The effect of a two-dimensional structure on the dielectric constant and photovoltaic characteristics

Abstract

Along with the recent intensive studies on improvements in organic solar cell efficiency, a new material design concept of organic semiconductors has become important for the practical realization of solar cells. Donor materials with a high dielectric constant that markedly boost the efficiency have been proposed, but theoretical material designs and/or experimental results are still scarce. In this paper, we report on a combined theoretical and experimental result of donors with high dielectric constants. Our design guidelines were to bind a conventional one-dimensional thiophene-based donor molecule in a two-dimensional (2D) manner, which would lead to extended π-conjugation and a large dielectric constant. Impedance spectroscopy measurements showed that the relative dielectric constant increased unilaterally with extending the 2D structure. Organic solar cells based on these materials in the combination of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) show that an increase in power-conversion efficiency was observed experimentally with increasing the extension of the molecular structure. These sults imply that this is a potentially promising design strategy for the 2D extension of the molecules, and it eventually leads to a high dielectric constant and to an efficient power conversion of the organic solar cell material.