Fluorenone-Based Molecules for Bulk-Heterojunction Solar Cells: Synthesis, Characterization, and Photovoltaic Properties

A series of four conjugated molecules consisting of a fluorenone central unit symmetrically coupled to different oligothiophene segments are conceptually designed and synthesized to provide new electroactive materials for application in photovoltaic devices. The combination of electron-donating oligothiophene building blocks with an electron-accepting fluorenone unit results in the emergence of a new band assigned to an intramolecular charge transfer transition that gives rise to the extension of the absorption spectral range of the resulting molecules. Detailed spectroscopic and voltammetric investigations show that all studied molecules have highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) level positions, which make them good candidates for the application as electron-donors in bulk-heterojunction photovoltaic cells, with (6,6)-phenyl-C61-butyric acid methyl ester (PCBM)-C-60 as electron acceptor component. Moderate device performances, with power conversion efficiencies (PCEs) comprised between 0.3 and 0.6%, were obtained with rigid molecules, containing either the bridging units between the thiophene rings, i.e., (2,7-bis(4,4'-dioctyl-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)-fluoren-9-one (SCPTF) and 2,7-bis(4-(dioctylmethylene)-cyclopenta[2,1-b:3,4-b']dithiophen-5-yl)-fluoren-9-one (MCPTF) or a vinylene unit 2,7-bis(5-[(E)-1,2-bis(3-octylthien-2-yl)ethylene])-fluoren-9-one (TVF), whereas with (2,7-bis-(3,3'''-dioctyl-[2,2';5',2 '';5 '',2''']quaterthiophen-5-yl)-fluoren-9-one (QTF) PCE up to 1.2% (under AM 1.5 illumination, 100 mW cm(-2), active area 0.28 cm(2)) was obtained. The strong pi-stacking interactions in the solid state for this oligomer leading to improved morphology could explain the good performances of QTF-based devices, which rank among the highest recorded for non-polymeric materials. Consequently, fluorenone-based non-polymeric molecules constitute highly attractive materials for solution-processable solar cell applications.