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Low Band Gap Polymer Solar Cells With Minimal Voltage Losses
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
Chalmers, Sweden.
Lund University, Sweden.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
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2016 (English)In: ADVANCED ENERGY MATERIALS, ISSN 1614-6832, Vol. 6, no 18, article id 1600148Article in journal (Refereed) Published
Abstract [en]

One of the factors limiting the performance of organic solar cells (OSCs) is their large energy losses (E-loss) in the conversion from photons to electrons, typically believed to be around 0.6 eV and often higher than those of inorganic solar cells. In this work, a novel low band gap polymer PIDTT-TID with a optical gap of 1.49 eV is synthesized and used as the donor combined with PC 71 BM in solar cells. These solar cells attain a good power conversion efficiency of 6.7% with a high open-circuit voltage of 1.0 V, leading to the E-loss as low as 0.49 eV. A systematic study indicates that the driving force in this donor and acceptor system is sufficient for charge generation with the low E-loss. This work pushes the minimal E-loss of OSCs down to 0.49 eV, approaching the values of some inorganic and hybrid solar cells. It indicates the potential for further enhancement of the performance of OSCs by improving their V-oc since the E-loss can be minimized.

Place, publisher, year, edition, pages
WILEY-V C H VERLAG GMBH , 2016. Vol. 6, no 18, article id 1600148
National Category
Energy Systems
Identifiers
URN: urn:nbn:se:liu:diva-133010DOI: 10.1002/aenm.201600148ISI: 000387132200002OAI: oai:DiVA.org:liu-133010DiVA, id: diva2:1054672
Note

Funding Agencies|Swedish Research Council; Swedish Research Council Formas; Swedish Energy Agency; Chalmers Area of Advance Energy and Materials Science; EU projects SUNFLOWER-"SUstainable Novel FLexible Organic Watts Efficiently Reliable" [287594]; Knut and Alice Wallenberg foundation; program for the Excellent Doctoral Dissertations of Guangdong Province [ybzzxm201114]; China Scholarship Council; National Natural Science Foundation of China [21504066, 21534003]; Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy [DE-AC02-05CH11231]

Available from: 2016-12-08 Created: 2016-12-07 Last updated: 2017-11-15
In thesis
1. Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics
Open this publication in new window or tab >>Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The great tunability of structure and electronic properties of π-conjugated organic molecules/polymers combined with other advantages such as light weight and flexibility etc., have made organic-based electronics the focus of an exciting still-growing field of physics and chemistry for more than half a century. The application of organic electronics has led to the appearance of wide range of organic electronic devices mainly including organic light emitting diodes (OLED), organic field effect transistors (OFET) and organic solar cells (OSC). The application of the organic electronic devices mainly is limited by two dominant parameters, i.e., their performance and stability. Up to date, OLED has been successfully commercialized in the market while the OSC are still on the way to commercialization hindered by low efficiency and inferior stability. Understanding the energy levels of organic materials and energy level alignment of the devices is crucial to control the efficiency and stability of the OSC. In this thesis, energy levels measured by different methods are studied to explore their relationship with device properties, and the strategies on how to design efficient and stable OSC based on energy level diagrams are provided.

Cyclic Voltammetry (CV) is a traditional and widely used method to probe the energy levels of organic materials, although there is little consensus on how to relate the oxidation/reduction potential ((Eox/Ered) to the vacuum level. Ultraviolet Photoelectron Spectroscopy (UPS) can be used to directly detect vertical ionization potential (IP) of organic materials. In this thesis, a linear relationship of IP and Eox was found, with a slope equal to unity. The relationship provides for easy conversion of values obtained by the two techniques, enabling complementarily use in designing and fabricating efficient and stable OSC. A popular rule of thumb is that the offset between the LUMO levels of donor and acceptor should be 0.3 eV, according to which a binary solar cell with the minimum voltage losses around 0.49 V was designed here.

Introduction of the ternary blend as active layer is an efficient way to improve both efficiency and stability of the OSC. Based on our studied energy-level diagram within the integer charge transfer (ICT) model, we designed ternary solar cells with enhanced open circuit voltage for the first time and improved thermal stability compared to reference binary ones. The ternary solar cell with minimum voltage losses was developed by combining two donor materials with same ionization potential and positive ICT energy while featuring complementary optical absorption. Furthermore, the fullerene acceptor was chosen so that the energy of the positive ICT state of the two donor polymers is equal to the energy of negative ICT state of the fullerene, which can enhance dissociation of all polymer donor and fullerene acceptor excitons and suppress bimolecular and trap-assistant recombination.

Rapid development of non-fullerene acceptors in the last two years affords more recipes of designing both efficient and stabile OSC. We show in this thesis how non-fullerene acceptors successfully can be used to design ternary solar cells with both enhanced efficiency and thermal stability. Besides improving the efficiency of the devices, understanding of the stability and degradation mechanism is another key issue. The degradation of conjugated molecules/polymers often follow many complicated pathways and at the same time many factors for degradation are coupled with each other. Therefore, the degradation of non-fullerene acceptors was investigated in darkness by photoelectron spectroscopy in this thesis with the in-situ method of controlling exposure of O2 and water vapor separately.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. p. 84
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1893
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Condensed Matter Physics Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-143025 (URN)10.3384/diss.diva-143025 (DOI)9789176853931 (ISBN)
Public defence
2017-12-08, Schrödinger, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
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Supervisors
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-22Bibliographically approved

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Xia, Yuxin

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