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–C60 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 C60. 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.