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Advancing tandem solar cells by spectrally selective multilayer intermediate reflectors

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Abstract

Thin-film silicon tandem solar cells are composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell, stacked and connected in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is required. Up to date, single-layer intermediate reflectors of limited spectral selectivity are applied to match the photocurrents of the top and the bottom cell. In this paper, we design and prototype multilayer intermediate reflectors based on aluminum doped zinc oxide and doped microcrystalline silicon oxide with a spectrally selective reflectance allowing for improved current matching and an overall increase of the charge carrier generation. The intermediate reflectors are successfully integrated into state-of-the-art tandem solar cells resulting in an increase of overall short-circuit current density by 0.7 mA/cm2 in comparison to a tandem solar cell with the standard single-layer intermediate reflector.

© 2014 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 (a) Scheme of a tandem thin-film silicon solar cell with intermediate reflector. (b) The external quantum efficiency (EQE) of a tandem thin–film silicon solar cell without (black line) and with intermediate reflector (blue). Full lines give the EQE of the sub cells, the sum of the EQEs is shown in dashed lines.
Fig. 2
Fig. 2 (a) Reflectance R into an a-Si:H half space for an increasing number of µc–SiOx:H and ZnO:Al layers. The reflectance of a single layer of 110 nm ZnO:Al (n = 2) is shown in blue. The reflectance of an alternating stack of N = 100 (IR100)/ N = 3 IR3) layers is depicted in black/ red, respectively. The reflectance of the flat IR3 stack (dashed, red line) is compared to an IR3 on rough substrate (full, red) in (b).
Fig. 3
Fig. 3 The transmittance into the non-absorbing µc-Si:H halfspace is shown for the IR3 and ZnO IR simulated by rigorous optical simulations.
Fig. 4
Fig. 4 (a) Measured external quantum efficiency EQE and absorptance A = 1-R of a flat tandem solar cell without (black) and with our IR3 (red line). (b) EQE and absorptance A = 1-R for the various IR designs in a textured tandem solar cell on AsahiVU substrate. The EQE ratio path enhancement in the top cell in the flat case EQEtop,IR / EQEtop,w/o is shown in (c) while (d) shows the EQE ratio EQEsum,IR / EQEsum,w/o for the textured tandem solar cells.

Tables (1)

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Table 1 Photovoltaic parameters short circuit current density Jsc of the sub cells and sum, open-circuit voltage Voc, fill factor FF, as well as conversion efficiency η of the solar cells with the studied IR designs measured in a sun simulator at AM1.5 spectrum. Thicknesses of the intrinsic layers were 330 nm (a-Si:H) and a 3.2 µm (µc-Si:H).

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