Dispersion-orientation effects of fulleropyrrolidine in zone annealed block-copolymer films toward optimizing OPV interfaces

Introduction of an insulating polystyrene block (BCP) polymethylmethacrylate copolymer (PS-b-PMMA) layer between the poly poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) and the photoactive layer (fullerene or functionalized fullerene/poly-3-hexylthiophene) has been reported to improve solar cell performance. We explore how the morphological structure of this ordered interfacial BCP layer may be modified with novel synthesized electron accepting fulleropyrrolidine nanoparticles (f-NP), processed via a novel dynamic zone-annealing (ZA) method. N-methyl-2-(4-nitro phenyl) fulleropyrrolidine and N-methyl-2-(4-cyano phenyl) fulleropyrrolidine were synthesized by 1,3-dipolar cycloaddition of azomethineylides to fullerene and characterized by 1H NMR, 13C NMR, MALDI-TOFMS, cyclic voltammetry, and thermogravimetry. The newly synthesized f-NP's exhibited higher thermal stability and equivalent electronic properties compared to conventionally used [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) for photoactive layer. f-NP filled PS-b-PMMA thin films processed using uniform oven annealing promoted phase segregation driven aggregation of f-NP located at the defect junction points of the block copolymer films. In contrast, ZA of f-NP filled BCP films led to the homogeneous dispersion of f-NPs within the films, however the f-NP had a synergistic orientation effect on BCP films, switching PMMA cylinders from vertical to parallel orientation in the ZA films. This effect is presumably due to a lowering of thermal conductivity of the BCP film by nanoparticles that scatter the phonons thereby decreasing the mean free path length of phonon propagation. These results may be important for the self-assembly of thermally stable and interfacially insulating BCP films for improved solar cell devices.

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