Quantitative anomalous powder diffraction analysis of cation disorder in kesterite semiconductors

Kesterite type compound semiconductors, containing copper and zinc, have photovoltaic properties depending on cation distribution in the crystal structure. Anomalous diffraction allows discrimination of isoelectronic cations, in principle allowing a straightforward determination of site occupation factors from data collected at multiple energies close to the X ray absorption edges of copper and zinc. However, extremely strong correlation between structural parameters precludes this. We present a recipe based on the direct dependency between refined occupation factors and atomic scattering power, which allows to lift the correlations and to detect issues of individual diffraction patterns or assumptions in the model, thereby allowing for reliable quantitative analysis of the Cu Zn distributio

Structural characterization of off stoichiometric kesterite type Cu2ZnGeSe4 compound semiconductors from cation distribution to intrinsic point defect density

The substitution of Ge4 for Sn4 in Cu2ZnSn S,Se 4 CZTSSe kesterite type absorber layers for thin film solar cells has proven to enhance the opto electronic properties of the material. By cationic substitution, in general, the optical bandgap can be more readily designed for the purpose of bandgap engineering, and the substitution of Sn4 by Ge4 , in particular, widens the optical bandgap such that it can be employed both for photovoltaics as well as solar fuel quarrying by photocatalytic water splitting. This work is an experimental study of intrinsic point defects in off stoichiometric kesterite type Cu2ZnGeSe4 CZGSe by means of neutron powder diffraction. We revealed the existence of copper vacancies VCu , various cation antisite defects CuZn, ZnCu, ZnGe, CuGe as well as interstitials Cui, Zni in a wide range of off stoichiometric polycrystalline material synthesized by solid state reaction. In addition to the off stoichiometry type specific defects, Cu Zn disorder is always present in the kesterite type CZGSe phase. While compositional changes are clearly reflected by the tetragonal deformation c 2a, the lattice parameters a, c seem differently responding to point defect types and concentration variations, respectively. The CuGe antisite defect which is known to greatly deteriorate the opto electronic properties exists only in Cu rich CZGSe, but appears already in CZGSe with Cu Zn Ge amp; 8776;1. Furthermore we showed by diffuse reflectance hyperspectral imaging a widening of the energy bandgap in off stoichiometric kesterite type CZGSe with decreasing Cu Zn G

Facile Bulk Synthesis of pi Cubic SnS

The cubic modification of binary tin sulfide SnS has gained significant interest as an earth abundant, low toxicity solar absorber material with a band gap close to the optimal value for the conversion of sunlight. We herein report a simple synthesis for the metastable material, which will allow more elaborate characterization methods to be used on this material, and present a full powder refinement of the material along with some preliminary results on the optical and thermal stability propertie

Cu2ZnSnS4 thin film solar cells grown by fast thermal evaporation and thermal treatment

Cu2ZnSnS4 thin films have been produced via rapid thermal evaporation of off-stoichiometric kesterite powder followed by annealing in an Ar atmosphere. Different heating rates were applied during the thermal treatments. The chemical composition and structural properties of the deposited layers as well as the distribution of the elements through the kesterite thin film have been investigated. The initial growth of a SnS secondary phase during evaporation led to the formation of this secondary phase next to the Mo back contact. Solar cell power conversion efficiencies were limited to values about 3 % due to this secondary phase. Furthermore, an increased open circuit voltage was demonstrated by using a Zn(O,S) buffer layerThis work was supported by DAAD project (INTERKEST, Ref: 57050358), Marie Curie-ITN (KESTCELLS, GA: 316488) and MINECO project (SUNBEAM, ENE2013-49136-C4-3-R). RC and ES acknowledge financial support from Spanish MINECO within the Ramón y Cajal program (RYC-2011-08521) and (RYC-2011-09212) respectively. SG also thanks the Government of Spain for the FPI fellowship (BES-2014-068533)

Analysis of grain orientation and defects in Sb2Se3 solar cells fabricated by close spaced sublimation

The performance of a superstrate TiO2 Sb2Se3 solar cell, fabricated by close spaced sublimation technique CSS , was improved after the deployment of a seed layer. The seed layer caused columnar Sb2Se3 film growth with texture coefficient analysis TC showing increased presence of crystal planes, which are inclined towards the [001] crystal direction. Given the highly anisotropic properties of Sb2Se3, preferential growth of Sb4Se6 n rib bons along the [001] direction is best suited for effective charge collection. Hence, grain orientation of Sb2Se3 films was studied more closely via measurement of pole figures by XRD and orientation distribution maps by electron backscatter diffraction EBSD . Although the measurements did not reveal strong preferred orientation, it was observed that the columnar Sb2Se3 growth enhanced texture along the [001] direction. Temperature dependent admittance spectroscopy TAS and capacitance voltage CV profiling were performed on the seed assisted TiO2 Sb2Se3 solar cell to evaluate carrier density and deep defects in the Sb2Se3 absorber. TAS study revealed a deep defect with activation energy of 0.39 eV. CV profiles indicated that the density of defects could be as high as 1017 c

Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

Formamidinium lead iodide FAPbI3 can be used in its cubic, black form as a light absorber material in single junction solar cells. It has a band gap 1.5 eV close to the maximum of the Shockley Queisser limit, and reveals a high absorption coefficient. Its high thermal stability up to 320 C has also a downside, which is the instability of the photo active form at room temperature RT . Thus, the black amp; 945; phase transforms at RT with time into a yellow non photo active amp; 948; phase. The black phase can be recovered by annealing of the yellow state. In this work, a polymorphism of the amp; 945; phase at room temperature was found as synthesized amp; 945;i , degraded amp; 945; amp; 948; and thermally recovered amp; 945;rec . They differ in the Raman spectra and PL signal, but not in the XRD patterns. Using temperature dependent Raman spectroscopy, we identified a structural change in the amp; 945;i polymorph at ca. 110 C. Above 110 C, the FAPbI3 structure has undoubtedly cubic Pm[3 with combining macron]m symmetry high temperature phase amp; 945;HT . Below that temperature, the amp; 945;i phase was suggested to have a distorted perovskite structure with Im[3 with combining macron] symmetry. Thermally recovered FAPbI3 amp; 945;rec also demonstrated the structural transition to amp; 945;HT at the same temperature ca. 110 C during its heating. The understanding of hybrid perovskites may bring additional assets in the development of new and stable structure

Navigating in the Landscape of Care: A Critical Reflection on Theory and Practise of Care and Ethics

The theory and practise of care is defined and enacted differently in different national as well as cultural contexts, illuminating how differently constructed the personal and societal structures in Europe are. A common trait is however that care work paid or non-paid, private or public is identified with women. To navigate in the landscape of care and ethics requires taking into account the constitutive relation between one’s identity, embodiment and position. The author suggests conceiving care as an existential condition of life demanded from all human beings. This will free care from the identification with women and pave a way towards a more gender equal and just society with less gender segregation in the labour market and at the arena of education

Rudolf Otto’s ‘The Absolute Other’ and a radical postsecular urban contextualization

This article proposes an idea of radical urban contextualisation that follows Rudolf Otto’s discussion on an encounter with the Absolute Other. The article critically reviews current applications of postsecularism to urban theory formulated in a general framework of Jurgen Habermas’ intervention in the early 21st century. The article argues that contemporary postsecular urban theory cannot fully answer fundamental challenges that contemporary cities are facing – both political and environmental – mostly because it focuses on linguistic and cultural aspects of a city. The article proposes the ‘radicalization’ of postsecularism, engaging directly with the ‘religious experience’ defined by Rudolf Otto as an encounter with The Absolute Other – the unknown and unpredictable. The Absolute Other notion allows to ultimately contextualize every urban situation in order to formulate conditions for future-oriented (post-capitalist) urbanism

Expressions 1980

Expressions contains selected work from the 1980 Creative Writing Contest winners and honorable mentions, Campus Chronicle Photography Contest entrants, and the Commercial Art students at Des Moines Area Community College. Design, typography and layout was done by Journalism students.https://openspace.dmacc.edu/expressions/1002/thumbnail.jp

Unsettling planning theory

Recent political developments in many parts of the world seem likely to exacerbate rather than ameliorate the planetary-scale challenges of social polarization, inequality and environmental change societies face. In this unconventional multi-authored essay, we therefore seek to explore some of the ways in which planning theory might respond to the deeply unsettling times we live in. Taking the multiple, suggestive possibilities of the theme of unsettlement as a starting point, we aim to create space for reflection and debate about the state of the discipline and practice of planning theory, questioning what it means to produce knowledge capable of acting on the world today. Drawing on exchanges at a workshop attended by a group of emerging scholars in Portland, Oregon in late 2016, the essay begins with an introduction section exploring the contemporary resonances of ‘unsettling’ in, of and for planning theory. This is followed by four, individually authored responses which each connect the idea of unsettlement to key challenges and possible future directions. We end by calling for a reflective practice of theorizing that accepts unsettlement but seeks to act knowingly and compassionately on the uneven terrain that it creates