Manipulating the Charge Transfer Absorption for Narrowband Light Detection in the Near-Infrared

Charge generation and recombination processes at interfaces between electron donating (donor, D) and accepting molecules (acceptor, A) are mediated by intermolecular charge-transfer (CT) states. Since organic photovoltaic and photodetecting devices rely on D-A interfaces, an understanding of the molecular and morphological aspects governing CT state properties is crucial. In this paper, we synthesize a novel series of bi(thio)pyranylidene donor molecules and show how the interplay of molecular structure and energy levels in a D-C-60 blend affect the line shape of the CT absorption cross section. By rationally designing the molecule 2,2',6,6'-tetra-(2-methylthienyl)-4,4'-bithiopyranylidene, we achieve a 2 times stronger CT absorption peak than the literature-known molecule 2,2',6,6'-tetraphenyl-4,4'-bipyranylidene when blended with C-60. The low CT state energy combined with relatively strong CT absorption of this new material blend is exploited by fabricating near-infrared, cavity enhanced narrowband detectors. The photodetectors cover an impressive wavelength range from 810 to 1665 nm with line widths between 30 and 50 nm.