PATTERNED GLASS SUBSTRATES FOR ENHANCED SOLAR ENERGY HARVESTING IN THIN FILM SOLAR CELLS

Yu-Lin Tsai1, Ting-Gang Chen2, Min-An Tsai3, Chih-Wei Hsu2, Ping-Chen Tseng2, Hsun-Wen Wang3, Hao-Wei Han4, Liang-Hao Jin2, Peichen Yu2*, Jia-Min Shieh2 and Hao-Chung Kuo2

1Department of Photonics & Display Institute, National Chiao Tung University2 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University

3 Department of Electro-physics, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C. 4 Institute of Photonic System, National Chiao Tung University, Tainan 711, Taiwan, R.O.C * yup@faculty.nctu.edu.tw, NSC-96-2221-E-009-095-MY3 and NSC-97-2120-M-006-009

ABSTRACT

The enhanced photoelectric conversion is demonstrated innanostructured glass substrates for a-Si thin film solar cell. The nanostructured glass substrates were fabricated using nanosphere lithography and RIE technique. Thenanostructure substrates provide antireflective and light scattering characteristics, which enhance the broadband light absorption, especially in near-infrared range. Finally we demonstrate the patterned glass substrates (nipple shape arrays) which are useful in light absorption of a-Si thin film solar cell. Compared to a flat glass substrate cell and Asahi-U glass substrate cell, the power conversion efficiency enhancement achieved 48.4% and 3.1%, the Jscenhancement achieved 51.6% and 8%.

INTRODUCTION

The amorphous silicon (a-Si) thin film solar cells are considered for future generation of photovoltaics. Enhancing the power conversion efficiency of a-Si solar cells has been spotlighted over the past few decades. However, because of the thin active layer (<1um), it shows a low light absorption at near-infrared. Improving light harvesting is a very important issue in high efficiency a-Si thin film solar cell.

There are two ways to improve light harvesting in the cells, the first one is to minimize the Fresnel reflection at the interface, and the second one is to achieve longer optical path length in a-Si thin film. Single layer and multi-layer antireflection coating (ARC) are commonly used to reduce the interface reflection [1]. Light scattering at textured surface permits to achieve longer light paths in a-Si thin film, and enhance the light absorption in a-Si thin film. Several studies of sub-micrometer gratings (SMG) structure demonstrate the SMG structure offer a graded refraction index interface, and shown ultra-low reflectivity at normal incidence and low reflectivity at large angles of incidence (AOI) was also demonstrated with a single wavelength [2], another feature of SMG structure is diffraction of light, to get longer optical path length, further to enhance the light absorption of a-Si thin film [3-5]. There are several approaches to fabricate pattern glass, such as e-beam lithography, nano-imprinting lithography and nanosphere lithography. Directly patterned glass substrates via large-area production approaches are

desirable due to reduce discontinuity between air/glass interfaces.

In this work, patterned glass substrates were employed to fabricate a-Si thin film solar cells, and the nanostructured glass substrate were produced by nanosphere lithography and reactive-ion-etching technique. Finally, we investigate the light trapping characteristic of the patterned glass substrate via integrating sphere.

EXPERIMENT

Figure 1 Schematic diagram of the fabrication process of the patternd glass substrate a-Si thin film solar cell.

Figure 2 The scanning electron microscope (SEM)image of (a) monolayer & hexagonal polystyrene (PS) nanosphere arrangement and (b) patterned glass substrate.

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Figure 1 shows the fabrication flows. Self-assembled polystyrene (PS) spheres with a diameter 600 nm were used as sacrificial masks to fabricate sub-wavelength structures which were firstly spread on a glass substrate with proper speed of the spin coater and the surfactant concentration. Figure 2a shows the close packaging PS sphere. Then the glass substrates covered with a monolayer of PS spheres were etched using a reactive ion etching (RIE) technique with CF4 gas injection for 12 minutes, which resulted in nipple shape arrays with 500nm in height, the cross section of a scanning electron microscopic (SEM) image are shown in figure 2b. After patterning the glass substrate, a typical single junction a-Si:H solar cell was deposited by very high-frequency plasma-enhanced chemical vapor deposition (VHF-CVD),which consists of an 80-nm-thick indium tin oxide (ITO) layer as front electrode, a 430-nm-thick a-Si:H active layer (p-i-n, 12/400/20 nm), an 80-nm-thick ITO as the back contact buffer layer of electrode, and a 1000-nm-thick Al acts not only a back electrode but also a back reflector [6].

RESULTS AND DISCUSSIONS

Fig.3 shows the reflectance of patterned glass substrate solar cell and flat glass substrate solar cell and Asahi-Usubstrate cell, the result indicates the anti-reflective characteristic of glass substrate with nipple shape array is greater than flat glass substrate and Asahi-U glass substrate (λ<560 nm), it just mapping to the highest power of solar spectrum (λ=550 nm). The lower reflectancemeans the higher light absorption, which is good beneficialfor enhancing the power conversion efficiency.

