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Tress, Wolfgang
Publications (10 of 15) Show all publications
Schwarze, M., Tress, W., Beyer, B., Gao, F., Scholz, R., Poelking, C., . . . Leo, K. (2016). Band structure engineering in organic semiconductors. Science, 352(6292), 1446-1449
Open this publication in new window or tab >>Band structure engineering in organic semiconductors
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2016 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 352, no 6292, p. 1446-1449Article in journal (Refereed) Published
Abstract [en]

A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

Place, publisher, year, edition, pages
AMER ASSOC ADVANCEMENT SCIENCE, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-130126 (URN)10.1126/science.aaf0590 (DOI)000377975400045 ()27313043 (PubMedID)
Note

Funding Agencies|DFG [LE-747/44-1]; European Communitys Seventh Framework Programme [EP7-267995]; Knut and Alice Wallenberg Foundation through a Wallenberg Scholar grant; Swedish Research Council (VR) [330-2014-6433]; European Commission, Made Sklodowska-Curie Actions [INCA 600398]; Bundesministerium fur Bildung und Forschung [FKZ 03EK3503B]; Dr Isolde-Dietrich-Stiftung

Available from: 2016-07-12 Created: 2016-07-11 Last updated: 2017-11-28
Bergqvist, J., Tress, W., Forchheimer, D., Melianas, A., Tang, Z., Haviland, D. & Inganäs, O. (2016). New method for lateral mapping of bimolecular recombination in thin film organic solar cells. Progress in Photovoltaics, 24(8), 1096-1108
Open this publication in new window or tab >>New method for lateral mapping of bimolecular recombination in thin film organic solar cells
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2016 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 24, no 8, p. 1096-1108Article in journal (Refereed) Published
Abstract [en]

The best organic solar cells are limited by bimolecular recombination. Tools to study these losses are available; however, they are only developed for small area (laboratory-scale) devices and are not yet available for large area (production-scale) devices. Here we introduce the Intermodulation Light Beam-Induced Current (IMLBIC) technique, which allows simultaneous spatial mapping of both the amount of extracted photocurrent and the bimolecular recombination over the active area of a solar cell. We utilize the second-order non-linear dependence on the illumination intensity as a signature for bimolecular recombination. Using two lasers modulated with different frequencies, we record the photocurrent response at each modulation frequency and the bimolecular recombination in the second-order intermodulation response at the sum and difference of the two frequencies. Drift-diffusion simulations predict a unique response for different recombination mechanisms. We successfully verify our approach by studying solar cells known to have mainly bimolecular recombination and thus propose this method as a viable tool for lateral detection and characterization of the dominant recombination mechanisms in organic solar cells. We expect that IMLBIC will be an important future tool for characterization and detection of recombination losses in large area organic solar cells.

Place, publisher, year, edition, pages
John Wiley & Sons, 2016
Keywords
Organic photovoltaics, imaging, photocurrent, bimolecular recombination, light beam induced current, LBIC, intermodulation
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-123033 (URN)10.1002/pip.2770 (DOI)000380164100007 ()
Note

Funding agencies|Swedish Research Council; Swedish Energy Agency; the Knut and Alice Wallenberg foundation through a Wallenberg Scholar grant to O.I

At the time for thesis presentation publication was in status: Manuscript

Available from: 2015-12-02 Created: 2015-12-02 Last updated: 2017-12-01Bibliographically approved
Tress, W., Marinova, N., Inganäs, O., Nazeeruddin, M. K. K., Zakeeruddin, S. M. & Graetzel, M. (2015). Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination. ADVANCED ENERGY MATERIALS, 5(3), 1400812
Open this publication in new window or tab >>Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination
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2015 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 5, no 3, p. 1400812-Article in journal (Refereed) Published
Abstract [en]

n/a

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2015
Keywords
perovskites; solar cells; open-circuit voltage; luminescence; radiative recombination
National Category
Biological Sciences
Identifiers
urn:nbn:se:liu:diva-116836 (URN)10.1002/aenm.201400812 (DOI)000350565400002 ()
Note

Funding Agencies|Knut and Alice Wallenberg foundation (Sweden); European Union [604032]; European Research Council (ERC) [ARG 247404]

Available from: 2015-04-07 Created: 2015-04-07 Last updated: 2015-04-07
Gao, F., Tress, W., Wang, J. & Inganäs, O. (2015). Temperature Dependence of Charge Carrier Generation in Organic Photovoltaics. Physical Review Letters, 114(12), 128701
Open this publication in new window or tab >>Temperature Dependence of Charge Carrier Generation in Organic Photovoltaics
2015 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 12, p. 128701-Article in journal (Refereed) Published
Abstract [en]

