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Understanding structure-property relationship of bulk hetero-junction polymer-fullerene blends

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Title: Understanding structure-property relationship of bulk hetero-junction polymer-fullerene blends
Authors: Matrone, Giovanni Maria
Item Type: Thesis or dissertation
Abstract: Progress over the last few years designing new materials for organic bulk heterojunction (BHJ) solar cells has been enormous with efficiencies reaching more than 13%1 . However, a multitude of combinations exist on how to blend donors and acceptors, with a wide variety of processing options, rendering materials selection and testing an intricate task. Indeed, detailed fundamental understanding between phase behaviour and BHJ properties is still lacking, drastically limiting, among many things, device reproducibility and general progress in the field2 . Despite the apparently chaotic scenario, a simple pathway is here presented to predict BHJs microstructure and phase morphology in OPV using PCE11 (i.e. PffBT4T-2O) and poly(2,5-bis(3-tetradecyllth-iophen-2-yl) thieno[3,2,-b]thiophene) (i.e. pBTTT) :fullerene blends as model systems. We confirm that differential scanning calorimetry assist in establishing temperature/composition diagrams. These can be read as solidification road-maps to predict BHJ morphologies, by far analogy with metallurgy3,4. Coexistence of relatively phase-pure fullerene4 domains together with a finely intermixed phase has already been proved via several visualisation techniques and it is regarded as a requirement for high performances5,6 . Exploring a range polymer-to-fullerene combinations, different morphology scenarios have been targeted: complete phase separation, co-crystal and fullerene intercalation, disordered intermix. Thereby the investigative focus is on the role played by the intermixed phases, discerning between ordered and disordered ones. Indeed, considering the results on the prototypical P3HT blends and the presented pBTTT, the proposed approach is extended to the class of high-performance D-A polymers system, such as PCE11: PCBM ([6,6]-Phenyl-C61-butyric acid methyl ester). Undeniably phase diagrams may predict phase separation, though hereby an approach is offered to manipulate the length scale of each component-rich domain and the intermixed matrix surrounding it. In this frame the solidification processes are investigated via in-situ spectroscopy, in order to reveal the -HJ aggregates formation mechanisms. “Rebus sic stantibus”, this work offers a platform to disentangle effects of miscibility, processing and final device performance.
Content Version: Open Access
Issue Date: Aug-2019
Date Awarded: Dec-2019
URI: http://hdl.handle.net/10044/1/84806
DOI: https://doi.org/10.25560/84806
Copyright Statement: Creative Commons Attribution NonCommercial Licence
Supervisor: Stingelin, Natalie
Sponsor/Funder: European Commission
Funder's Grant Number: mmre p55608
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses

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