Towards efficient NFA-based selective near-infrared organic photodetectors: impact of thermal annealing of polymer blends

Abstract

For wavelength-selective photodetection or color discrimination, organic photodetectors (OPDs) can provide significant advantages as solution processability, chemical versatility and functionality. To eliminate the need for commonly used filters, the development of a narrowing approach that simultaneously achieves a selective detection range of less than 50 nm bandwidth and a spectral response greater than 20%, especially for wavelengths designed in the near-infrared, remains a real challenge. Herein, we demonstrate a bulk-heterojunction (BHJ) organic blend exhibiting a filter-free visible-blind near-infrared light responsive characteristic. An indacenodithienothiophene-based non-fullerene acceptor (NFA) of the ITIC family incorporating fluorine atoms, namely ITIC-4F, is mixed with PM6 donor polymer to form ultra-thick active layers in OPDs. A systematic comparison between inverted versus normal architectures was performed using different pairs of hole and electron extraction layer materials. Depending on the thermal annealing temperature of PM6:ITIC-4F blends (from 100 °C up to 200 °C), distinct light responses were observed. While the spectral response of normal structures changes from narrow to broadband with the temperature, inverted structures improve their spectral tuning. A detailed analysis of structure and morphology of blends upon thermal annealing was performed by atomic force microscopy (AFM), contact water angle (CWA) measurement, Raman analysis and 2D grazing-incidence X-ray diffractometry (2D-GIXRD). The transport properties of electrons and holes were further investigated in blends following a Space-Charge-Limited current (SCLC) protocol. We found a marked misbalance between hole and electron mobility values at 200 °C together with highly crystalline films. As a result, the optimized OPD exhibits a high-selective spectral response following the charge collection narrowing (CCN) principle with an external narrowband quantum efficiency of 24.5% and a bandwidth of 42 nm centered at 807 nm. A specific detectivity as high as ∼6 × 10¹² Jones is also achieved due to a low dark current density.