272591 Synthesis and Crystallization of All-Conjugated Block Copolymers
The nanoscale structure of the active layer plays a key role in determining the efficiency of photon-to-electricity conversion in bulk heterojunction (BHJ) organic photovoltaics (OPVs), which are typically comprised of a blend of p-type (hole-conductive) and n-type (electron-conductive) organic semiconductors. While some degree of phase separation is desired for creating continuous charge transport pathways, large-scale phase separated domains are unfavorable due to reduced interfacial area for efficient charge separation. This presents significant challenges for all-polymer OPVs, which are made up of a blend of a p-type and n-type conjugated polymers; large-scale phase separation is commonly observed due to the reduced entropy of mixing for polymer blends. While the best performance in all-polymer OPVs (~ 2%) is significantly lower compared with state-of-the art polymer-fullerene BHJs, reducing or eliminating large-scale phase separation in all-polymer OPVs may dramatically improve performance. Advantages of all-polymer OPVs over polymer-fullerene OPVs include a typically higher Voc and broader absorbance due to the presence of two polymeric semiconductors in the active layer.
All-conjugated block copolymers with p- and n-type blocks represent a promising approach to improving the performance of all-polymer OPVs. Phase separation can be avoided and block copolymer self-assembly may lead to ideal structures for charge dissociation and transport. However, the synthesis of well-defined, high molecular weight all-conjugated block copolymers is challenging, and a broad understanding of crystallization and micro-phase segregation in all-conjugated block copolymers is lacking. In this work, we present new synthetic approaches which enable the preparation of all-conjugated block copolymers with little or no homopolymer impurities. Additionally, these approaches enable the preparation of a systematic series of all-conjugated block copolymers with molecular weights greater than 50,000 g/mol in some cases. In one approach, a poly(alkyl thiophene) polymer is coupled to a poly(9,9 dioctyl fluorene) polymer or copolymer through copper-catalyzed azide-alkyne “click” coupling. End-group control in high molecular weight polymers is achieved through the use of an functionalized, external initiator. In a second approach, the Suzuki polymerization of various poly(fluorene) copolymers in the presence of a poly(alkyl thiophene) macroreagent enables the preparation of high molecular weight (> 50,000 g/mol) all-conjugated block copolymers in two steps. This latter approach is particularly useful for synthesizing all-conjugated block copolymers with both p- and n-type polymer blocks.
Figue 1. Reaction scheme for the “click” coupling of two conjugated polymer to make a block copolymer and SEC chromatograph of the starting homopolymers and final block copolymer.
Crystallization and micro-phase segregation in all-conjugated block copolymers is analyzed through a combination of atomic force microscopy (AFM) and grazing-incidence x-ray scattering (GIXS). While previous work with poly(3-hexylthiophene) (P3HT) block copolymers has found that crystallization of P3HT typically dominates the final morphology, GIXS measurements show that P3HT crystalllinity can be suppressed for a large second block (see Figure below). Further evidence is provided by AFM and DSC measurements.
As systems relevant to organic photovoltaics, we investigate block copolymers with P3HT and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (PFOTBT). The limited solubility of PFOTBT precludes the preparation of high molecular weight block copolymers, but we find that the addition of P3HT-b-PFOTBT block copolymers can improve the performance of P3HT/PFOTBT all-polymer photovoltaics. A PCE of 1.5 % is achieved in an all polymer system with 20 wt % added block copolymer, compared with a PCE of just 0.5 % for an all-polymer blend.
This work demonstrates new methods for synthesizing all-conjugated block copolymers and provides strong evidence for improved performance in block copolymer OPVs.
This work was carried out with support from the Welch Foundation for Chemical Research (Grant #C-1750), the Shell Center for Sustainability, and Louis and Peaches Owen. Use of the Center for Nanoscale Materials and Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. J. A. H. acknowledges support from the Century Scholars program.
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