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Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Studies of Hole-Selective Molybdenum Oxide Contacts in Silicon Solar Cells
摘要： In this study, sub-stochiometric hole-selective molybdenum oxide (MoOx) contacts in crystalline silicon (c-Si) solar cells were investigated by a combination of transmission electron microscopy (TEM) and spatially-resolved electron energy-loss spectroscopy (SR-EELS). It was observed that a ≈ 4 nm SiOx interlayer grows at the MoOx/c-Si interface during the evaporation of MoOx over c-Si substrate. SR-EELS analyses revealed the presence of 1.5 nm diffused MoOx/ITO (indium tin oxide) interface in both as-deposited and annealed samples. Moreover, the presence of a 1 nm thin layer with a lower oxidation state of Mo was detected at SiOx/MoOx interface in as-deposited state which disappears upon annealing. Overall, it was evident that no hole-blocking interlayer is formed at MoOx/ITO interface during annealing and homogenization of the MoOx layer takes place during the annealing process. Furthermore, device simulations revealed that efficient hole collection is dependent on MoOx work function and that reduction in work function of MoOx results in loss of band bending and negatively impacts hole-selectivity.
关键词： silicon,electron energy-loss spectroscopy (EELS),hole-selective,transmission electron microscopy (TEM),molybdenum oxide (MoOx)
更新于2025-09-12 10:27:22
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Nanoscale temperature measurement during temperature controlled in situ TEM using Al plasmon nanothermometry
摘要： Over recent years, the advent of microelectromechanical system (MEMS)-type microheaters has pushed the limits of temperature controlled in situ transmission electron microscopy (TEM). In particular, by enabling the observation of the structure of materials in their application environments, temperature controlled TEM provides unprecedented insights into the link between the properties of materials and their structure in real-world problems, a clear knowledge of which is necessary for rational development of functional materials with new or improved properties. While temperature is the key parameter in such experiments, accessing the precise temperature of the sample at the nanoscale during observations still remains challenging. In the present work, we have applied aluminium plasmon nanothermometry technique that monitors the temperature dependence of the volume plasmon of Al nanospheres using electron energy loss spectroscopy for in situ local temperature determination over a MEMS microheater. With access to local temperatures between room temperature to 550°C, we have assessed the spatial and temporal stabilities of the microheater when it operates at different setpoint temperatures both under vacuum and in the presence of a static H2 gas environment. Temperature comparisons performed under the two environments show discrepancies between local and setpoint temperatures.
关键词： in situ transmission electron microscopy,electron energy loss spectroscopy,volume expansion of metal,local temperature determination,volume plasmons shift
更新于2025-09-12 10:27:22
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Engineering Charge-Transfer States for Efficient, Low-Energy-Loss Organic Photovoltaics
摘要： Charge transfer (CT) between donors and acceptors following photoexcitation of organic photovoltaics (OPVs) gives rise to bound electron–hole pairs across the donor–acceptor interface, known as CT states. While these states are essential to charge separation, they are also a source of energy loss. As a result of reduced overlap between electron and hole wavefunctions, CT states are influenced by details of the film morphology and molecular structure. Here, we describe several important strategies for tuning the energy level and dynamics of the CT state and approaches that can enhance their dissociation efficiency into free charges. Furthermore, we provide an overview of recent physical insights into the key parameters that significantly reduce the Frenkel-to-CT energy offset and recombination energy losses while preserving a high charge-generation yield. Our analysis leads to critical morphological and molecular design strategies for achieving efficient, low-energy-loss OPVs.
关键词： molecular structure,energy loss,organic photovoltaics,charge transfer states,film morphology
更新于2025-09-12 10:27:22
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Size-dependent dielectric function for electron-energy-loss spectra of plasmonic nanoparticles
摘要： A size-dependent complex dielectric function is proposed to describe the impact that size effects have on the dielectric response for electron energy loss spectroscopy (EELS) of plasmonic nanoparticles. Our implementation is based on experimental bulk complex refractive index and the modification of the Lorentz-Drude model. Our theoretical framework is verified and analysed by performing numerical simulation comparisons of EELS for Au spherical nanoparticles of different sizes. The results show that finite-size effects cannot be neglected for a broader size range of up to at least 200 nm for Au spherical nanoparticles. Moreover, the EELS regions in which contributions of surface or bulk energy loss are dominant are confirmed by the optical extinction spectra of Au spherical nanoparticles of different sizes, which takes into account the size-dependent dielectric function. The results provided here provide a suitable and versatile framework for the design of plasmonic elements on the nanometre scale.
