Nanocrystalline charge transport

In Dr. M. Gratzel s plenary lecture at IPS-2000,103 he presented the following research topics to improve DSC. 1) Mastering the interfaces, electron transfer dynamics, control of dark current. 2) Charge transport in nanocrystalline films. 3) Panchromatic sensitizers, dye cocktail, quantum dot charge injection. 4) Light management, mixed metal oxide films, core-shell metal oxide films. 5) New... 

Fig. 10.28. Model of charge carrier separation and charge transport in a nanocrystalline film. The electrolyte has contact with the individual nanocrystallites. Illumination produces an electron-hole pair in one crystallite. The hole transfers to the electrolyte and the electron traverses several crystallites before reaching the substrate. Note that the photogenerated hole always has a short distance (about the radius of the particle) to pass before reaching the semiconductor/electrolyte interface wherever the electron-hole pair is created in the nanoporous film. The probability for the electron to recombine will, however, depend on the distance between the photoexcited particle and the tin-coated oxide back-contact. (Reprinted with permission from A. Hagfeldt and Michael Gratzel, Light-Induced Redox Reactions in Nanocrystalline Systems Chem. Rev. 95 49-68, copyright 1995, American Chemical Society.)...

Charge transport in nanocrystalline electrodes is clearly strongly influenced by the inter-penetration of the solid and liquid phases. If electron hole pairs are generated by band to band excitation, it is usually observed that one type of carrier is transferred to the solution, while the other is transported to the substrate contact. In the case of the dye sensitized nanocrystalline systems, an electron is injected into the conduction band from the photoexcited dye and is then transported to the substrate. The dye is regenerated by reaction of its oxidised state with a supersen-sitiser such as 1 as shown in Fig. 8.25. 

Hagfeldt A., Lindquist S. E. and Gratzel M. (1994), Charge carrier separation and charge transport in nanocrystalline junctions , Sol. Energy Mat. Sol. Cells 32, 245-257. 

Green A. N. M., Palomares E., Haque S. A., Kroon J. M. and Durrant J. R. (2005), Charge transport versus recombination in dye-sensitized solar cells employing nanocrystalline Ti02 and Sn02 films , J. Phys. Chem. B 109, 12525-12533. 

Charge transport in nanocrystalline ZnO is less well studied than in Ti02, but we may make a few relevant comments ... 

J. van de Lagemaat, N. G. Park, A. J. Frank, I nfluence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline Ti02 solar cells a study by electrical impedance and optical modulation techniques, J. Phys. Chem. B 2000, 104(9), 2044-2052. 

Litzelman, S.J. (2008) Modification of space charge transport in nanocrystalline ceriiun oxide by heterogeneous doping. Ph.D. thesis, Massachusetts Institute... 

Charge transport in solid-state DSSCs based on nanocrystalline TiOa appears to be electron-limited at short-circuit, meaning that the transport of electrons through the mesoporous Ti02 is slower than the transport of holes through the... 

Usami A, Ozaki H (2001) Computer simulations of charge transport in dye-sensitized nanocrystalline photovoltaic cells. J Phys Chem B 105 4577 583... 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

DSSCs convert sunlight to electricity by a different mechanism than conventional p-n junction solar cell. Light is absorbed directly at the solid/liquid interface by a monolayer of adsorbed dye, and initial charge separation occurs without the need of exciton transport.42,43 Following the initial charge separation, electrons and holes are confined in two different chemical phases electrons in the nanocrystalline... 

This reaction occurs in about 10 ns when R is an iodide ion in the 0.5 M concentration range [5]. Diffusion of 2 through the nanocrystalline Ti02 film to the substrate Sn02 electrode and diffusion of the oxidized redox species, R +, through the solution to the counterelectrode allow both charge carriers to be transferred to the external circuit where useful work is performed. The transport of electrons [7,24-29] and redox species [30] will not be considered further except insofar as they relate to the interfacial processes that are the focus of this chapter. 

Solar cells, or photovoltaic devices, have been studied for many years [3], Most of the current work is focused on dye-sensitized nanocrystalline solar cells. These provide a technical and economically viable alternative to present-day photovoltaic devices. In contrast to conventional systems, in which the semiconductor assumes both the task of light absorption and charge carrier transport, the two functions are separated in dye-sensitized nanocrystalline solar cells [54] (cf. OPCs). Light is absorbed by the dye sensitizer, which is anchored to the surface of a wide-band-gap semiconductor. Charge separation takes place at the interface via photoinduced electron injection from the dye into the conduction band of the... 

As we shall see later, electron transport in nanocrystalline films is necessarily accompanied by charge-compensating cations because the holes are rapidly injected into the flooded electrolyte phase. This provides opportunities for studying ion transport processes in mesoporous media, that are coupled to electron motion. Ion insertion also has practical consequences as in energy storage device applications [296]. 

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