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Dye Sensitized Solar cells: synthesis and applications of a novel sensitizer and pentacene as hole conductor material

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23 The electron concentration in the conduction band and the Fermi level for an n-type semiconductor can be approximated according to the following equations: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ += == −− C D CF kTEE CDc N N kTEE eNNn FC ln /)( (10, 11) where N D is the effective donor intensity. With typical values for N C =10 20 cm -3 and N D =10 17 cm - 3 , the difference between E C and E F can be calculated to 0.2 eV. The hole concentration in the valence band and the Fermi level for a p-type semiconductor can be approximated in a similar way. Crystal surfaces and grain boundaries are two other examples of defects that cause energy levels in the band gap. Energy levels in the band gap are often referred to as traps or recombination centres, depending on the electron lifetime in the state. The band gap states can play an important role in the charge transport and recombination dynamics. [9] Depending on the energy distance from the conduction band, traps are divided into shallow and deep traps. The probability for an electron in a shallow trap to be thermally excited to the conduction band is relatively large whereas the probability is low for an electron in a deep trap. In a semiconductor, electrons in the conduction band and holes in the valence band (electron vacancies) are the charge carriers. In a n-type semiconductor, electrons dominate the carrier transport (majority carrier). In a p-type semiconductor, holes are the majority carrier. Under the influence af an applied electric field, a randomly moving free electron would accelerate in a direction opposite to the field. Due to collisions with atoms an electron in a crystal lattice would not continue to accelerate for very long. The velocity of electrons between collision caused by the electric field is called the drift velocity. The electron mobility µ n is defined as this electron drift velocity in an electric field of unit strenght and is inversely proportional to the effective mass of the electron. The mobility is a material constant. Electron mobilities are tipically in the range of 100-1000 cm 2 V -1 s -1 for semiconductors. The conductivity κ of electrons is proportional to the electron mobility and electron concentration in the conduction band. The conductivity κ of holes is proportional to the holes mobility and holes concentration in the valence band. Hole mobilities are typically in the range 1-1000 cm 2 V -1 s -1 for semiconductors.

Anteprima della Tesi di Vito Sgobba

Anteprima della tesi: Dye Sensitized Solar cells: synthesis and applications of a novel sensitizer and pentacene as hole conductor material, Pagina 12

Tesi di Dottorato

Dipartimento: Ingegneria dell'Innovazione

Autore: Vito Sgobba Contatta »

Composta da 156 pagine.

 

Questa tesi ha raggiunto 5584 click dal 02/03/2005.

 

Consultata integralmente 10 volte.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.