ABSTRACT
[6,6]-Phenyl-C61-butyric acid methyl ester (PCBM), a fullerene derivative, is the most widely used electron acceptor in bulk-heterojunction (BHJ) organic photovoltaics, and its concentration is usually tuned to achieve optimal device performance. However, PCBM loading can significantly impair the thermal transport performance of the BHJs due to its ultra-low thermal conductivity (0.03-0.07 W m(-1) K(-1)). In this work, we study the thermal conductivity of BHJs as a function of the PCBM concentration using time domain thermoreflectance. The thermal conductivities of BHJs composed of PCBM blended with donor polymers from the PBDTTT family with different side chains systematically deviate from those predicted by effective medium theory. Evidence presented in this work indicates that for these copolymers, only when the polymer concentration reaches a threshold value (â¼30 to 35% volumetric fraction), does the thermal conductivity BHJ film start to increase, possibly due to the formation of high thermal conductivity percolation pathways.
ABSTRACT
We have used coherent X-ray diffraction experiments to characterize both the 1-D and 2-D foci produced by nanofocusing Kirkpatrick-Baez (K-B) mirrors, and we find agreement. Algorithms related to ptychography were used to obtain a 3-D reconstruction of a focused hard X-ray beam waist, using data measured when the mirrors were not optimally aligned. Considerable astigmatism was evident in the reconstructed complex wavefield. Comparing the reconstructed wavefield for a single mirror with a geometrical projection of the wavefront errors expected from optical metrology data allowed us to diagnose a 40 µrad misalignment in the incident angle of the first mirror, which had occurred during the experiment. Good agreement between the reconstructed wavefront obtained from the X-ray data and off-line metrology data obtained with visible light demonstrates the usefulness of the technique as a metrology and alignment tool for nanofocusing X-ray optics.
