While many research groups investigate the working principles of DSSCs and new developments have been achieved concerning their efficiency and large scale applications, new companies founded in the meanwhile try to carry DSSC technology in market place evaluating all process steps are needed for industrial production. Experimental results for small size solar cells cannot directly applied in large scale DSSCs as the efficiencies measured for small size solar cells cannot be repeated in large scale (Spath et al., 2003). This is caused by the high internal resistance of FTO substrates which eliminates their efficiency and it is found to be drastically decreased. Below are referred some of the main issues which have to be taken into account before DSSCs go to a production line:
• Large area deposition of TiO2 layers. The layers have to be homogeneous and uniform
• New methods for dye staining and electrolyte filling
• Electrical interconnection of individual cells. A major factor for limited efficiency of the DSSC is the ineffective contacts on FTO glass. The external connections of the individual cells are also a problem.
• Sealing process for modules in case of liquid electrolytes
• Long-term stability of at least 10 years for outdoor use.
• Evaluation costs, which is believed to be approximately 10% of that needed for silicon solar cells
Moreover, for the durability of the cells, it was considered necessary to replace the liquid electrolyte with quasi-solid state, solid, polymer electrolytes or p-type inorganic semiconductors as the manufacturing cost and convenience of preparation is highly simplified. However, the low manufacturing cost by using the recently developed roll-to – roll coating process especially for DSSCs creates the need of replacing the glass substrate with light weighted flexible plastic electrodes, expanding this way the area of DSSCs’ applications. Three basic structures for large scale DSSCs are proposed either for high current (cells in parallel) or for high voltage (cells in series) collection: