Abstract
In this paper, we show how light absorption in a plasmonic grating nanosurface can be calculated by means of a simple, analytical model based on a transmission line equivalent circuit. The nanosurface is a one-dimensional grating etched into a silver metal film covered by a silicon slab. The transmission line model is specified for both transverse electric and transverse magnetic polarizations of the incident light, and it incorporates the effect of the plasmonic modes diffracted by the ridges of the grating. Under the assumption that the adjacent ridges are weakly interacting in terms of diffracted waves, we show that the approximate, closed form expression for the reflection coefficient at the air-silicon interface can be used to evaluate light absorption of the solar cell. The weak-coupling assumption is valid if the grating structure is not closely packed and the excitation direction is close to normal incidence. Also, we show the utility of the circuit theory for understanding how the peaks in the absorption coefficient are related to the resonances of the equivalent transmission model and how this can help in designing more efficient structures.
Highlights
Light trapping is an optical phenomenon that many photovoltaic cells employ to increase performance
We have shown how light absorption in a plasmonic grating nanosurface can be calculated approximately by means of a simple analytical model based on a transmission line equivalent circuit
The transmission line model is specified for both transverse electric (TE) and transverse magnetic (TM) polarizations of the incident light
Summary
Light trapping is an optical phenomenon that many photovoltaic cells employ to increase performance. One of the most common plasmonic structures employed to reduce the physical thickness of the film material is a 1-D corrugated metallic film on the back surface of a thin photovoltaic absorber layer This grating structure can couple sunlight into Surface Plasmon Polariton (SPP) modes supported at the metal-semiconductor interface as well as into cavity modes in the semiconductor slab ( called photonic or guided modes). The weak-coupling assumption is valid if the grating structure is not closely packed and if the impinging light is arriving from a direction close to normal incidence Even with these constraints, the circuit theory results are useful to understand how the peaks in the absorption coefficient are related to the resonances of the equivalent transmission model. We demonstrate how the circuit theory allows us to interpret the peaks in the absorption spectrum in terms of an equivalent open circuit condition at the air-Si interface, which can be used as a guideline to design more efficient structures
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