon Solar Cells Enabled by Nucleated Silver Nanoparticles Xi Chen,† Baohua Jia,*,† Jhantu K. Saha,† Boyuan Cai,† Nicholas Stokes,† Qi Qiao,‡ Yongqian Wang,‡ Zhengrong Shi,‡ and Min Gu*,† †

Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, P.O. Box 218, Hawthorn, 3122 Victoria, Australia ‡ Suntech Power Holdings Co., Ltd., 9 Xinhua Road, New District, Wuxi, Jiangsu Province 214028, China S Supporting Information *

ABSTRACT: Recently plasmonic effects have gained tremendous interest in solar cell research because they are deemed to be able to dramatically boost the efficiency of thin-film solar cells. However, despite of the intensive efforts, the desired broadband enhancement, which is critical for real device performance improvement, has yet been achieved with simple fabrication and integration methods appreciated by the solar industry. We propose in this paper a novel idea of using nucleated silver nanoparticles to effectively scatter light in a broadband wavelength range to realize pronounced absorption enhancement in the silicon absorbing layer. Since it does not require critical patterning, experimentally these tailored nanoparticles were achieved by the simple, low-cost and upscalable wet chemical synthesis method and integrated before the back contact layer of the amorphous silicon thin-film solar cells. The solar cells incorporated with 200 nm nucleated silver nanoparticles at 10% coverage density clearly demonstrate a broadband absorption enhancement and significant superior performance including a 14.3% enhancement in the short-circuit photocurrent density and a 23% enhancement in the energy conversion efficiency, compared with the randomly textured reference cells without nanoparticles. Among the measured plasmonic solar cells the highest efficiency achieved was 8.1%. The significant enhancement is mainly attributed to the broadband light scattering arising from the integration of the tailored nucleated silver nanoparticles. KEYWORDS: Broadband enhancement, silicon thin film solar cells, nucleated silver nanoparticles, light scattering

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hin-film amorphous silicon solar cells have stimulated enormous research interest as a cheap alternative to bulk crystalline silicon solar cells.1 However, the significantly reduced thickness of the silicon layer leads to insufficient sunlight absorption and inevitably the low energy conversion efficiency. Therefore light trapping technology is of paramount importance to increase the performance of thin-film solar cells to make them competitive with crystalline silicon solar cells, which are still dominant in the solar cell market. Recently the silver nanoparticle-enabled plasmonic effect has been proposed and demonstrated as a promising approach to achieving light trapping in thin-film solar cells because the relative scattering efficiency of silver nanoparticles is higher than that of other noble metals in the visible range.2−5 The nanoparticles can be used to effectively scatter the incident light into the intrinsic absorbing layer, increasing the optical path length in the thinfilm solar cells. It can also excite the surface plasmon modes to improve the light absorption within the absorbing layer.2,3 However, due to the resonant nature of the plasmonic effect useful absorption enhancements can only be realized at certain resonant scattering wavelengths determined by the nanoparticle size, shape, and their local dielectric environment. Even worse, these enhancements are generally offset by the detrimental particle absorption at other wavelengths therefore hardly leading to any substantial efficiency boost of the solar cells.6 © XXXX American Chemical Society

Therefore, broadband absorption enhancement is the primary challenge to tackle before the plasmonic solar cells can take off from laboratories. To achieve a broadband absorption enhancement in the silicon layer, the plasmonic nanostructures need to be able to strongly scatter the incident solar light into a large angle range so that light can be best trapped inside the silicon layer while keep the particle absorption minimum. Previous attempts mainly focused on the theoretical modeling of regularly patterned particle arrays or gratings with rigorous geometry precision requirement.7−9 Although it is promising theoretically to achieve considerable efficiency enhancement, these attempts rely on sophisticated and expensive semiconductor lithography equipment and thus are less attractive for industrial in-line mass production. In comparison, the selfassembly method has been proved to be a simple and less expensive option.10−13 However, it does not offer the essential flexibility in controlling the nanoparticle size, shape and particle patterning. Most important of all, it is challenging to arbitrarily manipulate the nanoparticle coverage density, in particular less than 30%.10 These limitations determine that the self-assembly Received: October 4, 2011 Revised: January 28, 2012

