Date of Award

Fall 2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Electrical & Computer Engineering

Program/Concentration

Electrical Engineering

Committee Director

Gon Namkoong

Committee Member

Abdel Tarek Fattah

Committee Member

Helmut Baumgart

Call Number for Print

Special Collections LD4331.E55 S96 2008

Abstract

Conjugated polymer/fullerene based solar cells represent an exciting alternative to inorganic ones because of their low production costs, flexibility and low weight. At present, commercialization of organic photovoltaic (OPV) cells is limited due to their relatively low efficiency in comparison to silicon ones. In order to understand the operation of OPV cells and to increase their efficiency, more information about the structure of the absorber layer is needed. In particular, the connection between structure and properties of the absorber layer is of great importance. Performance of bulk heterojunction organic solar cells based on poly (3-hexylthiophene) as donor and a soluble fullerene PCBM (C6o or C70) as acceptor is strongly influenced by active layer morphology. Optimized high temperature annealing under nitrogen atmosphere leads to a well-defined nanoscale (< 5 nm) interpenetrating network. Films made of organic blend by mixing P3HT, PCBM are annealed at different temperatures. Improvement in film morphology upon annealing results in enhanced photovoltaic device performance. This enhanced device performance is thought to be due partly to the improvement in the interfacial contact between the metal electrode and the active layer. Upon annealing the roughness of the active layer increases and becomes highest at 150°C. To elucidate the nanomorphology, atomic force microscopy (AFM) is used. Morphology studies and roughness analysis of the blend films (P3HT:PCBM-C70) indicated that excellent miscibility between polymer and PCBM favors exciton separation and improved absorption spectrum. The aim of this work was to study the correlation between structural, optical and transport properties of conjugated polymer/fullerene films. We start by investigating of pristine polythiophene films. From spectroscopic ellipsometry studies it follows, that the polythiophene films consist of a highly ordered interface layer. A correlation between the surface roughness and absorption was shown. After this, the polythiophene/fullerene films were investigated. We found that annealing of such films supports the formation of polythiophene crystallites due to enhanced diffusion of fullerene at elevated temperatures. The crystallization of polythiophene leads to an increased optical absorption in the visible region due to stronger interchain interaction between polythiophene molecules. The observed increase of the efficiency of the polythiophene/ fullerene solar cells after annealing was explained by improved optical absorption together with improved hole mobility.

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DOI

10.25777/bzby-5r26

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