Concomitant development of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 Å(-1)), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing). These two competing kinetics result in local phase separation with space-filling PCBM and P3HT nanodomains less than 20 nm in size when annealing temperature is kept below 180 °C. Accompanying the morphological development is the synchronized increase in electron and hole mobilities of the BHJ thin-film solar cells, revealing the sensitivity of the carrier transport of the device on the structural features of PCBM and P3HT nanodomains. Optimized structural parameters, including the aggregate size and mean spacing of the PCBM aggregates, are quantitatively correlated to the device performance; a comprehensive network structure of the optimized BHJ thin film is presented.
Presented are transmission electron microscopic observations of micron-sized single crystals
of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) prepared from thin films in the melt state. A preliminary
determination of unit cell dimensions and molecular packing (orthorhombic, a = 2.56 nm, b = 2.34 nm,
c = 3.32 nm, 8 chains, with space group P212121 and density 1.041 g mL-1) was made via combined
considerations of the selected-area electron diffraction (SAED) pattern obtained along the [00l] zone of
the single crystals, the SAED “fiber” pattern obtained from shear-oriented films, and the “powder” pattern
from X-ray diffraction of melt-crystallized thick films in the absence of preferred orientation. In this
model, PFO backbones are generally separated by transversely extended alkyl side chains, consistent
with the dominance of single-chromophore emissions and the general lack of interbackbone delocalization
of PFO chains in this ordered state as indicated by earlier photophysical studies. In addition, microscopic
evidence for the presence of a vast number of nanograins was presented, and its implications in the
crystallization process of semirigid PFO chains were discussed.
Melt crystallization of racemic polylactide (equimolar
PLLA/PDLA)
blend upon slow cooling (1 °C/min from 270 °C) was studied
via a combination of wide-angle X-ray scattering (WAXS), differential
scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy
(FTIR). Results indicated extensive development of racemic (32/31) helical pairs below 220 °C, followed
by emergence of a broad mesomorphic peak in the WAXS profile below
190 °C; the intensity of this mesophase peak started to decrease
at 150 °C, with concomitant emergence of WAXS- or DSC-discernible
formation of stereocomplex (βc) crystals. Isothermal
measurements at 200 vs 170 °C revealed the presence of low vs
high populations of helical pairs; βc crystals were
observed to develop only at 170 °C but not at 200 °C, indicating
the need for adequate population of racemic helical pairs for formation
of their mesomorphic clusters in the melt matrix as precursors of
βc nuclei. The clear change in the melt structure well before the formation of incipient βc crystals reflects strong driving force under large supercooling
toward transformation, but the transformation process is kinetically
suppressed: only after extensive development of racemic helices and
emergence of mesomorphic clusters in the melt matrix may nucleation
occur. These observations suggest that the nucleation process proceeds
in elementary units of preformed helical pairs in the melt matrix,
with an intermediate stage of clustered helical pairs before incipience
of βc crystals.
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