Charge dynamics at heterojunctions for PbS/ZnO colloidal quantum dot solar cells probed with time-resolved surface photovoltage spectroscopy

Time-resolved laser-pump X-ray-photoemission-probe spectroscopy of a ZnO (101 ⎯ ⎯ 0 101¯0 ) substrate with and without PbS quantum dots (QDs) chemically linked to the surface is performed, using laser photon energies resonant with and below the band gap energy of the substrate (λ = 372 and 640 nm, hν = 3.33 and 1.94 eV). Charge injection from the photoexcited QDs to ZnO is demonstrated through the change in the surface photovoltage of the ZnO substrate observed when the heterojunction is illuminated with 1.94 eV radiation. The measured carrier dynamics are limited by the persistent photoconductivity of ZnO, giving dark carrier lifetimes of the order of 200 μs in a depletion layer at the interface. The chemical specificity of soft X-rays is used to separately measure the charge dynamics in the quantum dots and the substrate, yielding evidence that the depletion region at the interface extends into the PbS QD layer

Crystalline morphologies at the surface of PET/PEN random copolymer films

A series of PET (poly(ethylene terephthalate))/PEN (poly(ethylene 2,6‐naphthalate)) copolyesters were synthesized by molten transesterification, and the surface crystallization behavior of their thin films investigated by atomic force microscope with an in situ heating stage. Force‐distance measurements detected a surface glass transition (T gS) of the copolymers several tens of degrees below their bulk glass transition (T gB) obtained by differential scanning calorimeter. The surface crystalline morphologies as a function of annealing temperature and film thickness were summarized as surface morphology diagrams. The surface crystallization temperature (T cS) was found to be several degrees lower than the bulk crystallization (T cB), and the films thinner than ~100 nm showed significant increase in T cB. The lamellar crystalline morphology of copolymers with high randomness and short sequence length deviated from that of the homopolymers, reflecting the composition and degree of randomness. Highly random PET/PEN = 75/25 wt% copolymers exhibited unique lamellar curvature with arbitrary growth directions. Sharp boundaries between the crystals and amorphous suggested an absence of large amounts of rejected material at the growth front. In the case of copolymers with high randomness and short sequence length, no bulk crystallization morphology was observed even at 190°C, with the relatively thick surface crystalline layer totally covering the emergence of any bulk crystals. </p

The solvation of poly(vinyl alcohol)

The possible incorporation of water molecules within the crystal structure of poly(vinyl alcohol) is discussed. Modelling of the crystal structure suggested that water could be incorporated without severe disruption, and the effect on the X-ray powder diffraction trace was simulated. The effect of variation in tacticity is discussed in terms of the nature of the hydrogen bonding. Simulated traces are compared with experimental data from atactic samples in which a change in the diffraction peak intensities is observed for samples crystallised with water present. This is compared with samples produced from nonaqueous solutions

Crystalline morphologies at the surface of PET/PEN random copolymer films

A series of PET (poly(ethylene terephthalate))/PEN (poly(ethylene 2,6‐naphthalate)) copolyesters were synthesized by molten transesterification, and the surface crystallization behavior of their thin films investigated by atomic force microscope with an in situ heating stage. Force‐distance measurements detected a surface glass transition (T gS) of the copolymers several tens of degrees below their bulk glass transition (T gB) obtained by differential scanning calorimeter. The surface crystalline morphologies as a function of annealing temperature and film thickness were summarized as surface morphology diagrams. The surface crystallization temperature (T cS) was found to be several degrees lower than the bulk crystallization (T cB), and the films thinner than ~100 nm showed significant increase in T cB. The lamellar crystalline morphology of copolymers with high randomness and short sequence length deviated from that of the homopolymers, reflecting the composition and degree of randomness. Highly random PET/PEN = 75/25 wt% copolymers exhibited unique lamellar curvature with arbitrary growth directions. Sharp boundaries between the crystals and amorphous suggested an absence of large amounts of rejected material at the growth front. In the case of copolymers with high randomness and short sequence length, no bulk crystallization morphology was observed even at 190°C, with the relatively thick surface crystalline layer totally covering the emergence of any bulk crystals