Spectrally Switchable Photodetection with Near-Infrared-Absorbing Covalent Organic Frameworks

Most covalent organic frameworks
(COFs) to date are made from relatively
small aromatic subunits, which can only absorb the high-energy part
of the visible spectrum. We have developed near-infrared-absorbing
low bandgap COFs by incorporating donor–acceptor-type isoindigo-
and thieno­isoindigo-based building blocks. The new materials
are intensely colored solids with a high degree of long-range order
and a pseudo-quadratic pore geometry. Growing the COF as a vertically
oriented thin film allows for the construction of an ordered interdigitated
heterojunction through infiltration with a complementary semiconductor.
Applying a thieno­isoindigo-COF:fullerene heterojunction as the
photoactive component, we realized the first COF-based UV- to NIR-responsive
photodetector. We found that the spectral response of the device is
reversibly switchable between blue- and red-sensitive, and green-
and NIR-responsive. To the best of our knowledge, this is the first
time that such nearly complete inversion of spectral sensitivity of
a photodetector has been achieved. This effect could lead to potential
applications in information technology or spectral imaging.