Tuesday, June 28, 2011
4:00pm (206 JFB)
Title: Optical & Magnetic Resonance Studies of Organic Materials used in Photovoltaic Applications
Organic photovoltaic cells have recently attracted great interest and research due to their advantages over their inorganic counterparts. In this work we focused on the study of the electronic processes in active materials used to improve the power conversion efficiency of organic photovoltaics.
We studied films of P3HT – electron donor, PCBM acceptors, and their blend by means of optical (PA-photoinduced absorption, DIA-doping induced absorption and PL-photoluminescence) and magnetic resonance (PLDMR-PL detected magnetic resonance, PACDMR-PA detected magnetic resonance, and LESR-light induced electron spin resonance) probes. We used continuous wave spectroscopy, which allows us to study mainly long-lived photoexcitations such as triplet excitons, polarons, and polaron pairs. Most materials have been provided by our collaborators; Plextronics, Inc.
RR P3HT is a common electron donor used in organic solar cells. Since the charge transport is affected by region-regularity which, in turn is correlated with the polymer molecular weight, we studied RR P3HT with various molecular weight. We discovered that the photoinduced absorption (PA) spectrum can be used to differentiate between low and high molecular weight RR P3HT, by tracking the appearance of a polaron PA band at low photon energy. According to the PADMR results used to distinguish the spin nature of PA bands, the PA band in RRa P3HT is of triplet exciton nature. By means of DIA we concluded that in the highest molecular weight RR P3HT the PA band is due to a delocalized triplet exciton.
The study of PCBM films that also included XRD shows the existence of microcrystalline aggregation.
In the study of polymer/fullerene blends we mixed RR P3HT with PCBM. The PA spectrum is dominated by polaron bands in both donors and acceptors matrices. This is supported by the LESR measurements that show tow spin ½ species, one in the polymer and the other in PCBM. In RRa P3HT we could not detect LESR of spin ½ carriers, and this is in agreement with the poor performance of OPV solar cells based on RRa-P3HT/PCBM blend.