posted on 2023-01-09, 16:05authored bySaikat Bag, Himadri Sekhar Sasmal, Sonu Pratap Chaudhary, Kaushik Dey, Dominic Blätte, Roman Guntermann, Yingying Zhang, Miroslav Položij, Agnieszka Kuc, Ankita Shelke, Ratheesh K. Vijayaraghavan, Thalasseril G. Ajithkumar, Sayan Bhattacharyya, Thomas Heine, Thomas Bein, Rahul Banerjee
The synthesis of homogeneous covalent
organic framework (COF) thin
films on a desired substrate with decent crystallinity, porosity,
and uniform thickness has great potential for optoelectronic applications.
We have used a solution-processable sphere transmutation process to
synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2
cm2 TiO2-coated fluorine-doped tin oxide (FTO)
surface. This process controls the nucleation of COF crystallites
and molecular morphology that helps the nanospheres to arrange periodically
to form homogeneous COF thin films. We have synthesized four COF thin
films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional
backbones. In a close agreement between the experiment and density
functional theory, the TpEtBr COF film showed the lowest optical band
gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all
four COF films. Hence, the TpEtBr COF film can participate in efficient
charge generation and separation. We constructed optoelectronic devices
having a glass/FTO/TiO2/COF-film/Au architecture, which
serves as a model system to study the optoelectronic charge transport
properties of COF thin films under dark and illuminated conditions.
Visible light with a calibrated intensity of 100 mW cm–2 was used for the excitation of COF thin films. All of the COF thin
films exhibit significant photocurrent after illumination with visible
light in comparison to the dark. Hence, all of the COF films behave
as good photoactive substrates with minimal pinhole defects. The fabricated
out-of-plane photodetector device based on the TpEtBr COF thin film
exhibits high photocurrent density (2.65 ± 0.24 mA cm–2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10–3 cm2 V–1 S–1) compared
to other as-synthesized films, indicating the best photoactive characteristics.