Why Gratzel’s cell works so well

In Gratzel’s cell, the electrons injected by the photo-excitation of dye molecules, anchored to a mesoporous TiO2 film, efficiently diffuse to the back contact achieving solar energy conversion at efficiencies exceeding 10%. The mesoporous TiO2 surface constituted of randomly arranged nanocrystallites with a roughness factor of the order 1000 is heavily populated with traps, defects and adsorbed species which act as recombination centers. Nevertheless, the cell functions, mitigating recombination expected to occur via the interaction electrons at the surface. Evidence based mainly on 1/f noise measurements is presented to show that dye bonded to the TiO2 surface passivates recombination centers. Furthermore the suppression of trapping–detrapping events at the surface increases the diffusion coefficient of the electrons through the nanocrystalline matrix facilitating electron transport to the back contact. The Gratzel cell is also unique, none of the high bandgap oxide materials other than TiO2 yield energy conversion and quantum efficiencies as high as that of the cells based on TiO2. 1/f noise measurements also reveal a distinct difference between TiO2 and ZnO mesoporous films suggesting that the films made from the latter material are more intensely populated with surface states that mediate recombination.