| 1. |
- Wang, Le, et al.
(författare)
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Issue of spatial coherence in MQW based micro-LED simulation
- 2021
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Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 29:20, s. 31520-31526
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Tidskriftsartikel (refereegranskat)abstract
- In existing flip-chip LED simulations, the light extraction efficiency is related to the multiple quantum well (MQW) to metal reflector distance because of optical interference. We calculate the contrast using several typical light intensity distributions among the several QWs in MQW. The coherence is obtained analytically. When the luminosity of each QW is equal, the contrast is ∼0, meaning the light is incoherent, contrary to traditional studies. The spatial coherence is important only when the light emission comes from just one QW. As the MQW has a not negligible thickness, the traditional single-dipole model is no longer accurate.
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| 2. |
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| 3. |
- Chen, Yongzhen, 1990-, et al.
(författare)
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Experimental and Theoretical Investigation into the Polaron Structure of K-Doped Polyfluorene Films
- 2021
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:1, s. 937-945
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of the electronic structure and optical transition upon n-doping of poly(9,9-dioctylfluorene) (PFO) films is elucidated with photoelectron spectroscopy, optical absorption, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations. Optical absorption measurements extending into near infrared show two low-energy absorption features at low doping ratios and an additional peak at a higher energy of similar to 2.2 eV that disappears with increasing doping ratios. A gap state (i.e., polaronic state) close to the Fermi level and a significantly destabilized highest valence band appear in the experimentally measured ultraviolet photoelectron spectra. These experimental results are interpreted by the TD-DFT calculations, which show that the lower energy peaks originate from the excitation from polaronic states to the conduction band, while the higher energy peak mainly originates from the destabilized valence band to conduction band transitions and only appears at low doping ratios (c(red) <= 50%, 0.5 potassium atom per fluorene monomer). The DFT calculations further indicate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. Comparing the results of doped glassy and beta-phase films, we find that the ordered segments in the beta-phase film disappear due to the dopant (potassium) insertion, resulting in a similar polaronic structure.
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| 4. |
- Chen, Yongzhen, 1990-
(författare)
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Image dipoles and polarons in organic semiconductors
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The rapid development of organic electronics depends on the synthesis of new π-conjugated molecules/polymers and the exploration of fundamental physics. However, most of the efforts have been concentrated on the former, leading to a lack of thorough understanding of many important concepts, which will become the ultimate limiting factor for overall performance and applications. Thereinto, the interface energetics in multilayer stacked optoelectronic devices and the electronic structure of doped organic semiconductors are two of the most complicated and yet inevitable topics in this field. A better understanding of them can provide needed additional insights into the operation of devices and give valuable guidance for device and molecule design. Hence, the aim of this thesis is to investigate these two fundamental issues using various spectroscopic characterizations and supported by computational modeling.The cathode/organic interface plays a critical role in achieving balanced charge transport and improved stability in organic electronic devices. Employing stable cathode materials, however, typically results in large electron injection barriers and sub-optimal devices. Using small-molecule electron transport materials (ETMs) as interlayers is an effective approach to reduce the electron injection barriers, but the working mechanism is still under debate. By studying the energy level alignment behavior of ETMs on different types of substrates, we find the work function of the substrate is reduced by an extra “image” dipole formed at the interface and within the first layer of the ETM film. The use of non-reactive substrates and the results from X-ray photoelectron spectroscopy core level analysis exclude the orbital hybridization theory, which states that an ETM-metal complex may form at the interface. The characterization results on molecular orientation disqualify an explanation using intrinsic molecular dipole moments. Instead, experiments demonstrate that the interface dipole depends on the areal density and direction of lone electron pairs on the heteroatoms in ETMs, which is similar to previous observations in tertiary aliphatic amines. This behavior is well described by the so-called “double dipole step” model, where one dipole formed by the nitrogen nuclei and the lone pairs in the organic side points from the substrate surface to the organic film and the other one formed by their image charges in the electrode side shows the same direction.The polaron charge carrier is another important concept involved in multiple (opto)electronic processes during device operation, such as charge transport, exciton recombination/dissociation. Although numerous experimental and theoretical efforts have been made, there is still a lack of comprehensive studies on the electronic structure of negative polarons due to their high air sensitivity, including correlation between the valence band structure measured by ultraviolet photoelectron spectroscopy (UPS) and the optical band gap derived from the UV−vis−NIR absorption. In the present work, we are able to integrate the optical and electrical measurements with photoelectron spectroscopy to collect all information without breaking an ultra-high vacuum. Negative polarons formed in alkali metal-doped polymers are detected with new polaronic states below the Fermi level and lower energy absorption bands arising from the excitation from polaronic states to unoccupied states. In addition, the Fermi level shifts toward the conduction band with increasing the doping ratio, and the doubly-occupied polaronic state shows slightly lower energy than the topmost valance band peak of the neutral polymer. These observations are supported by the density functional theory (DFT) simulations, from which we also demonstrate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. By comparison of different polymer and dopant systems, we find the polymer-dopant interaction and the polaron delocalization are dependent on the distortion of the polymer backbone and the size of the dopant, properties that in turn affect the conductivity and air stability of the n-doped materials.I hope that the findings presented in the thesis can greatly promote the understanding of the energetics of the ETMs/cathode interface and the electronic structures of negative polarons in alkali metal-doped polymers, contributing to further providing new guidelines for the molecular design and improve the device performance.
