By Andreas Hinsch
Dye Solar Cells
Despite the strong market growth in photovoltaic application as roof tops, only a very small portion of the photovoltaic modules is sold in the building integrated market so far. The ideal case of a building integrated PV module, as being a glass element that also generates electrical power, can be realized with the DSC technology. Dye Solar Cells (DSC) printed on glass uniquely offer a high degree in freedom for the design and the optical appearance of a building integrated module. DSC, representing a completely new type of solar cells, are also regarded as one of the potentially disruptive solar technologies in terms of costs.
Dye solar cells are photoelectrochemical solar cells. The conversion process is similar to photosynthesis. In principle, DSC are simple to manufacture and present a prime example for the research behind and the realization of functionalizing nanomaterials. DSC are based on a thin nanocrystalline carrier layer made of porous titanium dioxide (TiO2) whose large inner surface is chemically bonded with dye molecules (Figure 1). After photoexcitation of the dye molecules, electrons are injected and collected in the TiO2 photoelectrode. An electrolyte is used for transporting of the remaining positive charge carriers to the counter electrode. The manufacturing of DSC is based on printing (Figure 2) and does not require expensive vacuum technology. The required amount of active material per area is low, which means less than 1 g/m2 for the dye and less than 30 ml/m2 for the electrolyte.
DSC have been under intensive investigation as a new type of solar cell technology for over 15 years. In contrary to earlier expectations, the transfer of the technology from laboratory stage to a module production phase has turned out to be complex and time demanding. Despite the ease of the basic manufacturing principle of DSC, a completely new set of materials, cell concepts and manufacturing methods had to be developed which are still undergoing further optimization at various research groups worldwide.
With respect to a first market introduction, one advantage of dye solar modules is the combination of PV solar electricity with decorative aspects which makes DSC attractive for building integrated PV. During the last years, a growing effort has been undertaken towards the piloting of dye solar cell modules1)-4). A successful up-scaling and future certification of DSC modules requires the choice of suitable module concepts and manufacturing schemes5). In particular, the internal spatial diffusion of electrolyte species under long-term operation of a DSC module has to be prohibited6). Simultaneously, methods for quality control gain of importance which requires the analysis of the underlying microscopic situation.
DSC Module Development at Fraunhofer ISE
At Fraunhofer ISE, research and development of DSC is carried out for more than 10 years. Emphasis is put on up-scaling related issues of laboratory processes, the optimization of materials in cooperation with industrial partners and the development of characterization methods for quality control.
Due to their working principle, DSC contain a small amount of a liquid or gelified electrolyte. The development of suitable module interconnection concepts and materials is, therefore, strongly related to the sealing of the electrolyte. Fraunhofer ISE has succeeded in developing DSC modules with internal interconnection of so-called meander type using a unique glass frit sealing technology which is up-scalable. The glass frit sealing, stable over the long-term, is also applied by a screen printing process.
Currently, on 10 cm x 10 cm DSC modules a solar efficiency of 7.1% has been reached on the active area (75%). These modules are internally 2-fold interconnected. In the frame of a European project, several accelerated ageing tests similar to standard photovoltaic module certification, i.e., 1,000 hours at 85°C, 1,000 hours illumination at 1 sun and -40°C to 85°C thermal cycling, have been performed at these modules with promising results. A degradation of less than 10% loss in efficiency after the tests has been achieved already for the small modules. Based on the same manufacturing process a successful up-scaling of the module size has been undertaken with the aim to proof the suitability for integration into glass facades. Stability testing of large area DSC module are planned to start this year as soon as a sufficient number of prototypes is produced.
The manufacture of the 60 x 100 cm² module prototypes (Figure 3) at Fraunhofer ISE is carried out using industry-relevant procedures and machines (Figure 4). For applying the dye and the electrolyte, a customized, in-house development was necessary. Therefore, in cooperation with the Fraunhofer IAO in Stuttgart, Fraunhofer ISE developed a station for automatically filling and final sealing the large area dye solar cell modules. A precise control of the liquid flow and leak detection in the seal is possible. With this apparatus, the further manufacture of modules for future demonstration projects and field testing is guaranteed, and a decisive step towards a pilot processing line has been accomplished.
