Wide-Linear-Dynamic-Range Polymer Photodiode with a New Benzo[1,2-b:4,5-b′]dithiophene-Copolymer: The Role of Crystalline Orientation

A new polymeric semiconductor, which can effectively extend linear dynamic ranges (LDRs) of organic photodiodes (OPDs), was developed. Copolymers based on alkylthio-substituted benzo­[1,2-b:4,5-b′]­dithiophene (BDT) are synthesized in conjunction with fluorinated terthiophene (BDT-Th-3T) or alkylterthiophene (BDT-Th-3AT). When deposited onto an ITO/polyethylenimine ethoxylated (PEIE) substrate, both copolymer thin films render apparent face-on orientations as indicated in the two-dimensional grazing incidence X-ray diffraction (2D-GIXD) results, and especially among them, BDT-Th-3T thin films show much improved crystalline properties. This is because the molecular structure of BDT-Th-3T shows a higher molecular planarity induced by noncovalent intramolecular interaction and small steric hindrance, which are proved from UV–vis-NIR absorption and Raman spectroscopy studies, respectively. BDT-Th-3T polymers effectively maintain their crystalline properties when blended with nonfullerene acceptors and form a bulk heterojunction (BHJ) of a percolating network composed of face-on-oriented donors and acceptors, which is favorable for charge carrier transports in a vertical direction of the device. As a result, the optimized OPD shows a high specific detectivity over 1013 Jones and an unprecedentedly wide LDR of 232 dB. It is demonstrated that the wide LDR is originated from the high charge transporting property of the polymer, which induces a high saturation photocurrent of BHJ.