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Figure 3 The reflectance of patterned glass substrates and flat glass substrates and Asahi-U glass substrates.

Each structure has produced eight cells for comfirming the consequence of cells. The current density-voltage (J-V)was measured under AM1.5g illumination. Fig.4 shows the measured current density-voltage curve, which the JSC of patterned glass substrate shows 8% enhancement and 51.6% enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively, and the power

conversion efficiency shows 3.1% enhancement and 48.4% enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively (table 1). The results indicate the patterned glass substrate improves the light harvesting. It should be noted, the poor open circuit voltage (Voc) of patterned glass substrate cell may be caused by insufficient thickness of the front ITO eletrode. Since the height of the nipple shape is about 500 nm, and the front ITO eletrode thinkness is 80nm, it may create some crack on ITO surface. The crack of ITO will decline the eletrical property. We believe, if the poor Voc could be solved, the patterned glass substrate cell will show a higher power conversion efficiency enhancement.

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Figure 4 The J-V curve of the patterned glass and flat glass substrate and Asahi-U glass substrate a-Si thin film solar cells.

Voc(V) Jsc(mA/cm2) F.F. Eff.(%)Patterned glass 0.85 13.5 49 5.64

Flat glass 0.9 8.9 46 3.8Asahi-U glass 0.87 12.5 50 5.47

Table 1 the J-V measurement data of the patterned glass and flat glass substrate and Asahi-U glass substrate a-Si thin film solar cells.

In order to characterize the antireflective and light trapping mechanism of patterned glass substrate, the external quantum efficiency (EQE) was measured (Fig.5). It shows a significantly EQE enhancement in broadband, for the wavelength under 500 nm, EQE enhancement is resulted from antireflection effect. The nipple arrays on glass substrate offer a graded effective refraction index interface of glass and ITO, and reduce the Fresnel reflection. For the long wavelength range (λ>500 nm), the photon can’tbe totally absorb in thin active layer, so the EQE enhancement was resulted from both antireflection mechanism and light trapping effect. The nipple arrays on glass substrate scatter the incident photon, enhancing the optical path length, and make the light with low energy could be absorbed effectively. We believe the patterned glass substrates are also good for other application, such

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as organic photovoltaics. Finally we demonstrate the patterned glass substrate (nipple shape arrays) which is useful in enhancing the power conversion efficiency of a-Si thin film solar cell. The short circuit current of patterned glass substrate shows 8% enhancement and 51.6%enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively, and the power conversion efficiency shows 3.1% enhancement and 48.4% enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively.

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Figure 5 The external quantum efficiency (EQE) ofpatterned glass & flat glass substrate for a-Si thin film solar cells.

CONCLUSION

In conclusion, we demonstrate the nipple arrays glass substrates by using nanosphere lithography and RIE technique, and the measured EQE shows a significantly broadband enhancement. The glass substrates with nipple arrays exhibit a notable antireflection and light scattering property, and these two mechanisms are beneficial for light harvesting. Finally, the JSC of patterned glass substrate cell shows 8% enhancement and 51.6%enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively, and the power conversion efficiency of patterned glass substrate cell shows 3.1% enhancement and 48.4% enhancement compared to Asahi-U glass substrate and flat glass substrates cell respectively.In the future, we will improve the electrical property by adjusting the height of arrays and front ITO thickness to prevent the active layer from cracking, and optimize the optical property of patternd glass substrate structure (shape& height& period) to maximize the power conversion efficiency.

REFERENCE

[1] P. Lalanne and G. M. Morris, “Design, fabrication and characterization of subwavelength periodic structures for semiconductor anti-reflection coating in the visible domain,” Proc. SPIE 2776, 300, 1996.

[2] C. C. Striemer and P. M. Fauchet, “Dynamic etching of silicon for broadband antireflection applications” Appl. Phys. Lett. 81, 2980, 2002.

[3] Karin Söderström and Franz-Josef Haug, Jordi Escarré, Oscar Cubero, and Christophe Ballif, “Photocurrent increase in n-i-p thin film silicon solar cells by guided mode excitation via grating coupler” Appl. Phys. Lett. 96, 213508, 2010.

[4] Rahul Dewan, Marko Marinkovic, Rodrigo Noriega, Sujay Phadke,” Light trapping in thin-film silicon solar cells with submicron surface texture” Opt. Exp. 17, 23058, 2009.

[5] C. H. Chang, Peichen Yu,ı and C. S. Yang “Broadband and omnidirectional antireflection from conductiveindium-tin-oxide nanocolumns prepared by glancing-angle deposition with nitrogen” Appl. Phys. Lett.94, 051114, 2009.

[6] Benjamin Curtin, Rana Biswas, and Vikram Dalal ”Photonic crystal based back reflectors for light management and enhanced absorption in amorphous silicon solar cells” Appl. Phys. Lett. 95, 231102, 2009.ı

978-1-4244-9965-6/11/$26.00 ©2011 IEEE 000947