bluuebunny/crossref_metadata_2025 huggingface dataset:
Functionalized Magnetic 3D Printing of High Performance Electrodes for Lithium Battery
Authors: Sarkar Abhishek , Shrotriya Pranav , Nlebedim Ikenna C
Published: December 2020

With the trend for rapid prototyping of parts and systems, additive manufacturing process like three-dimensional (3D) printing can advance the fabrication of shape-specific architectures for anisotropic permanent magnets involving expensive and critical rare-earth elements1,2. Although in-situ alignment will open new horizons for manufacturing of complex permanent magnets, integration of alignment systems into 3D printing processes is still a challenge. In our former works, we have demonstrated a mechanism to address this challenge accomplished by integration of bonded magnet production, 3D printing, finite element modeling and system development3.

Alignment of electrode pores, (e.g. using magnetic field) allow from better electrochemical transport of electrolyte through the electrode, thereby improving cell performance. The formerly developed magnetic field alignment model for 3D printed material in my postdoctoral program will be modified and used to predict the degree-of-alignment in electrodes for lithium-ion battery. The relationship between pore tortuosity and degree-of-alignment was modeled using a power law with the effect of exponent compared in Fig. 1a.

A coupled electrochemical-thermal-mechanical modeled is developed to predict the cell capacity loss due to above-mentioned anodic degradation mechanisms. Keeping the porosity, the tortuosity is altered between aligned and non-aligned states, to evaluate the electrochemical transport properties like diffusivity and ionic conductivity in the electrode. The model predicts an increase in cell capacity for lower tortuosity (aligned) electrodes, which is further amplified at faster charging conditions (Fig. 1b). It is generally observed that fast charging induces a steep non-uniform charge distribution in the unaligned electrodes. With increase in alignment, the electrolyte has easier pathways leading to a more uniform charge distribution and therefore a larger capacity. An immediate impact of a more uniform charge distribution is the lowering of tendency for lithium-ions to deposit in metallic form on the anode (called lithium plating).

These results provide an indication that alignment of electrodes will be beneficial towards designing a higher capacity and faster charging battery. Experimental validations are the next step towards validating the model predictions.

Huber, C. et al. 3D print of polymer bonded rare-earth magnets, and 3D magnetic field scanning with an end-user 3D printer. Appl. Phys. Lett.
109, 162401 (2016).

Li, L., Post, B., Kunc, V., Elliott, A. M. & Paranthaman, M. P. Additive manufacturing of near-net-shape bonded magnets: Prospects and challenges. Scr. Mater. (2017) doi:10.1016/j.scriptamat.2016.12.035.

Sarkar, A. et al. Functionalizing magnet additive manufacturing with in-situ magnetic field source. Addit. Manuf.
34, 101289 (2020).

Figure 1
(Invited) Nanostructured Rocksalt-Based Positive Electrode Materials for Rechargeable Li/Na Batteries
Authors: Yabuuchi Naoaki
Published: December 2020

In the past decade, lithium-enriched compounds, Li2MO3 (M = Mn4+, Ru4+
etc.), have been extensively studied for high-capacity positive electrode materials of lithium batteries. Although the origin of high reversible capacities was a debatable subject for a long time, recently it has been evidenced that charge compensation is partly achieved by solid-state redox of non-metal anions, i.e., oxide ions (anionic redox), coupled with redox reaction of transition metal ions (cationic redox), which is the basic theory used for classical lithium/sodium insertion materials. Recently, many cation-disordered rocksalt oxides have been proposed as a new series of electrode materials, which utilize reversible anionic redox. Nevertheless, insufficient electrode kinetics for the cation-disordered rocksalt system limits its use for practical applications. One simple strategy is to synthesize nanosized materials, which overcomes a problem of electrode kinetics (for electrons, holes and ions), and indeed electrode kinetics is significantly improved through nanosizing even for a non-lithium-excess and stoichiometric system.1) Moreover, Nanosized Li/Na-excess oxides also deliver large reversible capacities even at room temperature,2, 3) which shows much better electrode kinetics compared with as-prepared submicrometer-sized samples.

From these findings, we discuss the advantages/disadvantages of “nanostructured” rocksalt-based electrode materialsto develop high energy Li/Na-ion batteries in the future.

References

Sato et al., and N. Yabuuchi., Journal of Materials Chemistry A, 6, 13943 (2018).

Kobayashi et al., and N. Yabuuchi, Small, 15, 1902462 (6 pages) (2019).

Kobayashi et al., and N. Yabuuchi, Materials Today, in-press, DOI: 10.1016/j.mattod.2020.03.002
A Simple One-Dimensional Transport Model to Explain Inverse Signatures in Lithium-Metal Batteries
Authors: Uppaluri Maitri , Subramaniam Akshay , Subramanian Venkat R.
Published: December 2020

Lithium metal as the negative electrode in lithium batteries provides high theoretical capacity (3,860 mAh g–1) and negative electrochemical potential (-3.040 V vs standard hydrogen electrode) 1. However, using lithium metal poses several challenges, including dendritic electrodeposition and accumulation of dead lithium. These phenomena can cause the cell to short, and also contribute to poor cyclability 2.

Lithium deposition and stripping can be observed on the cell’s voltage response as unique signatures. These signatures can give insight on the morphology of the deposit 3. Physics-based continuum models can be used to understand lithium ion transport effects on deposition and stripping, and on the emergent voltage signatures.