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| 5. |
- Chen, Yongzhen, 1990-, et al.
(författare)
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Image-force effects on energy level alignment at electron transport material/cathode interfaces
- 2020
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Ingår i: Journal of Materials Chemistry C. - : ROYAL SOC CHEMISTRY. - 2050-7526 .- 2050-7534. ; 8:1, s. 173-179
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Tidskriftsartikel (refereegranskat)abstract
- Electron transport materials (ETMs) are widely used as interlayers to lower the cathode electrode work function in organic solar cells and organic light-emitting diodes, for example. The usual interpretation for their operating principle is a chemical interaction between the ETM and the electrode, inducing partial or integer charge transfer or collectively an intrinsic dipole moment caused by preferential molecular orientation. Herein, we systematically explore the commonly used ETM bathophenanthroline (BPhen) deposited on a series of conducting substrates. The energetics at the BPhen interface follows the typical integer charge transfer (ICT) model with an extra displacement of the vacuum level by up to -1.4 eV. The extra displacement is ascribed to the "double dipole step" formed by the positive and negative charged species and their induced image charges when they are close to the surface of substrates. After n-type doping the displacement is further increased to -1.8 eV, yielding a larger work function modification than obtained using typical electrolytes and zwitterions as cathode interlayer.
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| 6. |
- Chen, Yongzhen, et al.
(författare)
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In Situ Spectroscopic and Electrical Investigations of Ladder-type Conjugated Polymers Doped with Alkali Metals
- 2022
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 55:16, s. 7294-7302
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Tidskriftsartikel (refereegranskat)abstract
- Ladder-type conjugated polymers exhibit a remarkable performance in (opto)electronic devices. Their double-stranded planar structure promotes an extended pi-conjugation compared to inter-ring-twisted analogues, providing an excellent basis for exploring the effects of charge localization on polaron formation. Here, we investigated alkali-metal n -doping of the ladder-type conjugated polymer (polybenzimidazobenzophe-nanthroline) (BBL) through detailed in situ spectroscopic and electrical characterizations. Photoelectron spectroscopy and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy indicate polaron formation upon potassium (K) doping, which agrees well with theoretical predictions. The semiladder BBB displays a similar evolution in the valence band with the appearance of two new features below the Fermi level upon K-doping. Compared to BBL, distinct differences appear in the UV-vis-NIR spectra due to more localized polaronic states in BBB. The high conductivity (2 S cm(-1)) and low activation energy (44 meV) measured for K-doped BBL suggest disorder-free polaron transport. An even higher conductivity (37 S cm(-1)) is obtained by changing the dopant from K to lithium (Li). We attribute the enhanced conductivity to a decreased perturbation of the polymer nanostructure induced by the smaller Li ions. These results highlight the importance of polymer chain planarity and dopant size for the polaronic state in conjugated polymers.
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| 7. |
- Chen, Yongzhen, 1990-, et al.
(författare)
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Understanding Interface Dipoles at an Electron Transport Material/Electrode Modifier for Organic Electronics
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:39, s. 47218-47225
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Tidskriftsartikel (refereegranskat)abstract
- Interface dipoles formed at an electrolyte/electrode interface have been widely studied and interpreted using the "double dipole step" model, where the dipole vector is determined by the size and/or range of motion of the charged ions. Some electron transport materials (ETMs) with lone pairs of electrons on heteroatoms exhibit a similar interfacial behavior. However, the origin of the dipoles in such materials has not yet been explored in great depth. Herein, we systematically investigate the influence of the lone pair of electrons on the interface dipole through three pyridine derivatives B2-B4PyMPM. Experiments show that different positions of nitrogen atoms in the three materials give rise to different hydrogen bonds and molecular orientations, thereby affecting the areal density and direction of the lone pair of electrons. The interface dipoles of the three materials predicted by the "double dipole step" model are in good agreement with the ultraviolet photoelectron spectroscopy results both in spin-coated and vacuum-deposited films. These findings help to better understand the ETMs/electrode interfacial behaviors and provide new guidelines for the molecular design of the interlayer.
