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Chargeand Light Management in Organic Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solar cells based on organic materials are extensively investigated both in academia and industry, owing to their potential of being inexpensive, lightweight, flexible and suitable for roll-to-roll production. Performance of organic solar cells has been rapidly improving, and the state of the art organic solar cells have an efficiency over 10%.

This thesis aims to provide a comprehensive study of device engineering and device physics of organic solar cells. The ambition has been to investigate and optimize the conditions for light in-coupling and charge extraction in organic solar cells with alternative geometries. The ultimate goal is to propose methods that could potentially get the power conversion efficiency of organic solar cells with industrially compatible geometries over the current limit of 10%.

To achieve such a goal, we employed a conjugated polymer as an interface modifier to improve the surface and electrical properties of substrate electrodes for solar cells with active layers based on blends containing conjugated polymers and fullerenes. A possibility of improving photovoltaic performance of easily fabricated reverse geometry solar cells (bottom metal cathode/active layer/top  transparent anode) has been demonstrated via this simple interface modification. Systematic investigations revealed that the enhanced device performance was not only related to the modified active layer/electrode interface but also changes in the bulk of the active layer. Consequently, the device performance of a reversed solar cell was found to be as good as that of a standard solar cell. The understanding of the effects of the interlayer modification on the performance of reversed solar cells, and the methodology provided in this thesis is a guideline for further studying and developing interface materials for highly efficient organic solar cells.

An ITO electrode was further converted into a cathode by the polymer interlayer to construct semi-transparent solar cells with great potentials for building integrated photovoltaic applications. By stacking semi-transparent solar cells on top of each other, we showed that light can be more efficiently harvested compared to a conventional architecture solar cell. It has also been demonstrated that semitransparent solar cells combined with macroscopic light trapping elements can also be highly efficient.

However, this thesis is not only about papers that I have published as a graduate student. The first five chapters have been included to provide background in the field of organic photovoltaics.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 79 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1567
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-103930ISBN: 978-91-7519-419-6 (print)OAI: oai:DiVA.org:liu-103930DiVA: diva2:693150
Public defence
2014-03-07, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2014-02-03Bibliographically approved
List of papers
1. Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells
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, 554-558 p.Article 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
2. 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: 2017-01-11Bibliographically approved
3. Semi-Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency
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, 1467-1476 p.Article 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
Keyword
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
4. Light trapping with total internal reflection and transparent electrodes in organic photovoltaic devices
Open this publication in new window or tab >>Light trapping with total internal reflection and transparent electrodes in organic photovoltaic devices
2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 16Article in journal (Refereed) Published
Abstract [en]

Herein, we demonstrate a method to build highly efficient light trapping structures for printed organic solar cells and modules, compatible with roll to roll manufacturing. Echelle grating structures in combination with semitransparent electrodes allow for efficient light trapping via means of total internal reflection. With this method, we demonstrate an increased cell photocurrent response up to 24%, compared to a standard cell configuration with a planar reflector. The demonstrated light trapping approach is expected to be even more useful for photovoltaic modules, where light hitting "dead areas" in between the sub-cells comprising the module will now be utilized.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87228 (URN)10.1063/1.4759125 (DOI)000310669300075 ()
Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06
5. Light Trapping with Dielectric Scatterers in Single- and Tandem-Junction Organic Solar Cells
Open this publication in new window or tab >>Light Trapping with Dielectric Scatterers in Single- and Tandem-Junction Organic Solar Cells
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2013 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 3, no 12, 1606-1613 p.Article in journal (Refereed) Published
Abstract [en]

Efficient dielectric scatterers based on a mixture of TiO2 nanoparticles and polydimethylsiloxane are demonstratedfor light trapping in semitransparent organic solar cells. An improvement of 80% in the photocurrent of an optimized semitransparent solar cell is achieved with the dielectric scatterer with approximate to 100% diffuse reflectance for wavelengths larger than 400 nm. For a parallel tandem solar cell, the dielectric scatterer generates 20% more photocurrent compared with a silver mirror beneath the cell; for a series tandem solar cell, the dielectric scatterer can be used as a photocurrent balancer between the subcells with different photoabsorbing materials. The power conversion efficiency of the tandem cell in series configuration with balanced photocurrent in the subcells exceeds that of an optimized standard solar cell with a reflective electrode. The characteristics of polydimethylsiloxane, such as flexibility and the ability to stick conformably to surfaces, also remain in the dielectric scatterers, which makes the demonstrated light trapping configuration highly suitable for large scale module manufacturing of roll-to-roll printed organic single- or tandem-junction solar cells.

Place, publisher, year, edition, pages
Wiley-VCH Verlag, 2013
Keyword
light trapping; light scattering; organic solar cells; semitransparent solar cells; tandem solar cells
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-103295 (URN)10.1002/aenm.201300524 (DOI)000328337500012 ()
Available from: 2014-01-17 Created: 2014-01-16 Last updated: 2014-02-03
6. Light trapping in thin film organic solar cells
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, 389-396 p.Article, 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

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