The manufacturing of the DSC modules follows a nine step approach as is illustrated in Figure 5. As substrates, low-cost flat glass which is coated with a Transparent Conducting Oxide (TCO) is used. The substrates are drilled and the TCO layer is electrically isolated by laser. A special type of laser is used in order to avoid microcracking of the glass. All layers including the glass frit paste are screen printed and subsequently dried. The organic residues of the printed layers are then removed in a simple sintering step. Then, the plates are positioned and fused in an oven at temperatures which allow the glass frit to join and to form a tight external and internal seal. During the fusing process, the mechanical stress in the glass is released and the plates are settling at a constant distance which in the ideal case results in the mechanical properties of a single glass pane. In the next steps, dye solution and electrolyte are purged through the filling holes which are sealed afterwards. The advantage of this filling method is that the chemicals are easily handled in a closed tube system which is essential for long-term stability as the uptake of water vapour in the module is prevented.
Interestingly, the glass substrate, the glass handling and the thermal treatments are similar to the ones which are already well established in the cost-effective production of thin-film solar modules; although substantial lower investment costs can be assumed for DSC, as no vacuum equipment is necessary.
Applications and Product Development
In parallel to the technology and module development, cooperation with leading architects from the field of Building Integrated Photovolatics (BIPV) was set up. For that purpose, expert interviews and workshops were conducted to evaluate different applications of BIPV and the requirements for new photovoltaic products such as DSC-modules. The results were summed up in the three scenarios wall cladding, structural glazing and rooflights that were further detailed. An important outcome of the workshops was that many applications of DSC-based BIPV can also be financially competitive if total system cost like substituted facade material and saved energy are taken into account. From the investigation, an additional 100 €/m2 can be estimated as the tolerable cost contribution of a DSC module to the sales price of a glass facade element.
In a recent European project ‘Robust-DSC’, a Life Cycle Analysis (LCA) has been carried out for glass-based DSC modules of Fraunhofer ISE. The LCA is a tool for analyzing the ecological impact of new products, materials and production processes. It is shown, that more than 70% of the impact on the indicators─human health, ecosystem quality and resources depletion─are originating from the glass substrates alone. The additional contribution of the DSC specific materials and processes is, therefore, low and does not represent a high initial load in the LCA balance. This is also true for the energy payback time which, including the glass, for a south-oriented DSC facade is determined to be 0.5 to 1.5 years depending on the geographic location and the assumed module efficiency. In other words, if a DSC module is integrated into a glass facade which spares at least one glass substrate, the energy which is associated with the production and construction of the facade can be returned back at a very good ecological value during the lifetime.
In 1994, Dr. Andreas Andreas started his professional career as project manager R&D Dye Solar Cells at the Swiss glass company Glas Trosch. From 1997 to 2001 he was senior researcher at the Energy Research Institute of the Netherlands ECN where he coordinated EU projects on dye solar cell devices and stability. In 2001, he established a research group on dye and organic solar cells at Fraunhofer ISE (www.fraunhofer.de). From 2007 on, he has been involved in commercialization related activities for the product development of building integrated dye solar modules. Andreas is the inventor of seven patents and author of more than 40 scientific publications on DSC and OPV which have been cited over 800 times.
1) H. Matsui et al., ‘Thermal stability of dye sensitized solar cells with current collecting grid’, Solar Energy Materials & Solar Cells. 93, 2009, pp.1110-1115.
2) N. Kato et al., ‘Degradation analysis of dye sensitized solar module after long-term stability test under outdoor working condition’, Solar Energy Materials & Solar Cells. 93, 2009, pp.893-897.
3) A. Fukui et al., ‘Dye-sensitized photovoltaic module with conversion efficiency of 8.4%’, Appl. Phys. Express. 2, 2009, pp.82202-05.
4) A. Hinsch et al., ‘Dye solar modules for facade applications: recent results from project ColorSol’, Solar Energy Materials & Solar Cells. 93, 2009, pp.820-824.
5) A. Hinsch et al., ‘Material development for dye solar modules: results from an integrated approach’, Progress in Photovoltaics: Research and Applications. 16, 2008, pp.489-501.
6) R. Sastrawan, ‘Photovoltaic modules of dye solar cells’, Dissertation, University Freiburg, Germany, available on-line, 2006
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