Understanding Polymers in Thin Films to Improve Photovoltaic Properties and Surface Bound Polymerization

Organic photovoltaics offer the long-term potential as an economically viable alternative for large-scale power generation. Most organic photovoltaics are based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl)(P3HT) with fullerene derivatives such a [6,6]-phenyl-C₆₁-butyric-acid-methyl-ester (PCBM). These bulk heterojunction systems take advantage of phase segregation between P3HT and PCBM to facilitate charge separation and transport. Raman spectroscopy and thermal analysis are used to monitor the ordering and aggregation state of P3HT in P3HT/PCBM thin films as a function of composition and thermal history. Comparison of Raman and X-ray data lead to the conclusion that the “aggregated” species observed in Raman spectroscopy are not crystalline domains as surmised in the literature, but actually correspond to P3HT segments with an intermediate amount of order that are connected to the surface of the P3HT crystal. PCBM is also shown to behave as a plasticizer for the P3HT as it mixes with the amorphous phase, and can dissolve small P3HT crystals. Examination of bilayer systems demonstrates that P3HT domains formed by crystallizing P3HT prior to PCBM deposition creates crystals that are impervious to PCBM. The information supplied from these results provides guidelines to rationally design improved organic photovoltaic active layers.

Further studies were completed to improve the polymerization process of ethyl cyanoacrylate when developing latent fingerprints by the cyanoacrylate fuming method (CFM). During forensic investigations, recovery and identification of latent fingerprints is vital in recreating a crime scene. As a fingerprint ages, however, changes to the chemical nature of the print residue results in a decrease in the quality of the impression obtained using CFM. Attempts to re-hydrate aged prints by exposing prints to water vapor hindered the fuming process, exhibiting a deteriorated print quality after fuming. The water vapor appears to degrade the print, washing away part of the residue. Further experiments demonstrate that fuming at lower temperatures improves the growth of poly(ethyl cyanoacrylate) during the fuming process. An optimum temperature of ~ 10 °C is found, as this temperature balances looser ion pairs and sufficient energy to allow the reaction of monomer to the propagating chain end.