Antagonistic effects in structural design of sulfur-based polyimides as shielding layers for solar cells

Abstract

A series of polyimides (PIs) was synthesized from 4,4′-diaminodiphenyl sulfide and various aromatic or aliphatic dianhydrides to obtain shielding layers for solar cells. Structural design strategy involved combination of alicyclic or fluorinated chemical sequences together with aromatic units and thioether bridges to optimize the balance between thermal stability, optical transparency and refractive index. Molecular modeling revealed significant differences in the spatial molecular conformations between PIs based on aromatic dianhydrides and those containing aliphatic ones. The transmittance spectra reflect the effects of electron-acceptability, polarizability and symmetry of dianhydride moiety, which contribute distinctly to charge transfer complex interactions. Light dispersion in the PI films allowed evaluation of optical reflection losses at interface with ZnO or ZnO:Sn. In terms of transparency and refraction characteristics, it was concluded that PIs derived from either aromatic sulfur-based dianhydride or non-symmetric cycloaliphatic dianhydride render the best features that match the demands for shielding layers in solar cells.