Our past work studied one-dimensional models for lithium symmetric cells to model charge transport and capture the transitions in voltage observed experimentally 4. In this work, we propose a simple diffusion-based one-dimensional moving boundary that shows inverse signatures of lithium ion concentration and overpotential for the lithium deposition reaction. We show that using a transport model alone can produce unique inverse voltage signatures. The deposition of lithium is modeled as a moving interface.

The model was tested with different design parameters such as salt concentration, and different applied current densities and ion diffusivities to study their effect on the formation of signatures. These parameters affect the interface motion and ion transport in the electrolyte. The interplay between these two phenomena then determines characteristic signatures of the overpotential and the cell voltage.

Acknowledgement

This work at the University of Texas at Austin was supported by U.S. DOE award DEAC05- 76RL01830 through PNNL subcontract 475525 and the assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy (DoE) through the Advanced Battery Materials Research Program (Battery500 Consortium).

References

1. W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, and J. G. Zhang, Energy Environ. Sci., 7, 513–537 (2014).

2. D. Lin, Y. Liu, and Y. Cui, Nat. Nanotechnol., 12, 194–206 (2017).

3. K. N. Wood, M. Noked, and N. P. Dasgupta, ACS Energy Lett., 2, 664–672 (2017).

4. A. Subramaniam, J. Chen, R. M. Kasse, M. F. Toney, T. Jang, and N. R. Geise, J. Electrochem. Soc., 166, A3806–A3819 (2019).
(Invited) Scaling of Copper Interconnection As Opposed to Alternative Conductors
Authors: Nogami Takeshi
Published: December 2020

Cu/low-k interconnection is facing to challenges in its extension per scaling in the 3nm node and beyond where the metal pitch is less than 32 nm. As is listed in Table.1, the difficulties lie in securing (1) Cu gap fill capability in scaled via in the dual damascene process flow due to the limited step coverage of PVD Cu seed layer, (2) Vx-Mx breakdown voltage and TDDB reliability due to scaled spacing and misalignment in lithography, (3) Electromigration reliability when the barrier/liner is scaled, (4) Low line resistance due to decreased Cu volume when the barrier/liner scaling is limited, (5) Low via resistance when the barrier/liner scaling is limited, (6) Selectivity in selective CVD deposition of cobalt capping layer which is required for electromigration reliability and (7) Low interconnect parasitic capacitance when thickness of the plasma-induced damage of the intra-layer low-k dielectric becomes comparable to the inter-metal spacing per scaling.

Because of these difficulties in extension of dual damascene Cu interconnection scheme, various alternative metallization unit processes, materials (conductors and dielectrics) and schemes (alternative to dual damascene) have been investigated. These new processes and materials include both (a) those for extension of Cu technology such as Co_Cu hybrid metallization (i.e., Co for via and Cu for wires) and (b) those for entire replacement of Cu to other conductors. Basically extension of Cu interconnects is the preference in the industry rather than entire replacement to alternative conductors and/or metallization schemes, because entire replacement needs to be accompanied with alignment of all unit processes to the new metal schemes such as CMP and/or RIE of the new conductor, deposition of the conductor, wet cleaning compatible to the conductor, and nucleation/adhesion layer formation for the conductor on dielectrics, in all fine dimensions. In addition, even when fine Cu lines are replaced to alternative conductors such as Co, wide power rails need to stay with Cu for the required low line resistance (i.e., Cu_Co composite metallization).

In this talk, process candidates which have potential to resolve the difficulties listed in Table 1 and enable extension of Cu interconnection scheme are reviewed such as (i) via pre-fill processes with vertical ALD of low-k dielectrics [1], (ii) plasma damage resistant low-k dielectric materials [2], (iii) Fully Aligned Via integration process [3], (iv) PVD-assisted ALD barrier formation [4, 5], (v) through cobalt self-forming barrier process barrier/liner scaling processes without loss in their integrity [1, 6 - 8], (vi) graphene capping material on top of Cu interconnects for electromigration reliability 10]. Then, the talk covers (vii) the composite metallization scheme [11] which is an enabler for the post-Cu alternative conductor metallization.

Figure 1
Laser Debonding of Ultrathin Wafers and Packages: A Technology for the New Generation of Electronic Devices
Authors: Bravin Julian , Burggraf Jurgen , Pichler Mathias , Jung Stefan , Brandl Elisabeth
Published: December 2020

The continuous shrinkage of the device wafers thickness is driving the current mature debonding technologies to their limits. The challenging form factor demands for memory (≤ 30 µm) or new generations of power devices applications are just two examples for the need of lowering the device thickness.

Slide-off as well as mechanical debonding are methods currently in use in high volume production, but these two technologies are not capable to support the new requirements in terms of thermal and mechanical stability, handling, throughput (affecting the cost of ownership - CoO), and process margin. Therefore, device manufactures are looking for a more versatile, universal debonding technology. State of the art UV laser debonding is a very promising candidate which can help bringing the required process margin and flexibility. By combining laser debonding and thermoset materials with a high thermal budget the process window can be significantly enhanced.

Most of the thermoset materials of interest which are used to form a bond and to embed the device topography cannot be stimulated directly with UV light. In order to enable a debonding process for these materials, an additional layer is commonly deposited on the carrier. The post bond uniformity (quantified typically by the total thickness variation - TTV) of the used bulk layer of thermoset adhesive has to be very well controlled in order to enable a defect free thinning of the device wafer: this becomes crucial particularly when the final device layer has to be thinned down below 10 µm.

Besides the optimization of the bond layer thickness uniformity, suitable carriers for different devices and applications were evaluated. Due to the significant variety of the available glass carriers, a customized temporary bonding process can be developed by selecting the most appropriate adhesive and carrier combinations suitable even for very challenging substrates, like reconstituted wafers.

