The effect of structural order on solar cell parameters, as illustrated in a SiC-organic junction model

Abstract

To understand the title topic a model system of single crystal SiC, modified with an interfacial molecular monolayer of alkyl siloxane molecules, with polycrystalline pentacene deposited on it, was fabricated. In this way a change in the length of the alkyl chain could change the structural order of the pentacene film by changing the surface's hydrophobicity, while no significant variation was found in the surface potential, and, thus, in the surface dipole. The pentacene film grown on top of the monolayers showed, with increasing alkyl chain length, increased lateral order and decreased band gap state density, as observed by X-ray diffraction and surface photovoltage spectroscopy. The V_oc, J_sc and fill factor of solar cells, made with these material combinations, improved with increasing alkyl chain length. We explain this as a result of increased 2D film growth with increasing alkyl chain length of the monolayer, as the surface becomes more hydrophobic, which increases ordering of the pentacene film. Thus, this model system illustrates the role of ordering in charge separation and recombination.