|SEMICONDUCTOR NON-TRADITIONAL ENERGY SOURCES|
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).
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.
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..
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.
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.
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.
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.