Reconstituted wafers consist of a number of different layers and materials with different thermal expansion coefficients, which makes the stress management and the handling of such substrates extremely difficult. However, the choice of the suitable carrier enables the processing independent of the chosen integration process flow (RDL first; Chip first). Even a carrier flip flop to enable the processing of the back side of the very thin package is possible and this is enabling the creation of ultra-thin and smart packages.

The scope of the current work was to address the post bond adhesive thickness uniformity as well as the stress management. Experimental results will be presented for the process optimization: figure 1 shows two bow measurement results for two different carrier materials and figure 2 shows an example of post-bonding thermoset adhesive uniformity measurement

Figure 1
Mechanical Behavior of Na-Ion Battery Anode Material
Authors: Pakhare Akshay , Nadimpalli Siva
Published: December 2020

Na-ion batteries (NIB) are been developed as an alternative to Li-ion batteries due to cost and scarcity of Li metal, but one of the major issues is capacity fade due to stress generation in electrodes during sodiation/desodiation process. Stress generation occurs due to volume expansion of Na when it alloys and dealloys with the anode. Here, we chose Ge as a representative anode material and measured its stress response during sodiation/desodiation cycling. The mechanical behavior of Ge due to sodiaiton/desodiation is compared and contrasted with that due to lithation/delithation. In this talk the interesting observations from this in-situ measurements and their implications will be presented.
(Invited) Evaluation of 2" Power-Grade GaN Substrates Produced By the Near Equilibrium Ammonothermal (NEAT) Method
Authors: Hashimoto Tadao , Letts Edward , Key Daryl , Jordan Benjamin , Shang Eric , Dudley Michael , Raghothamachar Balaji , Liu Yafei
Published: December 2020

This paper provides evaluation of 2" GaN substrates produced by the near equilibrium ammonothermal (NEAT) method. Ammonothermal growth is a proven method of producing low-dislocation GaN crystals with practical productivity. [1, 2] By choosing a near-equilibrium growth condition, consistent growth with maintained crystal quality has been achieved. [3] Several tens of bulk GaN crystals were grown simultaneously, yielding many 2" GaN substrates in one growth batch. Figure 1 is an example of bulk GaN crystals after shaping and Figure 2 is an example of 2" GaN wafer.

We have developed an X-ray mapping technique which presents the distribution of full width half maximums (FWHMs) of rocking curves from 002 and 201 diffractions over a 2" wafer. The X-ray mapping revealed superior microstructure quality of NEAT GaN substrates. NEAT GaN substrates typically show an average full width half maximum (FWHM) of the 002 X-ray rocking curve of 30 arcsec or better, as shown in Figure 3.

The X-ray mapping data were compared with other characterization techniques, including X-ray topography. X-ray topography confirmed that a dislocation density of typical NEAT GaN substrate was about 2 x 105 cm-2. Wide area X-ray topography also revealed flatter lattice of a NEAT GaN substrate compared with a GaN substrate fabricated by hydride vapor phase epitaxy (HVPE).

Additionally, recent progress in oxygen reduction yielded NEAT GaN substrates with reduced residual donors, coloration and optical absorption. The oxygen concentration was about 2 x 1018 cm-2. An absorption coefficient of 1.3 cm-1 was obtained at 450 nm.

The usability of NEAT GaN substrates was also confirmed through device fabrication. Multiple collaborators successfully fabricated high-power diodes with breakdown voltage over 1000V.

References

[1] E. Letts, Y. Sun, D. Key, B. Jordan, and T. Hashimoto, J. Cryst. Growth 501 (2018) 13.

[2] R. Dwilinski, R. Doradzinski, J. Garczynski, L. Sierzputowski, R. Kucharski, M. Zajac, M. Rudzinski, R. Kudrawiec, W. Strupinski, and J. Misiewicz, Phys. Stat. Sol. A 208 (2011) 1489.

[3] T. Hashimoto, E.R. Letts, D. Key, and B. Jordan, Jpn. J. Appl. Phys 58 (2019) SC1005. Acknowledgement

This work was supported by U.S. Department of Energy (DOE), Advanced Research Program Agency, Energy (ARPA-E), SWITCHES program (grant number DE-AR0000445), US DOE ARPA-E OPEN 2018 programs (DE-AR0001008 and DE-AR0001008), U.S. DOE, the Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office (AMO), Small Business Innovation Research (SBIR) program (grant number DE-SC0013791), US DOE EERE AMO FY18/FY19 Lab Call (grant number DE-LC-000L059), and the Office of Naval Research (grant number N00014-19-1-2069).

Figure 1
Photocurrent Measurements of AlGaN/GaN Hemt Under X-Ray Irradiation
Authors: Miyata Atsuki , Hoshii Takuya , Wakabayashi Hitoshi , Tsutsui Kazuo , Kakushima Kuniyuki
Published: December 2020

The photocurrent of AlGaN/GaN HEMT under x-ray exposure was measured. A sharp response in the photocurrent was observed with the exposure and the current showed linear relationship with the x-ray tube current. An increase in the photocurrent was measured, especially for high intensity exposure, and a tail in the current was observed after shutting the exposure. The transient response in the photocurrent may have relationship with the deep defects in the epitaxial wafer.

Introduction

Gallium nitride (GaN) HEMTs are widely used in the devices for power and RF applications. Due to the epitaxial growth of the layers on foreign substrates, lots of defects are incorporated in the epitaxial wafer. In this study, we measured the photocurrent of the AlGaN/GaN HEMT under x-ray exposure, and its transient response was observed.

