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Schwäbisch Hall, Germany

Ott T.,Ulm University of Applied Sciences | Schonberger F.,Ulm University of Applied Sciences | Walter T.,Ulm University of Applied Sciences | Hariskos D.,Center for Solar Energy and Hydrogen Research | And 3 more authors.
Thin Solid Films

Previous studies of Cu(In,Ga)Se2thin film solar cells showed that the long-term stability critically depends on the bias across the junction. As a result of a dark anneal the current-voltage (IV)-characteristics in the dark showed a blocking behavior with increasing anneal time. In the final stage the device exhibits an open circuit voltage (Voc) which is independent from the illumination intensity, a crossover of the dark and illuminated IV-characteristics and Vocsaturation for decreasing temperatures. These characteristics also occur in the initial state prior to the endurance test, however, at low temperature (< 200 K) measurements. We suggested a phototransistor model to explain the observed characteristics. The prerequisite of this model is the existence of a Schottky barrier at the back contact. In this contribution more insights into this phototransistor model and its experimental verification will be given and discussed. Finally we suggest how to avoid the effects of the back barrier with the help of a CuGaSe2layer at the back of the absorber and a Ga gradient through the absorber. These measures will be verified with simulations and compared to measurements on co-evaporated devices. © 2014 Elsevier B.V. Source

Adams J.,Bavarian Center for Applied Energy Research | Vetter A.,Bavarian Center for Applied Energy Research | Hoga F.,Bavarian Center for Applied Energy Research | Fecher F.,Bavarian Center for Applied Energy Research | And 4 more authors.
Solar Energy Materials and Solar Cells

CuInGaSe2 (CIGS) thin film solar modules, despite their high efficiency, may contain three different kinds of macroscopic defects referred to as bulk defects, interface defects and interconnect defects. This occurs due to film's sensitivity to inhomogeneities during the manufacturing process. The result is a decrease of electrical power output from a cell or module. In this paper, we present the influence of macroscopic defects on the electrical behavior of CIGS thin film solar cells. To accomplish this, we investigated the relation between the IR-signal emitted of a defect in a cell (measured using illuminated lock-in thermography ILIT) and the respective open circuit cell voltage (Voc,cell) under low light conditions (< 100 W/m©2014 Elsevier B.V. All rights reserved. Source

Ott T.,Ulm University of Applied Sciences | Schonberger F.,Ulm University of Applied Sciences | Walter T.,Ulm University of Applied Sciences | Hariskos D.,Center for Solar Energy and Hydrogen Research | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering

CIGS is the most promising technology for thin-film solar cells with record efficiencies of 20.4 % on laboratory scale and 17.8 % aperture area efficiency on a 900 cm- module. Another important factor besides the cell efficiency is the reliability and long term stability of the manufactured modules, which can be assessed by accelerated ageing. In this contribution the accelerated ageing of CIGS mini modules has been investigated. Therefore, modules were dark annealed under dry heat conditions at different temperatures. During the endurance test a positive or negative bias was applied to the cells. In regular intervals the IV- and CV-characteristics were measured at room temperature. After an overall stress time of 3500 h the IV-characteristics were determined under different illumination conditions (intensity, spectral illumination). Our previous publications suggest a barrier at the back contact to explain the observed parameter drifts. This contribution is focused on the influence of different bias conditions during the endurance test on the generation of a back diode and on the change of the acceptor concentration. These parameter drifts have an impact on the open circuit voltage, fill factor and on the appearance of a cross over between dark and illuminated IV-characteristics. The interpretation of the observed parameter drifts was supported by SCAPS simulations based on the above mentioned back barrier model. As an outcome of the simulations signatures for the existence of a back barrier diode were established. IVmeasurements, temperature dependent Voc measurements and SunsVoc measurements are helpful means to detect such back diodes. © 2013 SPIE. Source

Misic B.,Julich Research Center | Pieters B.E.,Julich Research Center | Theisen J.P.,Manz CIGS Technology GmbH | Gerber A.,Julich Research Center | Rau U.,Julich Research Center
Physica Status Solidi (A) Applications and Materials Science

The influence of the i-ZnO/CdS buffer layer on intentionally produced defects in Cu(In,Ga)Se2 (CIGS) mini-modules is investigated by electroluminescence (EL) imaging. Macroscopic shunts of the dimension of 100 μm in length and several μm in width were produced by mechanically removing locally one or several of the layers during the module production process. After creating the defects the modules were finished in the usual way. It is found that heavy shunts were produced whenever the doped ZnO:Al came into contact with the Mo back contact. The decline of photovoltaic performance is seen by a decrease of the EL intensity of the damaged cell. In contrast, considerable shunt mitigation was observed whenever the i-ZnO/CdS buffer combination was present. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Ott T.,Ulm University of Applied Sciences | Walter T.,Ulm University of Applied Sciences | Hariskos D.,Center for Solar Energy and Hydrogen Research | Kiowski O.,Center for Solar Energy and Hydrogen Research | Schaffler R.,Manz CIGS Technology GmbH
Conference Record of the IEEE Photovoltaic Specialists Conference

The long-term stability of solar cells is a crucial factor for the competitiveness of a technology. In this study, the accelerated aging of CIGS solar cells was studied, and the influence of an applied bias during the endurance test on the open-circuit voltage Voc and fill factor (FF) was investigated. Time constants for parameter drifts of the open-circuit voltage and the associated activation energy were determined. The observed parameter drifts will be discussed, and a model will be proposed based on SCAPS simulations, explaining the observed behavior of the electrical characteristics of the solar cells. Therefore, cells were dark annealed under dry conditions at two different temperatures and different voltage biases were applied to the cells. Our study revealed that the application of a positive bias, which is similar to light soaking, first leads to an improvement and stabilization of the open-circuit voltage and FF followed by a slow decrease of these parameters. This long-term decrease can be explained in terms of a back barrier or phototransistor, as simulated with SCAPS. However, applying a positive bias enhances the long-term stability of these devices. The appearance of a back barrier is associated with a time constant exceeding 30 years. Therefore, this degradation mechanism is not critical. © 2012 IEEE. Source

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