118 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)
Interpretation of admittance signatures in Cu(In,Ga)Se2 solar cells
No full textIn this work we use numerical simulations to investigate the origin of the capacitance step commonly observed in thermal admittance spectra of CIGS solar cells. We analyze what are the effects on the frequency- and temperaturedependent capacitance curves of: (a) the conduction band offset at the CdS/CIGS interface; (b) an interfacial defect distribution between CIGS and CdS; (c) a Schottky barrier at the rear contact; (d) a grain-boundary (GB). We extract the depletion region width from the simulated capacitance step height and compare it with experimental data
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
Alkali treatments of Cu(In,Ga)Se2 thin‐film absorbers and their impact on transport barriers
No full textpeer reviewe
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
Investigation of Elemental Distribution, Microstructure and Optoelectronic Properties of Cu(In,Ga)Se2 Thin Films for Solar Cells
No full textEfficient and Stable NIR-Transparent Perovskite Solar Cells for Thin-Film Tandem Photovoltaics
No full textZUR CHRONOLOGIE DER MENIPPEEN DES VARRO
No full textThe author of this article analyzes three fragments of Varro’s Menippean Satires (Frg. 183, 334 and 104 Buecheler) which are supposed to contain further information about the dating of these lines. From the interpretation of these fragments he tries to establish more precisely their chronology
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