Diarylethene-Type Photochromic Electron Traps Extending Narrow Linear Dynamic Range of Organic Photodetectors. Design Principles and Fate After Electron Transfer. Self-Destructive or Self-Protective?

Linear dynamic ranges (LDRs) of organic photodetectors are typically narrow due to the low charge mobility of organic semiconductors. They have been recently extended by doping the photoactive layer of a p-type organic semiconductor (poly-3-hexylthiophene; P3HT) with a small amount of diarylethene (DAE) photochromic switches. A speculated mechanism is that DAE accept photoelectrons from P3HT to their lowest unoccupied molecular orbitals (LUMOs), i.e., act as n-type traps, only in their aromatic closed-ring form (DAE-c), which is predominant under strong sunlight, and stop such action in their less aromatic open-ring form (DAE-o), which is predominant under dim light. The fate of such n-type dopants after trapping electrons, i.e., the ring-opening energy profile of the DAE-c anion calculated herein also supports the speculated mechanism: DAE anions with trapped electrons tend to be more stable in their closed form (DAE-c) than in their open form (DAE-o), contrary to neutral DAE. The equilibrium is thus shifted from neutral DAE-o to anionic DAE-c after trapping electrons. Therefore, electron-trapped DAE-c anions would be preserved long enough (in a self-protective mechanism) to accumulate near a positively coated electrode and to induce band bending, Schottky barrier thinning, and photomultiplication-type hole injection near the electrode, achieving ultrahigh (≫100%) external quantum efficiency. When the DAE-c anions return to the neutral state by transferring the trapped electrons to (or accepting the injected holes from) the electrode, the equilibrium would be shifted back to the open form (DAE-o) to get ready for the next operation cycle. This application is a rare example that utilizes a photoswitchable electron transfer to the LUMO of DAE-c. Typical DAE derivatives, which have often been used as p-type traps utilizing their photoswitchable highest occupied MO (HOMO) energies, may not be the best candidate for this application. We thus herein virtually screen suitable DAE n-type dopants according to a new set of design rules embracing the reversed roles of their HOMO and LUMO energies. The best candidate found among 133 compounds indeed succeeded in a significant LDR extension in a recent experiment.