The variation of the sodium content in low-temperature grown Cu(In,Ga)Se2 (CIGSe) absorbers has a strong influence on the doping level and the photoelectrical properties of the bare CIGSe films and corresponding solar cells. The negative impact of excessive sodium contents is studied for samples from CIGSe processes with co-evaporation of NaF during absorber deposition. By a combined analysis of temperature dependent current–voltage, capacitance-voltage and quantum efficiency measurements we conclude that the dominating recombination takes place in the bulk of the absorber for all samples independent of the sodium content. The optimum sodium content with respect to solar cell performance results from a tradeoff between the beneficial influence of sodium on open circuit voltage (VOC) and the negative impact of high sodium contents on short circuit current (ISC) and fill factor (FF). The change of VOC, ISC and FF can be explained by a concurrent increase of net doping and deep defect density for increasing sodium content and the presence of barriers at the CIGSe/CdS and the CIGSe/Mo interface.

In order to increase the efficiency of thin film Cu(In,Ga)Se2 (CIGSe) solar cells in space applications, it is necessary to reduce the working temperature of the cells under these special conditions. The most promising approach to achieve this is to increase the thermal emissivity. In this work, a new alumina/silica double layer coating system with a thickness of less than 1 µm that provides high values of emissivity on flexible CIGSe solar cells is presented. The coatings were applied by dip coating using a polysilazane, poly(alkyliminoalane) and an alumina sol as precursors. As the temperature for the thermal treatment of the coatings is limited to about 280 °C, due to thermal stability of the CIGSe device, the applied precursor based coatings must show good adhesion to the substrate and with each other as well as a high reactivity at low temperatures to form oxides. Conversion behaviour, coating thickness, morphology, mechanical properties, optical and thermal properties are investigated. Results of the optical and thermal measurements show that the coating system is transparent and can increase the emissivity remarkably from about 0.38 (uncoated CIGSe) to more than 0.7. Thus the developed double layer system is a promising candidate as high-ε coating on solar cells for space applications.

The behavior of solar cells under reverse bias conditions is of crucial importance for solar module design and stability. For flexible, low temperature deposited Cu(In,Ga)Se2 (CIGSe) solar cells indications for tunnel breakdown at operation temperatures around 300K are found using temperature and net carrier concentration dependent measurements of the breakdown voltage VBR. For temperatures below 200K, the temperature coefficient of the breakdown voltage becomes positive, indicating a change of the breakdown mechanism to avalanche breakdown. A reduction of VBR caused by the illumination with blue photons is demonstrated, which coincides well with results and models for high temperature deposited CIGSe on glass. Furthermore, spectrally resolved measurements of light emission during breakdown indicate a first ReBEL (reverse bias electroluminescence) signature for CIGSe solar cells.

Sodium is known to play an important role for the performance optimization of Cu(In,Ga)Se-2 (CIGSe) thin film solar cells, i.e. it is found to significantly increase the open-circuit voltage (Vac).For this work CIGSe absorbers were produced in a low temperature deposition process on polyimide substrates with varying sodium content. In agreement with previous studies we find an increase of V-OC and overall solar cell performance with increasing sodium content and a decrease of the overall performance for very high sodium content.At the same time time-resolved photoluminescence measurements indicate that the minority carrier lifetime decreases from 50 ns to about 5 ns for increasing sodium content in the absorber. This correlation was found to be valid for three different types of low-temperature CIGSe deposition processes.The results can be explained by an increase in deep defect density with increasing Na content, proportional to the increased net charge carrier density as measured by capacitance-voltage profiling. For very high sodium content this leads to an actual decrease in the overall performance of the solar cells. (C) 2013 Elsevier B.V. All rights reserved.

Long term stability is crucial to maturing any photovoltaic technology. We have studied the influence of sodium, which plays a key role in optimizing the performance of Cu(In,Ga)Se2 (CIGSe) solar cells, on the long-term stability of flexible CIGSe solar cells on polyimide foil. The standardized procedure of damp heat exposure (85% relative humidity at 85 °C) was used to simulate aging of the unencapsulated cells in multiple time steps while they were characterized by current-voltage analysis, capacitance-voltage profiling, as well as electroluminescence imaging. By comparing the aging process to cells that were exposed to heat only, it could be confirmed that moisture plays the key role in the degradation process. We found that cells with higher sodium content suffer from a more pronounced degradation. Furthermore, the experimental results indicate the superposition of an enhancing and a deteriorating mechanism during the aging process. We propose an explanation based on the corrosion of the planar contacts of the solar cell.

Thin films of Cu(In,Ga)Se-2 (CIGS) absorber layers for thin film solar cells have been manufactured on polyimide foil in a low temperature, ion beam assisted co-evaporation process.In the present work a set of CIGS thin films was produced with varying selenium ion energy. Solar cell devices have been manufactured from the films and characterized via admittance spectroscopy and capacitance-voltage profiling to determine the influence of the selenium ion energy on the electric parameters of the solar cells. It is shown that the impact of energetic selenium ions in the CIGS deposition process leads to a change in the activation energy and defect density and also in the spatial distribution of electrically active defects. For the interpretation of the results two defect models are taken into account. (C) 2011 Elsevier B.V. All rights reserved.

This German joint project is directed towards the development of a flexible Cu(In, Ga)Se-2 (CIGSe) thin film solar cell technology on a polyimide (PI) substrate for space applications. A group of partners with an academic and/or industrial background in the field of chalcopyrite based thin film solar cells work together to ingrain space technology in production facilities for terrestrial PV. The three production technologies batch type multi-stage co-evaporation, in-line co-evaporation and roll-to-roll co-evaporation are investigated. So far, a maximum total area solar cell efficiency of 15.5 % has been achieved for lab scale devices (0.5 cm(2), AM 1.5, no AR). On a large area, standardized device an active area efficiency of 12.7 % has been achieved (25.9 cm(2), AM 1.5, no AR) based on an industrial roll-to-roll production process.