Localized plasmonic losses at metal back contacts of thin-film silicon solar cells

Abstract

Investigations of optical losses induced by localized plasmons in protrusions on silver back contacts of thin-film silicon solar cells are presented. The interaction of electromagnetic waves with nanoprotrusions on flat silver layers is simulated with a three-dimensional numerical solver of Maxwell's equations. Spatial absorption profiles and spatial electric field profiles as well as the absorption inside the protrusions are calculated. The results presented here show that the absorption of irradiated light at nanorough silver layers can be strongly enhanced by localized plasmonic resonances in Ag nanoprotrusions. Especially, localized plasmons in protrusions with a radius below 60 nm induce strong absorption, which can be several times the energy irradiated on the protrusion's cross section. The localized plasmonic resonances in single protrusions on Ag layers are observed to shift to longer wavelengths with increasing refractive index of the surrounding material. At wavelengths above 500 nm localized plasmonic resonances will increase the absorption of nanorough μc-Si:H/Ag interfaces. The localized plasmon induced absorption at nanorough ZnO/Ag interfaces lies at shorter wavelengths due to the lower refractive index of ZnO. For wavelengths above 500 nm, a high reflectivity of the silver back contacts is essential for the light-trapping of thin-film silicon solar cells. Localized-plasmon induced losses at silver back contacts can explain the experimentally observed increase of the solar cell performance when applying a ZnO/Ag back contact in comparison to a μc-Si:H/Ag back contact.

Comparison of Light Trapping in Silicon Nanowire and Surface Textured Thin-Film Solar Cells

“…A very small drop of both parameters will be observed. However, the situation is different for the solar cell with textured contact layers [27][28][29][30][31][32][33][34][35]. The formation of surface plasmon polaritons and localized plasmon polaritons can lead to a distinct drop of the quantum efficiency for long wavelengths.…”

“…The optical losses are most sensitive to metal textures with dimensions ranging from 30 to 100 nm [33,35]. The optical losses can be reduced, but not prevented by inserting a dielectric layer with a low refractive index between the metal back reflector and the silicon solar cell [3,4,6,27,35]. In the current study, and in most studies in the literature, a sputtered metal oxide film, e.g., zinc oxide, is inserted between the metal back reflector and the silicon solar cell.…”

“…The formation of surface plasmon polaritons and localized plasmon polaritons can lead to a distinct drop of the quantum efficiency for long wavelengths. Detailed experimental and simulation studies have shown that the loss can account for up to 30% of the total short circuit current density [27]. The optical losses increase with increasing roughness of the back contact and decreasing thickness of the solar cell [30][31][32][33].…”

Current Approach in Surface Plasmons for Thin Film and Wire Array Solar Cell Applications

“…The observed increase in absorption compared to the smooth silver could be explained by surface plasmon absorption. This phenomenon was further explained by Paetzold et al , in 2010 through simulations with a three-dimensional numerical solver of Maxwell’s equations [44]. In 2011, Feltrin et al , studied light scattering and parasitic absorption in thin film silicon solar cells containing metal nanoparticles [45].…”

Amorphous and micromorph Si solar cells: current status and outlook

“…Accordingly, the major efforts to increase power conversion efficiency of thin-film Si solar cells are mainly focused on exploration of new concepts for better light trapping and improved contacts. In line with the former direction, employment of 1D and 2D photonic crystals as surface antireflection intermediate reflectors for improved broadband angular light acceptance [75,76], distributed Bragg reflectors [77], and plasmonic nanostructure gratings [78][79][80][81][82] as intermediate and back reflectors are the major research directions. Novel approaches to contact materials aim to improve contact transparency for incident light through enhancement of optical bandgap energy and adjustments of refractive index over a considerable wavelength range while keeping high electrical conductivity.…”