Disentangling the effects of structure and lone-pair electrons in the lattice dynamics of halide perovskites.

Halide perovskites show great optoelectronic performance, but their favorable properties are paired with unusually strong anharmonicity. It was proposed that this combination derives from the ns2 electron configuration of octahedral cations and associated pseudo-Jahn-Teller effect. We show that such cations are not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in these materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and molecular dynamics to contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds that are structurally similar but with the former lacking ns2 cations with the propensity to form electron lone pairs. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint of anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This highlights the role of structure in perovskite lattice dynamics, providing design rules for the emerging class of soft perovskite semiconductors.

Alloying Effects on Charge-Carrier Transport in Silver-Bismuth Double Perovskites.

Alloying is widely adopted for tuning the properties of emergent semiconductors for optoelectronic and photovoltaic applications. So far, alloying strategies have primarily focused on engineering bandgaps rather than optimizing charge-carrier transport. Here, we demonstrate that alloying may severely limit charge-carrier transport in the presence of localized charge carriers (e.g., small polarons). By combining reflection-transmission and optical pump-terahertz probe spectroscopy with first-principles calculations, we investigate the interplay between alloying and charge-carrier localization in Cs2AgSbxBi1-xBr6 double perovskite thin films. We show that the charge-carrier transport regime strongly determines the impact of alloying on the transport properties. While initially delocalized charge carriers probe electronic bands formed upon alloying, subsequently self-localized charge carriers probe the energetic landscape more locally, thus turning an alloy's low-energy sites (e.g., Sb sites) into traps, which dramatically deteriorates transport properties. These findings highlight the inherent limitations of alloying strategies and provide design tools for newly emerging and highly efficient semiconductors.

Optically Induced Long-Lived Chirality Memory in the Color-Tunable Chiral Lead-Free Semiconductor (R)/(S)-CHEA4Bi2BrxI10-x (x = 0-10).

Hybrid organic-inorganic networks that incorporate chiral molecules have attracted great attention due to their potential in semiconductor lighting applications and optical communication. Here, we introduce a chiral organic molecule (R)/(S)-1-cyclohexylethylamine (CHEA) into bismuth-based lead-free structures with an edge-sharing octahedral motif, to synthesize chiral lead-free (R)/(S)-CHEA4Bi2BrxI10-x crystals and thin films. Using single-crystal X-ray diffraction measurements and density functional theory calculations, we identify crystal and electronic band structures. We investigate the materials' optical properties and find circular dichroism, which we tune by the bromide-iodide ratio over a wide wavelength range, from 300 to 500 nm. We further employ transient absorption spectroscopy and time-correlated single photon counting to investigate charge carrier dynamics, which show long-lived excitations with optically induced chirality memory up to tens of nanosecond timescales. Our demonstration of chirality memory in a color-tunable chiral lead-free semiconductor opens a new avenue for the discovery of high-performance, lead-free spintronic materials with chiroptical functionalities.

Probing the Disorder Inside the Cubic Unit Cell of Halide Perovskites from First-Principles.

Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms in the prototypical cubic CsPbBr3. Our investigation based on first-principles molecular dynamics calculations finds that the motions of neighboring Cs-Br atoms interlock, which appears as the most likely Cs-Br distance being significantly shorter than what is inferred from an ideal cubic structure. This form of dynamic Cs-Br coupling coincides with very shallow dynamic potential wells for Br motions that occur across a locally and dynamically disordered energy landscape. We reveal an interesting dynamic coupling mechanism among the atoms within the nominal unit cell of cubic CsPbBr3 and quantify the important local structural fluctuations on an atomic scale.

Transversal Halide Motion Intensifies Band-To-Band Transitions in Halide Perovskites.

Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites (HaPs) exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the finite-temperature optoelectronic properties in the prototypical cubic CsPbBr3 , using first-principles molecular dynamics based on density-functional theory. It is shown that the dynamic flexibility associated with HaPs enables the so-called transversality, which manifests as a preference for large halide displacements perpendicular to the Pb-Br-Pb bonding axis. The authors find that transversality is concurrent with vibrational anharmonicity and leads to a rapid rise in the joint density of states, which is favorable for photovoltaics since this implies sharp optical absorption profiles. These findings are contrasted to the case of PbTe, a material that shares several key properties with CsPbBr3 but cannot exhibit any transversality and, hence, is found to exhibit much wider band-edge distributions. The authors conclude that the dynamic structural flexibility in HaPs and their unusual vibrational characteristics might not just be a mere coincidence, but play active roles in establishing their favorable optoelectronic properties.

Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells.

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages ( VOC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the VOC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

Caracterización del Comportamiento Sexual y Reproductivo y Acceso A los Servicios de Salud, de los Estudiantes de la Facultad de Ciencias Médicas de la Universidad de Cuenca / Characterization of Sexual and Reproductive Behavior and Access to Health Services, of the Students of the Faculty of Medical Sciences of the University of Cuenca

ANTECEDENTES: La población universitaria es principal-mente joven y por lo tanto expuesta a factores que puede generar efectos adversos en salud sexual y re-productiva, que conllevan a un bajo rendimiento académico e inclusive a la deserción estudiantil. OBJETIVO: Caracterizar el comportamiento sexual y re-productivo y las necesidades en salud de los en estudiantes de la Facultad de Ciencias Médicas de la Universidad de Cuenca. METODOLOGÍA: Tipo de estudio descriptivo. Universo: 3 265 estudiantes de la Facultad de Ciencias Médicas de la Universidad de Cuenca, en el período lectivo septiembre 2014- Febrero 2015. Muestra: fue aleatorizada por conglomerados (carrera y cursos) en las tres escue-las de la facultad de Ciencias Médicas. RESULTADOS: De los estudiantes encuestados, el 44.7% fueron de la escuela de medicina de 27.7% de enfermería, 27.7% tecnología médica; 68.6% mujeres y 31.4% varones, con una edad media de 21 años para ambos grupos; la mayoría son solteros 87.7%, mestizos 94.1% y heterosexuales 97.2%, que residen en zona urbana 81.5%. El 45.7% de las mujeres y el 70.1% de los varones son sexualmente activos, 15% son padres de familia, el 81,6% ha usado un método anticonceptivo MAC, sin embargo el uso consistente es de 55.1%. 66.2% de los encuestados ha acudido a un médico el último año; el 77.3% maneja suficiente información para la prevención de un embarazo y el 79.7% para la prevención de ITS. CONCLUSIONES: Pese a que los estudiantes tienen in-formación en sexualidad y acceden a los servicios de salud, los efectos adversos en salud sexual y reproduc-tiva son identificados como un problema que conlleva a deserción estudiantil y bajo rendimiento académico. Las principales soluciones para combatir los efectos adversos, incluyen: mejorar la calidad de información, fortalecer el acceso a servicios integrales de salud, generar espacios saludables.

OBJECTIVE: To characterize sexual and reproductive be-havior and health needs of students in the Faculty of Medical Sciences of the University of Cuenca. METHOD: descriptive study type. Universe: 3265 students of the Faculty of Medical Sciences of the University of Cuenca, in the academic year in September 2014 in February 2015. Sample: was randomized by cluster (ca-reer and courses) in the three schools of the Faculty of Medical Sciences. RESULTS: Of all the surveyed students, 44.7% were from medical school, nursing 27.7%, 27.7% medical technology; 68.6% female and 31.4% male, with a mean age of 21 years; most are unmarried 87.7%, 94.1% are heterosexual, mestizos 97.2% residing in urban areas 81.5%. 45.7% of the women and 70.1% of males are sexually active, 15% are parents, 81.6% have used a contraceptive, however the consistent use of contra-ceptives is 55.1%. 66.2% of respondents have visited a doctor last year; 77.3% handle sufficient information for the prevention of pregnancy and 79.7% for STD prevention. CONCLUSIONS: Although students have information on sexuality and access to health services, adverse effects on sexual and reproductive health are identified as a problem that leads to student dropout and low aca-demic performance. The main solutions to combat the adverse effects include: improving the quality of infor-mation, enhance access to comprehensive health ser-vices, generating healthy spaces.