| 1. |
- Wang, Chuanfei, 1986-
(författare)
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Electronic Structure of π-Conjugated Materials and Their Effect on Organic Photovoltaics
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 2. |
- Wang, Chuanfei, 1986-, et al.
(författare)
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Ternary Organic Solar Cells with Minimum Voltage Losses
- 2017
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Ingår i: Advanced Energy Materials. - : Wiley. - 1614-6832 .- 1614-6840. ; 7:21
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Xu, Xiaofeng, 1984, et al.
(författare)
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Pyrrolo[3,4-g]quinoxaline-6,8-dione-based conjugated copolymers for bulk heterojunction solar cells with high photovoltages
- 2015
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Ingår i: Polymer Chemistry. - : ROYAL SOC CHEMISTRY. - 1759-9954 .- 1759-9962. ; 6:25, s. 4624-4633
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Chen, Shangzhi, et al.
(författare)
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Conductive polymer nanoantennas for dynamic organic plasmonics
- 2020
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Ingår i: Nature Nanotechnology. - London : Nature Publishing Group. - 1748-3387 .- 1748-3395. ; 15
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Tidskriftsartikel (refereegranskat)abstract
- Being able to dynamically shape light at the nanoscale is oneof the ultimate goals in nano-optics1. Resonant light–matterinteraction can be achieved using conventional plasmonicsbased on metal nanostructures, but their tunability is highlylimited due to a fixed permittivity2. Materials with switchablestates and methods for dynamic control of light–matterinteraction at the nanoscale are therefore desired. Here weshow that nanodisks of a conductive polymer can supportlocalized surface plasmon resonances in the near-infraredand function as dynamic nano-optical antennas, with their resonancebehaviour tunable by chemical redox reactions. Theseplasmons originate from the mobile polaronic charge carriersof a poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf)polymer network. We demonstrate complete and reversibleswitching of the optical response of the nanoantennasby chemical tuning of their redox state, which modulatesthe material permittivity between plasmonic and dielectricregimes via non-volatile changes in the mobile chargecarrier density. Further research may study different conductivepolymers and nanostructures and explore their usein various applications, such as dynamic meta-optics andreflective displays.
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| 5. |
- Karki, Akchheta, et al.
(författare)
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Electrical Tuning of Plasmonic Conducting Polymer Nanoantennas
- 2022
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Ingår i: Advanced Materials. - : Wiley. - 1521-4095 .- 0935-9648. ; 34:13
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructures of conventional metals offer manipulation of light at the nanoscale but are largely limited to static behavior due to fixed material properties. To develop the next frontier of dynamic nano-optics and metasurfaces, this study utilizes the redox-tunable optical properties of conducting polymers, as recently shown to be capable of sustaining plasmons in their most conducting oxidized state. Electrically tunable conducting polymer nano-optical antennas are presented, using nanodisks of poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) as a model system. In addition to repeated on/off switching of the polymeric nanoantennas, the concept enables gradual electrical tuning of the nano-optical response, which was found to be related to the modulation of both density and mobility of the mobile polaronic charge carriers in the polymer. The resonance position of the PEDOT:Sulf nanoantennas can be conveniently controlled by disk size, here reported down to a wavelength of around 1270 nm. The presented concept may be used for electrically tunable metasurfaces, with tunable farfield as well as nearfield. The work thereby opens for applications ranging from tunable flat meta-optics to adaptable smart windows.
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| 6. |
- Wang, Chuanfei, 1986-, et al.
(författare)
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Effects of water vapor and oxygen on non-fullerene small molecule acceptors
- 2019
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry. - 2050-7526 .- 2050-7534. ; 7:4, s. 879-886
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Tidskriftsartikel (refereegranskat)abstract
- Due to the rapid development of non-fullerene acceptors (NFAs), the efficiency of organic solar cells is steadily being improved. The stability of organic solar cells also is expected to be enhanced with the introduction of the NFAs, yet the stability of NFAs themselves have been less investigated to date. In this paper, the stability of a set of typical NFAs was studied in situ employing photoelectron spectroscopy. The studied molecules show higher resistance to water vapor and thermal stress compared to fullerenes. For water vapor exposure, the highest occupied molecular orbital (HOMO) of NFAs undergoes only minor and reversible changes and the NFAs/substrate work function stays constant. Exposure to oxygen gas significantly modified the electronic structure of the NFAs and the effect was only partially reversible by annealing. However, the presence of water vapor was shown to slow down the degradation caused by oxygen. This is in stark contrast to fullerenes that undergo irreversible degradation upon water vapor exposure.
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| 7. |
- Wang, Chuanfei, 1986-, et al.
(författare)
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Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics
- 2018
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Ingår i: Solar RRL. - : Wiley-Blackwell. - 2367-198X. ; 2:9
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Tidskriftsartikel (refereegranskat)abstract
- Ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV) are employed to measure energy levels for charge transport in organic semiconductor films. A series of classical molecules/polymers used in organic bulk heterojunction solar cells are deposited on platinum substrates/electrodes to form thin films and a linear relationship of vertical ionization potential (IP) measured by UPS and relative oxidation potential (Eox) obtained by CV is found, with a slope equal to unity. The intercept varies with the different reference redox couples and repeated potential sweep numbers during experiment processes. The relationship provides for an easy conversion of values obtained by the two techniques and correlates well with device parameters. The precision in the CV-derived IP values is not sufficient, however, to enable precise design of energy level alignment at heterojunction and the approach does not improve upon the current ?best practice? for obtaining donor ionization potential?acceptor electron affinity gaps at heterojunctions.
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