Thermal activation energy on electrical degradation process in BaTiO3 based multilayer ceramic capacitors for lifetime reliability.

For a high capacitance and high lifetime reliability of multilayer ceramic capacitors for automotive applications, the activation energy on thermal activation process can typically be calculated by using Arrhenius based Prokopowicz-Vaskas equation as a method for lifetime prediction. In this study, it is clearly observed that the activation energy shows to be constant in the range of ~ 1.5 eV for the prototype MLCCs, higher than the activation energy values of ~ 1.0 eV related to the motion or diffusion of oxygen vacancies reported in the previous literature. The activation energy value of ~ 1.5 eV for three prototype MLCCs is close to a half the energy band gap (Eg/2 ≈ 1.6 eV) of BaTiO3 obtained from specific environment, where oxygen vacancies are stabilized by external containment such as the effect of rare earth oxide additives. Due to an obvious difference in activation energy values, it difficult to explain the conduction mechanism for failure by only oxygen vacancy migration. Therefore, the concepts of electronic processes and oxygen vacancy should be considered together to understand conduction mechanism for failure of BaTiO3-based MLCCs in thermal activation processes. It can be useful as an indicator for future MLCC development with high lifetime reliability.

Defect-Induced in Situ Atomic Doping in Transition Metal Dichalcogenides via Liquid-Phase Synthesis toward Efficient Electrochemical Activity.

Transition metal dichalcogenides (TMDs), due to their fascinating properties, have emerged as potential next-generation semiconducting nanomaterials across diverse fields of applications. When combined with other material systems, precise control of the intrinsic properties of the TMDs plays a vital role in maximizing their performance. Defect-induced atomic doping through introduction of a chalcogen vacancy into the TMDs lattices is known to be a promising strategy for modulating their characteristic properties. As a result, there is a need to develop tunable and scalable synthesis routes to achieve vacancy-modulated TMDs. Herein, we propose a facile liquid-phase ligand exchange approach for scalable, uniform, and vacancy-tunable synthesis of TMDs films. Varying the relative molar ratio of the chalcogen to transition metal precursors enabled the in situ modulation of the chalcogen vacancy concentrations without necessitating additional post-treatments. When employed as the electrocatalyst in the hydrogen evolution reaction (HER), the vacancy-modulated TMDs, exhibiting a synergetic effect on the energy level matching to the reduction potential of water and optimized free energy differences in the HER pathways, showed a significant enhancement in the hydrogen production via the improved charge transfer kinetics and increased active sites. The proposed approach for synthesizing tunable vacancy-modulated TMDs with wafer-scale synthesis capability is, therefore, promising for better practical applications of TMDs.

Origin of the luminescence spectra width in perovskite nanocrystals with surface passivation.

Though halide perovskite nanocrystal (PeNC) based blue light emitting devices have been improved in the last few years, and the reasons for the improvements have been successfully explained, the origin of the narrow emission spectra of PeNCs have not been studied much. Here, the factors that affect the width of the emission spectra of PeNCs are analyzed with controlled synthesis and surface passivation treatment. The overall spectra are governed by the size of PeNCs; however, the width could be narrowed by surface passivation treatment. The anion passivation effect of the surface passivation improved most of optoelectronic properties, but had less effect on the emission spectra width. The narrower emission spectra of PeNCs are obtained by ligand passivation effect of the surface passivation. Light emitting devices with enhanced optoelectronic properties are successfully fabricated and narrow (0.094 eV, 16.72 nm) blue electroluminescence emission spectra (∼470 nm) are obtained.

High colloidal stability ZnO nanoparticles independent on solvent polarity and their application in polymer solar cells.

Significant aggregation between ZnO nanoparticles (ZnO NPs) dispersed in polar and nonpolar solvents hinders the formation of high quality thin film for the device application and impedes their excellent electron transporting ability. Herein a bifunctional coordination complex, titanium diisopropoxide bis(acetylacetonate) (Ti(acac)2) is employed as efficient stabilizer to improve colloidal stability of ZnO NPs. Acetylacetonate functionalized ZnO exhibited long-term stability and maintained its superior optical and electrical properties for months aging under ambient atmospheric condition. The functionalized ZnO NPs were then incorporated into polymer solar cells with conventional structure as n-type buffer layer. The devices exhibited nearly identical power conversion efficiency regardless of the use of fresh and old (2 months aged) NPs. Our approach provides a simple and efficient route to boost colloidal stability of ZnO NPs in both polar and nonpolar solvents, which could enable structure-independent intense studies for efficient organic and hybrid optoelectronic devices.

