Solar cells charge transport

The low efficiencies could be due to lack of intimate contact (interface) between the sensitizer (which is hydrophilic) and the spirobifluorene (which is hydrophobic). Moreover, the surface charge also plays a significant role in the regeneration of the dye by the electrolyte.98 In an effort to reduce the charge of the sensitizer and improve the interfacial properties between the surface-bound sensitizer and the spirobifluorene hole-carrier, amphiphilic heteroleptic ruthenium(II) complexes ((48)-(53)) have been used as sensitizers. These complexes show excellent stability and good interfacial properties with hole-transport materials, resulting in improved efficiencies for the solar cells. 

Yang, N. Zhai, J. Wang, D. Chen, Y. Jiang, L., Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells. ACS Nano 2010, 4, 887-894. 

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

In dye sensitized solar cells (or Gratzel cells [180, 181]), a redox mediator is required to allow charges to be transported from the mesoporous and light sensitive Ti02 film to the cathode. Although other systems have been studied, the equilibrium potential, mobility, and stability of the h j system are most suitable for this application and most cells developed to date employ the iodine redox system in an organic solvent environment. 

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... 

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

In photo-catalytic and solar energy conversion devices, the absorption of a photon results in the generation of electrons and holes, which, upon separation, can provide an electric potential or trigger chemistry. The efficiency of these devices is frequently determined by the transport of the charges following photo-generation. In particular, for TiC>2-based dye-sensitized solar cells, it has been demonstrated that the efficiency is limited by electron transport through TiC>2 nanoparticles [1]. 

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... 

---