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| 8. |
- Fan, Yongzhen, et al.
(författare)
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OC-SMART : A machine learning based data analysis platform for satellite ocean color sensors
- 2021
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Ingår i: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 253
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a new platform, Ocean Color - Simultaneous Marine and Aerosol Retrieval Tool (OC-SMART), for analysis of data obtained by satellite ocean color sensors. OC-SMART is a multi-sensor data analysis platform which supports heritage, current, and possible future multi-spectral and hyper-spectral sensors from US, EU, Korea, Japan, and China, including SeaWiFS, Aqua/MODIS, SNPP/VIIRS, ISS/HICO, Landsat8/OLI, DSCOVR/EPIC, Sentinel-2/MSI, Sentinel-3/OLCI, COMS/GOCI, GCOM-C/SGLI and FengYun-3D/MERSI2. The products provided by OC-SMART include spectral normalized remote sensing reflectances (R-rs values), chlorophyll_a (CHL) concentrations, and spectral in-water inherent optical properties (IOPs) including absorption coefficients due to phytoplankton (a(ph)), absorption coefficients due to detritus and Gelbstoff (a(dg)) and backscattering coefficients due to particulates (b(bp)). Spectral aerosol optical depths (AODs) and cloud mask results are also provided by OC-SMART. The goal of OC-SMART is to improve the quality of global ocean color products retrieved from satellite sensors, especially under complex environmental conditions, such as coastal/inland turbid water areas and heavy aerosol loadings. Therefore, the atmospheric correction (AC) and ocean IOP algorithms in OC-SMART are driven by extensive radiative transfer (RT) simulations in conjunction with powerful machine learning techniques. To simulate top of the atmosphere (TOA) radiances, we solve the radiative transfer equation pertinent for the coupled atmosphere-ocean system. For each sensor, we have created about 13 million RT simulations and comprehensive training datasets to support the development of the machine learning AC and in-water IOP algorithms. The results, as demonstrated in this paper, are very promising. Not only does OC-SMART improve the quality of the retrieved water products, it also resolves the negative water-leaving radiance problem that has plagued heritage AC algorithms. The comprehensive training datasets created using multiple atmosphere, aerosol, and ocean IOP models ensure global and generic applicability of OC-SMART. The use of machine learning algorithms makes OC-SMART roughly 10 times faster than NASA's SeaDAS platform. OC-SMART also includes an advanced cloud screening algorithm and is resilient to the contamination by weak to moderate sunglint and cloud edges. It is therefore capable of recovering large amounts of data that are discarded by other algorithms (such as those implemented in NASA's SeaDAS package), especially in coastal areas. OC-SMART is currently available as a standalone Python package or as a plugin that can be installed in ESA's Sentinel Application Platform (SNAP).
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| 9. |
- Zhang, Qilun, et al.
(författare)
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In situ near-ambient pressure X-ray photoelectron spectroscopy reveals the effects of water, oxygen and light on the stability of PM6:Y6 photoactive layers
- 2023
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Ingår i: Journal of Materials Chemistry C. - : ROYAL SOC CHEMISTRY. - 2050-7526 .- 2050-7534. ; 11:8, s. 3112-3118
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Tidskriftsartikel (refereegranskat)abstract
- The power conversion efficiency of organic solar cells (OSCs) has taken a further leap in the past three years owing to the emergence of Y6; however, their inferior stability hinders commercialization. Understanding the ambient degradation mechanism of photovoltaic materials is a key component to address this challenge. In this study, we first used in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate the effects of water, oxygen and absorbed photons on the stability of PM6 and Y6. The studied materials PM6 and Y6 show instability to oxygen and water, respectively, possibly due to the weak interaction between PM6 backbone sulphur and oxygen, and Y6 end cyano groups show instability to water. In addition, the stability of Y6 in blended PM6:Y6 films is enhanced, which is confirmed by the performance of OSCs with blended or quasi-bilayer PM6:Y6 photoactive layers. Our findings reveal PM6 and Y6 degradation on ambient exposure and predict a possible way to prevent the degradation of Y6 in OSCs.
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