The charge generation mechanism in organic photovoltaics is a fundamental yet heavily debated issue. All the generated charges recombine at the open-circuit voltage (VOC), so that investigation of recombined charges at VOC provides a unique approach to understanding charge generation. At low temperatures, we observe a decrease of VOC, which is attributed to reduced charge separation. Comparison between benchmark polymer: fullerene and polymer: polymer blends highlights the critical role of charge delocalization in charge separation and emphasizes the importance of entropy in charge generation.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Biological Sciences
Identifiers
urn:nbn:se:liu:diva-117377 (URN)10.1103/PhysRevLett.114.128701 (DOI)000351735600018 ()
Note

Funding Agencies|National Basic Research Program of China [2015CB932200]; Natural Science Foundation of Jiangsu [BK20131413]; National Natural Science Foundation of China [11474164]; Swedish Research Council (VR); European Commission Marie Sklodowska-Curie actions; Swedish Energy Agency; Knut and AliceWallenberg foundation (KAW)

Available from: 2015-04-24 Created: 2015-04-24 Last updated: 2017-12-04
Tang, Z., Tress, W., Bao, Q., Jafari, M. J., Bergqvist, J., Ederth, T., . . . Inganäs, O. (2014). Improving Cathodes with a Polymer Interlayer in Reversed Organic Solar Cells. Advanced Energy Materials, 4(15), Article ID 1400643.
Open this publication in new window or tab >>Improving Cathodes with a Polymer Interlayer in Reversed Organic Solar Cells
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2014 (English)In: Advanced Energy Materials, ISSN 1614-6832, Vol. 4, no 15, article id 1400643Article in journal (Refereed) Published
Abstract [en]

The effects of cathode modification by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer, which is further related to an improved built-in electric field and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding efficient exciton dissociation and photocurrent extraction in the modified solar cells. Hence, internal quantum efficiency is found to be significantly higher than that of their unmodified counterparts, while optically, the modified and unmodified solar cells are identical. Moreover, the deep highest occupied molecular orbital (HOMO) of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fill factor.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2014
Keywords
organic solar cells; polymer solar cells; interlayer modification; interfaces; selectivity
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-112636 (URN)10.1002/aenm.201400643 (DOI)000344368500014 ()
Note

Funding Agencies|KAW; Science Council (VR); Swedish Energy Agency; Knut and Alice Wallenberg Foundation KAW through the project Power Papers; Wallenberg Scholar grant

Available from: 2014-12-08 Created: 2014-12-05 Last updated: 2017-01-11Bibliographically approved
Tang, Z., Tress, W. & Inganäs, O. (2014). Light trapping in thin film organic solar cells. Materials Today, 17(8), 389-396
Open this publication in new window or tab >>Light trapping in thin film organic solar cells
2014 (English)In: Materials Today, ISSN 1369-7021, E-ISSN 1873-4103, Vol. 17, no 8, p. 389-396Article, review/survey (Refereed) Published
Abstract [en]

A major issue in organic solar cells is the poor mobility and recombination of the photogenerated charge carriers. The active layer has to be kept thin to facilitate charge transport and minimize recombination losses. However, optical losses due to inefficient light absorption in the thin active layers can be considerable in organic solar cells. Therefore, light trapping schemes are critically important for efficient organic solar cells. Traditional light trapping schemes for thick solar cells need to be modified for organic thin film solar cells in which coherent optics and wave effects play a significant role. In this review, we discuss the light trapping schemes for organic thin film solar cells, which includes geometric engineering of the structure of the solar cell at the micro and nanoscale, plasmonic structures, and more.

Place, publisher, year, edition, pages
Elsevier, 2014
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-103929 (URN)10.1016/j.mattod.2014.05.008 (DOI)000344209800018 ()
Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2017-12-06Bibliographically approved
Tress, W. (2014). Organic Solar Cells: Theory, Experiment, and Device Simulation. Springer
Open this publication in new window or tab >>Organic Solar Cells: Theory, Experiment, and Device Simulation
2014 (English)Book (Refereed)
Abstract [en]

This book covers in a textbook-like fashion the basics or organic solar cells, addressing the limits of photovoltaic energy conversion and giving a well-illustrated introduction to molecular electronics with focus on the working principle and characterization of organic solar cells. Further chapters based on the author’s dissertation focus on the electrical processes in organic solar cells by presenting a detailed drift-diffusion approach to describe exciton separation and charge-carrier transport and extraction. The results, although elaborated on small-molecule solar cells and with focus on the zinc phthalocyanine: C60 material system, are of general nature. They propose and demonstrate experimental approaches for getting a deeper understanding of the dominating processes in amorphous thin-film based solar cells in general.