Experiments

AlGaN/GaN HEMT on a Si substrate was fabricated by Ohmic contact to the 2DEG with top electrode (Ti/Al/Ni/Au) with thermal treatment[1], followed by mesa isolation by dry etching. Finally, backside contact was formed by Al evaporation. The area of the exposure was 5×5 cm2. A bias voltage of 40 V was applied to the substrate, where the dark current was as low as 0.5 nA. The x-ray source was WKα and tube current was changed from 10 to 150 mA.

Results and discussions

Fig.1 shows the photocurrent response to the x-ray exposure. A sharp response of the photocurrent was obtained when the x-ray is exposed to the devices. A linear relationship with the photocurrent and the tube current was obtained, as shown in fig. 2. However, a gradual increase in the photocurrent was observed, especially at high tube current. Moreover, a tail photocurrent was also observed after shutting the x-ray. The magnitude of the tail current was high with large tube current. The gradual increase and the tail in the photocurrent might be caused by the trapping of photogenerated carriers.[2]

Conclusion

A gradual increase in the photocurrent was observed under high intensity x-ray exposure to the AlGaN/GaN HEMT. Also, a tail current was measured after shutting the exposure. These photocurrent transient responses might have some relationship with the defects in the epitaxial wafer.

References

[1]Z. X. Qin, et al., Appl. Phys. A 78, 729-731 (2004)

[2]Jean-Yves Duboz, et al.
Phys. Lett.
92, 263501 (2008)

Figure 1
Electrochemical Purification of Molten Chloride/Fluoride Salt for Silicon Electrorefining
Authors: Tseng Mao-Feng , Ricci Lewis , Tao Meng
Published: December 2020

With the ever-increasing demand for solar modules nowadays, there is a growing interest in developing an alternative, eco-friendly and low-cost manufacturing process for solar-grade silicon. Silicon electrorefining in a molten salt is a promising technique for producing solar-grade silicon directly from metallurgical-grade silicon with low carbon emission and low energy consumption. Due to the stringent requirement in the purity of the silicon for solar applications (at least 99.9999% or 6N), a high-purity molten salt electrolyte is necessary for the solar-grade silicon electrorefining process. In this report, an electrochemical purification process for the eutectic CaCl2-CaF2 (CaCl2:CaF2 = 86.2:13.8 by weight) molten salt at 900℃ has been studied. The impurity level in the salt was in-situ examined by cyclic voltammetry (CV). After the electrochemical purification process, a decrease in peak current of the CV is observed, indicating the removal of the impurities. According to energy-dispersive x-ray spectroscopic results, Fe, Mn, Mg and Al were successfully extracted from the salt. This electrochemical purification process and in-situ salt purity examination technique are useful for purity control of the molten salt in the solar-grade silicon electrorefining process.
(Keynote) Efficient, Stable and Scalable Perovskite Solar Cells
Authors: Seo Jangwon
Published: December 2020

Power conversion efficiencies (PCE) of perovskite solar cells (PSCs) has rising from the initial 3.8% to the state-of-the-art 25.2% within the past few years. Current PSCs intended for commercialization in the near future require both high efficiency and good long-term stability. Most highly efficient PSCs utilize an n-type layer of mesoporous titanium dioxide or tin oxide in an n-i-p device configuration, in which organic conductors are widely used to transport holes into an adjoined metal. Thus far, a variety of efforts have been devoted to achieve a defect-less perovskite film with high-quality morphologies to realize reduced loss-in-potential outcomes and enhanced efficiency levels. In this talk, we will discuss several challenges that need to be addressed in improving the photovoltaic performance up to 26% and enhancing the stability of the perovskite solar cells, i.e. (1) preparation of high crystalline film of (FAPbI3)1-x(MAPbBr3)x with controlled carrier mobility and light harvesting, (2) development of selective charge-transporting layers (CTL) with physically and electrically tuned property, (3) interfacial control between the perovskite and the CTLs for reducing the surface defect and preventing the interfacial recombination, and (4), new hole-transporting materials for thermal stability. Finally, our recent works on scale-up of PSCs will be discussed; 20.6%-efficient flexible cells and large-area sub-modules. Our strategy as presented in this work will offer new directions for those involved in the fabrication of highly efficient, stable and scalable PSCs.
(Invited) Interfacial Engineering for High Voltage Performance of Perovskite Photovoltaic Cells
Authors: Miyasaka Tsutomu , Zhanglin Guo , Jena Ajay , Kim Gyu Min , Takei Izuru
Published: December 2020

Since our first discovery of perovskite solar cell (PSC) in 2009, enormous efforts have been put into different aspects of PSCs and the progress has been incredibly fast on all fronts. The background, on-going R&Ds, and future direction of PSC research have been reported by our group as a comprehensive review.1 Although efficiency level has reached 25%, PSCs face serious challenges of practical stability and durability required for industrialization. Compositional engineering of lead halide perovskites by mixing different cations and anions, using modulator molecules and mixing 2D and 3D structures have improved the stability of perovskites against heat and moisture. However, organic cations in halide perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs is highly desired.2 We have conducted the device fabrication in this direction using CsPbI3 and CsPbI2Br as solution-processed perovskite films. The perovskite absorbers were made into junction with dopant-free HTMs to develop thermally stable PSCs.By chemical engineering to improve the quality of interfacial structure at the hetero-junction, all-inorganic and dopant-free PSCs yielded power conversion efficiency up to 15%. Further we could enhance the open-circuit voltage (Voc) of this PSC up to 1.42V, which is the highest Voc ever achieved with a single perovskite cell.3 In preparation of all-inorganic perovskites, a big challenge should also be directed to development of lead-free perovskite materials for environmental safety in practical applications.2

Among extensive applications of PSCs for outdoor and indoor power generation, use of PSCs in space environments is promising because thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to high energy particle irradiations (proton and electron beams).4 Thin absorbers (<500 nm) avoid accumulation of particles and due to intrinsic defect tolerant nature of perovskites, radiation-induced collision damage is highly suppressed. Our current efforts in making PSCs based on both lead and lead-free perovskites, and future perspectives of perovskite photovoltaics will be presented.

