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 10⁹ Jones at 1240 nm, and the detectivity estimated from the shot noise is ≈10¹¹ Jones, independent of the bias voltage.