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Vandewal, Koen
Publications (10 of 12) Show all publications
Ma, Z., Sun, W., Himmelberger, S., Vandewal, K., Tang, Z., Bergqvist, J., . . . Wang, E. (2014). Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells. energy and environmental science, 17(1), 361-369
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, p. 361-369Article 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
Keywords
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
Wang, E., Bergqvist, J., Vandewal, K., Ma, Z., Hou, L., Lundin, A., . . . Andersson, M. R. (2013). Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene-Quinoxaline Copolymer. ADVANCED ENERGY MATERIALS, 3(6), 806-814
Open this publication in new window or tab >>Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene-Quinoxaline Copolymer
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2013 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 6, p. 806-814Article in journal (Refereed) Published
Abstract [en]

The side-chain architecture of alternating copolymers based on thiophene and quinoxaline (TQ) is found to strongly influence the solubility and photovoltaic performance. In particular, TQ polymers with different linear or branched alkyloxy-phenyl side chains on the quinoxaline unit are compared. Attaching the linear alkyloxy side-chain segment at the meta- instead of the para-position of the phenyl ring reduces the planarity of the backbone as well as the ability to order. However, the delocalisation across the backbone is not affected, which permits the design of high-performance TQ polymers that do not aggregate in solution. The use of branched meta-(2-ethylhexyl)oxy-phenyl side-chains results in a TQ polymer with an intermediate degree of order. The reduced tendency for aggregation of TQ polymers with linear meta-alkyloxy-phenyl persists in the solid state. As a result, it is possible to avoid the decrease in charge-transfer state energy that is observed for bulk-heterojunction blends of more ordered TQ polymers and fullerenes. The associated gain in open-circuit voltage of disordered TQ:fullerene solar cells, accompanied by a higher short-circuit current density, leads to a higher power conversion efficiency overall. Thus, in contrast to other donor polymers, for TQ polymers there is no need to compromise between solubility and photovoltaic performance.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2013
Keywords
polymer solar cell; solubility; side-chain geometry; backbone twisting; charge-transfer state; thiophene-quinoxaline copolymer
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-96487 (URN)10.1002/aenm.201201019 (DOI)000319888000017 ()
Available from: 2013-08-23 Created: 2013-08-20 Last updated: 2015-05-29
Tang, Z., Andersson, M., George, Z., Vandewal, K., Tvingstedt, K., Heriksson, P., . . . Inganäs, O. (2012). Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells. Advanced Materials, 24(4), 554-558
Open this publication in new window or tab >>Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells
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2012 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 24, no 4, p. 554-558Article in journal (Refereed) Published
Abstract [en]

Inverted polymer solar cells with a bottom metal cathode modified by a conjugated polymer interlayer show considerable improvement of photocurrent and fill factor, which is due to hole blocking at the interlayer, and a modified surface energy which affects the nanostructure in the TQ1/[70]PCBM blend.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-74183 (URN)10.1002/adma.201104579 (DOI)
Available from: 2012-01-20 Created: 2012-01-20 Last updated: 2017-12-08
Andersson, M., Hsu, Y.-T., Vandewal, K., Sieval, A. B., Andersson, M. R. & Inganäs, O. (2012). Mixed C60/C70 based fullerene acceptors in polymer bulk-heterojunction solar cells. Organic electronics, 13(12), 2856-2864
Open this publication in new window or tab >>Mixed C60/C70 based fullerene acceptors in polymer bulk-heterojunction solar cells
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2012 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 13, no 12, p. 2856-2864Article in journal (Refereed) Published
Abstract [en]

Different mixtures of identically substituted C60 and C70 based fullerens have been used as acceptors in three polymer: fullerene systems that strongly express various performance limiting aspects of bulk heterojunction solar cells. Results are correlated with, and discussed in terms of e.g. morphology, charge separation, and charge transport. In these systems, there appears to be no relevant differences in either mobility or energy level positions between the identically substituted C60 and C70 based fullerenes tested. Examples of how fullerene mixtures influence the nano-morphology of the active layer are given. An upper limit to the open circuit voltage that can be obtained with fullerenes is also suggested.

Place, publisher, year, edition, pages
Elsevier, 2012
Keywords
Bulk-heterojunction solar cell; Fullerene; Polymer
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87210 (URN)10.1016/j.orgel.2012.08.028 (DOI)000311681600012 ()
Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06
Vandewal, K., Tvingstedt, K. & Inganäs, O. (2012). Polarization anisotropy of charge transfer absorption and emission of aligned polymer: fullerene blend films. Physical Review B. Condensed Matter and Materials Physics, 86(3), 035212
Open this publication in new window or tab >>Polarization anisotropy of charge transfer absorption and emission of aligned polymer: fullerene blend films
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 3, p. 035212-Article in journal (Refereed) Published
Abstract [en]