References:

A. K. Jena, A. Kulkarni, and T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.

T. Miyasaka, A. Kulkarni, G. M. Kim, S. Oez and A. K. Jena, Adv. Energy Mat., 2019, 1902500.

G. Zhanglin, T. Miyasaka, et al. J. Am. Chem. Soc.
2020, 142, 21, 9725–9734.

Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, and T. Miyasaka, iScience
2018, 2, 148-155.
(Invited) All Inorganic Metal Halide Perovskite Solar Cells
Authors: Heo Jin Hyuck , Im Sang Hyuk
Published: December 2020

Metal halide perovskite solar cells have attracted great attention because of their excellent properties such as high absorption coefficient, small exciton binding energy, convenient bandgap tunability, long charge carrier’s diffusion length, and solution processability. Owing to the intensive studies, to date their record efficiency reached to 25.2 % at 1 sun condition. However, the organic/inorganic hybrid metal halide perovskite solar cells have intrinsically weak stabilities against moisture and heat. Therefore, all inorganic metal halide perovskite materials have been of great interest because they exhibit better thermal stability than the hybrid perovskite materials even though they have wider bandgap than the hybrid perovskite materials. Therefore, it is still challenging to improve the efficiency of all inorganic metal halide perovskite solar cells to find commercial applications. Here, we would like to share recent progress of all inorganic metal halide perovskite solar cells.
Interlayer Solid-Phase Reaction in Halide Perovskite Thin Films Fabricated Via Alternate Infrared Laser Deposition
Authors: Matsuki Nobuyuki , Iida Yuki , Shimada Takahiro , Abe Yuta , Sato Tomomasa
Published: December 2020

It is now widely pursued to investigate the growth mechanism of halide perovskite thin films in detail to elucidate the degradation mechanism and to develop the thin films with superior photovoltaic properties. We have applied infrared laser molecular beam deposition (IRL-MBD), which can control the deposition of halide perovskite at the atomic layer level in ultra-high vacuum condition [1]. In our IRL-MBD process, CH3NH3PbI3 (MAPbI3) layers are formed by alternately depositing CH3NH3I (methylammonium iodide: MAI) and PbI2 layers, and subsequent interlayer solid-phase reaction at room temperature. In this study, we focus on the effect of the layer thickness on the interlayer solid-phase reaction and the properties of the formed MAPbI3 thin films.

A synthetic silica substrate cleaned using organic solvents was introduced into the IRL-MBD chamber via a load lock. After that, PbI2 and MAI molecular beams were alternately irradiated onto the substrate by evaporating MAI and PbI2 sources using an infrared laser beam (wavelength: 808 nm, input power: 2~3 W). The deposition rate of PbI2 was controlled at 0.1 nm/s using a quartz crystal microbalance (QCM) thickness monitor and manual laser power control. The evaporation rate of MAI was manually adjusted so that the pressure in the chamber was kept at 4×10-4 Pa during the irradiation of MAI onto the substrate. Figure 1 shows a schematic layer structure of the samples A, B and C consisting of 1, 5 and 10 pair(s) of [MAI/PbI2] bilayer, respectively. The irradiation time of MAI and the nominal thickness of PbI2 were A: [80 min/300 nm]×1, B: [16 min/60 nm]×5 and C: [8 min/30 nm]×10.

Figure 2 shows the X-ray diffraction (XRD) patterns obtained from the samples. The sample A shows two diffraction peaks corresponding to PbI2(001) and MAPbI3(110), while the samples B and C exhibits only single peak corresponding to MAPbI3(110) in the shown diffraction angle range. Figure 3 shows the schematic structure of the samples inferred from the results described above. In the sample A, PbI2 layer, which is not completely reacted with MAI, remains in the thin film. In contrast, in the samples B and C, only the phase of MAPbI3 is observed, indicating that PbI2 and MAI react completely by the interlayer solid-phase reaction. The results suggest that PbI2 has a critical film thickness that causes a complete interlayer solid-phase reaction with MAI at room temperature between 60 nm and 300 nm. The critical film thickness is likely comparable to the penetration depth of MAI into PbI2 and is supposed to be dependent on the temperature during the interlayer solid-phase reaction.

[1] K. Kawashima, Y. Okamoto, O. Annayev, N. Toyokura, R. Takahashi, M. Lippmaa, K. Itaka, Y. Suzuki, N. Matsuki and H. Koinuma, Sci. and Tech. of Adv. Mater.
18, 307-315 (2017).

Figure 1
Highly Purified Materials for Sn-and Pb-Based Perovskite Solar Cells
Authors: Wakamiya Atsushi
Published: December 2020

Perovskite solar cells are promising cost-effective next generation printable photovoltaics. These solar cells have attracted the interest of many researchers. Thanks to tremendous effort of the researchers in this field, the power conversion efficiencies (PCEs) have been substantially increased in a short period, which is mainly due to improvements of the fabrication protocols for the perovskite layer as well as the development of new materials for buffer layers.

