Squaraine Dyes for Organic Photomultiplication Photodetectors with 220% External Quantum Efficiency at 1240 nm.

Near-infrared (NIR) light detection at wavelengths λ > 1100 nm is essential in modern science and technology. Emerging organic semiconductors are promising for solution-processed, flexible, and large-area NIR organic photodetectors (OPDs), but only a few organic chromophores with peak absorption beyond the silicon bandgap are available. Furthermore, the external quantum efficiency (EQE) and specific detectivity (D) of NIR OPDs are restricted by insufficient exciton dissociation and high dark/noise current. Here, the combination of strong electron-accepting and -donating groups is used to synthesize a selection of novel NIR squaraine dyes with superior redshifted absorptions, peaking at 1165 nm in solution and extending to 1240 nm in a blend film. To overcome the tradeoff between long wavelength absorption and high photoresponse, NIR photons are detected utilizing a gain OPD design, where photomultiplication occurs via squaraine hole trap-induced injection of external charges. The OPD can achieve an EQE of 220% at 1240 nm and still maintains 25% in the absorption tail at 1400 nm, thereby surpassing existing NIR OPDs in a broad wavelength range beyond 1100 nm. The measured maximum D equals 10 Jones at 1240 nm, and the detectivity estimated from the shot noise is ≈10 Jones, independent of the bias voltage.