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Figure 1. Schematic drawings of silver nanoparticle scattering in the ZnO:Al layer of amorphous silicon solar cells (left side of each panel) and calculated scattering intensity (I) versus scattering angle patterns (right side of each panel, logarithmic plot) of light spectra from 300 to 800 nm by the nanoparticles based on the Mie theory19 for (a) 20 nm, (b) 100 nm, (c) 200 nm, and (d) 400 nm silver nanoparticles embedded in ZnO layer.

Figure 2. Schematic drawings showing the scattering patterns of (a) a small nanoparticle; (b) a large nanoparticle of 200 nm; (c) a nucleated large nanoparticle of 200 nm (color arrows are used to illustrate the broadband scattering of the nucleated nanoparticles only); and (d) the calculated scattering intensity versus scattering angle pattern for a nucleated large nanoparticle of 200 nm under the illumination of a total-field-scattered-field plane wave from 300 to 800 nm with the FDTD method.21.

combined with small particle nucleation to effectively scatter light in a broad spectrum range with large oblique angles and in the meantime minimizing the detrimental particle absorption. These nanoparticles were realized by the simple and low-cost

method is lack of controllable mechanism to achieve broadband absorption enhancement. In this paper, we propose and demonstrate a novel nanoparticle geometry based on large silver nanoparticles B

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domain (FDTD) software from Lumerical21 was employed to calculate the scattering pattern of a 200 nm large nanoparticle covered with 40 nm half-truncated small particles under the illumination of a total-field-scattered-field plane wave from 300 to 800 nm, as shown in Figure 2d. As expected, the nucleated nanoparticle presents a dramatically different scattering pattern as that for the smooth particle of the same size (200 nm). In fact, the scattering pattern is similar to those in Figure 1a,b, confirming that large oblique angle has been achieved with this model. On the other hand, the scattering intensity of this particle is on the same order of the 200 nm smooth particle shown in Figure 1c with a scattering cross-section 1 order of magnitude higher than the absorption cross-section. The simulation result shown in Figure 2d confirms the feasibility of using the proposed nucleated large particles to achieve large angle broadband scattering. The proposed nucleated nanoparticles can be realized by the simple, low-cost and industry favorable wet chemical method.14,15 Conventional silver nanoparticle synthesis based on the reduction method can routinely produce nanoparticles ranging from 5 to 100 nm. However these particles are isotropic during growth due to the use of a strong reductant, sodium borohydride (NaBH4). Therefore the particles exhibit almost a perfect spherical shape with small size deviations (150 nm), a weaker reductant, ascorbic acid, and an Ag+ ion abundant environment are required,22 which will lead to the large nanoparticle formation by continuous silver supply and the anisotropic growth along certain crystalline directions (see Supporting Information). The scanning electron microscopic (SEM) images of the resultant nanoparticles with sizes of 200 ± 20 nm (see Supporting Information) and 400 ± 20 nm are shown in the insets of Figure 3c,d. It can be clearly seen that these particles exhibit large surface roughness similar to the truncated small particles shown in Figure 2c. The size of the small particles on the surface of the 200 and 400 nm particles are approximately 40−50 nm and 80−90 nm, respectively, and can be well controlled by the growth conditions. Unlike the smaller nanoparticles, which possess only one distinct plasmonic resonance peak, the 200 and 400 nm silver nanoparticles C

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Figure 4. Schematic drawings of integrating silver nanoparticles into amorphous silicon solar cells.

