Florian Werner

Bio: Florian Werner is an academic researcher from University of Luxembourg. The author has contributed to research in topics: Passivation & Atomic layer deposition. The author has an hindex of 33, co-authored 84 publications receiving 3450 citations. Previous affiliations of Florian Werner include University of Göttingen & University of Bonn.

Improved quantitative description of Auger recombination in crystalline silicon

Abstract: An accurate quantitative description of the Auger recombination rate in silicon as a function of the dopant density and the carrier injection level is important to understand the physics of this fundamental mechanism and to predict the physical limits to the performance of silicon based devices. Technological progress has permitted a near suppression of competing recombination mechanisms, both in the bulk of the silicon crystal and at the surfaces. This, coupled with advanced characterization techniques, has led to an improved determination of the Auger recombination rate, which is lower than previously thought. In this contribution we present a systematic study of the injection-dependent carrier recombination for a broad range of dopant concentrations of high-purity $n$-type and $p$-type silicon wafers passivated with state-of-the-art dielectric layers of aluminum oxide or silicon nitride. Based on these measurements, we develop a general parametrization for intrinsic recombination in crystalline silicon at 300 K consistent with the theory of Coulomb-enhanced Auger and radiative recombination. Based on this improved description we are able to analyze physical aspects of the Auger recombination mechanism such as the Coulomb enhancement.

Electronic and chemical properties of the c-Si/Al2O3 interface

Abstract: Using aluminum oxide (Al2O3) films deposited by atomic layer deposition (ALD), the dominant passivation mechanisms at the c-Si/Al2O3 interface, as well as the chemical composition of the interface region, are investigated. The excellent surface passivation quality of thin Al2O3 films is predominantly assigned to a high negative fixed charge density of Qf = − (4 ± 1) × 1012 cm−2, which is located within 1nm of the Si/Al2O3 interface and is independent of the layer thickness. A deterioration of the passivation quality for ultrathin Al2O3 layers is explained by a strong increase in the interface state density, presumably due to an incomplete reaction of the trimethyl-aluminum (TMA) molecules during the first ALD cycles. A high oxygen-to-aluminum atomic ratio resulting from the incomplete adsorption of the TMA molecules is suggested as a possible source of the high negative charge density Qf at the Si/Al2O3 interface.

19.4%-efficient large-area fully screen-printed silicon solar cells

Abstract: We demonstrate industrially feasible large-area solar cells with passivated homogeneous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 × 125 mm2 p-type 2–3 Ω cm boron-doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen-printing of silver and aluminum paste and firing in a conventional belt furnace. We implement two different dielectric rear surface passivation stacks: (i) a thermally grown silicon dioxide/silicon nitride stack and (ii) an atomic-layer-deposited aluminum oxide/silicon nitride stack. The dielectrics at the rear result in a decreased surface recombination velocity of Srear = 70 cm/s and 80 cm/s, and an increased internal IR reflectance of up to 91% corresponding to an improved Jsc of up to 38.9 mA/cm2 and Voc of up to 664 mV. We observe an increase in cell efficiency of 0.8% absolute for the cells compared to 18.6% efficient reference solar cells featuring a full-area aluminum back surface field. To our knowledge, the energy conversion efficiency of 19.4% is the best value reported so far for large area screen-printed solar cells. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Global effects of land use on local terrestrial biodiversity

Abstract: Human activities, especially conversion and degradation of habitats, are causing global biodiversity declines. How local ecological assemblages are responding is less clear--a concern given their importance for many ecosystem functions and services. We analysed a terrestrial assemblage database of unprecedented geographic and taxonomic coverage to quantify local biodiversity responses to land use and related changes. Here we show that in the worst-affected habitats, these pressures reduce within-sample species richness by an average of 76.5%, total abundance by 39.5% and rarefaction-based richness by 40.3%. We estimate that, globally, these pressures have already slightly reduced average within-sample richness (by 13.6%), total abundance (10.7%) and rarefaction-based richness (8.1%), with changes showing marked spatial variation. Rapid further losses are predicted under a business-as-usual land-use scenario; within-sample richness is projected to fall by a further 3.4% globally by 2100, with losses concentrated in biodiverse but economically poor countries. Strong mitigation can deliver much more positive biodiversity changes (up to a 1.9% average increase) that are less strongly related to countries' socioeconomic status.

Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26

Abstract: The efficiency of silicon solar cells has a large influence on the cost of most photovoltaics panels. Here, researchers from Kaneka present a silicon heterojunction with interdigitated back contacts reaching an efficiency of 26.3% and provide a detailed loss analysis to guide further developments.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Abstract: Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

When group-III nitrides go infrared: New properties and perspectives

Abstract: Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...