Our approaches toward efficient lead and lead-free perovskite solar cells are as follows:

Development of highly-purified perovskite precursor materials

Development of solution process fabrication methods for the perovskite layer

Development of organic semiconductors for efficient charge-collection layers

We have synthesized lead-halide and tin halide complexes as purified precursors for perovskite materials, which were later commercialized by TCI and are now widely used in this field as standard precursor materials. We also focus on the intermediates formed during the film-growth and have developed efficient fabrication methods based on the formation mechanism. We have designed and synthesized novel pi-conjugated materials designed to control frontier orbital energy levels, molecular aggregation properties, and the interface with the perovskite layer.

In this talk, our recent progress on tin halide perovskite solar cells as well as lead halide perovskites will be introduced.
Transparent Hole-Transporting Materials Containing Partially Oxygen-Bridged Triphenylamine Skeletons for Efficient Perovskite Solar Cells
Authors: Minh Anh Truong , Murdey Richard , Wakamiya Atsushi
Published: December 2020

Transparent organic semiconductors are desirable for organic optoelectronic devices. While electrical performance is often improved by increasing the p-conjugation length, the resulting red-shifted absorption profile typically leads to loss of transparency. Therefore, it is a great challenge to design and develop organic semiconductor materials that possess both high electrical performance and transparency in visible light region. We have previously developed promising transparent hole-transporting materials (HTMs) based on the quasiplanar skeletons of partially oxygen-bridged triphenylamines.

In the present study, we have designed new transparent HTMs by introducing diarylamines into the 7-position of a quasiplanar partially oxygen-bridged triphenylamine. The combination of an oxygen-bridged triphenylamine strong donor and a diarylamine weak donor makes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) localize on the oxygen-bridged triphenylamine and diarylamine, respectively, leading to minimal orbital overlap between the HOMO and the LUMO, therefore, weak absorption. Three HTMs, namely, HND-Cbz, HND-DTP, and HND-NOMe, were synthesized by connecting a partially oxygen-bridged triphenylamine structure to carbazole, dithieno[3,2-b:2′,3′-d]pyrrole, and bis(4-methoxyphenyl)amine through Buchwald-Hartwig amination with yields of 91%, 70%, and 88%, respectively. All of three materials are amorphous and thermally stable with decomposition temperatures higher than 350 °C. The vacuum-deposited thin films are transparent, with absorption edges from 405 nm to 419 nm. The space-charge limited current (SCLC) hole mobilities of HND-Cbz, HND-DTP, and HND-NOMe are 1.3 x 10-
7, 3.5 x 10-
5 and 1.4 x 10-
4 cm2 V-
1 s-
1, respectively while the HOMO energy levels are –5.34, –5.30 and –5.08 eV. The HOMO levels lie above the corresponding valence band of typical perovskites (e.g. –5.45 eV for MAPbI3 or –5.56 for Cs0.05FA0.15MA0.80PbI2.75Br0.25), making these molecules potentially good candidates for hole-transporting materials in perovskite solar cells. To verify this, perovskite solar cells having the structures ITO/SnO2/Cs0.05FA0.15MA0.80PbI2.75Br0.25/HTM/Au for the solution processed HTM and ITO/SnO2/Cs0.05FA0.15MA0.80PbI2.75Br0.25/HTM/MoO3/Au/MoO3 for the vacuum processed HTM layer, were fabricated. With HND-Cbz, HND-DTP and HND-NOMe as the HTM, the best power conversion efficiency reached 13.7%, 15.0% and 17.2%, respectively.

Figure 1
Silver Recovery from Silver Fluoride Solution Using Electrowinning
Authors: Tezak Cooper , Ricci Lewis , Tao Coby , Tao Meng
Published: December 2020

As solar technology becomes more popular, more modules with limited lifetimes will be manufactured. By 2050 there are expected to be 78 million tons of waste modules, which are rarely recycled today. Each module contains approximately 6 grams of silver, which is the most precious metal found in silicon modules. Hydrofluoric acid is proposed as the leaching agent to remove silver and other metals from the surface of the silicon, while electrowinning is used to recover the silver from the leachate and plate it onto a cathode. The electrowinning setup uses a three-electrode configuration with a pseudo reference electrode to control the half-cell potential. A silver pseudo-reference electrode was found to be sufficient for achieving a high mass recovery rate. Variable voltage trials were conducted and 0.7 V with reference to silver achieved a high mass recovery rate of 96.4% with a high purity of silver. Current efficiency is below 7% for all the voltages tested, which suggests parasitic reactions in the reaction vessel such as O2(g) + 4H+ + 4e− ⇌ 2H2O. The trials at 0.7 V vs silver in a nitrogen purge environment eliminated corrosion at the vapor-liquid interface on the cathode but did not improve the current efficiency. It is possible that the nitrogen purge environment still contains residual oxygen.
Compositional Engineering of FA-Based Lead Halide Perovskitesfor Shear Coating Processes
Authors: Kim Hansol , Gu Hyewon , Chung Choong-Heui , Song Chang Eun , Hong Ki-Ha
Published: December 2020

Recently Organic-inorganic halide perovskite (OIHP) solar cells have become one of the most promising next-generation photovoltaic materials due to their excellent electrical and optical properties. Formamidinium lead triiodide (HC(NH2)2PbI3, FAPbI3) exhibits the narrowest band gap (~1.45 eV) among Pb-based halide perovskites, which is closer to Shockley-Queisser limit than that (~1.55 eV) of Methylammonium Lead Iodide (CH3NH3PbI3, MAPbI3). Therefore, FAPbI3 has been widely adopted for recently developed highly efficient photovoltaic cells.

