Fabrication and simulation of antireflective nanostructures on c-Si solar cells

Ming-Hsuan Kao1, Ting-Gang Chen2, Min-An Tsai3, Hsin-Chu Chen2, Fang-I Lai1, Shou-Yi Kuo4*,

Yu2 and Hao-Chung Kuo2 1 Department of Photonics Engineering, Yuan Ze University, Taoyuan 32003, Taiwan, R.O.C.

2 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C. 3 Department of Electro-physics, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C.

4 Department of Electronic Engineering, Chang Gung University,Taoyuan 333, Taiwan, R.O.C Presenting authors e-mail address: sykuo@mail.cgu.edu.tw

Abstract: The enhanced photoelectric conversion is demonstrated in nanostructured photovoltaics using colloidal lithography and reactive-ion-etching (RIE) techniques. From the reflectance spectroscopy, trapezoid-cone arrays (TCAs) Si with SiNx passivation layer effectively suppress the reflection in the wavelength range from 400 nm to 1000 nm. The power conversion shows the TCAs Si solar cell with 120 nm thickness of SiNx passivation layer achieves 13.736%, which is 8.87% and 2.56% enhancement compared to the conventional KOH-textured photovoltaics and TCAs with 80-nm-thick SiNx, respectively. An optical simulation based on RCWA describes the optimized shape of nanostructure to further reduce reflectance for maximum light absorption.

1. Introduction

Solar cells of nanostructures have attracted much attention due to their potential for improving charge collection efficiency. In order to improve light harvesting in solar cells, it is mandatory to minimize the Fresnel reflection at the air/silicon interface for the range of the entire solar spectrum. Several studies of sub-micrometer gratings (SMG) on c-Si surfaces have shown ultra-low reflectivity at normal incidence and low reflectivity at large angles of incidence (AOI) for a single wavelength only [1-2].

In this work, we investigate the antireflective characteristics of SiNx passivated SMG nanotrapezoid. The experimental results were compared to KOH-etched textured silicon and showed excellent antireflective characteristics in terms of the wavelength. We found out the best antireflection characteristic (λ< 600nm) was achieved with a t-cone with 80-nm-thick SiNx. However, the best power conversion efficiency was found with TCAs with 120-nm-thick SiNx. The EQE measurement confirmed the inconsistency between spectroscopy and J-V measurement was caused by surface states, which inhibit the current generated from high energy photons.

2. Experiment

Figure 1 shows the fabrication flows. First, we spread PS spheres with a diameter of 600 nm on a p-type c-Si substrate by spin-coating method. The substrate covered with a monolayer of PS spheres was then etched by using a reactive ion etching (RIE) technique with Cl and O2 gas injection, which resulted in trapezoid-cone heights of 550 nm on the surface. The pn junction was formed via POCl3 injection in diffusion furnace, and different thickness of SiNx (80nm & 120nm) were deposited by using a plasma-enhanced chemical vapor deposition (PECVD). Ag and Al paste were printed at the top and the bottom of cell, respectively. Finally, co-firing was adopted to form the ohmic contacts.

3. Result and discussion

Figure 2a shows the normal incident reflectance spectra and figure 2b plots the external quantum efficiency. The J-V curves were characterized under AM1.5g illumination and the power conversion efficiency (Table 1.) achieved was 13.76% (t-cone with SiNx 120nm) which showed a 8.87% and 2.56% enhancement compared to a conventional KOH-textured with 80-nm-thick SiNx and t-cone with 80-nm-thick SiNx, respectively. The TCAs with 80-nm-thick SiNx shows the lowest reflectance (λ<600 nm). However, the result of J-V measurement cannot agree with reflectance spectroscopy, TACs with 120-nm-thick SiNx achieves the highest power conversion efficiency. In order to better understand, we analyzed this data from the EQE measurement, TCAs with SiNx 80nm doesn’t show an EQE enhancement on short wavelength (λ< 600nm), this result indicates the enhancement of electron-hole generation doesn’t contribute to short circuit current.

