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Synthesis and characterization of benzodithiophene-isoindigo polymers for solar cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
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2012 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 5, 2306-2314 p.Article in journal (Refereed) Published
Abstract [en]

Three new alternating polymers with the electron-deficient isoindigo group as the acceptor unit and benzo[1,2-b:4,5-b] dithiophene (BDT) or BDT flanked by thiophenes (or octylthiophenes) as the donor unit were designed and synthesized. All the polymers have good thermal stability, solubility and broad absorption spectra. Their photophysical, electrochemical and photovoltaic (PV) properties were investigated. To understand their different PV performance in the resulting polymer solar cells (PSCs), the morphology of their blends with fullerene derivatives was investigated by atomic force microscopy, and the molecular geometries as well as the molecular frontier orbitals were simulated by density functional theory calculations (Gaussian 09). The polymer PBDT-TIT, with BDT flanked by thiophenes as the donor unit and isoindigo as the acceptor unit, exhibits quite planar backbones and its blend with fullerene derivatives shows optimal morphology. As a result, the PSCs based on PBDT-TIT with a conventional device configuration of ITO/PEDOT: PSS/PBDT-TIT: PC(61)BM/LiF/Al showed a power conversion efficiency of 4.22%, with a short-circuit current density of 7.87 mA cm(-2), an open-circuit voltage of 0.79 V and a fill factor of 0.68 under the AM 1.5G illumination with an intensity of 100 mW cm(-2) from a solar simulator.

Place, publisher, year, edition, pages
Royal Society of Chemistry , 2012. Vol. 22, no 5, 2306-2314 p.
National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-74644DOI: 10.1039/c1jm14940gISI: 000298970700080OAI: diva2:489758

Funding Agencies|Swedish Energy Agency||

Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2015-05-29
In thesis
1. Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells
Open this publication in new window or tab >>Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis focuses on the two critical issues of bulk-heterojunction polymer solar cells: morphology of active layers and energy loss during charge transfer process at electron donor/acceptor interfaces. Both issues determine the performance of polymer solar cells through governing exciton dissociation, charge carrier recombination and free charge carrier transport.

The morphology of active layers (spatial percolation of the donor and acceptor) is crucial for the performance of polymer solar cells due to the limited diffusion length of excitons in organic semiconductors (5-20 nm). Meanwhile, the trade-off between charge generation and transport also needs to be considered. On the one hand, a finely mixed morphology with a large donor/acceptor interface area is preferred for charge generation because efficient exciton dissociation only occurs at the interface, but on the other hand, proper phase separation is needed to reduce charge carrier recombination and facilitate free charge carrier transport to the electrodes. In this thesis, morphologies of the active layers based on different polymeric donors and fullerene acceptors are correlated to the performance of solar cells with various microscopic and spectroscopic techniques including atomic force microscope, transmission electron microscope, grazing incidence x-ray diffraction, photoluminescence, electroluminescence and Fourier transform photocurrent spectroscopy. Furthermore, methods to manipulate the morphologies of solution processed active layers to achieve high performance solar cells are also presented. Processing solvents, chemical structures of the donor and the acceptor materials, and substrate surface properties are found critically important in determining the nanoscale phase separation and performance of polymer solar cells.

Optimizing morphology of active layers alone does not guarantee high performance devices. In addition to spatial percolation, energy arrangements of donors and acceptors are also essential due to contrary requests of the photocurrent and the photovoltage: Efficient exciton dissociation or charge transfer at donor/acceptor interfaces requires large enough energetic driving force, which is also known as energy loss for charge transfer. However, the energy loss due to charge transfer will unavoidably reduce the photovoltage. In this thesis the balance between the photocurrent and the photovoltage in polymer solar cells due to charge transfer at donor/acceptor interfaces is investigated for different active material systems. The driving force tuned by synthesizing series of polymers is determined by directly measuring the optical band gap via UV-Vis spectroscopy and probing the charge transfer recombination via electroluminescence measurements. Influences of driving force on the photocurrent and the photovoltage are characterized via field dependent photoluminescence and internal quantum efficiency measurements. The results correlated well with the performance of the solar cells.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. 53 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1545
National Category
Natural Sciences Engineering and Technology
urn:nbn:se:liu:diva-99430 (URN)10.3384/diss.diva-99430 (DOI)978-91-7519-509-4 (print) (ISBN)
Public defence
2013-11-14, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Available from: 2013-10-17 Created: 2013-10-17 Last updated: 2013-10-17Bibliographically approved

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