Very Good.
Maybe explain a little bit about what the various parameters e.g. Q v, are.
Keli Thurston
07/25/2012
The Morphological Effect of Chloronaphthalene Additive on PII2T BHJ Films
Introduction
The potential of solar energy as a plentiful, cheap, and renewable energy source had led
to a great drive to produce low-cost, efficient solar cells. Polymer-based bulk-heterojunction
(BHJ) devices hold great promise due to their flexibility, low production cost, and potential
ability to scale up. To date, BHJ cells suffer from low efficiencies relative to silicon at 6-10% vs.
30%. However, fine tuning of the polymer film’s morphological as well as chemical structure
results in great differences in device performance, and so a better understanding of their
crystallite growth may lead to superior devices. This fine tuning may be controlled by varying
growth conditions such as temperature, drying process, chemical content, etc. One of the more
tunable, processing additive concentration, can have a huge effect on the growth of BHJ film
polymer crystallites.
Significant past precedence exists regarding the impact of adding and altering the
concentration of process additives such as the work by Liang’s group in 20101. Their research
found great promise for the polymer PTBT; the addition of additive DIO to the PTBT resulted in
improved performance, likely in part due to the improved morphology the polymer film
displayed. Another promising new semiconducting polymer PII2T has been shown to respond
dramatically to the addition of additives, but its recent synthesis has resulted in little work
investigating its properties in this respect, making it of particular interest for experimentation.
However to study the changes in properties such as film crystallinity as a function of
additive concentration over time, a form of x-ray diffraction called grazing incidence x-ray
diffraction or GIXD must be used. As a beamline specializing in thin film diffraction using a
large area detector and fast collection rates relative to the film kinetics, BL 11-3 in the SSRL
facility of SLAC provides the proper imaging environment for these films.
1
(Liang, AM, 2010)
Experimental Methods
Materials and Equipment
Beamline 11-3 equiped with a standard 345 mm diameter MAR345 imaging plate was
used for all GIXD imaging of the samples. These samples consisted of films of PII2T that were
prepared through spincoating onto silicon wafers off-site and brought to beamline 11-3 for
testing and approximately measured 2 cm x 2 cm. When not being tested, the samples were kept
covered to prevent accidental light bleaching of their surfaces. To test, a sample was placed
within a sample chamber that could be completely sealed, providing a closed chamber through
which gaseous mixtures could be flowed through. In order to control the proportions of solvent
and additive being introduced with the helium gas flow, a system of flow controllers had to be
arranged with two bubblers, one for 40 mL of chloroform solvent and one for 40 mL of
chloronapthalene-1 additive. Three Brooks Instruments 5850SE mass flow controllers were used
to control the three streams of solvent, additive, and pure He, which in turn were controlled by a
Brooks 2540 electronic controller. The overall set-up is displayed in Figure 1.
Fig. 1: Diagram of the final mass gas flow arrangement: two bubblers with controlling mass flow controller
arranged in parallel with one another and with a pure helium flow line
GIXD and Experimentation
For each trial, one sample was placed within the sample chamber and loaded onto the
sample stage on which it was centered and affixed with vacuum. Once positioned, the sample
chamber was sealed and purged with a 1000 sccm stream of helium for 10 minutes to remove
potential gaseous scattering candidates. Once purged, the sample was aligned in the z (vertical)
direction and phi (angled). Once aligned, the sample was tilted 0.12o to result in total reflectance
off of the silicon to ensure all diffraction data was from the thin surface film.
Next, GIXD imaging and testing began. After the initial He purge, between 2 and 10
frames were taken to evaluate the condition of the films pre-treatment as He was continuously
circulated through. Once these frames were taken, the gas flow was altered to the specifications
of the trial, switching from the pure He line to set rates through the two bubbler lines as tabulated
below [To be inserted after testing]. Immediately upon switching gas flows, 10 to 20 frames of
exposure times ranging from 20 to 90 s depending on the trial were taken to capture the effect of
the solvent/additive mixture on the film. At the conclusion of this “exposure” phase, gas flow
was switched back to a 1000 sccm He purge during which a final 10 frames were taken to
capture the effects of drying on the film. While the precise number of images taken, exposure
time, and experimental conditions were altered between trials, the overall imaging pattern
remained the same.
GIXD Image Analysis
In order to gain useful information from the GIXD images, analysis was required. The Q
vs. Chi plots were opened and converted into the proper Qxy and Qz format using WxWindows
Diffraction Integration Tool v0.993. Next, regions of interest were examined in more detail. One
‘wedge’ from 75o to 105o was integrated to examine the Q v. intensity properties of the
diffraction peaks which in turn would indicate any changes in the spacing between the polymers.
Of interest was also a pair of narrow bands intersecting the horizon; by integrating Chi v.
intensity, these regions indicate a change in angular dependence of the crystallites. After
performing the integration in WxDiff, the data analysis program Igor 6.22 A was used to fit
Voigt distribution functions to the data to provide a method of comparison for the state of the
film over the set of frames.
The position, area, and full width half-max of the resulting Voigt functions indicate
changes in the lamellar spacing, relative crystallinity, and orderliness of the films. The resulting
values were plotted as a function of time and were then analyzed to determine how the specific
parameters of gas flow affected the properties of the film.
Solvent Calibration
One final procedure was to correlate flow rate with percent composition of the solvent
and additive. To do so, chloroform and chloronaphthalene were calibrated by measuring the
mass of liquid removed through bubbling over time at specific flow rates. Thus mass flow rate
could be converted into mass or volumetric percentages of chloronaphthalene relative to
chloroform. To perform this calibration, the system was set up within a fume hood in a manner
similar to one of the upper ‘branches’ shown in Figure 1: the He in source was linked to one
mass flow controller which then lead into one bubbler filled with 40 mL of one of the two
liquids, venting into the hood. He was then bubbled through the system for intervals of five
minutes, between which the bubbler was removed and weighed, and the mass difference was
recorded.
In the case of chloronaphthalene, too little was obtained through this method, so a cold
trap was set up. Rather than venting into the hood, the bubbler was then connected to a cold
finger immersed in liquid N2. [In process; will include more details in future drafts]