Single-Particle Spectroscopy on Conducting Polymer-Fullerene Composite Materials for Application in Organic Photovoltaic Devices - - Spectroscopy
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Single-Particle Spectroscopy on Conducting Polymer-Fullerene Composite Materials for Application in Organic Photovoltaic Devices


Spectroscopy
pp. 32-44


Figure 4
With this method, MEH-PPV and P3HT nanoparticles doped with 0 wt%, 5 wt%, 10 wt%, and 50 wt% PCBM were prepared and characterized for size, morphology, and photophysical properties. Dynamic light scattering experiments show that the MEH-PPV nanoparticles have a size of approximately 30 nm that is only minimally affected by doping with PCBM. However, for P3HT, the nanoparticle size is found to increase significantly with PCBM doping. Undoped P3HT nanoparticles have a similar size to those of MEH-PPV, but the 50 wt% PCBM-doped P3HT nanoparticles have a size of approximately 80 nm, indicative of a stronger tendency of PCBM to aggregate with itself in the presence of P3HT compared to MEH-PPV. TEM data depicted in Figure 4 show well-separated individual nanoparticles on the TEM grid. The electron-dense nature of the organics in the nanoparticles ensures that a reasonable contrast can be obtained during imaging. The nanoparticle size measured from TEM data corroborates the findings from dynamic light scattering experiments. The particles have a nearly spherical shape, and although not monodisperse, appear to have a reasonable size distribution.


Figure 5
The optical properties of the nanoparticles were studied by bulk solution UV-vis absorption and fluorescence spectroscopy, and by single-particle imaging and spectroscopy. Absorption spectra of MEH-PPV nanoparticles in water are shifted slightly compared to the absorption spectra of the molecular solutions in tetrahydrofuran (Figure 5a). A unique observation is that for different samples prepared at different times, the nanoparticle absorption spectra vary between blue shifting and red shifting absorbance maxima compared to the molecular solutions in tetrahydrofuran, indicative of variations in single polymer chain morphology upon aggregation into nanoparticles between the different samples. The blue-shifted UV-vis spectra have maxima ranging from 492 to 498 nm and are attributed to a polymer chain conformation with kinking and bending of the chain, leading to a blue shift due to the reduced conjugation length (26,27). The red-shifted UV-vis spectra have maxima ranging from 502 to 515 nm that previously have been assigned to intra and interchain interactions (26,27). This new finding is illustrative of how the single polymer chain conformation is frozen upon assembly into the material: a quasi-instantaneous collapse of the polymer chain onto itself results in either a polymer chain conformation with kinking and bending of the chain, leading to a blue shift due to the reduced conjugation length, or otherwise a polymer chain conformation with extended straight conjugated segments that allow for pi-stacking interchain contacts and thus show red-shifted spectroscopy (28,29). In comparison, the P3HT absorption spectra (Figure 5b) consistently red-shift by 50 nm compared to P3HT solutions in tetrahydrofuran, which is attributed to increased interchain interactions in the densely compacted nanoparticles. In addition, for the nanoparticle suspensions, a shoulder in the absorption spectrum at 600 nm is apparent, which is assigned to interplane interactions (30,31). Neither the MEH-PPV nor the P3HT samples showed obvious variations in absorption spectra for different PCBM doping levels outside of the appearance of the PCBM absorption peak at 330 nm. As such, there is no apparent ground state interaction between conducting polymer and fullerene in the composite nanoparticles.


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