2 Relating Processing Conditions to Bulk-Heterojunction
Morphology
One of the stranger lessons to be gleaned from the study of P3HT:PCBM is that the
keys to understanding and controlling P3HT:PCBM BHJ morphology were
published in 1993 [47] and 1994 [48], before the BHJ concept had been introduced
[32–34]. The 1993 paper showed a transmission electron microscopy (TEM) image
and crystal structure for a self-assembled P3HT nanoribbon. The 1994 paper
reported on the solvatochromic and thermochromic properties of P3HT and related
the folding of P3HT, due to reduced temperature or poor solvent, to the strong red
shift in the absorption spectrum.
It is very instructive that in 1993 the strong tendency for P3HT to form ribbonlike nanostructures was known. Also, the solvents necessary to crystallize P3HT,
the techniques to measure the fibers, and the technique to separate and coat the
fibers were all known. In 1994 the research community knew that poor solvents
caused P3HT to form supramolecular structures with coplanar P3HT chains and
that this resulted in a red shift of the absorption spectrum and formation of vibronic
structure. It was known that the same structures result from cooling a P3HT melt.
But nevertheless, the research community did not start using these ideas to control
P3HT:PCBM morphology until a decade later.
Why not?
The P3HT:PCBM processing conditions did not create a linear and obvious
change in morphology and so the relationship between processing conditions and
morphology to OPV performance was not obvious. The rest of this section points
out the various relationships between processing conditions and the final film
morphology. Many new OPV polymers exist that have higher efficiency than
P3HT:PCBM. In terms of processability, these newer polymers may be more or
less soluble, have lower or higher tendency to crystallize, or have differing mutual
solubility with the fullerene of choice. The discussion focuses on the processing
lessons that can be broadly applied to a variety of OPV materials.
2.1 The Fabrication Toolkit
This section was written from the perspective of a device physicist and is designed
to help graduate students with new OPV materials. From this perspective, polymers
and fullerenes arrive at the laboratory in small quantities and one wishes to learn
whether this new material might be a good candidate for OPV devices. Usually, the
synthesis group supplies basic information about a polymer, such as the band gap of
the dissolved polymer determined using a UV/vis spectrometer, the molecular
weight (M W ), and the oxidation/reduction levels of the polymer measured using
cyclic voltammetry (CV).
190
A.J. Moule ´ et al.
Morphology
One of the stranger lessons to be gleaned from the study of P3HT:PCBM is that the
keys to understanding and controlling P3HT:PCBM BHJ morphology were
published in 1993 [47] and 1994 [48], before the BHJ concept had been introduced
[32–34]. The 1993 paper showed a transmission electron microscopy (TEM) image
and crystal structure for a self-assembled P3HT nanoribbon. The 1994 paper
reported on the solvatochromic and thermochromic properties of P3HT and related
the folding of P3HT, due to reduced temperature or poor solvent, to the strong red
shift in the absorption spectrum.
It is very instructive that in 1993 the strong tendency for P3HT to form ribbonlike nanostructures was known. Also, the solvents necessary to crystallize P3HT,
the techniques to measure the fibers, and the technique to separate and coat the
fibers were all known. In 1994 the research community knew that poor solvents
caused P3HT to form supramolecular structures with coplanar P3HT chains and
that this resulted in a red shift of the absorption spectrum and formation of vibronic
structure. It was known that the same structures result from cooling a P3HT melt.
But nevertheless, the research community did not start using these ideas to control
P3HT:PCBM morphology until a decade later.
Why not?
The P3HT:PCBM processing conditions did not create a linear and obvious
change in morphology and so the relationship between processing conditions and
morphology to OPV performance was not obvious. The rest of this section points
out the various relationships between processing conditions and the final film
morphology. Many new OPV polymers exist that have higher efficiency than
P3HT:PCBM. In terms of processability, these newer polymers may be more or
less soluble, have lower or higher tendency to crystallize, or have differing mutual
solubility with the fullerene of choice. The discussion focuses on the processing
lessons that can be broadly applied to a variety of OPV materials.
2.1 The Fabrication Toolkit
This section was written from the perspective of a device physicist and is designed
to help graduate students with new OPV materials. From this perspective, polymers
and fullerenes arrive at the laboratory in small quantities and one wishes to learn
whether this new material might be a good candidate for OPV devices. Usually, the
synthesis group supplies basic information about a polymer, such as the band gap of
the dissolved polymer determined using a UV/vis spectrometer, the molecular
weight (M W ), and the oxidation/reduction levels of the polymer measured using
cyclic voltammetry (CV).
190
A.J. Moule ´ et al.