Design, Fabrication & Characterization of Organic Photovoltaic Devices

Abstract

<P> In this thesis, several methods of material integration into organic photovoltaic devices are investigated by fabricating solution processed and vacuum coated devices. Each of these methods is aimed at examining and improving one or more of the four critical factors that determine solar cell efficiency: (1) photovoltage, (2) light absorption, (3) exciton separation, and ( 4) charge collection. To investigate and improve photovoltage, the photovoltaic properties of different M-Phthalocyanine/Fullerene (M-Pc/C60 ) blends are measured and demonstrate an improved open circuit voltage (Voc) using trivalent-metal phthalocyanine. Rubrene is also added to the tl-Pc/C60 cells and shown to systematically increase the Voc. To improve light absorption, two new device structures are developed: the parallel tandem cell and the heteromorphic cell. The parallel tandem cell is demonstrated using both all-vacuum coated materials as well as a combination of vacuum and solution processed materials. Results show definitive and significant current contribution from the near-infrared (NIR) wavelengths, and concomitant increase in photocurrent and power conversion efficiency (PCE). The heteromorphic cell demonstrates the integration of two polymorphs of the same M-Pc, yielding a broader external quantum efficiency (EQE) spectrum in the IR region and an increase in the overall PCE. To investigate exciton separation and charge collection, time of flight photoconductivity studies are performed on varying compositions of solution processed polymer/fullerene films as well as pristine and blended M-Pc:C60 films. Results verify the necessity for balanced carrier transport in blended systems, and t he importance of carrier mobility for achieving high fill factors. Finally, the stability of a relatively new polythiophene (PQT-12) in an organic solar cell is investigated , and shown to significantly increase the device lifetime as compared to the standard P3HT polymer. </p>