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Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-99430DOI: 10.3384/diss.diva-99430ISBN: 978-91-7519-509-4 (print)OAI: oai:DiVA.org:liu-99430DiVA: diva2:656985
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
List of papers
1. An isoindigo-based low band gap polymer for efficient polymer solar cells with high photo-voltage
Open this publication in new window or tab >>An isoindigo-based low band gap polymer for efficient polymer solar cells with high photo-voltage
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2011 (English)In: CHEMICAL COMMUNICATIONS, ISSN 1359-7345, Vol. 47, no 17, 4908-4910 p.Article in journal (Refereed) Published
Abstract [en]

A new low band gap polymer (E-g = 1.6 eV) with alternating thiophene and isoindigo units was synthesized and characterized. A PCE of 3.0% and high open-circuit voltage of 0.89 V were realized in polymer solar cells, which demonstrated the promise of isoindigo as an electron deficient unit in the design of donor-acceptor conjugated polymers for polymer solar cells.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-67974 (URN)10.1039/c1cc11053e (DOI)000289523000012 ()
Available from: 2011-05-04 Created: 2011-05-04 Last updated: 2015-05-29
2. An Easily Accessible Isoindigo-Based Polymer for High-Performance Polymer Solar Cells
Open this publication in new window or tab >>An Easily Accessible Isoindigo-Based Polymer for High-Performance Polymer Solar Cells
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2011 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 133, no 36, 14244-14247 p.Article in journal (Refereed) Published
Abstract [en]

A new, low-band-gap alternating copolymer consisting of terthiophene and isoindigo has been designed and synthesized. Solar cells based on this polymer and PC(71)BM show a power conversion efficiency of 6.3%, which is a record for polymer solar cells based on a polymer with an optical band gap below 1.5 eV. This work demonstrates the great potential of isoindigo moieties as electron-deficient units for building donor-acceptor-type polymers for high-performance polymer solar cells.

Place, publisher, year, edition, pages
American Chemical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-71384 (URN)10.1021/ja206610u (DOI)000295193700025 ()
Note

Funding Agencies|Swedish Energy Agency||

Available from: 2011-10-14 Created: 2011-10-14 Last updated: 2017-12-08
3. Enhance performance of organic solar cells based on an isoindigo-based copolymer by balancing absorption and miscibility of electron acceptor
Open this publication in new window or tab >>Enhance performance of organic solar cells based on an isoindigo-based copolymer by balancing absorption and miscibility of electron acceptor
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 14, 143302- p.Article in journal (Refereed) Published
Abstract [en]

Superior absorption of PC(71)BM in visible region to that of PC(61)BM makes PC(71)BM a predominant acceptor for most high efficient polymer solar cells (PSCs). However, we will demonstrate that power conversion efficiencies (PCEs) of PSCs based on poly[N,N-bis(2-hexyldecyl)isoindigo-6, 6-diyl-alt-thiophene-2,5-diyl] (PTI-1) with PC(61)BM as acceptor are 50% higher than their PC71BM counterparts under illumination of AM1.5G. AFM images reveal different topographies of the blends between PTI-1:PC(61)BM and PTI-1:PC(71)BM, which suggests that acceptors miscibility plays a more important role than absorption. The photocurrent of 9.1 mA/cm(2) is among the highest value in PSCs with a driving force for exciton dissociation less than 0.2 eV.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
Keyword
photoconductivity, polymer blends, solar cells, solubility
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-72028 (URN)10.1063/1.3645622 (DOI)000295625100091 ()
Note

Funding Agencies|Swedish Energy Agency||Swedish Research Council (VR)||VINNOVA||

Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2017-12-08
4. Synthesis and characterization of benzodithiophene-isoindigo polymers for solar cells
Open this publication in new window or tab >>Synthesis and characterization of benzodithiophene-isoindigo polymers for solar cells
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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
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-74644 (URN)10.1039/c1jm14940g (DOI)000298970700080 ()
Note

Funding Agencies|Swedish Energy Agency||

Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08
5. Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells
Open this publication in new window or tab >>Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells
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2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 46, 24462-24468 p.Article in journal (Refereed) Published
Abstract [en]

Here, we demonstrate the correlation between the surface roughness of the ZnO interlayer used as an electron transporting interlayer (ETL) in organic inverted solar cells (ISCs) and the photovoltaic performance of the ISCs. Three different surfaces of the ZnO ETL are studied in ISCs with the polymer poly[2,3-bis-(3-octyloxyphenyl)-quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) mixed with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the active layer. The results obtained from these ISCs show that power conversion efficiency increases from 2.7% to 3.9% when the root-mean-square roughness of the ZnO layer decreases from 48 to 1.9 nm. Moreover, it is found that the short-circuit current density is higher in the ISC based on the smoother ZnO interlayer, with a larger donor/acceptor (D/A) interfacial area in the active layer that facilitates exciton dissociation. The reduced effective interfacial area between the photoactive layer and the ZnO interlayer with decreased ZnO surface roughness leads to an observed improvement in both fill factor and open circuit voltage, which is ascribed to a reduced concentration of traps at the interface between the ZnO interlayer and the active layer.

Place, publisher, year, edition, pages
American Chemical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86648 (URN)10.1021/jp308480u (DOI)000311461100008 ()
Note

Funding Agencies|Swedish Energy Agency (Energimyndigheten)||Swedish Research Council (VR)||VINNOVA||

Available from: 2012-12-20 Created: 2012-12-20 Last updated: 2017-12-06
6. Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage
Open this publication in new window or tab >>Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage
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2012 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 22, no 16, 3480-3490 p.Article in journal (Refereed) Published
Abstract [en]

In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (ED*) and the energy of the charge transfer state (ECT) will optimize the open-circuit voltage (Voc). In this work, this energy loss ED*-ECT is measured directly via Fourier-transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI-1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI-1 are similar (1.4 eV1.5 eV), the optimized photovoltaic devices show large differences in Voc and internal quantum efficiency (IQE). For P3TI:PC71BM blends a ED*-ECT of similar to 0.1 eV, a Voc of 0.7 V and an IQE of 87% are found. For PTI-1:PC61BM blends an absence of sub-gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher Voc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI-1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between ECT and qVoc due to radiative and non-radiative recombination are quantified for both material systems, indicating that for the PTI-1:PC61BM material system, Voc can only be increased by decreasing the non-radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (andlt;0.1 eV) and a high Voc.

Place, publisher, year, edition, pages
Wiley-VCH Verlag Berlin, 2012
Keyword
organic solar cell, fullerene, conjugated polymer, charge transfer state
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-82067 (URN)10.1002/adfm.201200608 (DOI)000307566200016 ()
Note

Funding Agencies|Swedish Energy Agency||Swedish Research Council (VR)||VINNOVA||Knut and Alice Wallenberg foundation||

Available from: 2012-10-01 Created: 2012-09-28 Last updated: 2017-12-07
7. Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells
Open this publication in new window or tab >>Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells
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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
Keyword
organic solar cell, fullerene, conjugated polymer, charge transfer state
National Category
Natural Sciences Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-99424 (URN)10.1039/c3ee42989j (DOI)000329550700025 ()
Available from: 2013-10-17 Created: 2013-10-17 Last updated: 2015-05-29Bibliographically approved

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