 The main focus is on the interpretation of the current-voltage characteristics (J-V curve). This very standard measurement technique for a solar cell reflects the electrical processes in the device. Comparing experimental to simulation data, the author discusses the reasons for S-Shaped J-V curves, the role of charge carrier mobilities and energy barriers at interfaces, the dominating recombination mechanisms, the charge carrier generation profile, and other efficiency-limiting processes in organic solar cells. The book concludes with an illustrative guideline on how to identify reasons for changes in the J-V curve.

 This book is a suitable introduction for students in engineering, physics, material science, and chemistry starting in the field of organic or hybrid thin-film photovoltaics. It is just as valuable for professionals and experimentalists who analyze solar cell devices.

Place, publisher, year, edition, pages
Springer, 2014. p. 464
Series
Springer Series in Materials Science, ISSN 0933-033X, E-ISSN 2196-2812 ; 208
National Category
Biochemistry and Molecular Biology Cell and Molecular Biology
Identifiers
urn:nbn:se:liu:diva-118966 (URN)10.1007/978-3-319-10097-5 (DOI)978-3-319-10096-8 (ISBN)978-3-319-10097-5 (ISBN)
Available from: 2015-06-05 Created: 2015-06-05 Last updated: 2018-01-25Bibliographically approved
Bergqvist, J., Lindqvist, C., Backe, O., Ma, Z., Tang, Z., Tress, W., . . . Müller, C. (2014). Sub-glass transition annealing enhances polymer solar cell performance. Journal of Materials Chemistry A, 2(17), 6146-6152
Open this publication in new window or tab >>Sub-glass transition annealing enhances polymer solar cell performance
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2014 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 17, p. 6146-6152Article in journal (Refereed) Published
Abstract [en]

Thermal annealing of non-crystalline polymer: fullerene blends typically results in a drastic decrease in solar cell performance. In particular aggressive annealing above the glass transition temperature results in a detrimental coarsening of the blend nanostructure. We demonstrate that mild annealing below the glass transition temperature is a viable avenue to control the nanostructure of a non-crystalline thiophene-quinoxaline copolymer: fullerene blend. Direct imaging methods indicate that coarsening of the blend nanostructure can be avoided. However, a combination of absorption and luminescence spectroscopy reveals that local changes in the polymer conformation as well as limited fullerene aggregation are permitted to occur. As a result, we are able to optimise the solar cell performance evenly across different positions of the coated area, which is a necessary criterion for large-scale, high throughput production.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-106302 (URN)10.1039/c3ta14165a (DOI)000333580700024 ()
Available from: 2014-05-06 Created: 2014-05-05 Last updated: 2015-12-03Bibliographically approved
Tang, Z., Tress, W., Bao, Q., Jafari, M. J., Bergqvist, J., Ederth, T., . . . Inganäs, O. (2014). Universal modification of poor cathodes into good ones by a polymer interlayer for high performance reversed organic solar cells.
Open this publication in new window or tab >>Universal modification of poor cathodes into good ones by a polymer interlayer for high performance reversed organic solar cells
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2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

In organic bulk-heterojunction solar cells, energy losses at the active layer/electrode interface are often observed. Modification of these interfaces with organic interlayers optimizes charge carrier injection and extraction and thus improves device performance. In this work, the effects of cathode modification by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer; further related to an improved built-in electric field and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding efficient exciton dissociation and photocurrent extraction in the modified solar cells. Hence, internal quantum efficiency is found significantly higher than that of their unmodified counterparts, while optically, the modified and unmodified solar cells are identical. Moreover, the deep HOMO of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fill factor. We demonstrate a possibility of improving photovoltaic performance of reversed solar cells via a simple and universal interface modification and provide the basic guidelines for development and characterization of interface materials for organic solar cells in general.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-103928 (URN)
Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2018-09-01Bibliographically approved
Tress, W., Merten, A., Furno, M., Hein, M., Leo, K. & Riede, M. (2013). Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance. ADVANCED ENERGY MATERIALS, 3(5), 631-638
Open this publication in new window or tab >>Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance
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2013 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 5, p. 631-638Article in journal (Refereed) Published
Abstract [en]

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge-carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non-geminate recombination is the dominating loss mechanism in these devices. Numerical drift-diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2013
Keywords
recombination, quantum efficiency, spectral mismatch, imbalanced mobilities, generation profile, zinc-phthalocyanine, C60, bulk heterojunction, organic solar cell, fill factor
National Category
Social Sciences
Identifiers
urn:nbn:se:liu:diva-94323 (URN)10.1002/aenm.201200835 (DOI)000318761500012 ()
Note

Funding Agencies|Bundesministerium fur Bildung und Forschung|13N9720|InnoProfile project|03IP602|Reiner Lemoine foundation||

Available from: 2013-06-24 Created: 2013-06-24 Last updated: 2013-06-24
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