Summary of Expected Project – Keli Thurston
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. This summer, I
will examine the effect of one such condition, processing additive concentration, on the growth
of BHJ film polymer crystallites.
The specific BHJ material that I will investigate consists of a particular PDPP2FT
polymer acting as an electron donor coupled with fullerene as an electron acceptor. For our
experiments, the films will be prepared through spin casting out of chloroform to with a small
amount of chloronapthalene additive has been added prior to being placed in the sample
chamber. Used in this manner, chloronapthalene has been shown to improve phase segregation
and hence improve device efficiencies. Additive concentration will then be altered by varying
the ratio of gaseous solvent and additive pumped thorough the chamber. These gaseous solutions
are constructed by bubbling helium through solvent. X-ray diffraction will then be used to
analyze sample crystallinity at intervals of at most 70 s to observe changes with crystallinity as
the films’ crystallinity develops over the timescale of an hour. Data analysis will then allow
observation of how crystallinity of the films depends on the concentration of the additive with
other variables held constant.
The primary challenges that I will face this summer are twofold. First, I will determine a
method of calibration for measuring the concentrations and amounts of solvent being deposited
onto the surfaces by the gaseous mixture. Second, I will analyze the growth of the BHJ
crystallites as a function of solvent and additive dosing and determine the relative crystallinity of
the films at given time intervals.
This project introduces the possibility of future expansion and work that will likely lie
beyond the scope of a summer project. One likely avenue for future studies: under similar
conditions, examining the crystallization response of a similar PDPP2FT polymer modified to
contain, for instance, a longer linear R group. Another includes analyzing the photovoltaic
properties of the constructed films to learn which growth conditions yielded a donor/acceptor
system that behaves most optimally for solar cell use. This project functions as a potentially
important step towards the overall goal of learning to fine tune or predict growth conditions that
yield ideal BHJ solar cells.