SEMICONDUCTOR NON-TRADITIONAL ENERGY SOURCES

PHOTOELECTRIC CONVERSION

The main physical phenomena during photoelectric conversion of solar energy
The photoelectric conversion of solar energy includes three main stages: • light absorption, • generation of charge carriers (electrons and holes), • separation of electrons and holes by internal field and generation of photocurrent in external circuit (in the load).

APSORPTION OF LIGHT
     In order to convert energy of light it must be absorped by solar cell. The main energy of solar radiation spread in wave length spectrum from 0.4 micrometer (quantum energy 3 eV) to 1 micrometer (quantum energy 1.24 eV). It means, that the fundamental absorption of semiconductor materials, which are used in SC, must cover this spectral region. An active layer has to have thickness enough for absorption solar photons. The region must not be transparent in the spectrum of photons> energy of which is transferring in electricity. Free photons path must be less, than the film thickness Lph(?) < 1/ α(λ)(α(λ) - коэффициент поглощения). Not all solar quantums are absorping, even if their energy higher, than energy gap of semiconductor hν> Eg. The unabsorped photons cause optical wastes of energy. There are two main reasons for optical wastes: reflection photons by solar cell surface and transmission thems by semiconductor active region. In order to reduce light reflection antireflection coverings are used. In order to reduce transparency of active region, the thickness of it made thicker and behind it made reflective surface. Photon absorption, reflection and passing through, demo

   The absorption of monochromatic дight is described by a Buger ratio:   Nф(λ)=N0exp[-α(λ)x] or Nф(λ)=N0exp[-x/Lф(λ)] , where  Nф(λ) - absorbed on depth x photon number, , N0 - falling on a frontal surface foton number, α(λ) - coefficient of absorption on given length of a wave, Lф(λ)=1/α(λ) - depth of absorption of light with given length of a wave (photon free run length). Further we shall consider absorption, only at  >Eg , because only them generate electricity.
    The dependence
α(λ) is determined by a structure of energy zones between which the optical transitions take place. The optical spectra of absorption may be divided into two large groups:
- spectra made by direct optical transitions between energy bands (without phonon participation).For instance in GaAs and CdTe. For this groop: α=Aexp(-Eg)3/2 , where A -some value with the week energy dependence.
- spectra caused by njn direct transitions between bands (with phonon paricipation).For instance, in Si, Ge.

GaAs absorption curve In right figure the spectral dependence of GaAs absorption coefficient  is shown. The continuous line corresponds to an experimental curve, dotted line corresponds to equation for direct transition (see above Eq.). The diagram shows, that near to energy Eg (long wave border) the factor of absorption is small, while for short-wave spectrum region it is extremely high.   

Generation of electron-hole pairs

In the photo-electric converter every absorbed in active area photon should create an electron-hole  pair, i.e. generate a charge 1.6 10-19coulomb. This charge  is generated by fotons with energy more Eg and less 3 Eg. The excess over Eg energy is waasted for conversion in electricety and transferd in heat. Therefore for increase of efficiency of conversion it is desirable, that every photon was absorbed in that place of an active layer, which width of the forbidden zone is approximately equal to photon energy.
Let's roughly estimate the maximal current, which can 1 m2 of solar converter generate in a terrestrial condition under solar radiation of the maximal intensity 1000   W/m2 (100   100 mW/cm2). We asumet, that all radiationis transformed in monochromatic one with wave length wave 1 micron (1,24 eV). Then on 1   cm2 should fall approximately 5·1017 photons.  (estimated account is very simple - capacity of radiation is divided into energy of quantums. The current approximately 0,08   A/cm2 corresponds to this value (5 10-17[1/(cm2]·1,6·10-19[C] = 0.08 [A/cm2], or 80   A/cm2. This is a high meaning. At the best modern photo-electric converters this value almost twice is less.
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Generation of electrons and holes in SC and their separation by pn-junction field The separation of electrons and holes in standard photoconverters is carried out by an pn-junction electrical field (see figure on the right). Only  carriers, which are generated not far from a barrier than their diffusion length, are separated and make contribution to current. Others generated caariers are recombine and their energy onle heats converter.It imposes the certain requirements on diffusion length of the photogenerated carriers.
According to a principle of a superposition the generated photocurrent must be equal to the sum of currents from three areas of an element: from n-type region, from p-type region, from barrier  region (depleted by carriers).
  Not all carriers created by light, participate in formationof a current.Their some part recombine in volume or on a surface. For the characteristic of efficiency of participation of photocarriers in creation of a current enter collection factor Q, representing the relation of number of carriers which are taking paaaart in an external circuit ( i.e. experimentally measured of short circuit  current), to complete number of carriers generated in unit of time in the semiconductor volume of solar cell:

It is asumed, that all carriers generated in dep;eted layer are collected.

Maximal solar cell efficiency

SC effeicency equals ratio of maximal load power to the power of falling solar cell radiation:

,

where Nph - complete photon number in a solar spectrum, Еср -  their average energy, Ump - voltage on the optimal loadшng. The maximal efficiency depends on width of the forbidden zone of semiconductor.

Расчётная зависимость к.п.д. однокаскадного СЭ от ширины запрещенной зоны п.п. материала

Energy losses in solar cell

Optical and electric losses limit maximum achivable efficiency (33%). Conventional single junction silicon solar cells today exhibit efficiencies between 11 - 25 %. The main external losses: reflection from front surface of the cell, shadowing by electrical contacts, and ohmic losses at the electrode  leads. The main processes, which reduce solar cell are shown on the right picture. 


The main losses(see figure on the right):
1 - thermalization loss,
2 - dissipation loss in barrier,
3 - loss in contacts,
4 - recombination losses.

Spectral characteristic of photocurrent
The SC spectral characteristic is describing dependence of a photocurrent of a wave  length. It is defined by the spectral characteristic of absorbed photons and efficiency of their division by an electrical field of a barrier. The photoreceivers frequently are characterized by ratio of photocurrent to
a monocromatic power of radiation, which generate current. S(λ)=Iф/Pпад [А/Вт]. , Ni- the number of separated electron-hole pairs. Accordingly it is possible to write down the following equaations, where Q - collection coefficient factor, which equals to the ratio of absorbed photons number to number of separated electron-hole pairs.

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