Functionalized PFN-X (X = Cl, Br, or I) for Balanced Charge Carriers of Highly Efficient Blue Light-Emitting Diodes.

All-inorganic perovskite nanocrystals (PeNCs), CsPbX3 (X = Cl, Br, or I), have been considered as one of the prospective emissive materials for display applications, which showed superior photoluminescence quantum yield and high color purity with narrow spectral line width. Recently, high-performance green and red perovskite light-emitting diodes (PeLEDs) were introduced; however, the efficiency of blue PeLEDs still lagged owing to PeNCs' deep HOMO energy level (∼6.0 eV), which is in discord with the adjacent organic interlayer. In this work, we demonstrated an interfacial engineering strategy with conjugated polyelectrolytes, functionalized PFN (poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) with halide anions, between the hole injection layer and PeNCs. By introducing PFN-X (X = Cl, Br, or I), they exhibit well-balanced charge carriers and resultant effective radiative recombination in the PeNC layer with reduced hole injection barrier and electron blocking behavior. Among them, in particular, the PFN-Cl-treated PeLEDs display a maximum external quantum efficiency of 1.34% at 470 nm electroluminescence emission with enhanced spectral operating stability.

Enhancement of sub-bandgap light absorption in perovskite semiconductor films via critical coupling.

Light absorption in semiconductors is a fundamental problem that has broad impact on a wide range of fields. However, it is intrinsically limited by the bandgap energy of the semiconductor. Herein, we study the enhancement of sub-bandgap light absorption in inorganic-organic hybrid perovskite semiconductor films via critical coupling. This is achieved at large incidence angles by balancing radiative and nonradiative decay rates in a planar multilayer structure. We found that a very small loss in the semiconductor layer can result in substantial light absorption. This simple but general method can be used to enhance the optical and optoelectronic responses of semiconductors below the bandgap energy.

Morphology-Dependent Hole Transfer under Negligible HOMO Difference in Non-Fullerene Acceptor-Based Ternary Polymer Solar Cells.

In the field of organic solar cells, it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photoinduced hole transfer due to the large binding energy of excitons in organic materials. In this work, we investigate polymeric donor:non-fullerene acceptor junctions in binary and ternary blend polymer solar cells, which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is dependent on morphology. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC70BM. High external quantum efficiency was achieved at long wavelengths, despite ITIC-to-PC70BM ratio of 1:9, which indicates efficient photoinduced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend compared to that of donor:ITIC binary blend.

Morphological and Optical Engineering for High-Performance Polymer Solar Cells.

We demonstrate morphological and optical engineering by using processing additives and optical spacers for polymer solar cells. Among various processing additives, introduction of diphenyl ether (DPE) into the active layer results in the smoothest surface roughness with uniform and well-distributed donor/acceptor domains, and the device with DPE shows the highest device efficiency of 10.22% due to enhanced charge collection efficiency and minimized recombination loss. Additional ZnO optical spacers on the active layer controls the distribution of the electric field in the whole device and enhances the light absorption within the active layer, thereby improving device efficiency up to 10.81%.

Nanoparticle-Enhanced Silver-Nanowire Plasmonic Electrodes for High-Performance Organic Optoelectronic Devices.

