Nano-Photonic Structures for Light Trapping in Ultra-Thin Crystalline Silicon Solar Cells

Abstract

Thick wafer-silicon is the dominant solar cell technology. It is of great interest to develop ultra-thin solar cells that can reduce materials usage, but still achieve acceptable performance and high solar absorption. Accordingly, we developed a highly absorbing ultra-thin crystalline Si based solar cell architecture using periodically patterned front and rear dielectric nanocone arrays which provide enhanced light trapping. The rear nanocones are embedded in a silver back reflector. In contrast to previous approaches, we utilize dielectric photonic crystals with a completely flat silicon absorber layer, providing expected high electronic quality and low carrier recombination. This architecture creates a dense mesh of wave-guided modes at near-infrared wavelengths in the absorber layer, generating enhanced absorption. For thin silicon (<2 μm) and 750 nm pitch arrays, scattering matrix simulations predict enhancements exceeding 90%. Absorption approaches the Lambertian limit at small thicknesses (<10 μm) and is slightly lower (by ~5%) at wafer-scale thicknesses. Parasitic losses are ~25% for ultra-thin (2 μm) silicon and just 1%-2% for thicker (>100 μm) cells. There is potential for 20 μm thick cells to provide 30 mA/cm² photo-current and >20% efficiency. This architecture has great promise for ultra-thin silicon solar panels with reduced material utilization and enhanced light-trapping.

Keywords: light-trapping; nano-photonics; scattering; solar cell.

Figure 1

Photon absorption length as a function of wavelength for crystalline silicon (c-Si) and nano- crystalline silicon (nc-Si) (using the complex refractive index (n, k) parameters of Reference [26]).

Figure 2

Proposed solar architecture consists of thin flat spacer titanium dioxide (TiO₂) layers on the front and rear surfaces of silicon, nanocone gratings on both sides with optimized pitch and height, and rear cones are surrounded by Ag metal reflector.

Figure 3

(a) Cell structure used for optimization of texture parameter using simulations. The structure consists of thin flat TiO₂ layers on the front and rear surfaces of flat silicon (1 μm). The cones are only on the front surface without rear cones and Ag metal reflector; (b) Weighted absorption, <A_w> and (c) Short-circuit current density, J_SC, as a function of cone height for different pitch values. Figure shows optimum pitch >750 nm and cone height >500 nm with an increasing trend of tolerance of optimum cone height at larger pitch.

Figure 4

Sequence of light trapping structures on flat silicon. (a) Grating-free cell with thin layers of 60 and 50 nm on front and rear surface, respectively, and flat Ag back-reflector; (b) Front-grating cell with only front cones of height of 600 nm and a pitch of 750 nm and flat Ag back-reflector; (c) Rear-grating cell with only rear cones of height of 200 nm and a pitch of 750 nm and corrugated Ag back-reflector; (d) Dual-grating cell with a combination of front and rear cones with optimized parameters used in ‘b’ and ‘c’ with corrugated Ag back-reflector.

Figure 5

Comparison of absorption spectra of planar silicon with 4n² absorption limit for different light trapping configurations shown in Figure 2 such as (a) grating-free cell, (b) front-grating cell, (c) rear-grating cell, and (d) dual-grating cell using the optimized grating parameter for 2 μm silicon; (e) Comparison of J_SC of planar cell for different light trapping configurations shown in Figure 2 with respect to silicon thickness using optimized grating parameters for 2 μm silicon; (f) J_SC of the cell for a particular thickness of 2 μm for four different configurations.

Figure 6

(a) Absorption spectra of the dual-grating cell (c-Si thickness = 2 μm) and (b) corresponding J_SC of the cell as a function of angle of incidence (AoI) in the range 0°–85° in steps of 5° (inset shows the J_SC of grating-free cell for p- and s-polarization and its average). The average absorption is more than 80% over a wide wavelength band. J_SC is independent of polarization and is less influenced until the AoI reaches 70°, showing the omni-directionality of the nanocone grating structures.

Figure 7

Electric field intensity distribution across the silicon absorber layer at an incident wavelength of (a) 500 nm and (b) 700 nm for the dual-grating cell with optimized parameters for the front and rear nano-cone arrays. The incident electric field intensity is normalized to 1.

Figure 8

Comparison of the simulated short-circuit photo-current J_SC as a function of the thickness of the Si absorber layer in the real-space method utilizing only the absorption in the Si layer, compared to the Fourier space method which includes the absorption in all layers. The difference between the real-space and Fourier space results is the parasitic absorption. The Lambertian limit is shown for comparison.

Figure 9

Schematic of the TiO₂ nanoimprinting process. (1) TiO₂ film is spin-coated on silicon substrate using a precursor titanium diisopropoxide bis(acetylacetonate); (2) The polydimethylsiloxane (PDMS) stamp having nanocups is placed on the spin-coated film with patterned side facing the film. The whole assembly is sandwiched between two glass slides and held together with binder clips (not shown here); (3) After keeping at ~170 °C for 15 min, the binder clips are released to reveal the inverse of PDMS nanocups on the TiO₂ film.

Figure 10

(a) Atomic force microscopy (AFM) image of the periodic array of nanocones imprinted on TiO₂ film. The AFM line scan shows the periodicity at ~750 nm and the average height of nanocones at ~35 nm; (b) Three-dimensional view of the structure showing titania nanocone arrays.