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Broadband photocurrent enhancement in a-Si:H solar cells with plasmonic back reflectors

Seweryn Morawiec, Manuel J. Mendes, Sergej A. Filonovich, Tiago Mateus, Salvatore Mirabella, Hugo Águas, Isabel Ferreira, Francesca Simone, Elvira Fortunato, Rodrigo Martins, Francesco Priolo, and Isodiana Crupi

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Abstract

Plasmonic light trapping in thin film silicon solar cells is a promising route to achieve high efficiency with reduced volumes of semiconductor material. In this paper, we study the enhancement in the opto-electronic performance of thin a-Si:H solar cells due to the light scattering effects of plasmonic back reflectors (PBRs), composed of self-assembled silver nanoparticles (NPs), incorporated on the cells’ rear contact. The optical properties of the PBRs are investigated according to the morphology of the NPs, which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs we show that the photocurrent enhancement achieved in the a-Si:H light trapping window (600 – 800 nm) stays in linear relation with the PBRs diffuse reflection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of Jsc and Voc are achieved in comparison to those previously reported in the literature for the same type of devices.

© 2014 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 (a) Schematic drawing of a plasmonic back reflector (PBR) with the structure: Glass substrate/100nm Ag mirror/40nm AZO spacer/Ag NPs/80nm AZO cover. (b) 70° tilted SEM image of the surface morphology of a PBR with NPs formed from a 8 nm thick Ag film annealed at 400 °C for 1 h, covered with a 80 nm AZO layer.

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Fig. 2
Fig. 2 (a-b) Planar and (c-d) 70° tilted SEM micrographs of uncovered NPs formed, respectively, from 8 and 12 nm thick Ag films annealed at 400 °C for 1 h. (e) Histogram of relative counts (counts normalized to the total number of NPs), as a function of the NPs’ in-plane diameter, for the NPs presented in the micrographs.

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Fig. 3
Fig. 3 (a) Total and (b) diffuse reflection of the plasmonic back reflectors (PBRs) with NPs formed from 8 (circles) and 12 nm (triangles) thick Ag films annealed at 400 °C for 1 h, before (solid symbols) and after (open symbols) the deposition of the AZO cover layer. The RTotal and RDiff of a flat BR reference (without NPs) are also shown (open diamonds) for comparison. The wavelength range important for light trapping in a-Si:H (600 – 800 nm) solar cells is indicated by the vertical dashed lines.

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Fig. 4
Fig. 4 Annealing time dependence of the average (a) total and (b) diffuse reflection in the 600 – 800 nm wavelength range for the plasmonic back reflectors (PBRs) with NPs formed from 8 (open symbols) and 12 nm (solid symbols) thick Ag films annealed at 200, 400 and 500 °C. The absolute differences in <RTotal>600-800nm and <RDiff>600-800nm observed between PBRs fabricated in distinct batches, with the same fabrication parameters, were below 2.3%. That is the main error associated to the data points in the plots.

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Fig. 5
Fig. 5 Structure of a-Si:H n-i-p solar cell with plasmonic back reflector (PBR) shown (a) schematically and (b) in SEM cross section at a tilt angle of 20°. (c-d) SEM of the honeycomb-like surface texture of the top IZO layer of the devices fabricated with NPs formed from 8 and 12 nm thick Ag films annealed at 500 °C for 1 h and 400 °C for 4 h, respectively.

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Fig. 6
Fig. 6 External quantum efficiency (EQE) curves of solar cells fabricated on two PBRs with NPs formed from 8 and 12 nm thick Ag films. The EQE of a reference cell with a flat back reflector is shown for comparison.

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Fig. 7
Fig. 7 Plot of short circuit current density enhancement as a function of the average diffused reflection ( < R Diff > 600 − 800nm ) of the PBRs in the 600 – 800 nm wavelength range, corresponding to the light trapping window of a-Si:H solar cells.

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Tables (1)

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Table 1 Electrical parameters of the solar cells fabricated on two PBRs with NPs formed from 8 and 12 nm thick Ag films, in comparison with the reference cell deposited on a flat back reflector (EQE curves shown in Fig. 6)*

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Equations (1)

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〈 R X 〉 λ 2 − λ 1 = ∫ λ 1 λ 2 R X dλ λ 2 − λ 1
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