Dye sensitization solar cells: a critical assessment of the learning curve

The learning curve for dye sensitized solar cells is analyzed and tentatively compared with the learning curve for classical solar cells. It appears that for one decade now the efficiency of nano-structured cells has been essentially stagnating despite significant research progress. This problem is analyzed and found to be mainly due to the exhausted exploitability of the surface roughness factor and the involvement of new mechanistic principles, which are scientifically still barely understood and therefore not well optimized. These are charge separation via irreversible kinetics and not via imprinted electrical fields, the role of metal-centered, coordination chemical electron transfer and of chemical back bonding of the sensitizer for increased reactivity and stability as well as the function of the front FTO contact as a vectorial, electron collecting interface. The parameters which determine long-term solar cell stability are also addressed. They will have to be explored in detail. Metal-centered sensitized electron transfer is definitely an advantage. In addition, the branching coefficient determined by the ratio of sensitizer regeneration and product formation is critically dependent on TiO2 surface states, which determine degradation kinetics. The analysis includes solid-state dye sensitization cells, which are presently faced with even more problems concerning efficiency and stability than liquid ones. (C) 2004 Elsevier B.V. All rights reserved.