Solar cells with broadband absorption and large acceptance angle are demonstrated by using two-dimensional core-shell structures, which are composed of silicon oxide shells and silicon cores. This study considers structure parameters such as core/shell thickness and periodicity. Finite difference time domain calculation (FDTD) is used in the simulation.  As Figure 1 shows, our core-shell structure is built of Si and SiOx with a feature size much smaller than the wavelength of light.  The core can be either amorphous or polycrystalline silicon, and the shell is silicon dioxide. Four structural parameters are considered: outer radius (r1), inner radius (r2), height (h), and periodicity (a) or symmetry. By tuning these four parameters, we are able to generate an antireflection effect that covers the entire silicon absorption spectrum.

Figure 2 (a) highlights the superior antireflective property of the core-shell layer from our study. An enhancement of absorption is observed between wavelengths of 400-1200 nm. Strong antireflection is achieved as a 0.1-um-thick core-shell layer is applied. The reflectivity drops significantly by 20% at wavelength λ=600 nm and 25% at λ=900 nm. Figure 2 (b) shows that the reflectivity reduces as the periodicities decrease. A huge drop of 20% in reflectivity is observed at wavelength of 0.7 um as the periodicities change from 0.7 um to 0.3 um. The reflectivity within this range is uniformly low. We note that the broadband absorption spectra are not sensitive to the light incident angles. The large acceptant angle is contributed by the continuous varied refractive indexes. Hence, by using the core-shell structure, we improve the efficiency of solar cells by minimizing the loss caused by surface reflection. The easy fabrication of the core-shell structure enables large-scale fabrication of highly efficient solar cells.

1. P. T. Lin, Y. Yi, X. Duan, and L. C. Kimerling, “Simulation and fabrication of two-dimensional core-shell structures for broadband solar cells absorption,” presented at MRS Fall Meeting & Exhibit, Boston, J7.20, 2011.

Two-dimensional photonic crystals (PhCs) are fabricated using dual-beam focused ion beam (FIB) in Er3+-TeO2 thin films and demonstrate broadband enhancement of PL emission at near Infrared (NIR). As Figure 1 shows, highly uniformed patterns with smooth surfaces and pattern resolution better than hundred nanometers are achieved. PhCs arrays with photonic lattice constants ranging from 350 nm to 1700 nm are explored to optimize the PL extraction efficiency. Strong photoluminescence around 1530 nm is observed using 488-532 nm laser pump. A confocal microscope with spectrometer is used to capture the broadband PL signals from individual PhC arrays.

The emission enhancement factor and spectral dependent extraction ratio were analyzed to find the interaction between PL emission and PhC structures. Figure 2 (a) shows that when the PhC structures are optimized, 1500 µm-1560 µm broadband PL is successfully converted between the PL in-plane and out-of-plane emission. As in Figure 2(b), a 60 % enhancement of surface extraction efficiency is achieved when PhC with periodicity a=800 nm is applied. When photonic lattice constants a are smaller than the critical periodicity of 600 nm, the PL light becomes confined inside the thin film layer. Two-dimensional finite difference time domain (FDTD) simulation explains the experimentally observed anisotropic PL enhancement as due to the photonic band gap. The broadband PL enhancement enables Er3+-TeO2 PhCs thin film as a potential light source for three- dimensional integrated photonic circuits.

1.  P. T. Lin, M. Vanhoutte, N. S. Patel, V. Singh, J. Hu, Y. Cai, R. Camacho-Aguilera, J. Michel, L. C. Kimerling, and Anu Agarwal, “Engineering broadband and anisotropic photoluminescence emission from rare earth doped tellurite thin film photonic crystals,” Optics Express, vol. 20, no. 3, pp. 2124-2135, 2012.