Fundamental limit of nanophotonic light trapping in solar cells

Abstract

Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n 2 ∕ sin 2 θ, where n is the refractive index of the active layer, and θ is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the conventional limit can be substantially surpassed when optical modes exhibit deep-subwavelength-scale field confinement, opening new avenues for highly efficient next-generation solar cells.T he ultimate success of photovoltaic (PV) cell technology requires great advancements in both cost reduction and efficiency improvement. An approach that simultaneously achieves these two objectives is to use light-trapping schemes. Light trapping allows cells to absorb sunlight using an active material layer that is much thinner than the material's intrinsic absorption length. This effect then reduces the amount of materials used in PV cells, which cuts cell cost in general, and moreover facilitates mass production of PV cells that are based on less abundant materials. In addition, light trapping can improve cell efficiency, because thinner cells provide better collection of photogenerated charge carriers, and potentially a higher open circuit voltage (1).The theory of light trapping was initially developed for conventional cells where the light-absorbing film is typically many wavelengths thick (2-4). From a ray-optics perspective, conventional light trapping exploits the effect of total internal reflection between the semiconductor material (such as silicon, with a refractive index n ∼ 3.5) and the surrounding medium (usually assumed to be air). By roughening the semiconductor-air interface (Fig. 1A), one randomizes the light propagation direction inside the material. The effect of total internal reflection results in a much longer propagation distance inside the material and hence a substantial absorption enhancement. For such light-trapping schemes, the standard theory shows that the absorption enhancement factor has an upper limit of 4n 2 ∕ sin 2 θ (2-4), where θ is the angle of the emission cone in the medium surrounding the cell. This limit of 4n 2 ∕ sin 2 θ will be referred to in this paper as the conventional limit. This form is in contrast to the 4n 2 limit, which strictly speaking is only applicable to cells with isotropic angular response, but is nevertheless quite commonly used in the literature.For nanoscale films with thicknesses comparable or even smaller than wavelength scale, some of the basic assumptions of the conventional theory are no longer applicable. Whether the conventional limit still holds thus becomes an important open question that is currently being pursu...

Light trapping beyond the 4n2 limit in thin waveguides

“…Despite the early work suggesting that the absorption enhancement in thin slabs is less than 4n 2 , recent work has shown that the enhancement may in fact exceed this limit through the use of photonic crystals, 5 plasmonic waveguides, 6 and high index claddings, 7,8 which all elevate the local density of optical states to achieve enhancment. 9 Here we revisit the case of a thin waveguide and determine that the 4n 2 limit can be exceeded for a number of structures that support large propagation constants and/or slow modal group velocities.…”

Resonances and absorption enhancement in thin film silicon solar cells with periodic interface texture

“…11 For nonsilicon absorbers with low refractive index it was proposed that embedding between high index cladding layers can enhance the absorption beyond the 4n 2 limit via coupling to evanescent fields. 12,13 In this contribution, we discuss the absorption enhancement in realistic thin film solar cells based on amorphous silicon (a-Si). Our considerations are based on the modal properties of an equivalent multilayer stack, using realistic values for the thicknesses and the refractive index dispersion of all components.…”

Complex photonic structures for energy efficiency

“…This can either lead to enhanced absorption, or can reduce the amount of required material and hence reduce fabrication costs. Nanophotonics offers interesting possibilities for improving solar cell absorption [14][15][16]. Standard optics is limited by an upper thermodynamical limit [17], which can however be surpassed by nanophotonic strategies in particular when the absorbing films are extremely thin [18].…”

“…Through different photonic architectures it is possible to augment the optical absorption by, for instance, trapping light within ultra-thin films [14], or even manipulate the photon density of state [15] or slowing down light in absorbing thin films. This can be achieved by periodic photonic structures [14,15] or disordered ones [19][20][21], as it has been recently proposed in a two-dimensional system [22] in which disorder modes are on the verge of light (Anderson) localization [23][24][25].…”