关键词： metallic nanoparticles,Plasmonics,size-dependent dielectric function,surface plasmon,electron energy loss spectroscopy (EELS)
更新于2025-09-12 10:27:22
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Energy losses and transition radiation in graphene traversed by a fast charged particle under oblique incidence
摘要： We perform fully relativistic calculations of the energy loss channels for a charged particle traversing a single layer of graphene under oblique incidence in a setting pertinent to a scanning transmission electron microscope (STEM), where we distinguish between the energy deposited in graphene in the form of electronic excitations (Ohmic loss) and the energy emitted in the far field in the form of transition radiation (TR). Our formulation of the problem uses a definition of two in-plane, dielectric functions of graphene, which describe the longitudinal and transverse excitation processes that contribute separately to those two energy loss channels. Using several models for the electric conductivity of graphene as the input in those dielectric functions enables us to discuss the effects of oblique incidence on several processes in a broad range of frequencies, from the terahertz (THz) to the ultraviolet (UV). In particular, at the THz frequencies, we demonstrate that the nonlocal effect in the graphene’s conductivity is not important in the retarded regime, and we show that the longitudinal and transverse contributions to the emitted TR spectra exhibit strongly anisotropic angular patterns that are readily distinguishable in a cathodoluminescence measurement in a STEM. Moreover, we explore the possibility of exciting the so-called transverse mode in the optical response of graphene at the mid-infrared (MIR) range of frequencies by means of a fast charged particle under oblique incidence. Finally, we demonstrate that, aside from the usual high-energy peaks in the longitudinal contribution to the Ohmic energy loss in the MIR to the UV frequency range, there may arise strongly directional features in the in-plane distribution of the transverse contribution to the Ohmic energy loss for an oblique trajectory, which could be possibly observed via momentum- and angle-resolved electron energy loss spectroscopy of graphene in STEM.
关键词： MIR,energy loss,STEM,THz,transition radiation,UV,graphene,cathodoluminescence,oblique incidence,Ohmic loss
更新于2025-09-11 14:15:04
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Tuning of structural and optical properties of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine thin films as a promising photovoltaic absorber material
摘要： Thin films of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (H2TPyP) dye have been prepared using thermal evaporation technique. The crystal structure has been studied using the field emission scanning electron microscopy and X-ray diffraction pattern. The structure investigation of H2TPyP exhibited a polycrystalline nature in the form of powder while the thin films revealed a nanocrystalline structure. The optical features of H2TPyP thin films have been examined using spectroscopic measurements of the absorbance, the transmittance, and the reflectance of those films within spectral range of 200–2500 nm. From such measurements, the optical constants, the dispersion parameters, and the related optical variables were calculated and interpreted. The absorbance spectra of the films revealed the four quasi-electronic states named Q-bands and more intense Soret (B) band located at UV–visible spectral region. In addition to other weaker bands labeled M, N, and L observed at UV spectrum. The electronic inter-band transition for the H2TPyP thin films revealed an indirect allowed transition with the onset optical band gap Eg of 1.78 eV. Urbach tail energy EU was observed beyond the Eg and was found to be 0.36 eV. Furthermore, the energy loss functions, dielectric constants and optical conductivities were also studied.
关键词： Thin films,Structure analysis,Optical properties,Energy loss functions,Porphyrins
更新于2025-09-11 14:15:04
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Revealing the Critical Role of the HOMO Alignment on Maximizing Current Extraction and Suppressing Energy Loss in Organic Solar Cells
摘要： For state-of-the-art organic solar cells (OSCs) consisting of a large-bandgap polymer donor and a near-infrared (NIR) molecular acceptor, the control of the HOMO offset is the key to simultaneously achieve small energy loss (Eloss) and high photocurrent. However, the relationship between HOMO offsets and the efficiency for hole separation is quite elusive so far, which requires a comprehensive understanding on how small the driving force can effectively perform the charge separation while obtaining a high photovoltage to ensure high OSC performance. By designing a new family of ZITI-X NIR acceptors (X = S, C, N) with a high structural similarity and matching them with polymer donor J71 forming reduced HOMO offsets, we systematically investigated and established the relationship among the photovoltaic performance, energy loss, and hole-transfer kinetics. We achieved the highest PCEavgs of 14.05 G 0.21% in a ternary system (J71:ZITI-C:ZITI-N) that best optimize the balance between driving force and energy loss.
