Nature of Photovoltaic Action in Dye-Sensitized Solar Cells

Abstract

We explain the cause for the photocurrent and photovoltage in nanocrystalline, mesoporous dye-sensitized solar cells, in terms of the separation, recombination, and transport of electronic charge as well as in terms of electron energetics. On the basis of available experimental data, we confirm that the basic cause for the photovoltage is the change in the electron concentration in the nanocrystalline electron conductor that results from photoinduced charge injection from the dye. The maximum photovoltage is given by the difference in electron energies between the redox level and the bottom of the electron conductor's conduction band, rather than by any difference in electrical potential in the cell, in the dark. Charge separation occurs because of the energetic and entropic driving forces that exist at the dye/electron conductor interface, with charge transport aided by such driving forces at the electron conductor/contact interface. The mesoporosity and nanocrystallinity of the semiconductor are important not only because of the large amount of dye that can be adsorbed on the system's very large surface, but also for two additional reasons: (1) it allows the semiconductor small particles to become almost totally depleted upon immersion in the electrolyte (allowing for large photovoltages), and (2) the proximity of the electrolyte to all particles makes screening of injected electrons, and thus their transport, possible.

Physical electrochemistry of nanostructured devices

“…78 The upper limitation to the photovoltage in DSC is given when the Fermi level meets the lower edge of the conduction band of TiO 2 . 3,199 In Fig. 22(b) we see that the extrapolation of D n to V oc (at 1 sun) remains lower than 10 À3 cm 2 s À1 , which is two orders of magnitude below the value expected for bulk crystals.…”

Fabrication and Characterisation of Titanium Dioxide Based Dye Sensitized Solar Cell using Flame of the Forest Dye

“…The maximum theoretical value for the photovoltage at open circuit condition is determined by the potential difference between the conduction band edge of the TiO 2 and the redox potential of the I -/I -3 -pair in the electrolyte (Cahen, 2000). The operation of the cell is regenerative since no chemical substances are neither consumed nor produced during the working cycle, as visualized in the cell reactions shown in figure 2.…”

Quantum modeling of two-level photovoltaic systems

“…In nanostructured solar cells, following the photon absorption, electron-hole pair creation occurs in a confined zone [13][14][15][16] and hence, the Coulomb interaction between the charge carriers and their possible recombination plays an essential role. Fundamentally different from conventional silicon-based photovoltaic devices, in nanostructured solar cells the charge carriers separation and transport rely on a high density of nanoscale interfaces.…”