(409d) Surface Passivation of Silicon Solar Cells Using Polymeric Interfaces

This work demonstrates a low-temperature, scalable approach using chemical vapor deposition (CVD) polymerization to passivate the surface of silicon solar cells using polymer films. Such a process of chemical passivation can reduce the surface recombination of minority carriers (i.e. increase minority carrier lifetimes), increase solar cell efficiency, and lower overall cost per watt for solar energy. The CVD approach covalently grafts the polymer film directly onto the silicon substrate by initiating a chemical reaction between the surface hydride bonds on silicon and the reactive vinyl groups of the monomer; thereby satisfying surface dangling bonds and “passivating” electrically active interface states. Following the passivation process, the minority carrier lifetimes of the silicon showed several orders of magnitude improvement (~ 5 milliseconds at 1x10¹⁵ cm⁻³ carrier injection levels) compared to bare silicon (~ 5 microseconds), and remained stable in air for over 200 hrs. The roles played by field effect charges and chemical bonding in the observed surface passivation were examined through measurements of fixed charge and density of interface states along with analysis of the functionalized silicon surface using x-ray photoelectron spectroscopy (XPS). The addition of the passivation layer on silicon reduced the amount of band bending, which suggests a very low density of interface states. Passivation quality also improved on grafting using aliphatic monomers compared to aromatic ones suggesting a reduction in steric effects in the former. Our observations thus guide the development of design rules for polymer based surface passivation of silicon solar cells.