Date of Award

Fall 12-7-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Polymer Science and Engineering

Committee Chair

Xiaodan Gu

Committee Chair School

Polymer Science and Engineering

Committee Member 2

Sarah Morgan

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Sergei Nazarenko

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Derek Patton

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Jeffrey Wiggins

Committee Member 5 School

Polymer Science and Engineering

Abstract

Conjugated polymers (CPs) and polymer blends harbor the potential for high-performance organic solar cells (OSCs) due to their short energy payback time, low cost, solution processability, lightweight attributes, and flexibility. However, OSCs suffer from poor thermal stability compared to their inorganic equivalents. This study explores the thermal instability of OSCs, focusing on phase separation of the photoactive layer under heat, resulting in morphology changes and degradation of power conversion efficiency (PCE). Utilizing atomic-force microscopy coupled with infrared spectroscopy (AFM-IR) and differential scanning calorimetry (DSC), we delve into thermal stability-morphology relationships to devise strategies to improve OSC blend durability under thermal stress. The study comprises four interconnected parts:

1) We used AFM-IR to decipher thin-film polymer blend surface morphology, employing isotope labeling to avoid overlapping infrared absorption bands to enable high fidelity compositional analysis of the polymer blends. This approach enables clear visualization of blend separation size and domain purity, a significant breakthrough in analyzing multiphase systems.

2) To examine the nanoscale topography and composition of complex polymer blends, we varied laser parameters in AFM-IR, achieving adjustable depth sensitivity. This allows for studying buried polymeric structures without tomography or destructive etching and understanding the vertical phase separation for polymer blends.

3) With AFM-IR and Flash-DSC, we examined the glass transition temperature and its impact on the morphology of a photoactive blend of PM6 and Y6 under varying thermal conditions. Results indicated that Y6 crystallization causes significant morphological changes contributing to bulk heterojunction (BHJ) layer instability in OPV devices.

4) Finally, we used thermally cleavable sidechains to enhance the thermal stability of CPs in P3ET:PCBM blends, implementing a two-step annealing process to improve the phase separation size and raise the glass transition temperature (Tg) to vitrify the system to reduce morphological shifts.

Overall, this work provides novel analytical methods for examining conjugated polymer blend compositions and illuminates morphological changes upon thermal stress. Understanding thermal stability-morphology relationships can guide the design of more thermally stable optoelectronic devices, promoting high-performance organic solar cells.

ORCID ID

0000-0001-6268-0652

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