In addition, FAPbI3 has better stability than MAPbI3, which can be easily decomposed at a high temperature or humid environment. The spin-coating has been widely adopted to make FAPbI3-based solar cells, whereas the large-area coating technologies have not been extensively studied, yet.

This study presents the compositional engineering research on the polymorph control of formamidinium-based lead halide perovskites for the large-area film coating. The polymorph control of FAPbI3 has been one of the most important issues since its first appearance in photovoltaic applications. Non-perovskite structure (δ-FAPbI3) is thermodynamically more stable than the three-dimensional perovskite structure (α-FAPbI3). We adopted the shear coating method to make perovskite layers and controlled the polymorphism through A-site mixing. MA, commonly used A-site molecule, is excluded because MA can be a weak spot at the humid condition. Br mixing is not applied due to the photo-induced phase separation issue.

Our results reveal that double-cation mixing (Cs-FA or Rb-FA) is not enough and triple-cation mixing (Rb-Cs-FA) cannot be avoided to make robust α-FAPbI3 for the shear coating process. We present the ternary phase diagram of (Rb-CS-FA)PbI3 to show the process window based on the XRD analysis of mixed perovskite films. The photovoltaic cell shows 15.8% power conversion efficiency (PCE) at the best compositional mixing point, which is the best PCE for FA-based perovskite solar cells fabricated by shear coating method without MA-Br mixing.
Wearable Photodiodes & Arduino Made on Fabrics for Smart Fashion
Authors: Lee Wonjae , Abdur Rahim , Singh Son , Kuddus Sheikh Md Abdul , Paik Kyung-wook , Lee Jaegab
Published: December 2020

Advances in the development of the flexible sensors, low power radio communication systems, and miniature electronic components allow for the application of wearable devices and the smart fashion. Moreover, healthcare has become a general awareness. People are urging to detect health issues as the earliest stage as possible. The use of conventional technologies are expensive and hindered the performance & wearable due to mechanical rigid & non-flexibility. Therefore, low cost and excellent flexibility with high mechanically stable & wearable health trackers are needed to monitor health issues based on optical detection due to their noninvasive tracking of vital health issues. . Here, we demonstrate hybrid perovskite based flexible photodetector for wearable sensor devices and we have designed a photo sensor module on fabrics which is able to transfer signals wirelessly to cell phones & PC systems. It is useful for wearable health monitoring & smart fashion cloths.

In this study, we discuss the design and experimental demonstration of hybrid perovskite based photodetector which is integrated into flexible photo sensor module on fabric. We have fabricated perovskite-based flexible photodetector consisting of ITO/CdS/perovskite/s-V2O5/Au, focusing on the conformal coating of perovskite on the planar CdS by addition of PbBr2 and PbCl2 to PbI2, which significantly increases the nucleation rate of perovskite, the leading to highly smooth surface morphology of perovskite. This enhances the performance of photodetectors. The devices are fabricated on flexible substrate which withstand many bending cycles at a submillimeter radius without degradation of optoelectronic properties. This wearable smart photo sensor module is able to monitor vital health issues including heart rate & oxygen saturation level in blood. Our findings provide a hybrid perovskite photo sensor module for photonic devices to improve their role in the area of flexible & mechanically stable and wearable optoelectronic systems.
Novel Processing of Tin Monosulfide Single Crystals
Authors: Nose Yoshitaro , Takemura Tomoki , Katsube Ryoji
Published: December 2020

Tin monosulfide (SnS) is an attractive semiconductor for electronic and optoelectronic applications such as thin-film photovoltaics, due to the optical and electrical properties, earth-abundance of the composing elements, and the layered crystal structure. One of issues under active discussion in SnS-based devices is how to prepare SnS without sulfur-rich secondary phases such as SnS2 and Sn2S3, which are easily formed in processes such as sulfurization of tin. In contrast, we have been established the processing to prepare single-phase SnS by distillation from Sn/SnS composites based on chemical thermodynamic calculations on the S-Sn binary system.

Sn/SnS composites were prepared by heat-treatment of tin shots and sulfur flakes in a evacuated quartz ampule. The composites were again sealed in an ampule, and the distillation was carried out using Bridgman furnace. SnS bulk single crystals were obtained by unidirectionally cooling from the opposite side of composites. The crystals can be easily cleaved along to the layer plane, and secondary phases were not confirmed. The electric properties in the plane of single crystals are a p-type conduction with high carrier mobility of 100 cm2V-1s-1, which is advantage for an application to solar cell devices.
Electric and Photoelectrochemical Properties of CuGaSe2 Single Crystal
Authors: Ikeda Shigeru , Fujita Wakaba , Nose Yoshitaro , Katsube Ryoji , Yoshino Kenji , Harada Takashi
Published: December 2020

The CuGaSe2 compound is a promising material as a photoabsorber of solar cell due to its p-type semiconductive characteristic and optimum band gap for sunlight absorption (1.68 eV). This compound is also attractive for the use of a photocathode for photoelectrochemical (PEC) water splitting because of its sufficient negative energy level of conduction band minimum (CBM) to induce reduction of water. For such applications, a thin-film form of the compound is usually used. In order to clarify and control its bulk and/or surface properties optimal for photovoltaic and PEC applications, we attempted to use a single crystalline form of the compound obtained by using a conventional vertical Bridgman technique.