关键词： HOMO alignment,energy loss,ZITI-X NIR acceptors,organic solar cells,current extraction
更新于2025-09-11 14:15:04
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European Microscopy Congress 2016: Proceedings || Measuring Charge Distribution in Nanoscale Magnesium Aluminate Spinel by Electron Energy-Loss Spectroscopy and Electron Holography
摘要： Charge distribution resulting in the formation of a space charge zone (SCZ) in ionic materials has a critical role on functional properties [1]. Even though significant advances in theoretical models have been accomplished, experimental evidence in nanoscale granular materials is indirect. Here, we investigated the distribution of cations and defects on the formation of a SCZ in a nanoscale granular model system of non-stoichiometric MgO?nAl2O3 (MAS, n= 0.95 and 1.07). The SCZ was investigated experimentally by electron energy-loss spectroscopy (EELS) and off-axis electron holography (OAEH). EEL spectra were collected along directions perpendicular to grain boundaries (GB’s), from which the magnesium-to-aluminum relative cation concentrations were calculated, as presented in Fig.1. We found that regardless of annealing processes, the vicinity of GB’s of the Mg rich spinel has excess Mg+2 cations while the vicinity of GB’s of the Al rich spinel has excess of Al+3 cations. Additionally, the cation distribution shows strong dependency on the grain size. For non-stoichiometric MAS, cation concentration is proportional to the defect concentration, because deviation from stoichiometry results in adjacent defects that compensate for the electric charge [2, 3, 4]. In both materials, the cation distribution is inhomogeneous for grains smaller than 40 nm. For larger grains, the defect concentration approaches the bulk value at the center of the grain. Furthermore, excess of Mg (Al) cations at the vicinity of the GB decreased with increase of grain size. Maier et al. [1] calculated that for grain size at the scale of the Debye length (estimated at 9nm for non-stoichiometric MAS studied here [7]), the GC is no longer electrically neutral, instead influenced by accumulation or depletion of charge at the boundaries. Due to the lack of accurate values for defect formation energy [5, 6], we applied OAEH to measure directly the electrostatic charge distribution in nano-sized MAS. We show that charge distribution and the buildup of electrostatic potential between GB and core are linked to the spatial distribution of defects rather than the overall composition of MAS (Fig. 2). At the vicinity of GB’s, excess Mg+2 or Al+3 cations accumulate depending on the composition, the magnitude of which increases with decreasing grain size. Indeed, the potential distributions show the relation between the excess cation species, grain size and the Debye length, in agreement with theoretical models [1].
关键词： Lattice ordering,Electron holography,Ionic nano-materials,Electron energy loss spectroscopy,Space charge potential
更新于2025-09-11 14:15:04
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Rational Design of Low Bandgap Polymers for Efficient Solar Cells with High Open-Circuit Voltage: The Profound Effect of Me and Cl Substituent with Similar Van Der Waals Radius
摘要： Generally, low bandgap materials-based photovoltaic devices have reduced open circuit voltage (VOC), and how to realize the trade-off between the low bandgap (Eg<1.6 eV) and high VOC (>0.9 V) could be critical to give efficient polymer solar cells, especially for high-performance semitransparent PSCs and tandem solar cells. Although lots of efforts have been made to address the issue, most results may be not gratifying. In this work, the polymer PTBTz-Cl based on the chlorination method and efficient thiazole-induced strategy was designed and synthesized, aiming at the deep HOMO energy level, and the enhanced backbone planarity caused by the weak noncovalent Cl···S interaction. In addition, the methyl-substituted polymer PTBTz-Me was constructed as the reference due to the similar van der Waals radius of side chain (CH3: 0.20 nm vs Cl: 0.18 nm). Encouragingly, in comparison with that of PTBTz-2, the newly synthesized polymers exhibit the red-shifted absorption spectra ranging from 300 to 770 nm, with obviously reduced Eg of ～1.6 eV. However, the function of Cl and Me substituent is different. Compared to the polymer PTBTz-Me, PTBTz-Cl exhibits a lower HOMO value, stronger crystallinity, and more compact intramolecular interactions. Consequently, the polymer PTBTz-Cl exhibits excellent photovoltaic performance with a notable VOC of 0.94 V and a PCE of 10.35%, which is ～11% higher than the 9.12% efficiency based on PTBTz-Me, and is also one of the highest values among polymer/fullerene solar cells. Moreover, a smaller photo energy loss (Eloss) of 0.64 eV is achieved, which is rare among the current high-performance polymer systems.
关键词： High open-circuit voltage,Low energy loss,Photovoltaic performance,Chlorination method,Polymer solar cell
更新于2025-09-11 14:15:04
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A non-fullerene acceptor based on alkylphenyl substituted benzodithiophene for high efficiency polymer solar cells with a small voltage loss and excellent stability
摘要： In this work, a new non-fullerene small molecule acceptor (NF-SMA) named BP-4F, based on benzo[1,2-b:4,5-b’]di(cyclopenta[2,1-b:3,4-b’]dithiophene) with 4-(2-ethylhexyl)phenyl conjugated side chains (BDT-P) as an electron-donating core, flanked with the strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile (2FIC) unit, is designed and synthesized for polymer solar cells application. BP-4F exhibits strong absorption in the 550 to 830 nm region with a narrow optical band gap of 1.49 eV, suitable energy levels with a lowest unoccupied molecular orbital (LUMO) of -3.90 eV and an effective electron mobility of 2.10×10-4 cm2 V-1 s-1. When blended with the wide bandgap polymer PM6 as the active layer, the polymer solar cells (PSCs) achieve an average power conversion efficiency (PCE) of 13.9% with an energy loss (Eloss) as low as 0.59 eV, which is of benefit to overcome the trade-off between Jsc and Voc. Furthremore, the BP-4F-based PSCs achieve an excellent PCE of 12.3% with a device area of 1.10 cm2. Notably, the devices show an excellent storage stability and photo-stability with retaining near 90% of the initial PCE in air under dark and 93.5% in glovebox under continuous illumination for 720 hours, respectively. These results indicate that BP-4F is an effective electron acceptor for high efficiency and stable polymer solar cells.
关键词： PM6,polymer solar cells,energy loss,stability,BP-4F,non-fullerene small molecule acceptor
更新于2025-09-11 14:15:04