(513f) 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 V_oc
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.

Acknowledgements

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.