RESUMO
We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.
RESUMO
This study examined the impact of Philadelphia's mandatory Medicaid case management program (HealthPASS) on adequacy of prenatal care and birth outcomes among enrollees. A sample of 217 deliveries for HealthPASS patients at the Hospital of the University of Pennsylvania (HUP) during 1988 was compared with a matched sample of 1988 deliveries at HUP for whom the payor was Pennsylvania's traditional fee-for-service Medicaid program. Inpatient charts for all 434 subjects were abstracted for information on sociodemographic characteristics, substance use during pregnancy (cigarettes, alcohol, and drugs), course and extent of prenatal care, and birth outcomes including birth weight, gestational age, and mortality. No significant differences were detected between HealthPASS and Medicaid groups, suggesting that the mandatory managed care program neither improved nor impeded access to needed services. These results were not surprising in view of the fact that HealthPASS actually did little to change provider or patient behavior with respect to obstetrical care. Both the HealthPASS and Medicaid groups experienced low rates of adequate prenatal care (39%) and high rates of low birth weight (20%). Also disturbing was the finding that at least 46% of women smoked during pregnancy, at least 20% drank alcohol, and at least 17% used cocaine. These findings support the need for continued efforts to improve both access to, and content of, prenatal care for the urban poor.
