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 cm 2 TiO 2 -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/TiO 2 /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 cm 2 V −1 S −1 ) compared to other assynthesized films, indicating the best photoactive characteristics.

We report a detailed high pressure study involving X-ray diffraction, Raman spectroscopy, and photoluminescence measurements on a model Pb-free solar cell material Cs3Bi2Br9 halide perovskite. The sample starts showing photoluminescence in a broad range of 550–900 nm above 1.4 GPa, due to an isostructural transition to a distorted unit cell. Further enhancement in intensity with pressure is found to be driven by an increase in distortion of BiBr6 octahedra. Electronic band structure calculations show the sample in the high pressure phase to be an indirect band gap semiconductor. The photoluminescence peak shows a kink at higher energy and a broad asymmetric peak at lower energies due to the recombination of free excitons and self-trapped excitons, respectively. The blue shift of the PL peaks until about 4.4 GPa can be related to the extensive structural distortion before the transition to lowest symmetry triclinic phase.

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The increasingly popular, lead-free perovskite, Cs3Bi2I9 has a vulnerable Bi3+ state under reductive potentials, due to the high standard reduction potential of Bi3+/Bi+ (0 < δ < 3). Contrary to...

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