Fluorinated Conjugated Polymers Enabled Enhanced Detectivity in Organic Near-Infrared Photodetectors

Halogen substitution is often used to regulate the photoelectric properties of conjugated polymers (CPs). In particular, fluorine substitution can not only regulate the energy level of CPs but also improve their crystallinity, trap density, and mobility. However, the correlation between the molecule structure of fluorinated CPs and performance of organic photodetectors (OPDs) still remains unclear and is influenced by complex factors. In this work, we have systematically investigated the effect of fluorinated building blocks in CPs on the performance of OPDs specifically considering the number of fluorine (F) atoms. We found that increasing the number of fluorine atoms in CPs gradually deepened their highest occupied molecular orbital (HOMO) energy levels. Notably, PFBDB-T-2F with four F substitutions exhibited a deep HOMO (−5.73 eV) close to the acceptor (−5.75 eV), effectively blocking minority carrier injection from the electrodes and inhibiting dark current (Jdark). Consequently, the OPDs based on PFBDB-T-2F demonstrated nearly 3 orders of magnitude higher detectivity (D* = 1.06 × 1013 Jones) compared to that without fluorination. Furthermore, a trade-off between D* and responsivity (R) was observed in OPDs based on the fluorinated CPs. The excessively deep HOMO level in PFBDB-T-2F and the large phase separation of the blend film reduce the carrier collection efficiency of the device and thus decrease the R of PFBDB-T-2F-based OPDs. These findings expose the importance of carefully managing the number of fluorine atoms in the backbone of CPs to achieve a balance between detectivity and responsivity in OPDs.