Improving metal reflectors by suppressing surface plasmon polaritons: a priori calculation of the internal reflectance of a solar cell

Abstract

Imperfect internal reflectance of near-bandgap light reduces the performance of all solar cells, and becomes increasingly detrimental as absorbers become thinner. We consider light incident on the silicon/dielectric/metal structure at the back of rear-passivated crystalline silicon solar cells with surface textures that are large enough for geometric optics. By calculating the absorbance in the metal as a function of the angle of incidence, we discover three results that are important for understanding and improving rear reflectors in many types of solar cells. First, significant parasitic absorption occurs in the metal layer in two cases: s-and p-polarized propagating modes (near-normal angles of incidence) when the dielectric thickness is adjusted to cause destructive interference of the reflected beams, and p-polarized evanescent modes (angles of incidence above the semiconductor/dielectric critical angle) that excite surface plasmon polaritons at the metal surface. Second, the latter loss dominates; a well-designed rear dielectric passivation layer must suppress the penetration of evanescent waves to the metal. Third, when used as an input in a simple analytical model, the average rear internal reflectance calculated by assuming a Lambertian angular distribution of light accurately predicts the total reflectance and absorbance of a solar cell. INTRODUCTIONThe current trend towards higher silicon solar cell efficiencies has brought rear-passivated designs-including PERC (passivated emitter and rear cell), 1 interdigitated back contact 2 and heterojunction 3 structures-to the forefront of both research and production. While the impetus for rear passivation is a reduction in surface recombination and a corresponding gain in open-circuit voltage (V oc ), the dielectric passivation layer (or transparent conductive oxide layer in silicon heterojunction cells) can be made to simultaneously serve an optical role, thus increasing short-circuit current density (J sc ) too. Several authors have observed that even high-conductivity metals like silver are poor reflectors on textured silicon solar cells, and that a dielectric buffer layer increases the rear internal reflectance of infrared (IR) light. 1,[4][5][6] In thin-film silicon solar cells that employ nanoscale textures and metallic rear reflectors, transparent conductive oxide (TCO) rear buffer layers suppress localized surface plasmons that are excited in the metal by the rough surfaces. 7-10 For alkaline-textured monocrystalline wafers with micrometer-sized pyramids, however, a different loss mechanism must be responsible for parasitic absorption in the metal. In optical simulations of planar Si/SiO 2 /Al structures, Campbell observed diminished internal reflectance both above and below the Si/SiO 2 critical angle for thin SiO 2 layers, revealing that evanescent as well as propagating modes may be absorbed in the metal. 4,5 In a recent

CH_3NH_3PbI_3 perovskite / silicon tandem solar cells: characterization based optical simulations

“…In a single-junction design, such cells exhibit high open-circuit voltages (V oc ) of up to 750 mV [33] thanks to high c-Si bulk lifetimes and excellent surface passivation. These cells are particularly well-suited for bottom cell applications from an electrical and optical perspective: Their high voltages are maintained at reduced illumination levels, and with proper light management [34] these cells exhibit excellent external quantum efficiency (EQE) response in the long-wavelength region [35]. Our results are also valid for other types of c-Si solar cells, such as diffused junction cells, as the a-Si layers and diffused regions influence the optical properties only marginally.…”

The operation mechanism of poly(9,9-dioctylfluorenyl-2,7-diyl) dots in high efficiency polymer solar cells

“…Introduction of dopants in active layer is easy to implement due to its simple process and low cost. Some organic molecules, 18,19 metal nanoparticles (NPs), 20,21 rare-earth NPs, 22,23 and inorganic quantum dots 24,25 have been doped into PSCs and further advanced their PCEs.…”

Optical Evaluation of the Rear Contacts of Crystalline Silicon Solar Cells by Coupled Electromagnetic and Statistical Ray-Optics Modeling

“…In so-called 'passivated emitter and rear cell' (PERC) c-Si solar cells, a patterned dielectric stack (e.g., consisting of a selection of SiO 2 , AlO x and SiN x layers), rather than TCO, is often used at the rear [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In both structures, on textured layers, the dominant optical losses from the rear contact are associated with coupling of the incident light to the metal electrode through evanescent coupling [14][15][16][17][18][19]. This is an electromagnetic effect; full-wave modelling of this part of the device by solving Maxwell's equations is thus necessary [18].…”

“…electron-and hole-collecting contacts [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The principal aim of such contacts is selective collection of one type of carrier, while avoiding recombination of the opposite carrier type [1,2].…”