Poly-crystalline silicon-oxide films as carrier-selective passivating contacts for c-Si solar cells

Abstract

The poly-Si carrier-selective passivating contacts (CSPCs) parasitically absorb a substantial amount of light, especially in the form of free carrier absorption. To minimize these losses, we developed CSPCs based on oxygen-alloyed poly-Si (poly-SiO_x) and deployed them in c-Si solar cells. Transmission electron microscopy analysis indicates the presence of nanometer-scale silicon crystals within such poly-SiO_x layers. By varying the O content during material deposition, we can manipulate the crystallinity of the poly-SiO_x material and its absorption coefficient. Also, depending on the O content, the bandgap of the poly-SiO_x material can be widened, making it transparent for longer wavelength light. Thus, we optimized the O alloying, doping, annealing, and hydrogenation conditions. As a result, an extremely high passivation quality for both n-type poly-SiO_x (J₀ = 3.0 fA/cm² and iVoc = 740 mV) and p-type poly-SiO_x (J₀ = 17.0 fA/cm² and iVoc = 700 mV) is obtained. A fill factor of 83.5% is measured in front/back-contacted solar cells with both polarities made up of poly-SiO_x. This indicates that the carrier transport through the junction between poly-SiO_x and c-Si is sufficiently efficient. To demonstrate the merit of poly-SiOx layers' high transparency at long wavelengths, they are deployed at the back side of interdigitated back-contacted (IBC) solar cells. A preliminary cell efficiency of 19.7% is obtained with much room for further improvement. Compared to an IBC solar cell with poly-Si CSPCs, a higher internal quantum efficiency at long wavelengths is observed for the IBC solar cell with poly-SiO_x CSPCs, thus demonstrating the potential of poly-SiO_x in enabling higher J_SC.