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Light trapping in ultrathin plasmonic solar cells

Vivian E. Ferry, Marc A. Verschuuren, Hongbo B. T. Li, Ewold Verhagen, Robert J. Walters, Ruud E. I. Schropp, Harry A. Atwater, and Albert Polman

Open Access Open Access

Abstract

We report on the design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts, which demonstrate enhanced short circuit current densities compared to cells having flat or randomly textured back contacts. The primary photocurrent enhancement occurs in the spectral range from 550 nm to 800 nm. We use angle-resolved photocurrent spectroscopy to confirm that the enhanced absorption is due to coupling to guided modes supported by the cell. Full-field electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with the experimental results. Furthermore, the nanopatterns were fabricated via an inexpensive, scalable, and precise nanopatterning method. These results should guide design of optimized, non-random nanostructured back reflectors for thin film solar cells.

©2010 Optical Society of America

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Figures (5)
Fig. 1
Fig. 1 Plasmonic light trapping solar cell design. (a) Schematic cross section of the patterned solar cell. Patterns are made on the rear glass substrate, and there is conformal deposition of all layers over the patterns through the top ITO contact. Incident blue and red arrows indicate that blue light is absorbed before reaching the back contact while red light interacts more with the back patterns. (b) Photograph of finished imprinted patterned solar cell substrate. Each colored square is a separate device, with different particle diameter and pitch. (c) SEM of Ag overcoated patterns showing 290 nm diameter particles with 500 nm pitch. (d) SEM image of a cross section of a fabricated cell, cut using focused ion beam milling. Note that the ultrathin a-Si:H layer constitutes only a small part of the cell.

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Fig. 2
Fig. 2 Surface topography of nanopatterned and randomly textured solar cells. Tapping-mode AFM images of the top ITO contacts for two of the cells compared in this study. The underlying Ag/ZnO:Al nanostructure is transferred through each layer conformally, so that both the front and back contacts are structured. (a) Patterned cell with 500 nm pitch, (b) Cell on randomly textured Asahi U-type glass substrate.

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Fig. 3
Fig. 3 Electrical measurements on plasmonic solar cells. Data are shown for a-Si:H with two different intrinsic layer thicknesses. (a) a-Si:H thickness 340 nm and (b) a-Si:H thickness 160 nm. Curves are shown for square grid patterns of 250 nm diameter plasmonic scatterers at pitches of 500 nm and 700 nm, the flat reference cell, and (in (b)) the randomly textured Asahi cell.

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Fig. 4
Fig. 4 External quantum efficiency spectra of nanopatterned and randomly textured cells from measurement and simulation. EQE spectra are shown in (a) for cells of thickness 160 nm, under one sun illumination at 0V bias. The primary enhancement in photocurrent over the flat reference cell occurs from 550 - 800 nm. The 500 nm pitch cell shows higher EQE than the randomly textured Asahi cell. The inset of (a) shows EQE measurements of these two cells at higher spectral resolution. Electromagnetic simulations of the generation rate spectra are shown in (b) for the same set of devices.

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Fig. 5
Fig. 5 Angle-resolved photocurrent spectroscopy. Measured EQE versus incident wavelength and incident angle for (a) the randomly textured Asahi cell and (b) the 500 nm pitch nanopatterned cell with 160 nm a-Si:H thickness. The Asahi cell shows a rather isotropic angular response, while the nanopatterned sample shows clear evidence of grating coupling to guided modes. The EQE enhancement for the nanopatterned sample, the ratio of (b) to (a), is shown in c; the calculated folded-zone dispersion diagram of the lowest-order TE and TM modes is superimposed.

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