An improved understanding of the electronic structure of interfacial charge transfer (CT) states is of importance due to their crucial role in charge carrier generation and recombination in organic donor-acceptor (DA) solar cells. DA combinations with a small difference between the energy of the CT state (E-CT) and energy of the donor exciton (E-D*) are of special interest since energy losses due to electron transfer are minimized, resulting in an optimized open-circuit voltage. In that case, the CT state can be considered as a resonance mixture, containing character of a fully ionic state (D+ A(-)) and of the local polymer excited state (D* A). We show that the D* A contribution to the overall CT state wave function can be determined by measurements of the polarization anisotropy of CT absorption and emission of polymer: fullerene blends with aligned polymer chains. We study two donor polymers, P3HT and TQ1, blended with fullerene acceptors with different ionization potentials, allowing variation of the E-D* -E-CT difference. We find that, upon decreasing E-D* -E-CT, the local excitonic D* A character of the CT state increases, resulting in a decreased fraction of charge transferred and an increased transition dipole moment. For typical polymer: fullerene systems, this effect is expected to become detrimental for device performance if E-D* - E-CT andlt; 0.1 eV. This however, depends on the electronic coupling between D* A and D+ A(-), which we experimentally estimate to be similar to 6 meV for the TQ1: PCBM system.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-81833 (URN)10.1103/PhysRevB.86.035212 (DOI)000306924500006 ()
Note

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

Available from: 2012-09-25 Created: 2012-09-24 Last updated: 2017-12-07
Vandewal, K., Ma, Z., Bergqvist, J., Tang, Z., Wang, E., Henriksson, P., . . . Inganäs, O. (2012). Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage. Advanced Functional Materials, 22(16), 3480-3490
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, p. 3480-3490Article 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
Keywords
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
Tang, Z., George, Z., Ma, Z., Bergqvist, J., Tvingstedt, K., Vandewal, K., . . . Inganäs, O. (2012). Semi-Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency. ADVANCED ENERGY MATERIALS, 2(12), 1467-1476
Open this publication in new window or tab >>Semi-Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency
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2012 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 2, no 12, p. 1467-1476Article in journal (Refereed) Published
Abstract [en]

Semi-transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA-1 modified ITO coated glass substrate as the ohmic electron-collecting cathode and PEDOT:PSS PH1000 as the hole-collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition (similar to 90%) and high transmittance (similar to 50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency (PCE) compared to a single ST solar cell can be constructed by connecting the stacked two ST sub-cells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking five separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell.

Place, publisher, year, edition, pages
Wiley-VCH Verlag Berlin, 2012
Keywords
polymer solar cells, semi-transparent solar cells, interface, conjugated polymers, tandem solar cells
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86894 (URN)10.1002/aenm.201200204 (DOI)000312035800008 ()
Note

Funding Agencies|Swedish Energy Agency||Swedish Research Council||Advanced Functional Materials initiative at Linkoping University||Knut and Alice Wallenberg Foundation (KAW)||KAW||VINNOVA||

Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2015-05-29
Wang, E., Ma, Z., Zhang, Z., Vandewal, K., Henriksson, P., Inganäs, O., . . . Andersson, M. R. (2011). An Easily Accessible Isoindigo-Based Polymer for High-Performance Polymer Solar Cells. Journal of the American Chemical Society, 133(36), 14244-14247
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, p. 14244-14247Article 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
Vandewal, K., Tvingstedt, K. & Inganäs, O. (2011). Charge Transfer States in Organic Donor-Acceptor Solar Cells. Semiconductors and semimetals, 85, 261-295
Open this publication in new window or tab >>Charge Transfer States in Organic Donor-Acceptor Solar Cells
2011 (English)In: Semiconductors and semimetals, ISSN 0080-8784, Vol. 85, p. 261-295Article in journal (Refereed) Published
Abstract [en]

For an efficient conversion of photons to electrons by organic materials used for photovoltaic applications, the presence of a material interface between an electron-donating and electron-accepting material is crucial. This chapter deals with the interfacial charge transfer states formed at such interfaces under solar illumination. Absorption of long-wavelength light, with energy lower than the optical gap of both donor and acceptor results in the direct formation of these charge transfer states. Decay of CT states to the ground state will result in weak light emission. Both CT absorption and emission will be linked to photovoltaic performance. The role of the CT state in determining the open-circuit voltage is discussed in detail. We will also elaborate on the efficiency of dissociation and photocurrent generation from thermally relaxed CT states. Based on thermodynamical considerations and in the absence of nonradiative recombination, upper limits for the efficiency of organic solar cells based on donor–acceptor interfaces are derived and possible improvements and future research directions are indicated.

Place, publisher, year, edition, pages
Academic Press, 2011
Keywords
conjugated polymer
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-76218 (URN)10.1016/B978-0-12-391060-8.00008-3 (DOI)000299294900008 ()
Note

Classified as article and book chapter in Web of Science.

ISBN (Book): 978-0-12-391060-8

Available from: 2012-03-30 Created: 2012-03-30 Last updated: 2017-12-07Bibliographically approved
Ma, Z., Wang, E., Vandewal, K., Andersson, M. R. & Zhang, F. (2011). Enhance performance of organic solar cells based on an isoindigo-based copolymer by balancing absorption and miscibility of electron acceptor. Applied Physics Letters, 99(14), 143302
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, p. 143302-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
Keywords
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
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