Unveiling the Role of tBP-LiTFSI Complexes in Perovskite Solar Cells.

Lead halide-based perovskite materials have been applied as an intrinsic layer for next-generation photovoltaic devices. However, the stability and performance reproducibility of perovskite solar cells (PSCs) needs to be further improved to match that of silicon photovoltaic devices before they can be commercialized. One of the major bottlenecks that hinders the improvement of device stability/reproducibility is the additives in the hole-transport layer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4- tert-butylpyridine (tBP). Despite the positive effects of these hole-transport layer additives, LiTFSI is hygroscopic and can adsorb moisture to accelerate the perovskite decomposition. On the other hand, tBP, the only liquid component in PSCs, which evaporates easily, is corrosive to perovskite materials. Since 2012, the empirical molar ratio 6:1 tBP:LiTFSI has been wildly applied in PSCs without further concerns. In this study, the formation of tBP-LiTFSI complexes at various molar ratios has been discovered and investigated thoroughly. These complexes in PSCs can alleviate the negative effects (decomposition and corrosion) of individual components tBP and LiTFSI while maintaining their positive effects on perovskite materials. Consequently, a minor change in tBP:LiTFSI ratio results in huge influences on the stability of perovskite. Due to the existence of uncomplexed tBP in the 6:1 tBP:LiTFSI mixture, this empirical tBP-LiTFSI molar ratio has been demonstrated not as the ideal ratio in PSCs. Instead, the 4:1 tBP:LiTFSI mixture, in which all components are complexed, shows all positive effects of the hole-transport layer components with dramatically reduced negative effects. It minimizes the hygroscopicity of LiTFSI, while lowering the evaporation speed and corrosive effect of tBP. As a result, the PSCs fabricated with this tBP:LiTFSI ratio have the highest average device efficiency and obviously decreased efficiency variation with enhanced device stability, which is proposed as the golden ratio in PSCs. Our understanding of interactions between hole-transport layer additives and perovskite on a molecular level shows the pathway to further improve the PSCs' stability and performance reproducibility to make them a step closer to large-scale manufacturing.

Three-Dimensional Nanoscale Mapping of State-of-the-Art Field-Effect Transistors (FinFETs).

The semiconductor industry has seen tremendous progress over the last few decades with continuous reduction in transistor size to improve device performance. Miniaturization of devices has led to changes in the dopants and dielectric layers incorporated. As the gradual shift from two-dimensional metal-oxide semiconductor field-effect transistor to three-dimensional (3D) field-effect transistors (finFETs) occurred, it has become imperative to understand compositional variability with nanoscale spatial resolution. Compositional changes can affect device performance primarily through fluctuations in threshold voltage and channel current density. Traditional techniques such as scanning electron microscope and focused ion beam no longer provide the required resolution to probe the physical structure and chemical composition of individual fins. Hence advanced multimodal characterization approaches are required to better understand electronic devices. Herein, we report the study of 14 nm commercial finFETs using atom probe tomography (APT) and scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS). Complimentary compositional maps were obtained using both techniques with analysis of the gate dielectrics and silicon fin. APT additionally provided 3D information and allowed analysis of the distribution of low atomic number dopant elements (e.g., boron), which are elusive when using STEM-EDS.

Role of 4-tert-Butylpyridine as a Hole Transport Layer Morphological Controller in Perovskite Solar Cells.

Hybrid organic-inorganic materials for high-efficiency, low-cost photovoltaic devices have seen rapid progress since the introduction of lead based perovskites and solid-state hole transport layers. Although majority of the materials used for perovskite solar cells (PSC) are introduced from dye-sensitized solar cells (DSSCs), the presence of a perovskite capping layer as opposed to a single dye molecule (in DSSCs) changes the interactions between the various layers in perovskite solar cells. 4-tert-Butylpyridine (tBP), commonly used in PSCs, is assumed to function as a charge recombination inhibitor, similar to DSSCs. However, the presence of a perovskite capping layer calls for a re-evaluation of its function in PSCs. Using TEM (transmission electron microscopy), we first confirm the role of tBP as a HTL morphology controller in PSCs. Our observations suggest that tBP significantly improves the uniformity of the HTL and avoids accumulation of Li salt. We also study degradation pathways by using FTIR (Fourier transform infrared spectroscopy) and APT (atom probe tomography) to investigate and visualize in 3-dimensions the moisture content associated with the Li salt. Long-term effects, over 1000 h, due to evaporation of tBP have also been studied. Based on our findings, a PSC failure mechanism associated with the morphological change of the HTL is proposed. tBP, the morphology controller in HTL, plays a key role in this process, and thus this study highlights the need for additive materials with higher boiling points for consistent long-term performance of PSCs.

Dynamically tracking the strain across the metal-insulator transition in VO2 measured using electromechanical resonators.

We study the strain state of doubly clamped VO2 nanobeam devices by dynamically probing resonant frequency of the nanoscale electromechanical device across the metal-insulator transition. Simultaneous resistance and resonance measurements indicate M1-M2 phase transition in the insulating state with a drop in resonant frequency concomitant with an increase in resistance. The resonant frequency increases by ~7 MHz with the growth of metallic domain (M2-R transition) due to the development of tensile strain in the nanobeam. Our approach to dynamically track strain coupled with simultaneous resistance and resonance measurements using electromechanical resonators enables the study of lattice-involved interactions more precisely than static strain measurements. This technique can be extended to other phase change systems important for device applications.

Tunable superlattice in graphene to control the number of Dirac points.

Superlattice in graphene generates extra Dirac points in the band structure and their number depends on the superlattice potential strength. Here, we have created a lateral superlattice in a graphene device with a tunable barrier height using a combination of two gates. In this Letter, we demonstrate the use of lateral superlattice to modify the band structure of graphene leading to the emergence of new Dirac cones. This controlled modification of the band structure persists up to 100 K.

One-year clinical outcomes of BioMatrix™-Biolimus A9™ eluting stent: the e-BioMatrix multicenter post marketing surveillance registry in India.

OBJECTIVE: The e-BioMatrix is a post marketing multicenter registry with an objective to evaluate the 2 year clinical safety and efficacy outcomes in patients treated with BioMatrix™ - Biolimus A9™ (BA9™) drug eluting stents (DES). BACKGROUND: Drug-eluting stents still have late-stage disadvantages that might be attributable to the permanent polymer. BioMatrix a new generation DES containing anti-proliferative drug Biolimus A9™ incorporating a biodegradable abluminal coating that leaves a polymer-free stent after drug release enhancing strut coverage while preventing neointimal hyperplasia. METHODS: This interim analysis consists of a total of 1189 patients with 1418 lesions treated with BioMatrix stent who entered this multicenter registry in India. We analyzed the incidence of major adverse cardiac events (MACE) and stent thrombosis (ST) at 1, 6, and 12 months with an extended follow-up of 2 years. Recommended antiplatelet regimen included clopidogrel and aspirin for 12 months. RESULTS: The mean age was 57.6 ± 10.9 years, 81.8% were males, comorbidity index was 1.20 ± 1.33, 68% presented with acute coronary syndrome, 49% had hypertension and 40.8% had diabetes mellitus. One-year clinical follow-up was completed in 987 patients at the time of interim analysis. The incidence of MACE is 0.45 for 1544 person-year follow-up. There were only 03 cases of ST (01 late ST) reported during this time. CONCLUSION: This registry demonstrates excellent one-year clinical safety and efficacy of BioMatrix stents. The 1-year result shows that BioMatrix stent may be a suitable alternative as compared to contemporary DESs which are currently available in the market for simple as well complex disease.