Carbon Dioxide Capture Chemistry of Amino Acid Functionalized Metal-Organic Frameworks in Humid Flue Gas.

Metal-organic framework-808 has been functionalized with 11 amino acids (AA) to produce a series of MOF-808-AA structures. The adsorption of CO2 under flue gas conditions revealed that glycine- and dl-lysine-functionalized MOF-808 (MOF-808-Gly and -dl-Lys) have the highest uptake capacities. Enhanced CO2 capture performance in the presence of water was observed and studied by using single-component sorption isotherms, CO2/H2O binary isotherm, and dynamic breakthrough measurements. The key to the favorable performance was uncovered by deciphering the mechanism of CO2 capture in the pores and attributed to the formation of bicarbonate as evidenced by 13C and 15N solid-state nuclear magnetic resonance spectroscopy studies. On the basis of these results, we examined the performance of MOF-808-Gly in simulated coal flue gas conditions and found that it is possible to capture and release CO2 by vacuum swing adsorption. MOF-808-Gly was cycled at least 80 times with full retention of performance. This study significantly advances our understanding of CO2 chemistry in MOFs by revealing how strongly bound amine moieties to the MOF backbone create the chemistry and environment within the pores, leading to the binding and release of CO2 under mild conditions without application of heat.

Water Bridges Substitute for Defects in Amine-Functionalized UiO-66, Boosting CO2 Adsorption.

The binary adsorption of CO2 and water on an amine-functionalized UiO-66 metal-organic framework (MOF) was studied experimentally and computationally. Grand canonical Monte Carlo simulations were used to investigate three additional UiO-66 MOFs with different functionalized linkers. Each MOF was studied in a defect-free form as well as two additional forms with precise linker defects. Binary adsorption isotherms are presented for CO2 at specific water loadings. While water loading in defect-free MOFs reduces the CO2 uptake, the defects slightly boost the CO2 uptake at low water loadings. It was found that water bridges form between the metal oxide cores, replacing the missing linkers. Effectively, this creates smaller pores that are more welcoming of CO2 adsorption. Experimental measurement of the binary isotherms for UiO-66-NH2 shows a behavior that is consistent with this enhancement.

Femtosecond Charge-Injection Dynamics at Hybrid Perovskite Interfaces.

With a power conversion efficiency (PCE) exceeding 22 %, perovskite solar cells (PSCs) have thrilled photovoltaic research. However, the interface behavior is still not understood and is a hot topic of research: different processes occur over a hierarchy of timescales, from femtoseconds to seconds, which makes perovskite interface physics intriguing. Herein, through femtosecond transient absorption spectroscopy with spectral coverage extending into the crucial IR region, the ultrafast interface-specific processes at standard and newly molecularly engineered perovskite interfaces in state-of-the-art PSCs are interrogated. Ultrafast interfacial charge injection occurs with a time constant of 100 fs, resulting in hot transfer from energetic charges and setting the timescale for the first step involved in the complex charge-transfer process. This is also true for 20 % efficient devices measured under real operation, for which the femtosecond injection is followed by a slower picosecond component. These findings provide compelling evidence for the femtosecond interfacial charge-injection step and demonstrate a robust method for the straightforward identification of interfacial non-equilibrium processes on the ultrafast timescale.

Influence of Perovskite Interface Morphology on the Photon Management in Perovskite/Silicon Tandem Solar Cells.

Perovskite/silicon tandem solar cells are considered as one of the cost-effective solutions for determining high energy conversion efficiencies. Efficient photon management allows improving light incoupling in solar cells by reducing optical losses. The optics relies upon the interface morphology, and consequently, the growth mechanism of the top cell on the bottom cell is crucial for the implementation of efficient perovskite/silicon tandem solar cells. To describe the interface morphologies of perovskite/silicon tandem solar cells, a three-dimensional surface algorithm is used that allows investigating the perovskite solar cells deposited on the textured crystalline silicon solar cells. We distinguish between two extreme cases in which the film grows only in the direction of the substrate normal or in the direction of the local surface normal. The growth mode has significant influence on the film roughness, the effective thickness of the film, the optics of the solar cell, and the photovoltaic parameters. The optics is investigated by finite-differencetime-domain simulations. The influence of the interface morphology on the photovoltaic parameters is discussed, and guidelines are provided to reach high short-circuit current density and energy conversion efficiency.

Atomic layer deposition of metal oxides for efficient perovskite single-junction and perovskite/silicon tandem solar cells.

Aluminum-doped and undoped zinc oxide films were investigated as potential front and rear contacts of perovskite single and perovskite/silicon tandem solar cells. The films were prepared by atomic layer deposition (ALD) at low (<200 °C) substrate temperatures. The deposited films were crystalline with a single-phase wurtzite structure and exhibit excellent uniformity and low surface roughness which was confirmed by XRD and SEM measurements. Necessary material characterizations allow for realizing high-quality films with low resistivity and high optical transparency at the standard growth rate. Spectroscopic ellipsometry measurements were carried out to extract the complex refractive index of the deposited films, which were used to study the optics of perovskite single junction and perovskite/silicon tandem solar cells. The optics was investigated by three-dimensional finite-difference time-domain simulations. Guidelines are provided on how to realize perovskite solar cells exhibiting high short-circuit current densities. Furthermore, detailed guidelines are given for realizing perovskite/silicon tandem solar cells with short-circuit current densities exceeding 20 mA cm-2 and potential energy conversion efficiencies beyond 31%.

Perovskite/Silicon Tandem Solar Cells: From Detailed Balance Limit Calculations to Photon Management.

