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.
Highlights
The current trend towards higher silicon solar cell efficiencies has brought rear-passivated designs—including PERC,[1] interdigitated back contact[2] and heterojunction[3] structures—to the forefront of both research and production
The thickness t of the dielectric layer was varied—this is the parameter that is easiest to change in a solar cell—and the influence of the angle of incidence h was studied
Maps of the calculated field intensities at the rear of a silicon solar cell are plotted in Figure 2 as a function of h and z for SiNx thicknesses of 20, 200 and 1000 nm
Summary
The 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 (Voc), the dielectric passivation layer (or transparent conductive oxide layer in silicon heterojunction cells) can be made to simultaneously serve an optical role, increasing short-circuit current density (Jsc) 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.[7,8,9,10] For alkaline-textured monocrystalline wafers with micrometer-sized pyramids, a different loss mechanism must be responsible for parasitic absorption in the metal. While only Si/ SiNx/Ag is considered here, the results apply well to other material systems, including, e.g., III-V solar cells, for which high internal reflectance of near-bandgap photons translates into a Voc as well as a Jsc gain because of photon recycling.[12]
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