produce enhanced broadband absorption features. This can be attributed to the unique structure combination of both the large core particles and the small surface particles, which can effectively excite the dipolar and quadrupolar plasmonic modes in a broad wavelength region, as predicated in Figure 2c. This has been confirmed by the comparison of nucleated nanoparticles with smooth nanoparticles of the same size (see Supporting Information). These tailored silver nanoparticles were integrated at the rear side of the solar cells (Suntech Power Holdings Co., Ltd.) before the fabrication of the silver back reflector with different coverage densities less than 30%, as suggested from the numerical calculation based on the FDTD simulations.21 The samples used in this work are thin-film amorphous silicon solar cells. The 700 nm SnO2:F coated glass is used as a transparent conductive oxide (TCO) substrate. The cells have a geometry of Glass/TCO/p-i-n a-Si:H/ZnO:Ag. The silicon layers (350 nm in total) are deposited by plasma enhanced chemical vapor deposition (PECVD) at a RF excitation frequency. Figure 4 presents the schematic of the integration process. Before the integration of the silver nanoparticles, the solar cell samples (2 cm2) were subjected to a 5 min exposure to ethanol solution under sonication. Silver nanoparticles were embedded inside the ZnO:Al layer at the rear side of the solar cells before the back contact layer by the deposition of the nanoparticle water suspension onto the ZnO:Al layer. The ZnO:Al layer thickness between the silver nanoparticles and the silicon layer was 20 nm to maximize the near-field coupling and in the mean time avoid the potential recombination. After the integration of the nanoparticles, another ZnO:Al layer was deposited by the RF sputtering on the top of the nanoparticles followed by the silver back contact layer. The influence of the silver nanoparticles on the performances of solar cells was investigated through the relationship between Jsc, a parameter directly related to the light trapping effect of the solar cells, and the sizes of the silver nanoparticles under different coverage densities, as shown in Figure 5. It is obvious that within the integrated particle sizes ranging from 20 to 200

Figure 5. The relationship between the Jsc enhancement and the silver nanoparticle size of the cells under different coverage densities from 5 to 20%.

nm, larger particles indeed exhibit a higher Jsc enhancement than the smaller ones do for all the coverage densities as predicated by the Mie theory calculation in Figure 1. When the 20 nm silver nanoparticles are integrated into the thin-film amorphous silicon solar cells, parasitic absorption in the silver nanoparticles is dominate because the small nanoparticles have larger absorption cross sections than the scattering cross sections in the visible wavelength range, which cannot lead to a substantial enhancement of the absorbance in the amorphous silicon layer.5 Consequently the integration of 20 nm silver nanoparticles decreases the Jsc value significantly as shown in Figure 5. For the 200 nm nucleated nanoparticles, the cells present consistent Jsc enhancement for all three coverage densities. The largest Jsc enhancement of 14.3% can be achieved at the 10% coverage. The observed pronounced enhancement in Jsc can be attributed to the increased optical path length in the semiconductor layer resulted from the dominate broadband scattering from the nucleated nanoparticles of the incident light into a wider distribution angles.2,3 It is interesting to note that D

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tion and quantum efficiency below 510 nm are almost unaffected because of the adequate absorption of the short wavelength light by the amorphous silicon layer. We noticed that Jsc estimated from the EQE measurement is slightly smaller than that obtained by the current density−voltage (J−V) measurement shown in Figure 7. This is due to the different

the cells integrated with 400 nm nanoparticles do not show the expected largest Jsc enhancement. We believe that this is most likely because the large particle size leads to the dominate excitation of multiple higher-order plasmonic modes, which are known to have smaller scattering/absorption ratio than the dipolar and quadrapolar modes,10 and therefore hardly lead to useful Jsc enhancement. It is thus clear that there exists an optimum nanoparticle size to achieve the maximum Jsc enhancement. In our case this optimized size is 200 nm. In addition, to make the experimental results comparable, consistent ZnO:Al thickness of 100 nm has been used for all the solar cells with and without nanoparticles. This means the thickness of the embedding ZnO:Al layer is less than the diameter of the silver nanoparticles in the case of 400 nm leading to some contact loss at the back surface. It can also be seen from Figure 5 that for the nanoparticle sizes ranging from 20 to 200 nm, 10% surface coverage provides the best photovoltaic properties of the solar cells among all the three coverage densities used in the experiment. This is consistent with our FDTD simulation results. The 5% coverage seems to be insufficient to cause any significant impact to Jsc. The observed changes in Jsc are within ±4%. In contrast the 20% surface coverage leads to obvious changes in Jsc. In particular when the nanoparticle size is 20 nm, the reduction in Jsc is almost 30% due to the massive particle absorption, which does not contribute positively to the Jsc enhancement. To further confirm the remarkable broadband scattering enhancement of the 200 nm nucleated nanoparticles, wavelength dependent absorption and external quantum efficiency (EQE) measurements (see Supporting Information) are presented in Figure 6. The inset of Figure 6 clearly

