Progress in fluorene-based wide-bandgap steric semiconductors

Molecular bulks are favorable for the thermal and morphological stability in organic wide-bandgap semiconducting polymers with potential applications in both information and energy electronics. In this review, we present our progress in the design of fluorene-based bulky semiconductors with a fractal four-element pattern. Firstly, we established one-pot methods to spirofluorenes, especially spiro[fluorene-9,9′-xanthene](SFX) serving as the next-generation spiro-based semiconductors. Secondly, we observed the supramolecular forces at the bulky groups and discovered the supramolecular steric hindrance(SSH) effect on polymorphisms, nanocrystals as well as device performance. Thus, a synergistically molecular attractor-repulsor theory(SMART) was proposed for the control of nanocrystal morphology, thin film phase and morphology. Thirdly, the third possible type of defects has been identified to generate green band(g-band) emission in widebandgap semiconductors by the introduction of molecular strain design of cyclofluorene. Finally, the first bulky polydiarylfluorene with highly crystalline and β conformation was achieved by an attractor-repulsor design of tadpole-shape monomer, which offered an effective platform to fabricate stable wide-bandgap semiconducting devices. All the discoveries offer the solid basis to break through bottlenecks of organic/polymer wide-bandgap semiconductors by the improvements of overall performances.     

Nano-confined and copper-defect in wide-bandgap semiconductors

Copper-doped cadmium iodide nanocrystals were synthesized and grown for an investigation of the optical nonlinear effects via the measurements of the second harmonic generation (SHG) and nonlinear transmittance (NLT). The second-order optical susceptibilities in dependences of the size of the nanocrystals as well as of their copper contents were calculated. The observed size dependence demonstrates the nanosized quantum-confined effect, where the quantum confinement dominates the material's electronic and optical properties, with a clear increase in the SHG with decreasing the thickness of the nanocrystals. A dramatic change of the second-order optical response with increasing copper content of the nanocrystals was also observed. Optical absorption of the nanocrystals studied using NLT technique shows a decrease of the two-photon absorption with increasing thickness of the nanocrystals. The obtained results are discussed in details by exploring the photo-induced electron-phonon anharmonic mechanism for noncentrosymmetry of the impure material processes involving quantization.