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.