Insights into the relationship between ferroelectric and photovoltaic properties in CsGeI3 for solar energy conversion.

Materials such as oxide and halide perovskites that simultaneously exhibit spontaneous polarization and absorption of visible light are called photoferroelectrics. They hold great promise for the development of applications in optoelectronics, information storage, and energy conversion. Devices based on ferroelectric photovoltaic materials yield an open-circuit voltage that is much higher than the band gap of the corresponding active material owing to a strong internal electric field. Their efficiency has been proposed to exceed the Shockley-Queisser limit for ideal solar cells. In this paper, we present theoretical calculations of the photovoltaic properties of the ferroelectric phase of the inorganic germanium halide perovskite (CsGeI3). Firstly, the electronic, optical and ferroelectric properties were calculated using the FP-LAPW method based on density functional theory, and the modern theory of polarization based on the Berry phase approach, respectively. The photovoltaic performance was evaluated using the Spectroscopic Limited Maximum Efficiency (SLME) model based on the results of first-principles calculations, in which the power conversion efficiency and the photocurrent density-voltage (J-V) characteristics were estimated. The calculated results show that the valence band maximum (VBM) of CsGeI3 is mainly contributed by the I-5p and Ge-4s orbitals, whereas the conduction band is predominantly derived from Ge-4p orbitals. It can be seen that CsGeI3 exhibits a direct bandgap semiconductor at the symmetric point of Z with a value of 1.53 eV, which is in good agreement with previous experimental results. The ferroelectric properties were therefore investigated. With a switching energy barrier of 19.83 meV per atom, CsGeI3 has a higher theoretical ferroelectric polarization strength of 15.82 µC cm-2. The SLME calculation also shows that CsGeI3 has a high photoelectric conversion efficiency of over 28%. In addition to confirming their established favorable band gap and strong absorption, we demonstrate that CsGeI3 exhibits a large shift current bulk photovoltaic effect of up to 40 µA V-2 in the visible region. Thus, this material is a potential ferroelectric photovoltaic absorbed layer with high efficiency.

Pregnancy outcomes in urban black South African women aged 35 years and older.

We measured maternal and fetal outcomes for black South African women of age 35 years and above, compared with younger controls. The study was carried out at the Chris Hani Baragwanath Maternity Hospital and the Soweto Community Health Centres, that provide maternity services for urban black pregnant women. This was a retrospective pregnancy cohort, comparing pregnant women aged 35 and above with a control group aged 20-29 years. Early pregnancy losses (<20 weeks) were excluded. There were 1047 women in each arm of the study. Frequencies of hypertension (22.8%), gestational diabetes (0.9%), and prelabour rupture of the membranes (6.9%) were significantly higher for older women. Perinatal mortality rates (42/1000 vs. 33/1000) did not differ significantly. Antepartum admission, induction of labour, caesarean section and neonatal admission were required more frequently in older women. We conclude that pregnancy at age 35 years and above is often difficult and expensive, requiring hospital based interventions to maintain a low perinatal mortality rate.