Quinoidal dicyanomethylene-endcapped cyclopentadithiophenes as vacuum-processable n-type semiconductors

Abstract

Current research in the field of organic photovoltaics is mainly focusing on the application of non-fullerene acceptors in solution-processed materials. Another promising area for their commercial application is all-vacuum-processed organic solar cells enabling not only the formation of homogenous thin films but also the construction of very complex multi-layered architectures. In this field fullerenes C₆₀ and C₇₀ still play the major role as acceptor materials, while alternatives with better optical features still remain rarely reported. One fascinating class of materials is quinoids, due to their interesting energetic properties, which enable not only n-type charge transport but also strong absorption in the visible spectral region even for low molecular weight molecules. Here, we report the synthesis of a series of new vacuum-processable organic pigments based on dicyanomethylene-endcapped cyclopentadithiophenes. This new class of materials is characterized by a high tinctorial strength and a low-lying LUMO level enabling n-type charge transport with mobilities of up to 10⁻² cm² V⁻¹ s⁻¹. Crystal engineering was performed by introducing different residues at the cyclopentadithiophene, which tunes the solid-state molecular packing and thin-film formation. Supramolecular interactions are the dominating structural forces, which help in explaining the OTFT performance along with the film morphologies. As a proof of principle, a power conversion efficiency of up to 0.62% was observed in a fully vacuum-processed planar heterojunction device architecture combining our new quinoidal non-fullerene acceptors with a merocyanine dye as the donor material.