40 research outputs found
Impact of front-side point contact/passivation geometry on thin-film solar cell performance
No full textIn this work, we perform an extensive campaign of three-dimensional numerical simulations of CIGS solar cell structures to investigate the effect of a surface-passivated CIGS with point contacts openings on the cell performance parameters (Jsc, Voc, FF and η). Detailed analysis of the combination of passivation thickness, point contact size and pitch is performed under the hypothesis of highly defective CIGS front surface and ideal chemical passivation: efficiencies close to the case of ideal (i.e., defect-free) CdS/CIGS interface can be achieved by optimized nanometer-scale point contact arrays. To account for field-effect passivation due to positive residual charge density, Qf, within the passivation layer, we vary Qf in the range 1010–1013 cm−2 under the two extreme scenarios of ideal or ineffective chemical passivation. Several examples of CIGS cells with different buffer layers (CdS, ZnO, ZnMgO, In2S3, Zn(O, S)) are also analyzed. We find that a positive Qf in the interval 1012– 5·1012 cm−2 can help completely recover the ideal cell efficiency, irrespective of the chemical passivation effect and even in the presence of unfavorable conduction band alignment at the buffer/CIGS heterojunction. This may help devising solutions with buffer materials alternative to CdS, boosting the performance of otherwise surface-limited cells. The effect of grain boundary defect density and position with respect to point contacts is also addressed, with a grain dimension of 750 nm
The use of HfO2 in a point contact concept for front interface passivation of Cu(In,Ga)Se-2 solar cells
No full textWe report on the use of a high bandgap metal-oxide at the front interface of Cu(In,Ga)Se-2 (CIGS) solar cells in a point contact concept for reduced interface recombination. Highly resistive HfO2 is applied on the CIGS surface by atomic layer deposition (ALD). Aspects of the surface passivating effect of HfO2 on CIGS were investigated by time-resolved photoluminescence (TRPL), electron beam induced current (EBIC) and capacitance-voltage (C-V) measurements. Two structuring methods for point contact formation are compared, a lithographic top-down and a simple bottom-up approach using NaCl as template. The former method employed a plasma etch step which was found to degrade the performance of solar cells when applied on the CIGS surface. The template method omitted sputtering and allowed patterning of HfO2 up to 10 nm thickness without adversely impacting the open-circuit voltage (V-OC). EBIC revealed an improved carrier collection due to the HfO2 coating and a long term stable PL decay was observed. Yet, the point contact concept with HfO2 was not significantly influencing the performance of a CIGS solar cell for the investigated parameter range.The work has received funding from the Swiss Federal Office of Energy under contract No SI/501145-01 and the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract No 15.0158. The work has received support from the European Union's Horizon 2020 research and innovation programme under grant agreement No 641004 (Sharc25)
Alkali-Templated Surface Nanopatterning of Chalcogenide Thin Films: A Novel Approach Toward Solar Cells with Enhanced Efficiency
No full textConcepts of localized contacts and
junctions through surface passivation layers are already advantageously
applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se<sub>2</sub> thin film solar cells, such concepts are generally not applied,
especially at the heterojunction, because of the lack of a simple
method yielding features with the required size and distribution.
Here, we show a novel, innovative surface nanopatterning approach
to form homogeneously distributed nanostructures (<30 nm) on the
faceted, rough surface of polycrystalline chalcogenide thin films.
The method, based on selective dissolution of self-assembled and well-defined
alkali condensates in water, opens up new research opportunities toward
development of thin film solar cells with enhanced efficiency
Refractive indices of layers and optical simulations of Cu(In,Ga)Se2 solar cells
No full textCu(In,Ga)Se2 -based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se2 does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances
Impact of compositional grading and overall Cu deficiency on the near-infrared response in Cu(In, Ga)Se2 solar cells
No full textHighly efficient thin film solar cells based on co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu-poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near-infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection-transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu-dependent modifications of the NIR-absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re-adjustments of the co-evaporation process and of the alkali-fluorides post-deposition treatments allow maintaining Voc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd
Micro- and macroscopic characterization of recombination losses in high efficiency Cu(In,Ga)Se2 thin film solar cells
No full textTiefengrundwasser – Vorkommen, Nutzungspotenzial und Schutzwürdigkeit
No full textTiefengrundwasser findet sich in allen grosstektonischen Einheiten der Schweiz. Die bekannten, teilweise genutzten Vorkommen von Tiefengrundwas-ser und insbesondere die chemische Zusammensetzung dieser Wässer wird für die verschiedenen Einheiten aufgezeigt. Im Hinblick auf mögliche Nutzungskon-flikte werden die gesetzlichen Grundlagen und Massnahmen zum Schutz dieser Ressource diskutiert
Energy Harvesting by Subcutaneous Solar Cells: A Long-Term Study on Achievable Energy Output
No full textActive electronic implants are powered by primary
batteries, which induces the necessity of implant replacement
after battery depletion. This causes repeated interventions in
a patients’ life, which bears the risk of complications and is
costly. By using energy harvesting devices to power the
implant, device replacements may be avoided and the device
size may be reduced dramatically. Recently, several groups
presented prototypes of implants powered by subcutaneous
solar cells. However, data about the expected real-life power
output of subcutaneously implanted solar cells was lacking so
far. In this study, we report the first real-life validation data
of energy harvesting by subcutaneous solar cells.
Portable light measurement devices that feature solar cells
(cell area = 3.6 cm2) and continuously measure a subcutaneous
solar cell’s output power were built. The measurement
devices were worn by volunteers in their daily routine in
summer, autumn and winter. In addition to the measured
output power, influences such as season, weather and human
activity were analyzed. The obtained mean power over the
whole study period was 67 uW (=19 uW cm-2), which is
sufficient to power e.g. a cardiac pacemaker
Traceability of the Micro Scale Pipe Viscometer for Traceable Calibration of Dynamic Viscosity
No full textCalibration of flow devices is important in several areas of pharmaceutical, flow chemistry and microfluidic applications where dosage of process liquids or accurate measurement of flow rate is important. The process-oriented liquid itself might influence the performance of a flow device and the simultaneous determination of dynamic viscosity under flow conditions might provide valuable information for process parameters. To offer simultaneous calibration of the dynamic viscosity of a process-oriented liquid at the corresponding flowrate, METAS built a pipe viscometer for the traceable inline measurement of dynamic viscosity in current flow facilities for low flowrates from 1 μL/min to 150 mL/min and pressure drops up to 10 bar. The traceability of all measuring quantities as well as geometrical dimensions of the microtube guarantee the traceability of the pipe viscometer to SI units. The most challenging part is the traceable determination of the inner diameter of the microtube. This can be achieved by measuring the pressure drop as a function of flowrate using a pipe viscometer and applying the Hagen–Poiseuille law with a traceable dynamic viscosity of a reference liquid (water) or performing measurements by utilizing the μ-CT facility at METAS, where the inner diameter is determined using X-ray diffraction. The validation of the stated measurement uncertainty of the pipe viscometer was performed by calibrating the dynamic viscosity of several reference liquids with traceable density and kinematic viscosity. The setup of the facility, traceability as well as uncertainty calculation of the pipe viscometer for inline measurement of dynamic viscosity are discussed in this paper
Correcting for interference effects in the photoluminescence of Cu(In,Ga)Se2 thin films
No full textpeer reviewe
