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Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
Department of Chemical and Biological Engineering/Polymer Technology, Chalmers University of Technology, Göteborg, Sweden.
Department of Material Science and Engineering, Stanford University, USA.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
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2014 (English)In: energy and environmental science, ISSN 1754-5692, Vol. 17, no 1, 361-369 p.Article in journal (Refereed) Published
Abstract [en]

A series of alternating oligothiophene (nT)-isoindigo (I) copolymers (PnTI) were synthesized to investigate the influence of the oligothiophene block length on the photovoltaic (PV) properties of PnTI:PCBM bulk-heterojunction blends. Our study indicates that the number of thiophene rings (n) in the repeating unit alters both, polymer crystallinity and polymer-fullerene interfacial energetics, which results in a decreasing open-circuit voltage (Voc) of the solar cells with increasing n. The short-circuit current density (Jsc) of P1TI:PCBM devices is limited by the absence of a significant driving force for electron transfer. Instead, blends based on P5TI and P6TI feature large polymer domains, which limit charge generation and thus Jsc. The best PV performance with a power conversion efficiency of up to 6.9% was achieved with devices based on P3TI, where a combination of favorable morphology and optimal interface energy level offset ensures efficient exciton separation and charge generation. The structure-property relationship demonstrated in this work is a valuable guideline for the design of high performance polymers with small energy losses during the charge generation process, allowing for the fabrication of efficient solar cells that combine a minimal loss in Voc with a high Jsc.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2014. Vol. 17, no 1, 361-369 p.
Keyword [en]
organic solar cell, fullerene, conjugated polymer, charge transfer state
National Category
Natural Sciences Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-99424DOI: 10.1039/c3ee42989jISI: 000329550700025OAI: oai:DiVA.org:liu-99424DiVA: diva2:656978
Available from: 2013-10-17 Created: 2013-10-17 Last updated: 2015-05-29Bibliographically approved
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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1545
National Category
Natural Sciences Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-99430 (URN)10.3384/diss.diva-99430 (DOI)978-91-7519-509-4 (ISBN)
Public defence
2013-11-14, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-10-17 Created: 2013-10-17 Last updated: 2013-10-17Bibliographically approved

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