Charge Transport and Back Reaction in Solid-State Dye-Sensitized Solar Cells:  A Study Using Intensity-Modulated Photovoltage and Photocurrent Spectroscopy

Abstract

Solid-state dye-sensitized solar cells employing spiro-MeOTAD [2,2′7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene] as a hole transport phase were studied by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) over a wide range of illumination intensity. The IMPS and IMVS responses provide information about charge transport and electronhole recombination, respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO 2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO 2 was found to be 4.4 × 10 -4 cm. This value was almost independent of light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity (proportional to I 0 0.64 ).

Physical electrochemistry of nanostructured devices

“…68,72,[125][126][127] It was found that both the effective electron diffusion coefficient, D n , and the effective electron lifetime, t n , 128,129 that are measured become a function of the steady state. 13,68,72,[129][130][131][132][133][134][135] Using quasi-equilibrium arguments, the variations of both diffusion coefficient and lifetime were attributed to the statistics of electrons in the material, which deviates from dilution, as described by thermodynamic factors. 136 The varying D n was recognized as a chemical diffusion coefficient, 55,127 and the correlation between variations of D n and t n , 135 was explained by a common origin of their variations in an exponential distribution in the bandgap.…”

“…13,68,72,[129][130][131][132][133][134][135] Using quasi-equilibrium arguments, the variations of both diffusion coefficient and lifetime were attributed to the statistics of electrons in the material, which deviates from dilution, as described by thermodynamic factors. 136 The varying D n was recognized as a chemical diffusion coefficient, 55,127 and the correlation between variations of D n and t n , 135 was explained by a common origin of their variations in an exponential distribution in the bandgap. 95,136 The application of the multiple trapping model in DSC will be described in detail in section 3.4.…”

The advent of mesoscopic injection solar cells

“…from a fraction of a per cent 2 to over 4%. 20 The main draw back of these cells has been fast interfacial electron-hole recombination reducing the diffusion length of the conduction band electrons to a few microns 21 as compared to 20-100 microns for the electrolyte based DSC. As a consequence, the film thickness employed in these cells is only 2 microns which is insufficient to harvest the sunlight by the adsorbed sensitizer, thus reducing the resultant photocurrent.…”

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

“…In nanoparticle films, the nature of electron transport in oxide nanoparticle films is well understood using timeresolved photocurrent and photovoltage measurements and the corresponding modeling study. [113,114,115] It is found that the electron transport in nanoparticle films is through a trapping-detrapping diffusion process, in which the photogenerated electrons interact with charge traps as they undertake a random walk through the nanoparticle network. In FF is the fill factor, which describes the power extraction efficiency of solar cells, and P in (mW/cm 2 ) is the overall illumination power incident on the solar cells.…”