Fabrication and Characterization Of Organic Semiconductors For Use In Photovoltaics

Organic photovoltaics (OPVs) are a potential solution to provide low-cost solar energy. OPV materials are unique in that film morphology and processing conditions drastically affect photon harvesting and charge transport; as such, these parameters must be understood and optimized. Current approaches to investigate these issues utilize device-level measurements to connect film processing with overall OPV performance.  In this work, we focus on the synthesis and characterization of bulk heterojunction (BHJ) solar cells composed of poly(3-hexylthiophene) (P3HT) as a donor and the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor.  BHJ devices were constructed using different processing conditions (e.g. P3HT/PCBM blend ratio, various annealing steps, etc.) and tested at the macro- and nanoscales.  Dark and light I-V measurements on full devices were combined with nanoscale characterization, via tunneling atomic force microscopy (TUNA) and confocal Raman spectroscopy, to evaluate how film morphology, charge transport, and solar conversion efficiency were related to processing. General conclusions of the work were: (1) polymer dissolution was critical in spinning high quality films, (2) slowing down solvent evaporation during and after spin coating fosters polymer chain organization, increasing charge transport, (3) a BHJ bi-layer device composed of P3HT/PCBM with a PCBM overlayer gave the best performance, and (4) oxidative damage to the conjugated polymer inhibited charge transport.  This work, which connected macroscopic performance with nanoscale physicochemical properties, could provide insight to better engineer OPV materials and devices.