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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

Spring 2014

First Advisor

Thomas P Russell

Second Advisor

Alejandro L Briseno

Subject Categories

Condensed Matter Physics | Polymer Chemistry | Statistical, Nonlinear, and Soft Matter Physics

Abstract

In bulk heterojunction (BHJ) thin film organic photovoltaics (OPV), morphology control is critical to obtain good device efficiency. Nanoscale phase separation that creates bicontinuous interpenetrating structure on a size scale commensurate with exciton diffusion length (~10 nm) is thought to be the ideal morphology. Results obtained from this work indicate that morphology can be affected by chemical structure of the polymer, processing conditions, blending ratio and post treatments. Physical properties of the material, such as crystallinity, crystal orientation, material interactions and miscibility, surface energy and particle aggregations are critical for determining the morphology and thus the device performance. Previous investigations on poly(3-hexylthiophene) (P3HT) based OPV study yielded a solid structure-property relation. However, different physical properties of polymers preclude the direct transfer of P3HT knowledge to better-performing low band gap polymer OPVs, for which the morphology is directly obtained from solvent casting. This thesis discovered commonalities of low band gap polymer based OPVs. Two important photoactive polymers (PTB7 and DPP) are chosen, each with specific properties. In particular, the function of additives in morphology controls was investigated. Fibril formation at the ~10 nanometer scale proved crucial for obtaining high performance in solar cells. Besides these typical crystalline structures, mixed regions also proved important. The mesh size of the fibril network largely determined the current of the device, and thus determined the power conversion efficiency. The aggregation behavior of polymer chains also influenced the BHJ morphology. Besides the fibril network picture, we also observed multi-length scaled morphology in PTB7 based OPV systems.

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