As well-known, the dry etching process introduces a critical damage on Si surface. When the light with short wavelength (λ<600nm) is absorbed, the electron-hole pairs are trapped by surface state, which inhibits the short circuit current generated from electron-hole pair generation. As mentioned above, we need to reduce the reflectance

Pei-Chen

ACOFT Presentation ● IQEC/CLEO Pacific Rim 2011 ● 28 August - 1 September 2011 ● Sydney, Australia

978-0-9775657-7-1 © 2011 AOS 1529

of the long wavelength (λ> 600nm) and reduce the electron-hole pairs which are trapped by surface states. From electromagnetism wave theory, the relation between dielectric material thickness and reflectance minimum wavelength are as following equation, D=λ/4n (D: dielectric material thickness; λ: reflectance minimum wavelength; n: refraction index) , in other words, the thicker dielectric material layer we choose, the longer reflectance minimum wavelength we get, so the TCAs with 120-nm-thick SiNx makes the reflectance minimum wavelength red shift and reduces the effect of surface state. The KOH textured cell was made by wet etching process, so the surface state is fewer than dry etching process.

4. Simulation

Different nanostructured antireflection surfaces were modeled using rigorous coupled wave analysis (RCWA) The nanotrapezoid arrays were approximated to a stack of cylinders with increasing radii from air to the substrate (Figure 3a). Meanwhile, the nanonipple arrays were used for comparison, the parabolic-based function is used to describe the variation of radius with height to produce a nipple shape (Figure 3b). The base diameters of these two nanostructure are fixed at 600 nm. The material dispersion is taken into account for wavelengths from 300 nm to 1100 nm, corresponding to the irradiation absorption of crystalline silicon.

We calculated the short-circuit current density under the AM1.5G illumination from simulated reflectance spectrum with an assumption of 100% charge conversion and collection. Figure 3c shows a contour map of the calculated short-circuit current density of nanotrapezoid arrays as a function of the periods and heights, with a fixed diameter of 600 nm. At the height under 100 nm, the current density is relatively low over the entire period range. At fixed diameter, the current density is enhanced as the height is increased. However, for larger base-diameter, the higher nanostructure is required for increase in short current density.

Changing the shape of nanostructure arrays has a dramatic effect on the reflectance behavior, result in the variation of short-circuit current density. The short-circuit current density of nanonipple arrays indicates a high current density region as shown in Figure 3d. It is believe that the diameter of the nanonipple arrays shrinks gradually from the root to the top, resulting in a graded transition of the effective refractive index, leading to reduced reflection over a broad range of wavelengths.

5. Conclusion

We demonstrated the broadband antireflective characteristics of passivated TCAs c-Si fabricated by colloidal lithography and RIE. The TCAs with 80-nm-thick SiNx reveals enhanced absorption in the short wavelength (λ< 600nm) absorption. However, the enhanced absorption of short wavelengths doesn’t contribute to short circuit current. The phenomenon causes by surface states due to the dry etching damage. Finally we demonstrate the optimized thickness of SiNx passivation layer for TCAs. The power conversion efficiency achieves 13.76 %, which shows an 8.87 % and 2.56 % enhancement compared to conventional KOH-textured and TCAs photovoltaics with 80-nm-thick SiNx, respectively. In the future, damage removal etching (DRE) will be adopted to recover the surface damage by RIE and optimized structure by optical reflectance simulation will be present for maximum current output.

6. Acknowledgment

The authors would like to gratefully acknowledge partial financial support from the National Science Council (NSC) of Taiwan under Contract No. NSC96-2221-E-155-071-MY3 and NSC99-2221-E-155-014-MY3.

Fig. 1. The fabrication process of TACs on p-type crystalline silicon substrates.

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Voc Jsc F.F. ɳ KOH & 80-nm SiNx 0.594 30.21 70.25 12.61 TACs & 80-nm SiNx 0.591 32.45 69.82 13.39 TACs & 120-nm SiNx 0.589 33.63 69.31 13.73

Tab. 1. The J-V measurement of KOH-textured with 80-nm-thick SiNx, t-cone with 80 & 120-nm-thick SiNx.

Fig. 2. The measured (a) reflectance spectra and (b) EQE of KOH-textured with 80-nm-thick SiNx, TACs with 80&120-nm-thick SiNx.

Fig. 3. (a)(b) Schematic diagram of nanotrapezoid and nanonipple arrays model in simulation. (c)(d) Contour plot for the calculated short-circuit

current density of nanotrapezoid and nanonipple arrays as a function of the periods and heights.

7. Reference [1] C. C. Striemer and P. M. Fauchet, “Dynamic etching of silicon for broadband antireflection applications” Appl. Phys. Lett. 81, 2980 (2002).

[2] 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).

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