Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The synthesis of homogeneous covalent organic framework (COF) thin films on the 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 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 bandgap (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. The visible light with a calibrated intensity of 100 mW cm-2 was used for the excitation of COF thin films. All the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all the COF films behave as good photoactive substrates with minimal pin hole 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.