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Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics
Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, Faculty of Science & Engineering.
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: urn:nbn:se:liu:diva-143025DOI: 10.3384/diss.diva-143025ISBN: 9789176853931 (print)OAI: oai:DiVA.org:liu-143025DiVA, id: diva2:1157283
Public defence
2017-12-08, Schrödinger, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-22Bibliographically approved
List of papers
1. Low Band Gap Polymer Solar Cells With Minimal Voltage Losses
Open this publication in new window or tab >>Low Band Gap Polymer Solar Cells With Minimal Voltage Losses
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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
National Category
Energy Systems
Identifiers
urn:nbn:se:liu:diva-133010 (URN)10.1002/aenm.201600148 (DOI)000387132200002 ()
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
2. Ternary organic solar cells with enhanced open circuit voltage
Open this publication in new window or tab >>Ternary organic solar cells with enhanced open circuit voltage
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2017 (English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 37, p. 24-31Article in journal (Refereed) Published
Abstract [en]

By introducing a non-fullerene small molecule acceptor as a third component to typical polymer donor: fullerene acceptor binary solar cells, we demonstrate that the short circuit current density (J(sc)), open circuit voltage (V-oc), power conversion efficiency (PCE) and thermal stability can be enhanced simultaneously. The different surface energy of each component causes most of the non-fullerene acceptor molecules to self-organize at the polymer/fullerene interface, while the appropriately selected oxidation/reduction potential of the non-fullerene acceptor enables the resulting ternary junction to work through a cascade mechanism. The cascade ternary junction enhances charge generation through complementary absorption between the non-fullerene and fullerene acceptors and aids the efficient charge extraction from fullerene domains. The bimolecular recombination in the ternary blend layer is reduced as the ternary cascade junction increases the separation of holes and electrons during charge transportation and the trap assistant recombination induced by integer charge transfer (ICT) state potentially reduced due to the smaller pinning energy of inserted non-fullerene acceptor, leading to an unprecedented increase in the open circuit voltage beyond the binary reference values.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2017
Keyword
Ternary organic solar cell; Self-organization; Higher open circuit voltage; Generality; More thermal stable
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-138887 (URN)10.1016/j.nanoen.2017.04.060 (DOI)000402704500004 ()
Note

Funding Agencies|Knut and Alice Wallenberg Foundation; Swedish Research Council [2013-4022]; Ministry of Science and Technology [2016YFA0200700]; NSFC [21504066, 21534003]; Goran Gustafsson Foundation for Research in Natural Sciences and Medicine; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkping University [2009 00971]; China Scholarship Council; Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-AC02-05CH11231]

Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2017-11-15
3. Ternary Organic Solar Cells with Minimum Voltage Losses
Open this publication in new window or tab >>Ternary Organic Solar Cells with Minimum Voltage Losses
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2017 (English)In: Advanced Energy Materials, ISSN 1614-6840, Vol. 7, no 21, article id 1700390Article in journal (Refereed) Published
Abstract [en]

A new strategy for designing ternary solar cells is reported in this paper. A low-bandgap polymer named PTB7-Th and a high-bandgap polymer named PBDTTS-FTAZ sharing the same bulk ionization potential and interface positive integer charge transfer energy while featuring complementary absorption spectra are selected. They are used to fabricate efficient ternary solar cells, where the hole can be transported freely between the two donor polymers and collected by the electrode as in one broadband low bandgap polymer. Furthermore, the fullerene acceptor is chosen so that the energy of the positive integer charge transfer state of the two donor polymers is equal to the energy of negative integer charge transfer state of the fullerene, enabling enhanced dissociation of all polymer donor and fullerene acceptor excitons and suppressed bimolecular and trap assistant recombination. The two donor polymers feature good miscibility and energy transfer from high-bandgap polymer of PBDTTS-FTAZ to low-bandgap polymer of PTB7-Th, which contribute to enhanced performance of the ternary solar cell.

Place, publisher, year, edition, pages
John Wiley & Sons, 2017
Keyword
binary equivalent, minimum voltage losses, same bulk and interface energy, ternary solar cells
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:liu:diva-143026 (URN)10.1002/aenm.201700390 (DOI)000414711100002 ()2-s2.0-85025441174 (Scopus ID)
Note

Funding agencies: Knut and Alice Wallenberg Foundation; Swedish Research Council [2016-05498]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; Goran Gustafsson Foundat

Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-12-05Bibliographically approved
4. Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages
Open this publication in new window or tab >>Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages
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2015 (English)In: Polymer Chemistry, ISSN 1759-9954, E-ISSN 1759-9962, Vol. 6, no 25, p. 4624-4633Article in journal (Refereed) Published
Abstract [en]

A new electron-deficient building block 5,9-di(thiophen-2-yl)-6H-pyrrolo[3,4-g]quinoxaline-6,8(7H)-dione (PQD) was synthesized via functionalizing the 6- and 7-positions of quinoxaline (Qx) with a dicarboxylic imide moiety. Side chain substitution on the PQD unit leads to good solubility which enables very high molecular weight copolymers to be attained. The fusion of two strong electron-withdrawing groups (Qx and dicarboxylic imide) makes the PQD unit a stronger electron-deficient moiety than if the unit had just one electron-withdrawing group, thus enhancing the intramolecular charge transfer between electron-rich and deficient units of the copolymer. Four PQD-based polymers were synthesized which feature deep-lying highest occupied molecular orbital (HOMO) levels and bathochromic absorption spectra when compared to PBDT-Qx and PBDT-TPD analogues. The copolymers incorporated with benzo[1,2-b:4,5-b]dithiophene (BDT) units show that the 1D and 2D structural variations of the side groups on the BDT unit are correlated with the device performance. As a result, the corresponding solar cells (ITO/PEDOT:PSS/polymer: PC71BM/LiF/Al) based on the four copolymers feature very high open-circuit voltages (V-oc) of around 1.0 V. The copolymer PBDT-PQD1 attains the best power conversion efficiency of 4.9%, owing to its relatively high absorption intensity and suitable film morphology. The structure-property correlation demonstrates that the new PQD unit is a promising electron-deficient building block for efficient photovoltaic materials with high V-oc.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-120071 (URN)10.1039/c5py00394f (DOI)000356298900009 ()
Note

Funding Agencies|Swedish Research Council; Swedish Energy Agency; EU [287594]; China Scholarship Council; program for the Excellent Doctoral Dissertations of Guangdong Province [ybzzxm201114]

Available from: 2015-07-06 Created: 2015-07-06 Last updated: 2017-11-15

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