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Spotlight: Si-CMOS compatible materials and devices for mid-IR microphotonics

Analysis of Threshold Current Behavior for Bulk and Quantum-Well Germanium Laser Structures

Single-Crystal Germanium Growth on Amorphous Silicon

Photonic crystal structures for light trapping in thin-film Si solar cells: Modeling, process and optimizations

Engineering broadband and anisotropic photoluminescence emission from rare earth doped tellurite thin film photonic crystals

Light Trapping for Thin Film Solar Cells

Thin film crystalline silicon solar cells have an active layer of only several micrometers thick and are believed to be a promising candidate for further cost reduction while maintaining the advantages of bulk silicon. However, the efficiency of thin film silicon solar cells critically depends on optical absorption in the silicon layer since silicon has low absorption coefficient in the red and near-infrared (IR) wavelength ranges due to its indirect bandgap nature. To address this problem, we propose a backside textured Bragg Reflector as a light trapping structure. The technique was firstly demonstrated using a lithographic fabrication method, and then further prototyped by a low-cost self-assembled approach.

Fig. 1. Schematic of a surface-textured Bragg Reflector, in the backside of a p-i-n Si solar cell. The textured surface acts to scatter incident sunlight into oblique, or grazing, angles.

Fig. 2. Performances of the solar cells with different design. The texture was made by lithography.

Fig. 3. Performances of the solar cells with different design. The texture was made by a self-assembled templating method.

Follow-up questions are raised for thin film silicon photovoltaics: what is the ultimate limit for the absorption in the thin film silicon, and what is the optimal design that can reach this limit? We developed an optimization based simulation method to further explore the optimal light trapping structure, either periodic or aperiodic. The preliminary results reveal that conventional Lambertian models are no longer valid if we need to explore the optimal texture for thin film silicon at normal incidence. Higher performances are achievable.

References:

  • L. Zeng, P. Bermel, Y. Yi, B. A. Alamariu, K. A. Broderick, J. Liu, C. Hong, X. Duan, J. Joannopoulos and L. C. Kimerling, "Demonstration of enhanced absorption in thin film Si solar cells with textured photonic crystal back reflector," Applied Physics Letters, 93 22 (2008).

  • X. Sheng, J. Liu, I. Kozinsky, A. M. Agarwal, J. Michel and L. C. Kimerling, "Design and Non-Lithographic Fabrication of Light Trapping Structures for Thin Film Silicon Solar Cells," Advanced Materials, 23 843 (2011).
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