Compositional and additive design for efficient and stable perovskite solar cells

School of Energy and Chemical Engineering (Energy Engineering)Crystalline silicon solar cells, which are most commonly deployed, have high performance with power conversion efficiency (PCE) of 26.7% in the lab. However, since a large amount of energy is required to manufacture high purity silicon, the price is high, and due to the nature of the material, it is not flexible. As next-generation solar cells to solve these shortcomings, organic and dye-sensitized solar cells have been studied, but it is still difficult to commercialize due to low efficiency. More recently, metal halide perovskite materials (represented by the general chemical formula ABX3, where A denotes an organic ammonium or inorganic cation such as methylammonium (MA+), formamidinium (FA+), or Cs+B denotes a metal cation such as Pb2+, or Sn2+and X denotes a halide (I-, Br-, and Cl-)) have recently emerged as a highly promising class of electrical functional materials. The exceptional structural tunability enables these materials to possess three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures at the molecular level. Each category of these materials has its unique physical and chemical properties with a variety of applications such as transistors, light emitting iodides (LEDs), x-ray detector, and solar cells. In particular, perovskite solar cells (PSCs) display unique promise to satisfy low costs, flexibility, and high efficiency. Currently, PSCs have already obtained comparable PCE to silicon in the lab, of 25.5%, with low manufacturing cost compared to silicon solar cells. During my Ph.D., I focused on improving efficiency and stability of PSCs by designing composition, additive, and device. PSCs are mainly fabricated by solution process so the properties of precursor solution are important as it can greatly affect the resulting device???s properties and performance. I identified the cause of the phenomenon that the properties of the halide perovskite precursor solution change over time and minimized the problem by adding a certain amount of elemental sulfur. This work was published in Advanced Energy Materials and selected as a cover picture. Subsequently, the research was conducted to improve the efficiency and stability of PSCs. High-performance PSCs exceeding 22% of efficiency are fabricated by using FAPbI3 as a light absorbing layer due to its suitable bandgap and relatively higher thermal stability. However, FAPbI3 readily transforms from the desired trigonal black ??-phase into the undesired wide-bandgap ??-phase with hexagonal symmetry under ambient conditions at room temperature. The method mainly used to suppress this drawback was to stabilize the phase of FAPbI3 by adding some amount of MAPbBr3. However, MA is highly volatile and week to heat, and Br cause undesired blue-shift of bandgap. In the point of this view, I successfully conducted research on the introduction of a very new additive (methylenediammonium dichloride, MDACl2) to stabilize the ??-phase while minimizing the bandgap variation of FAPbI3. By using this new composition, we achieved world???s highest certified current density (JSC) and efficiency based on the date of publication and the results was published in the journal Science. Also, I published another Science paper that significantly increase the efficiency and stability of PSCs by reducing lattice strain while maintaining the ??-phase of FAPbI3. And I also conducted research on morphology control of all-inorganic perovskite, CsPbI3 which can be used top-cell of tandem device with silicon or narrow-bandgap perovskites and achieved the world???s highest efficiency and published in Joule. Currently, research on reducing the external strain of perovskite thin films has been conducted and manuscripts are being prepared. Furthermore, the results of the world???s highest efficiency which currently included National Renewable Energy Laboratory (NREL) chart are under evaluation in high impact journal.ope

mRNA vaccine effectiveness against SARS-CoV-2 B.1.617.2 (Delta) and B.1.1.529 (Omicron) variant transmission from home care cases to household contacts in South Korea

Objectives Household contacts of confirmed cases of coronavirus disease 2019 (COVID-19) are exposed to a high risk of viral transmission, and secondary incidence is an important indicator of community transmission. This study analyzed the secondary attack rate and mRNA vaccine effectiveness against transmission (VET) for index cases (patients treated at home) confirmed to be infected with the Delta and Omicron variants. Methods The subjects of the study were 4,450 index cases and 10,382 household contacts. Logistic regression analysis was performed to compare the secondary attack rate by vaccination status, and adjusted relative risk and 95% confidence intervals were identified. Results The secondary attack rate of the Delta variant was 27.3%, while the secondary attack rate of the Omicron variant was 29.8%. For the Delta variant, groups with less than 90 days and more than 90 days after 2 doses of mRNA vaccination both showed a VET of 37%. For the Omicron variant, a 64% VET was found among those with less than 90 days after 2 doses of mRNA vaccination. Conclusion This study provides useful data on the secondary attack rate and VET of mRNA vaccines for household contacts of COVID-19 cases in South Korea

