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Nanoimprinted backside reflectors for a-Si:H thin-film solar cells: Critical role of absorber front textures

Yao-Chung Tsao, Christian Fisker, and Thomas Garm Pedersen

Open Access Open Access

Abstract

The development of optimal backside reflectors (BSRs) is crucial for future low cost and high efficiency silicon (Si) thin-film solar cells. In this work, nanostructured polymer substrates with aluminum coatings intended as BSRs were produced by positive and negative nanoimprint lithography (NIL) techniques, and hydrogenated amorphous silicon (a-Si:H) was deposited hereon as absorbing layers. The relationship between optical properties and geometry of front textures was studied by combining experimental reflectance spectra and theoretical simulations. It was found that a significant height variation on front textures plays a critical role for light-trapping enhancement in solar cell applications. As a part of sample preparation, a transfer NIL process was developed to overcome the problem of low heat deflection temperature of polymer substrates during solar cell fabrication.

© 2014 Optical Society of America

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Figures (8)
Fig. 1
Fig. 1 Schematic process flows of the negative, positive and transfer NIL processes.

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Fig. 2
Fig. 2 SEM images with a tilt angle of 45þ of all negative and positive imprints on PMMA surfaces.

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Fig. 3
Fig. 3 2D AFM topography of a 200 nm a-Si:H film on T(PH) (dashed lines) and N(PH) (solid lines) showing (a) short- and (b) long-axis cross-section profiles.

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Fig. 4
Fig. 4 SEM images with tilt angles of 0þ and 45þ of (a-h) the N(PH) and (i-p) the P(PH) BSRs with 0 – 300 nm a-Si:H films.

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Fig. 5
Fig. 5 Total (a-c) and diffuse (d-f) reflectance spectra of all the positive and negative NIL BSRs with 100 – 300nm a-Si:H films.

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Fig. 6
Fig. 6 (a) Average absorption between 300 – 800 nm of all replicated BSRs as a function of the a-Si:H layer thickness (b) Integrated quantum efficiency (IQE) of O(PH), P(PH), N(PH) and T(PH) samples under AM1.5G as a function of a-Si:H layer thickness. The red and blue dashed arrows illustrate the increase and the decrease of the IQE, respectively.

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Fig. 7
Fig. 7 AFM cross-sections of center dimples on the front surface of (a) O(PH) and (b) P(PH) with 100 – 300 nm a-Si:H coating. (c) Ratio of height to FWHM of O(PH) and P(PH) as a function of the a-Si:H layer thickness.

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Fig. 8
Fig. 8 FDTD simulations of periodic hexagonal cell structures based on the AFM cross-section scans of the four different BSRs, (a) P(PH), (b) O(PH), (c) T(PH) and (d) N(PH), with the 200 nm a-Si:H coating. The rectangular columns above the cross-sections profiles are the photon densities of the reflected light.

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Equations (2)

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IQE=  ∫ λ 1 λ 2 λ  I sun ( λ ) A tot ( λ )RQE( λ )dλ ∫ λ 1 λ 2 λ  I sun ( λ )dλ ,
ϕ p h ( r ) = ∫ λ 1 λ 2 λ h c | E ( r ) / E 0 | 2 I s u n d λ ,  
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