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

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. Scientists in Switzerland have analyzed the rear reflectors used in silicon solar cells. Zachary Holman and co-workers from the Swiss Federal Institute of Technology in Lausanne presented a method of calculating the internal reflection of a solar cell, and applied this approach to the rear dielectric–metal (SiNx–Ag) reflectors found in crystalline silicon solar cells. They found that when the dielectric layer is too thin, incident near-infrared light can excite surface plasmon polaritons in the metal, leading to increased parasitic absorption and thus considerable optical loss. The researchers say that a significant drop in reflector performance occurs when the thickness of the dielectric falls below 100 nm, and that this drop is most severe for p-polarized light. Their analysis approach may also be useful for exploring other material systems.