OMBM-ML: An efficient memory bandwidth management for ensuring QoS and improving server utilization

© 2018 IEEE.As cloud data centers are dramatically growing, various applications are moved to cloud data centers owing to cost benefits for maintenance and hardware resources. However, latency-critical workloads among them suffer from some problems to fully achieve the cost effectiveness. The latency-critical workloads should show latencies in a stable manner, to be predicted, for strictly meeting QoSs. However, if they are executed with other workloads to save the cost, they experience QoS violation due to the contention for the hardware resources shared with co-location workloads. In order to guarantee QoSs and to improve the hardware resourse utilization, we proposed a memory bandwidth management method with an effective prediction model using machine learning. The prediction model estimates the amount of memory bandwidth that will be allocated to the latency-critical workload based on a REP decision tree. To construct this model, we first collect data and train the model with the data. The generated model can estimate the amount of memory bandwidth for meeting the SLO of the latency-critical workload no matter what batch processing workloads are collocated. The use of our approach achieves up to 99% SLO assurance and improves the server utilization up to 6.8x on average.N

Relaxation of externally strained halide perovskite thin layers with neutral ligands

The preferred orientation (PO) and non-uniform strain of the perov-skite layer coated on the substrate can cause unwanted blue shifts in the band gap and several defects inside and on the grain bound-aries. Therefore, suppressing the anisotropic preferred growth of halide perovskite crystals while satisfying the trap passivation of grain boundaries is important to the fabrication of efficient and sta-ble perovskite solar cells (PSCs). In this study, the properties of spin-coated perovskite thin films were investigated by adding an appropriate amount of trioctylphosphine (TOP) as a neutral ligand to a formamidinium lead triiodide (FAPbI3) precursor solution. The TOP in the FAPbI3 precursor solution containing MACl significantly alleviated the PO by MACl and further induced a redshift of the band gap through the reduction of microstrain. Furthermore, TOP contributed to the passivation of defects via the surface termination of grain boundaries. PSCs based on these perovskite thin films ex-hibited an efficiency close to 25%

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Reconstructions at various observation directions of the composite hologram with occlusio

Stabilization of Precursor Solution and Perovskite Layer by Addition of Sulfur

Efficient perovskite solar cells (PSCs) are mainly fabricated by a solution coating processes. However, the efficiency of such devices varies significantly with the aging time of the precursor solution used to fabricate them, which includes a mixture of perovskite components, especially methylammonium (MA), and formamidinium (FA) cations. Herein, how the inorganic-organic hybrid perovskite precursor solution of (FAPbI(3))(0.95)(MAPbBr(3))(0.05) degrades over time and how such degradation can be effectively inhibited is reported on, and the associated mechanism of degradation is discussed. Such degradation of the precursor solution is closely related to the loss of MA cations dissolved in the FA solution through the deprotonation of MA to volatile methylamine (CH3NH2). Addition of elemental sulfur (S-8) drastically stabilizes the precursor solution owing to amine-sulfur coordination, without compromising the power conversion efficiency (PCE) of the derived PSCs. Furthermore, sulfur introduced to stabilize the precursor solution results in improved PSC stability

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Reconstructions at various depths of the composite hologram with occlusio

Stabilization of Lead-Tin-Alloyed Inorganic-Organic Halide Perovskite Quantum Dots

Recently, lead-tin-based alloyed halide perovskite quantum dots (QDs) with improved stability and less toxicity have been introduced. However, the perovskite QDs containing tin are still unstable and exhibit low photoluminescence quantum yields (PLQYs), owing to the presence of defects in the alloyed system. Here, we have attempted to introduce sulfur anions (S2-) into the host lattice (MAPb(0.75)Sn(0.25)Br(3)) as a promising route to stable alloyed perovskite QDs with improved stability and PLQY. In this study, we used elemental sulfur as a sulfur precursor. The successful incorporation of sulfur anions into the host lattice resulted in a highly improved PLQY (>75% at room temperature), which is believed to be due to a reduction in the defect-related non-radiative recombination centers present in the host lattice. Furthermore, we found that the emission property could be tuned between the bright green and cyan bluish regions without compromising on color quality. This work invigorates the perovskite research community to prepare stable, bright, and color-tunable alloyed inorganic-organic perovskite QDs without compromising on their phases and color quality, which can lead to considerable advances in display technology