Energy conversion efficiency losses and limits of perovskite/silicon tandem solar cells are investigated by detailed balance calculations and photon management. An extended Shockley-Queisser model is used to identify fundamental loss mechanisms and link the losses to the optics of solar cells. Photon management is used to minimize losses and maximize the energy conversion efficiency. The influence of photon management on the solar cell parameters of a perovskite single-junction solar cell and a perovskite/silicon solar cell is discussed in greater details. An optimized solar cell design of a perovskite/silicon tandem solar cell is presented, which allows for the realization of solar cells with energy conversion efficiencies exceeding 32%.

Optics of Perovskite Solar Cell Front Contacts.

The front contact has a major impact on the electrical and optical properties of perovskite solar cells. The front contact is part of the junction of the solar cell and must provide lateral charge transport to the terminals and should allow for an efficient light incoupling, while having low optical losses. The complex requirements of the perovskite solar front contact will be described and the optics of the front contact will be investigated. It will be shown that the front contact has a distinct influence on the short-circuit current and energy conversion efficiency. Metal oxide films were investigated as potential front contacts. The incoupling of light in the solar cell is investigated by three-dimensional finite-difference time-domain optical simulations and optical measurements of experimentally realized self-textured zinc oxide films. The zinc oxide films were prepared by metal-organic chemical vapor deposition at low temperatures. Furthermore, the influence of free carrier absorption of metal oxide films on the optics of low bandgap and/or tandem solar cells is investigated. Guidelines are provided on how to choose the doping concentration and thickness of the metal oxide films. Finally, it will be shown that by selecting an optimal front contact design the short-circuit current and energy conversion efficiency can be increased by at least 15%.

Full space device optimization for solar cells.

Advances in computational materials have paved a way to design efficient solar cells by identifying the optimal properties of the device layers. Conventionally, the device optimization has been governed by single or double descriptors for an individual layer; mostly the absorbing layer. However, the performance of the device depends collectively on all the properties of the material and the geometry of each layer in the cell. To address this issue of multi-property optimization and to avoid the paradigm of reoccurring materials in the solar cell field, a full space material-independent optimization approach is developed and presented in this paper. The method is employed to obtain an optimized material data set for maximum efficiency and for targeted functionality for each layer. To ensure the robustness of the method, two cases are studied; namely perovskite solar cells device optimization and cadmium-free CIGS solar cell. The implementation determines the desirable optoelectronic properties of transport mediums and contacts that can maximize the efficiency for both cases. The resulted data sets of material properties can be matched with those in materials databases or by further microscopic material design. Moreover, the presented multi-property optimization framework can be extended to design any solid-state device.

Characteristics of severely malnourished under-five children hospitalized with diarrhoea, and their policy implications.

AIM: Identify clinical and nutritional features, and complications among severely malnourished, under-five children in an urban diarrhoeal disease facility in Bangladesh. METHODS: For this case-control design, children of both sexes, aged 0-59 months were studied. Severely (< -3 z-score) underweight, stunted or wasted constituted cases and those with better nutritional status (z-score > or = -3) constituted controls. RESULTS: During 2000-2005, of the total 6881 children, 1103 (16%) were severely underweight, 705 (11%) severely stunted and 217 (3%) severely wasted. In logistic regression analysis, severely underweight children were more likely to be older than 11 months (OR 3.7, 95% CI 3.1-4.3, p < 0.001), non-breastfed (OR 1.5, 95% CI 1.3-1.8, p < 0.001), have illiterate mothers (OR 2.6, 95% CI 2.2-3.0, p < 0.001), non-sanitary toilet (OR 1.4, 95% CI 1.2-1.6, p < 0.001), a history of measles in preceding 6 months (OR 1.7, 95% CI 1.3-2.4, p = 0.001), dehydrating diarrhoea (OR 1.9, 95% CI 1.6-2.2, p < 0.001), abnormal findings in lung auscultation (OR 1.7, 95% CI 1.3-2.3, p < 0.001) and require hospitalization > or = 48 h (OR 2.2, 95% CI 1.8-2.5, p < 0.001). CONCLUSION: There thus is a need to incorporate appropriate, cost-effective and sustainable preventive strategies and improved management policies in the health systems as well as in social support systems in Bangladesh.

Young children non-immunized against measles: characteristics and programmatic implications.

AIM: To examine the presenting characteristics, including nutritional status, of young children without measles immunization and to suggest appropriate public health measures to improve immunization status. METHODS: In this retrospective case-control analysis, we studied 4075 children aged 12-23 mo of either sex, who attended ICDDR,B's Dhaka hospital during 1994-2003. Cases included children who reported to this facility without receiving measles vaccine, and the control children were those who received the vaccine. RESULTS: 3181 of 4075 (78%) children, including 1227 (39%) girls and 1954 (61%) boys, received measles immunization. The proportion of vaccinated children increased from 74% in 1997 to 82% in 2001. Some non-specific effects of measles immunization were observed. Fifty-one per cent of the children without measles immunization were stunted, 76% were underweight, and 48% were wasted. The non-immunized children were twice as likely to be stunted, underweight, and wasted than the immunized children; they were more often dehydrated (some or severe dehydration) (28% vs 22%, p<0.001), required longer duration (>72 h) of hospitalization (15% vs 10%, p<0.001), did not receive vitamin A capsule in the previous 6 mo (56% vs 36%, p<0.001), and had more frequent abnormal lung auscultation indicative of acute lower respiratory tract infections (8% vs 5%, p<0.001). Female children, illiterate mother, lack of vitamin A supplementation, and history of measles were significantly associated with non-immunization against measles after controlling for co-variables. Results were similar when different nutritional indicators (underweight, stunting, or wasting) were added separately to logistic regression models. CONCLUSION: Intervention strategies to enhance immunization coverage in infants should target illiterate mothers and their children, particularly the females and malnourished ones, provide them with measles immunization and vitamin A capsule, and encourage their periodic follow-up visits as part of a preventive nutritional programme.