Improved performance in plasmonic organic solar cells (OSCs) and organic light-emitting diodes (OLEDs) via strong plasmon-coupling effects generated by aligned silver nanowire (AgNW) transparent electrodes decorated with core-shell silver-silica nanoparticles (Ag@SiO2 NPs) is demonstrated. NP-enhanced plasmonic AgNW (Ag@SiO2 NP-AgNW) electrodes enable substantially enhanced radiative emission and light absorption efficiency due to strong hybridized plasmon coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) modes, which leads to improved device performance in organic optoelectronic devices (OODs). The discrete dipole approximation (DDA) calculation of the electric field verifies a strongly enhanced plasmon-coupling effect caused by decorating core-shell Ag@SiO2 NPs onto the AgNWs. Notably, an electroluminescence efficiency of 25.33 cd A-1 (at 3.2 V) and a power efficiency of 25.14 lm W-1 (3.0 V) in OLEDs, as well as a power conversion efficiency (PCE) value of 9.19% in OSCs are achieved using hybrid Ag@SiO2 NP-AgNW films. These are the highest values reported to date for optoelectronic devices based on AgNW electrodes. This work provides a new design platform to fabricate high-performance OODs, which can be further explored in various plasmonic and optoelectronic devices.

Formamidinium-based planar heterojunction perovskite solar cells with alkali carbonate-doped zinc oxide layer.

We herein demonstrate n-i-p-type planar heterojunction perovskite solar cells employing spin-coated ZnO nanoparticles modified with various alkali metal carbonates including Li2CO3, Na2CO3, K2CO3 and Cs2CO3, which can tune the energy band structure of ZnO ETLs. Since these metal carbonates doped on ZnO ETLs lead to deeper conduction bands in the ZnO ETLs, electrons are easily transported from the perovskite active layer to the cathode electrode. The power conversion efficiency of about 27% is improved due to the incorporation of alkali carbonates in ETLs. As alternatives to TiO2 and n-type metal oxides, electron transport materials consisting of doped ZnO nanoparticles are viable ETLs for efficient n-i-p planar heterojunction solar cells, and they can be used on flexible substrates via roll-to-roll processing.

Clean thermal decomposition of tertiary-alkyl metal thiolates to metal sulfides: environmentally-benign, non-polar inks for solution-processed chalcopyrite solar cells.

We report the preparation of Cu2S, In2S3, CuInS2 and Cu(In,Ga)S2 semiconducting films via the spin coating and annealing of soluble tertiary-alkyl thiolate complexes. The thiolate compounds are readily prepared via the reaction of metal bases and tertiary-alkyl thiols. The thiolate complexes are soluble in common organic solvents and can be solution processed by spin coating to yield thin films. Upon thermal annealing in the range of 200-400 °C, the tertiary-alkyl thiolates decompose cleanly to yield volatile dialkyl sulfides and metal sulfide films which are free of organic residue. Analysis of the reaction byproducts strongly suggests that the decomposition proceeds via an SN1 mechanism. The composition of the films can be controlled by adjusting the amount of each metal thiolate used in the precursor solution yielding bandgaps in the range of 1.2 to 3.3 eV. The films form functioning p-n junctions when deposited in contact with CdS films prepared by the same method. Functioning solar cells are observed when such p-n junctions are prepared on transparent conducting substrates and finished by depositing electrodes with appropriate work functions. This method enables the fabrication of metal chalcogenide films on a large scale via a simple and chemically clear process.

Optimal top electrodes for inverted polymer solar cells.

Although polymer solar cells (PSCs) have received a tremendous amount of attention in recent years, a number of criteria must be met in order for them to be suitable as practical and commercially feasible power sources, including high performance, good air stability and inexpensive manufacturing. In this contribution, we determine the optimal top electrode for practical PSC fabrication by investigating the influence of the electrode material on the optical properties and performance of PSC devices. The optical properties of eight metals were considered, out of which three metal electrodes (aluminum (Al), silver (Ag), gold (Au)) with the best optical properties were used to prepare inverted PSC devices comprising a blended polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Among the photovoltaic parameters, the short circuit current density (JSC) was most strongly affected by the optical properties of the top electrode. In the results of the experiment, the J(SC) of the Al and Ag electrode devices was found to be approximately 13% (13.4 → 15.1 mA cm(-2)) higher than the Au electrode device due to the significant parasitic absorption of light by Au at wavelengths below 600 nm. In contrast, Al and Ag electrodes have high reflectance throughout the visible spectrum, which leads to high J(SC). Ag electrodes have relatively good stability to ambient exposure, maintaining over 96% of the original efficiency after 170 hours; this stability is comparable to Au. These data lead to the conclusion that Ag is the optimal top electrode material for use in inverted devices.