Enhancing Transition Dipole Moments of Heterocyclic Semiconductors via Rational Nitrogen‐Substitution for Sensitive Near Infrared Detection

Designing ultrastrong near‐infrared (NIR) absorbing organic semiconductors is a critical prerequisite for sensitive NIR thin film organic photodetectors (OPDs), especially in the region of beyond 900 nm, where the absorption coefficient of commercial single crystalline silicon (c‐Si) is below 103 cm−1. Herein, a pyrrolo[3,2‐b]thieno[2,3‐d]pyrrole heterocyclic core (named as BPPT) with strong electron‐donating property and stretched geometry is developed. Relative to their analogue Y6, BPPT‐contained molecules, BPPT‐4F and BPPT‐4Cl, show substantially upshifted and more delocalized highest occupied molecular orbitals, and larger transition dipole moments, leading to bathochromic and hyperchromic absorption spectra extending beyond 1000 nm with very large absorption coefficients (up to 3.7–4.3 × 105 cm−1) as thin films. These values are much higher than those (104 to 1 × 105 cm−1) of typical organic semiconductors, and 1–2 orders higher than those of commercial inorganic materials, such as c‐Si, Ge, and InGaAs. The OPDs based on BPPT‐4F or BPPT‐4Cl blending polymer PBDB‐T show high detectivity of above 1012 Jones in a wide wavelength range of 310–1010 nm with excellent peak values of 1.3–2.2 × 1013 Jones, respectively, which are comparable with and even better than those commercial inorganic photodetectors. Pyrrolo[3,2‐b]thieno[2,3‐d]pyrrole heterocyclic semiconductors with stretched geometry show enhanced transition dipole moments, leading to large absorption coefficients of up to 4.3 × 105 cm−1 beyond 900 nm in thin films. The self‐powered organic photodetectors in this work show high detectivity (1012 to 2.2 × 1013 Jones) over a broad spectrum of 310–1010 nm, comparable with some commercial inorganic photodetectors.