Figure 7. J−V characteristic for the cells without (blue) and with (red) 200 nm nucleated silver nanoparticle at 10% coverage density.

angles of incidence of the illumination beams between the EQE system (focused beam illumination) and the solar simulator used for the J−V measurement (parallel beam illumination).23 The significant enhancement in Jsc leads to the overall efficiency enhancement of 23%, which can be evidenced from Figure 7 in which the J−V curves of solar cells with and without the integration of 200 nm nucleated silver nanoparticles with the 10% coverage density are shown. After the nanoparticle integration, the maximum achieved energy conversion efficiency was 8.1% among all the cells. It is worth noticing that the enhancement of the overall efficiency in Figure 7 is larger than that of Jsc. This is due to the contribution from the enhanced fill factor (FF) of 6.02%. In fact, the FF enhancement was consistently observed for high coverage densities (10 and 20%). In particular, in the case of the 20% coverage with 200 nm particle size the FF enhancement reaches almost 8%. The enhanced FF can be attributed to the reduced contact resistivity of the dielectric layer by incorporating silver nanoparticles at high coverage densities.24 In conclusion, we have proposed a simple but novel nanoparticle geometry to enable the broadband absorption enhancement in thin-film amorphous silicon solar cells. Using the industry-friendly wet chemical method, such nucleated silver nanoparticles were demonstrated and integrated with thin-film amorphous silicon solar cells. Significant gains in both the short-circuit current density of up to 14.3% and the energy conversion efficiency of up to 23% were achieved mainly via improved large angle, broadband light scattering arising from the integration of the tailored silver nanoparticles. The achieved highest conversion efficiency was 8.1%. The wet chemical method has been proved to be a simple, low cost, and effective method to synthesize and integrate nanoparticles for solar cell application because it provides unique control of the particle morphology and coverage density.

Figure 6. EQE for cells without (blue) and with (red) 200 nm nucleated silver nanoparticles at 10% surface coverage. Inset absorption enhancement of cells with and without the nucleated silver nanoparticle integration. (Inset up) proposed model for the nucleated nanoparticles; (inset down) the SEM images of the measured 200 nm nucleated nanoparticles, scale bar: 100 nm.

demonstrates a broadband absorption enhancement of up to 22% at the long wavelength range between 530 and 800 nm due to the integration of the 200 nm nucleated silver nanoparticles compared with a randomly textured reference cells without nanoparticles. This result is further validated by the EQE measurement in Figure 6, which consistently shows the broadband enhancement between 530 and 800 nm. These results clearly indicate that the broadband large angle scattered light from the nucleated nanoparticles is trapped inside the amorphous silicon layer and contributes significantly to Jsc (approximately 13.6% enhancement). In contrast, the absorpE

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(24) Ganguly, G.; Carlson, D. E.; Hegedus, S. S.; Ryan, D.; Gordon, R. G.; Pang, D. Appl. Phys. Lett. 2004, 85, 479.

ASSOCIATED CONTENT

S Supporting Information *

Description of silver nanoparticle synthesis, size distribution graph of 200 nm nucleated silver nanoparticles, and characterization of the silver nanoparticle integrated solar cells. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (M.G.) [email protected]; (B.J.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Victorian Government to establish the Victoria-Suntech Advanced Solar Facility (VSASF) under the Victoria Science Agenda (VSA) scheme. B.C. thanks Suntech Power Holdings Co. Pty. for his Ph.D. scholarship. B.J. thanks the Victorian Government for the support through the Victorian Fellowship.

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