Side-chain engineering for fine-tuning of molecular packing and nanoscale blend morphology in polymer photodetectors

A series of four low-bandgap polymers containing diketopyrrolopyrrole (DPP) and dithienopyrrole (DTP) functionalized with four different side chains of 4-phenyl, 4-octylphenyl, 4-(octyloxy)phenyl and 4-(octylthio)phenyl were synthesized. Subtle changes in the side chains of polymers are found to effectively affect the molecular stacking and crystallinity, film morphology, and photodetector performance. The best polymer photodetector displays an average external quantum efficiency (EQE) of 40% and a specific detectivity (D*) of over 1013 Jones in the spectral region of 350–960 nm under −0.1 V bias, which is among the best EQE and D* values of all the known UV–vis–NIR polymer photodetectors reported to date.