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 10 3 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 × 10 5 cm -1 ) as thin films. These values are much higher than those (10 4 to 1 × 10 5 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 10 12 Jones in a wide wavelength range of 310-1010 nm with excellent peak values of 1.3-2.2 × 10 13 Jones, respectively, which are comparable with and even better than those commercial inorganic photodetectors.