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Molecular Weight Dependence of the Morphology in P3HT:PCBM Solar Cells
journal contribution
posted on 2014-11-26, 00:00 authored by Feng Liu, Dian Chen, Cheng Wang, Kaiyuan Luo, Weiyin Gu, Alejandro L. Briseno, Julia W. P. Hsu, Thomas P. RussellIn
polymer-based photovoltaic devices, optimizing and controlling the
active layer morphology is important to enhancing the device efficiency.
Using poly(3-hexylthiophene) (P3HT) with well-defined molecular weights
(MWs), synthesized by the Grignard metathesis (GRIM) method, we show
that the morphology of the photovoltaic active layer and the absorption
and crystal structure of P3HT are dependent on the MW. Differential
scanning calorimetry showed that the crystallinity of P3HT reached
a maximum for intermediate MWs. Grazing-incidence wide-angle X-ray
diffraction showed that the spacing of the (100) planes of P3HT increased
with increasing MW, while the crystal size decreased. Nonlinear crystal
lattice expansions were found for both the (100) and (020) lattice
planes, with an unusual π–π-stacking enhancement
observed between 50 and 100 °C. The melting point depression
for P3HT, when mixed with [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM), and, hence, the Flory–Huggins interaction parameter
depended on the MW. PCBM was found to perturb the ordering of P3HT
chains. In photovoltaic devices, P3HT with a MW of ∼20K showed
the best device performance. The morphologies of these blends were
studied by grazing-incidence small-angle X-ray scattering (GISAXS)
and resonant soft X-ray scattering. In GISAXS, we observed that the
low-molecular-weight P3HT more readily crystallizes, promoting a phase-separated
morphology.