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

Abstract

We study absorption enhancement by light scattering at periodically textured interfaces in thin film silicon solar cells. We show that the periodicity establishes resonant coupling to propagating waveguide modes. Ideally, such modes propagate in the high index silicon film where they are eventually absorbed, but waveguide modes exist also in the transparent front contact layer if the product of its refractive index and thickness exceeds half the wavelength. Taking into account that the absorption coefficient of realistic transparent conducing films exceeds the one of silicon close to its band gap, certain waveguide modes will enhance parasitic absorption in the transparent front contact. From an analysis based on the statistic distribution of energy among the available waveguide and radiation modes, we conclude that conventional thin film silicon solar cells with thick and nonideal contacts may fail to reach the previously noted bulk limit of 4n 2Si ; instead, a more conservative limit of 4 n 2 Si À n 2 TCO À Á applies.

Light Trapping in Solar Cells: Can Periodic Beat Random?

“…Motivated by this observation, some of us suggested earlier that a lower enhancement limit must apply for absorbers embedded in sufficiently thick dielectric media to support guided modes at these energies. 33 We now discuss the random case. For periods much larger than the wavelength considered, which asymptotically includes the random case, the upper limit for the enhancement factor derived by Yu et al averages to 4n…”

“…However, when the absorber thickness is on the order of the wavelength, as in our case, the discrete nature of the modal structure must be taken into account. 32,33 Figure 4a presents the guided mode band structure as a function of the parallel component of the wave vector k ) for a flat a-Si:H waveguide calculated by determining the complex poles of the Fresnel coefficients 34 using the experimentally determined values of the complex refractive indices and layer thicknesses. In this diagram, we also show the light lines (or cones in three dimensions) defined by E = pc/n 3 k ) for air, the glass substrate, the transparent conductive oxide (TCO) electrodes (in our case, In 2 O 3 :H at the front and ZnO in the back), as well as a-Si:H. Here E is the energy, p the reduced Planck constant, c the speed of light, and n the refractive index of the respective material.…”

Self-Organized Nanoscale Roughness Engineering for Broadband Light Trapping in Thin Film Solar Cells

“…In view of light trapping applications employing the self-organized glass gratings, we stress that the scattering efficiency can be of predominant importance with respect to the anti-reflection behavior; enhanced light absorption in a PV device can in fact be achieved when scattered photons penetrate the absorber layer at the grazing angle and eventually are coupled to wave-guided modes by total internal reflection [34].…”

Thermodynamic limit to photonic-plasmonic light-trapping in thin films on metals

“…The ergodic approach has been extended to accommodate more general forms for the photonic density of states, including exact treatments of waveguide modes, 3 and of multiplelayer dielectric structures. 4,5 Non-ergodic treatments of periodic gratings imposed on the metal and the thin-film layers have indicated light-trapping larger than inferred from the 4n 2 limit. 6,7 In recent years there has been great interest in the possibility that electromagnetic excitations such as surface plasmons, localized or extended, might further facilitate light-trapping.…”