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Projects / Programmes source: ARIS

Short- and long-term internal processes during operation of dye-sensitized solar cells

Research activity

Code Science Field Subfield
2.03.03  Engineering sciences and technologies  Energy engineering  Renewable resources and technologies 

Code Science Field
T150  Technological sciences  Material technology 

Code Science Field
2.02  Engineering and Technology  Electrical engineering, Electronic engineering, Information engineering 
Keywords
-dye-sensitized solar cell -dye-sensitized solar module -processes -preconditioning -accelerated ageing -long-term stability -degradation
Evaluation (rules)
source: COBISS
Researchers (1)
no. Code Name and surname Research area Role Period No. of publicationsNo. of publications
1.  25420  PhD Marko Berginc  Metrology  Head  2011 - 2013  83 
Organisations (1)
no. Code Research organisation City Registration number No. of publicationsNo. of publications
1.  1538  University of Ljubljana, Faculty of Electrical Engineering  Ljubljana  1626965  27,758 
Abstract
In the proposed project I will study dye-sensitized solar cells (DSSCs) which were developed in 1991 and belong to the third solar cells generation. Although the DSSCs have remarkable efficiencies nowadays (11%) they have not fully enter the market since they need preconditioning after production which is currently not explored enough and have a relatively short lifetime. Therefore these issues will be the main objective of the proposed project. All studies will be based on DSSCs using a K77 dye (Ru(2,2’-bipyridine-4,4’-dicarboxylic acid)(4,4’-bis(2-(4-tert-butyloxy-phenyl)ethenyl) 2,2’-bipyridine)(NCS)2) and ionic liquid based electrolyte (a mixture of 1-propyl-3-methyl-imidazolium iodide and 1-ethyl-3-methyl-imidazolium tetracyanoborate). This DSSC has already passed a thermal stress test at 80 °C and light soaking test at 60 °C and has remarkable conversion efficiency (7.6%). However, it still has not passed the light soaking test at 85 °C yet.   The manufacturing procedures and materials for the DSSCs will be further optimized within the proposed project. Several different materials and techniques for primary and secondary sealing will be examined and the effect of the water content on the long-term stability of the DSSCs will be examined. Studies will define the manufacturing procedure and the most appropriate sealing material that will be used throughout the project. Onward a short-term transients taking place immediately after the production of the DSSCs will be studied. Illuminated DSSCs will be stabilized using different loads. The current-voltage characteristics and the impedance spectroscopy measurements will be used to evaluate the internal processes and performances. The systematic analysis will define the internal processes causing the initial transient of the performance. Finally a preconditioning protocol will be given for a fast and effective initial stabilization of the DSSC’s performance which is essential for certification in the accredited laboratories around the world. The preconditioning will be also used before studying the long-term stability and lifetime of the DSSCs. The long-term stability of the DSSCs will be systematically studied under different operation conditions (different loadings, cell temperatures, light sources) using special DSSCs geometries (segmented cells, additional layers, symmetric cells, etc.) standard measurements protocol (dark and illuminated current-voltage characteristics, impedance spectroscopy, quantum efficiency, light beam induced current technique), additional characterization techniques (UV-Vis spectroscopy, impedance spectroscopy and cyclic voltammetry of symmetric cell, open circuit voltage decay technique, etc.). Each combination of cell temperature and type of illumination it will define (i) the change in performance parameters (JSC, VOC, FF and ?), (ii) which internal processes are affected and (ii) which materials or interfaces introduce unstable response of the DSSC. These materials will be gradually replaced with more stable materials. This will improve the stability of the DSSCs and it will also generalize the knowledge about the stability of the DSSCs using different materials. The purpose of solar cells is to generate the electrical energy therefore dye-sensitized solar modules using the most stable materials will be manufactured as well. Their performances and processes taking place during long-term operation will be examined too. All these findings and knowledge will make a significant contribution for faster and more successful entrance of the DSSCs technologies into the market.
Significance for science
The results of the postdoc project showed that together with the electrolyte also small iodine based crystals get introduced into the dye-sensitized solar cells (DSSCs) during the production. Heating the cells to 80 °C for 1 h dissolve the crystals, increase the conversion efficiency and prevent a further grow of the crystals which stabilize operation of the DSSCs therefore the heating after the production is recommended. The results also showed significant degradation of the cells that were aged under short-circuit condition. On contrary, the cells were remarkably stable when aged under open-circuit condition and especially in maximum-power point, i.e. the condition where the cells normally operate. After the production the small crystals were observed again and they can reversibly grow or dissolve which strongly and reversibly influence also the performance. Therefore the conclusions about DSSC’s instability found in the literature are sometimes unjustified. The crystals gradually grow in the dark and in the cold environment and reduce the performance. The growth is even accelerated when a forward bias is applied. On contrary the crystals dissolve at elevated temperatures (T ) 60 °C) meaning that the DSSCs are capable restoring their performances during operation outdoors. The conclusions obtained with the project are not related only to the field of dye-sensitized solar cells. They can be also applied to the photochromic devices which have integrated low-power DSSCs to colour the device under illumination. The results were presented at the conferences and they were or will be published in the scientific journals. The results are interesting therefore numerous citations are expected in the following years. In addition, new collaborations with foreign laboratories were established.
Significance for the country
Photovoltaic (PV) is one of the cleanest ways to generate electricity since it directly converts solar into electrical energy. The PV is a valuable technology which will reduce the CO2 emissions, mitigate the global climate change and contribute to a sustainable energy supply. Additionally, the PV will reduce pollution, noise and waste therefore the development of PV has very important impact on the quality of our lives. As outlined in numerous reports and studies, the PV will become an important source of energy in the long run. Thus the postdoc project dealt with the dye-sensitized solar cells (DSSCs) which are potentially the most cost-effective technology. However their stability remains an issue therefore this was a main subject of the postdoc project. The results showed that performances of DSSCs inevitably vary but variations can be reversible. Moreover initial performance of degraded cells can be sometimes completely renewed under certain conditions therefore instability of the DSSCs is often unjustified. One of such conditions showing healing effect is heating of the cells to 80 °C which demonstrates that the DSSCs are even capable of self-healing during operation outdoors. In addition, the results of the project showed that heating the cells (80 °C) after the production slightly increases initial performance and stabilized further operation therefore heating after production is highly recommended. Most probably the DSSCs will never provide sustainable energy supply. However, improved stability and their low-cost might be an opportunity for Slovenian industry to produce specific products which demand low-power DC electricity and does not demand stability over several decades (e.g. chargers for self-phones, tablets, power supply for calculators, watches, PV for backpacks, tents, etc.). The results can be applied also to the photochromic devices which changes their transparency according to the outdoor illumination and that is why a low-power DSSC needs to be integrated. The DSSCs are appropriate also for education in high schools since their technology is relatively easy.
Most important scientific results Annual report 2011, 2012, final report, complete report on dLib.si
Most important socioeconomically and culturally relevant results Annual report 2011, 2012, final report, complete report on dLib.si
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