Surface plasmon enhancement has been proposed as a way to achieve higher absorption for thin-film photovoltaics, where surface plasmon polariton(SPP) and localized surface plasmon (LSP) are shown to provide dense near field and far field light scattering. Here it is shown that controlled far-field light scattering can be achieved using successive coupling between surface plasmonic (SP) nano-particles. Through genetic algorithm (GA) optimization, energy transfer between discrete nano-particles (ETDNP) is identified, which enhances solar cell efficiency. The optimized energy transfer structure acts like lumped-element transmission line and can properly alter the direction of photon flow. Increased in-plane component of wavevector is thus achieved and photon path length is extended. In addition, Wood-Rayleigh anomaly, at which transmission minimum occurs, is avoided through GA optimization. Optimized energy transfer structure provides 46.95% improvement over baseline planar cell. It achieves larger angular scattering capability compared to conventional surface plasmon polariton back reflector structure and index-guided structure due to SP energy transfer through mode coupling. Via SP mediated energy transfer, an alternative way to control the light flow inside thin-film is proposed, which can be more efficient than conventional index-guided mode using total internal reflection (TIR).

The anti-reflection coating(ARC) based on dielectric nano-particles has been recently proposed as a new way to achieve the low reflectance required for solar cell front surfaces. In this scenario, the Mie modes associated with the dielectric nano-particles are utilized to facilitate photon forward scattering. In this work, versatile designs together with systematically optimized geometry are examined, for the ARCs based on dielectric scatterers. It is found that the Si3N4-TiO2 or SiO2-TiO2 stack is capable of providing low reflectance while maintaining a flat and passivated ARC-semiconductor interface which can be beneficial for reduced interface recombination and prevent V(OC) degradation associated with topography on the active materials. It is also confirmed that the plasmonic nano-particles placed at the front side of solar cells is not a preferred scheme, even with thorough geometrical optimization. At the ultimate design based on mixed graded index(GI) Mie-scattering, the averaged reflectance can be as low as 0.25%. Such a low reflectance is currently only achievable by ultra-long silicon nano-tips, but silicon nano-tips introduce severe surface recombination. On the other hand, the mixed GI Mie design preserves a flat and passivated ARC-silicon interface, with total thickness reduced to 279.8 nm, much thinner than 1.6 μm for silicon nanotips. In addition, the light trapping capability of mixed GI Mie design is much better than silicon nanotips. In fact, when compared to the state-of-art TiO2 light trapping anti-reflection coating, the mixed GI Mie design provides same light trapping capability while providing much lower reflectance.

In this work, a shape-optimized periodic pattern design is employed to boost the short circuit current of solar cells. A decent result of an additional 16.1% enhancement in short circuit current is achieved by solely patternwise optimization, compared to the baseline structure that is already under full parameter optimization. The underlying physics is that the shape-optimized pattern leads to optimal quasi-guided mode excitations. As a result of the pattern design, a single strongly confined quasi-guided mode is replaced with several weakly confined modes, to cover a broader spectral range. Previous works of optimized periodic gratings result in gradually varied grating heights and require grayscale lithography leading to high process complexity. Using randomized pattern for isotropic Lambertian light trapping, on the other hand, leads to an overly large simulation domain. The proposed pattern design methodology achieves the optimal balance between the slow-light enhancement strength and the enhancement spectral range for nanophotonic light trapping using quasi-guided modes.

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