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Morphology-performance relationships in polymer/fullerene blends probed by complementary characterisation techniques – effects of nanowire formation and subsequent thermal annealing

Jong Soo Kim, Sebastian Wood, Safa Shoaee, Steve J. Spencer, Fernando A. Castro, Wing Chung Tsoi Orcid Logo, Craig E. Murphy, Myungsun Sim, Kilwon Cho, James Durrant Orcid Logo, Ji-Seon Kim

Journal of Materials Chemistry C, Volume: 3, Issue: 35, Pages: 9224 - 9232

Swansea University Authors: Wing Chung Tsoi Orcid Logo, James Durrant Orcid Logo

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DOI (Published version): 10.1039/C5TC01720C

Abstract

We report detailed analysis of the thin film morphology (molecular packing, molecular conformational order, and vertical phase separation) – performance (charge transport, photocurrent generation, and photovoltaic performance) relationships under nanowire formation and subsequent thermal annealing i...

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Published in: Journal of Materials Chemistry C
ISSN: 2050-7526 2050-7534
Published: 2015
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URI: https://cronfa.swan.ac.uk/Record/cronfa32051
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Abstract: We report detailed analysis of the thin film morphology (molecular packing, molecular conformational order, and vertical phase separation) – performance (charge transport, photocurrent generation, and photovoltaic performance) relationships under nanowire formation and subsequent thermal annealing in polymer:fullerene blends. Nanowires of poly(3-hexylthiophene) (P3HT) are formed by controlled precipitation from solution and blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to form bulk heterojunction thin films. The formation of nanowires and further thermal annealing result in increased molecular order of the P3HT, where the short-range conformational order is maximised by annealing at 100 °C and decreases when annealed at higher temperatures, but the quality of long-range molecular packing and lamellar packing distance increase with annealing temperature up to 150 °C. The long-range order correlates strongly with an increase in hole mobility, but the reduction in short-range conformational order indicates a slight reduction in planarity of the conjugated backbone in this aggregated polymer morphology. Photoconductive atomic force microscopy reveals enhanced connectivity of the hole transporting nanowire network as a result of thermal annealing. Additionally, we find that the nanowire morphology results in a favourable vertical phase separation, with PCBM enrichment at the electron-extracting surface in the conventional architecture, which is contrary to the non-nanowire case. This effect is further encouraged by thermal annealing, resulting in an enhancement of open-circuit voltage, and represents a morphological advantage over conventional P3HT:PCBM devices. Our study identifies an important interplay between long-range and short-range molecular order in charge generation, transport, extraction, and hence solar cell device performance.
College: College of Engineering
Issue: 35
Start Page: 9224
End Page: 9232