Perovskite Materials for Energy and Environmental Applications. Группа авторов

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alt="Schematic illustration of the different recombination processes in semiconductors."/>

      Radiative recombination may be understood as the opposite of optical generation. This recombination takes place if a free electron drops out of the conduction band and recombine with a free hole in the valance band and a photon of energy equivalent to the difference in starting and ending energy states is released. For direct bandgap semiconductors, radiative recombination is very significant, but not as relevant for indirect semiconductors such as silicon, since a photon must also be absorbed or expelled for an electron to complete the transmission.

      Due to radiative processes the net recombination rate is expressed as:

      (1.52)image

      Where B is a material constant.

      For n-type semiconductors under low injection (p0pn0) the net rate of recombination can be given as:

      (1.53)image

      (1.54)image

      B. Auger recombination

      Auger recombination process is relatively similar to the process of radiative recombination with the exception that the transitional energy is transferred to an electron or hole that is excited in the respective conduction or valance band (Figure 1.1). Due to Auger recombination the net recombination is expressed as:

      (1.55)image

      Where Cn and Cp are the electron and hole Auger coefficients, respectively.

      The most widely used Auger coefficient values were calculated by Dziewior and Schmid (Cn = 2.8 × 10–31 cm6s–1 and Cp = 0.99 × 10–31 cm6s–1) for Silicon having a doping concentration more than 5 × 1018 cm–3.

      Auger recombination is relevant in semiconductors with high carrier concentration or also under high level injection (e.g. concentrator solar cells).

      C. Shockley-Read-Hall recombination (SRH)

      The Shockley-Read-Hall recombination process generally occupies the net rate of recombination in low quality materials having high density of defects.

      For single-energy level traps the SRH rate of volume recombination, USRH, is expressed as:

      (1.56)image

      Where τn0 and τp0 are the electron mean carrier lifetime and hole mean carrier lifetime, respectively, and are associated to the charge carrier thermal velocity, vth, the concentration of defect, NT, and the capture cross-sections of electron and capture cross-sections of hole of the specific defect, σn and σp as

image

      (1.57)image

      According to definition, n1 and p1 are the free-electron and free-hole concentrations when the Fermi level (EF) lies at the trap energy level (ET).

      Actually, all three recombination processes deliberated here occur simultaneously.

      The net recombination can be found by addition of the three rates of recombination as:

      (1.58)image

      The ASA program is intended to simulate devices based on amorphous and crystalline semiconductors. The ASA program resolves one dimensional elementary semiconductor equations (Poisson equation and electron continuity equation and hole continuity equation) and utilizes variables such as concentration of free electrons, n, and that of holes, p, and electrostatic potential. Additionally, in order to describe explicit device operation and optoelectronic properties of the material it employs several advanced physical.

      The ASA one dimensional (1-D) device simulator is very appropriate for simulating thin film structures of silicon solar cells. The program fulfils the usual requirements for simulating thin-film solar cell devices [3].

      From the optical perspective, in order to achieve greater light conversion efficiencies, both the effective use of solar spectrum and light distribution in the solar cells are significant. Light regulation is accomplished in thin film solar cells through the application of techniques for light trapping. The techniques for trapping of light are focused on substrates with textured surface being introduced and using special (back-) reflector layers. The substrates with textured surface provide rough interfaces to the solar cells. The light incident at rough interfaces is scattered and the simulation of solar cells must consider the rough interface scattering mechanisms so as to precisely assess the generation profile for charge carriers within the solar cell. It needs the design of optical models which take into account propagation of both coherent nonscattered (specular) light and incoherent distributed (diffused) light through a system.

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