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Indium tin oxide subwavelength nanostructures with surface antireflection and superhydrophilicity for high-efficiency Si-based thin film solar cells

Jung Woo Leem and Jae Su Yu

Open Access Open Access

Abstract

We fabricated the parabola-shaped subwavelength grating (SWG) nanostructures on indium tin oxide (ITO) films/Si and glass substrates using laser interference lithography, dry etching, and subsequent re-sputtering processes. The efficiency enhancement of an a-Si:H/μc-Si:H tandem thin film solar cell was demonstrated theoretically by applying the experimentally measured data of the fabricated samples to the simulation parameters. Their wetting behaviors and effective electrical properties as well as optical reflectance properties of ITO SWGs, together with theoretical prediction using a rigorous coupled-wave analysis method, were investigated. For the parabola-shaped ITO SWG/ITO film, the solar weighted reflectance (SWR) value was ~10.2% which was much lower than that (i.e., SWR~20%) of the conventional ITO film, maintaining the SWR values less than 19% up to a high incident angle of 70° over a wide wavelength range of 300-1100 nm. Also, the ITO SWG with a superhydrophilic surface property (i.e., water contact angle of 6.2°) exhibited an effective resistivity of 2.07 × 10−3 Ω-cm. For the a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, the conversion efficiency (η) of 13.7% was theoretically obtained under AM1.5g illumination, indicating an increased efficiency by 1.4% compared to the device with the conventional ITO film (i.e., η = 12.3%).

©2012 Optical Society of America

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Figures (5)
Fig. 1
Fig. 1 Schematic illustration of process steps for the fabrication of the parabola-shaped ITO SWGs on ITO films/Si and glass substrates. The side- and top-view SEM images of ITO SWGs as a function of re-sputtering time for OT and RBdP are also shown.

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Fig. 2
Fig. 2 Contour plots of the calculated reflectance variation as a function of wavelength at different (a) OT and (b) RBdP for ITO SWG nanostructures. The 2D and 3D simulation models used in these calculations are also shown in (a) and (b).

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Fig. 3
Fig. 3 (a) Measured reflectance spectra at different incident angles of θi = 8-70° for the parabola-shaped ITO SWG/ITO film on the Si substrate and (b) contour plots of the variation of calculated reflectance spectra of the (i) ITO film and (ii) parabola-shaped ITO SWG/ITO film on Si substrates as a function of the incident angle of light. For comparison, the reflectance spectra of Si substrate (θi = 8°) and conventional 460 nm-thick ITO film on Si substrate (θi = 8-70°) are also shown in the left inset of (a). The estimated SWR values of the ITO film and ITO SWG/ITO film on Si substrates as a function of incident angle are shown in the right inset of (a). The 3D simulation models of the corresponding structures are shown in (b).

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Fig. 4
Fig. 4 Photographs of (a) conventional ITO film and parabola-shaped ITO SWG/ITO film on Si substrates and (b) water droplets on the samples and (c) 30°-tilted oblique-view SEM images of the parabola-shaped ITO SWG/ITO film. The side-view SEM image of the parabola-shaped ITO SWG/ITO film is shown in the inset of (c).

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Fig. 5
Fig. 5 (a) (i) Schematic diagram of a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, (ii) calculated effective refractive index profile of the ITO SWG, and (iii) the refractive index (n) and extinction coefficient (k) of the ITO used in this simulation and (b) J-V characteristics of a-Si:H/μc-Si:H tandem thin film solar cells with the conventional ITO film and parabola-shaped ITO SWG/ITO film as an antireflective electrode layer. The parameters of a-Si:H/μc-Si:H tandem thin film solar cells with the ITO film and parabola-shaped ITO SWG/ITO film are summarized in the inset of (b), respectively.

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Tables (1)

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Table 1 Effective Electrical Properties of Conventional ITO Film and Parabola-shaped ITO SWG/ITO Film

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Equations (1)

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sin θ r,m = mλ Λn ,
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