Several crystallographic and compositional analyses indicated successful formation of the CuGaSe2 single crystal. Electronic properties of the thus-obtained single crystal measured after processing into a wafer form suggested a p-type semiconductive characteristic responsible for the use of photocathode. However, due to the insufficient acceptor concentration, the PEC property of the sample was not enough as was expecting it to be. After various trial, post annealing in a Se vapor atmosphere was found to be effective for improvement of its PEC functions. Details on effects of the annealing on the photovoltaic and the PEC hydrogen evolution properties were examined.
(Invited) Improved Copper Electrode Integration for Thin Film Electronics on Glass
Authors: Kim Hoon , Zhu Bin , Huang Ming-Huang , Vaddi Rajesh , Manley Robert G.
Published: December 2020

Increasing demand for higher resolution and larger displays continues to push research of thin film transistor materials, process, and devices. Especially, as display size become larger and pixel density increase, the signal delay becomes more critical due to high resistance of interconnects due to longer line length and smaller current carrying cross-sections. With high refresh rate and higher definition displays, the decrease of the pulse time for each pixel requires higher conductance of the interconnects to avoid non-uniformity of display performance. Thus, Cu is used as a conductor because it has low resistivity and high reliability. Incumbent Cu interconnect technologies require adhesion layers, such as Ti, Mo, or variant alloys, due to poor adhesion of Cu to glass or SiNx barriers. However, these technologies have limits such as higher resistivity of the interconnect lines due to diffusion of adhesion or barrier metal into Cu. They can also result in under-cut during pattering due to Galvanic effects in etching chemistry. The cost of processing and fabrication is another key consideration for direction of process research. Cu interconnects with the metal adhesion layer requires two step etching process to avoid the under-cut. Thus, single Cu interconnects without the use of a metal adhesion layer is promising not only because of process simplicity, but also low processing cost. A Cu alloy is a promising approach to address the issues. Most of the Cu alloy study results have high resistivity Cu interconnects due to some residue of alloy elements in the Cu lattice or grain boundary. In this study, we demonstrate single layer Cu interconnects using thin CuMn alloy as temporally adhesion layer. After short annealing of a 10 nm CuMn alloy with 500 nm of Cu film, CuMn alloy layer is converted to pure Cu and the Mn is reacted with glass to form MnOx. The formed MnOx serves as adhesion layer, binding the thin film Cu electrode to the glass substrate. After optimization of Mn concentration, alloy thickness, annealing environment, time and temperature, a resistivity lower than 1.8 µOhm·cm of the electrode was achieved. Based on this results, one step Cu etching with low resistivity Cu interconnects is feasible. We also evaluated the effect of glass surface treatment to confirm the sensitivity of this process depending on substrate preparing conditions. This process has been shown to be very stable for different display glasses with treatments. It is also compared with that of Ti/Cu stack as reference.
Electric Field Thermopower Modulation Analyses of the Operation Mechanism of Transparent Amorphous SnO2 Thin-Film Transistor
Authors: Liang Doudou , Zhang Yu-qiao , Cho Hai Jun , Ohta Hiromichi
Published: December 2020

Transparent amorphous oxide semiconductors (TAOSs) based transparent thin film transistors (TTFTs) with high field effect mobility (μ
FE) are necessary to develop advanced flat panel displays. Currently, amorphous (a-) InGaZnO4 is widely applied as the TAOS of the TFT channel of the commercially available OLED displays with a μ
FE is ~10 cm2 V−
1 s−
1[1]. Among TAOSs, amorphous (a-) SnO2 has several advantages against current a-InGaZnO4 such as higher μ
FE >100 cm2 V–1 s–1[2], which is one order of magnitude higher than that of a-InGaZnO4 and indium free. Although there are several researches on a-SnO2 TTFT, the operation mechanism has not been clarified thus far due to the strong gas sensing characteristics of SnO2.

We prepared a bottom-gate top-contact type TTFT by pulsed laser deposition technique using 4.2-nm-thick a-SnO2 as the channel without any surface passivation. We measured the transistor characteristics and thermopower(S) in air and vacuum. The resultant a-SnO2 TTFT showed clear transfer (I
d–V
g) characteristics (FIG. a) with the on-to-off current ratios of ~105. All the TTFTs show clear pinch-off in the output characteristics. The threshold voltage (V
th) was −14 V in air whereas it shifted dramatically to −23 V in vacuum shows a strong gas sensing characteristic. An almost linear relationship with a slope of ~120 μV K−1 decade−1 was observed in the –S–log n
s plot (FIG. b) when n
s exceeded 2.5 × 1012 cm−2 in air and a slope of ~84 μV K−1 decade−1 was observed in the –S–log n
s plot when n
s exceeded 2.9 × 1012 cm−2 in vacuum indicating that the E−k relation at the bottom of the conduction band is parabolic shaped. The t
eff ≡ n
s/n
3D of the conducting a-SnO2 channel were always ~1.7 ± 0.4 nm, insensitive to the gas atmosphere (FIG. c). From the thickness of the a-SnO2 film (4.2 nm) and the t
eff (1.7 ± 0.4 nm), the carrier depletion depth at the top surface was concluded to be 2.5 nm of the a-SnO2 film, which shows the similar depletion length as reported data.[4] The present results would provide a fundamental design concept to develop a-SnO2 TTFT.

[1] K. Nomura et al., Nature, 432, 488 (2004).

[2] C. W. Shin et al., Sci. Rep.
6, 19023 (2016).

[3] N. Yamazoe, Sensor Actuat. B-Chem.
5, 7 (1991).

[4] D. Liang et al., Appl. Phys. Lett.
116, 143503 (2020)

Figure 1