Atomic-layer tailored organic photodetectors: harnessing intermolecular charge-transfer absorption for expanded spectral sensitivity up to the telecommunication band†

The intermolecular charge transfer (CT) states within organic donor–acceptor blends are essential for absorbing photon energy below the bandgaps of separate donor and acceptor materials, which could significantly broaden the response spectrum of organic photodetectors (OPDs). However, CT absorption's inefficiency in the near-infrared (NIR) spectrum limits photocurrent generation, restricting detectable wavelengths. Herein, by incorporating an atomic-thick interfacial layer, we have effectively minimized the dark current of ZnPc:C₆₀ OPD, enabling the device to sense light with wavelengths extending up to the telecommunication band of 1550 nm. Raman spectroscopy analysis reveals that engineering the interfacial layer, particularly in terms of material type and layer thickness, is crucial for fully blocking the detrimental chemical reaction between ITO and ZnPc while simultaneously maximizing the photocurrent performance. Responsivity and detectivity of the optimized device can reach 45 mA W⁻¹ and 3.2 × 10¹¹ jones, respectively, under illumination of an 850 nm light source, which are comparable to those of other CT-based OPDs. In addition, the proposed device exhibits a swift response speed of 39 ns, and the response speed at CT absorption wavelengths surpasses that at short-wavelengths attributed to intrinsic absorption. The delayed response speed at short-wavelengths stems from the exciton diffusion process as well as the electron transfer process, and electron transfer process between ZnPc and C₆₀ was confirmed through transient absorption spectroscopy. This work not only overcomes the traditional limitations of CT absorption in the NIR regions but also opens new horizons for high-speed OPDs in various applications.