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119-122 The working principle of solar cells and basic characteristics

Author: Source: Datetime: 2017-01-22 14:53:30

5.1.2 The working principle of solar cells and basic characteristics


5.1.2.1 The working principle of solar cells


Solar cells how to convert light energy into electricity? The following simple example to single-crystal silicon solar cells.

The working principle of solar cells is based on the semiconductor P-N junction of the photovoltaic effect. The photovoltage effect, in short, is an effect of the electromotive force and current produced when the state of charge distribution in an object changes as the object is exposed to light. When the sunlight or other light P-N junction of the semiconductor, it will appear on both sides of the P-N junction voltage, known as the photovoltaic voltage. This phenomenon, that is, the photovoltaic effect. So that P-N junction short circuit, it will produce current.

Figure-5-2-structure-diagram-of-silicon-atom

There are four electrons in the outermost electron shell of a silicon atom, as shown in Fig. 5-2. The outer electrons of each atom have a fixed position and are constrained by the nucleus. They are excited by external energy, such as by solar radiation, it will get rid of the nucleus of the bondage and become free electrons, while in its original place to leave a vacancy, the hole. Since the electrons are negatively charged, the holes are positively charged. In the pure silicon crystal, the number of free electrons and holes is equal. If boron, aluminum, gallium, or indium, which are capable of trapping electrons, are doped into the silicon crystal, a hole-type semiconductor, or P-type semiconductor, is formed. If the silicon crystal can be incorporated into the release of phosphorus, arsenic or antimony and other impurity elements, constitute the electronic semiconductor, referred to as N-type semiconductor. If these two kinds of semiconductor together, due to the diffusion of electrons and holes in the interface will be formed at the P-N junction, and the formation of built-in electric field on both sides of the junction, also known as barrier potential electric field. Since this resistance is particularly high, it is also referred to as a barrier layer. When the solar radiation PN junction, in the semiconductor atoms to obtain the light energy and the release of electrons, at the same time corresponding to the electron-hole pairs, and in the barrier electric field under the action of electrons are driven to the N-type region , The holes are driven to the P-type region, so that the N-type region has excess electrons, P-type area has excess holes.Then, in the vicinity of the PN junction with the barrier electric field in the opposite direction of the electric field, 5-3. A portion of the photo-generated electric field counteracts the barrier electric field, and the remainder causes the P-type zone to be positively charged and the N-type zone to be negatively charged; thus, a thin layer between the N-type area and the P-type area generates an electromotive force, . When the external circuit is connected, there is power output. This is the basic principle of P-N junction contact type monocrystalline silicon solar cell power generation. If the dozens, hundreds of solar cells in series, in parallel, the composition of solar modules, in the sunlight, you can get a considerable amount of power output power.
For a better understanding of the reader, here are a few semiconductor physics terms involved in the brief.

Figure-5-3-Energy-Levels-of-Solar-Cells

(1) The energy band
The energy band is an important physical concept used in solid-state theory to describe the state of an electron in a crystal. In an isolated atom, electrons can only move in some specific orbit, the electron energy in different orbit is different. Therefore, the atoms in the electron can only take some of the specific energy value, known as an energy level. Crystal is composed of a large number of regularly arranged atoms, in which each atom has the same energy level, due to interaction in the crystal energy into a slightly different energy level, looks like a band, known as the band. The outer electrons of the atoms are in a higher energy band in the crystal and the inner electrons are in a lower band. Electrons in the energy band are not around the nuclei of their closed-cell orbital motion, but for the common atoms in the whole crystal movement.
(2) Carriers
Carrier means the particle carrying the current. Conductor or semiconductor, its conductive role is through the charged particles in the electric field under the action of directional movement (the formation of current) to achieve, this charged particles, known as the carrier. The carriers in the conductor are free electrons.
There are two kinds of carriers in the semiconductor, that is, negatively charged electrons and positively charged holes. If the number of electrons in a semiconductor is much larger than the number of holes, electrons are important for conduction, and electrons are called majority carriers, and holes are called minority carriers. Conversely, the hole is called the majority carrier, electron called minority carrier.
(1)
A hole is a carrier in a semiconductor. It is equal to the charge of electrons, but the opposite polarity. The band occupied by electrons in the crystal is called full band or valence band, and the band not occupied by electrons is called empty band or conduction band. The gap between conduction band and valence band is called energy gap or band gap. If the electrons in the valence band get energy and jump to the conduction band with higher energy due to external effects (such as heat, light, etc.), a very interesting effect arises: after the electrons leave, they stay in the valence band The next slot. According to the principle of neutrality, this gap should be positively charged, its power and electronic equivalent. When the electrons near the acupoint move to fill the vacancy, it is equivalent to the vacant position to move in the opposite direction. Its role is very similar to the positive charge of the particle movement, usually called positive holes, referred to as holes. Therefore, under the action of the external electric field, the conduction in the semiconductor, not only from the electronic movement, but also includes the hole movement.
(2) donor
Where a pure semiconductor doped with the role of impurities is to provide conductive electrons, known as the donor impurity, referred to as donor. For silicon, if the incorporation of phosphorus, arsenic, antimony and other elements, their role is the donor.
(3) the receiver
Where a pure semiconductor doped with impurities in the role of accepting electronic, or provide holes, is called acceptor impurities, referred to as acceptors. For silicon, such as the incorporation of boron, gallium, aluminum and other elements, their role is to accept.
(4) P-N junction
In a semiconductor wafer, a P-type (hole-conductive) and a N-type (electronically conductive) part of a P-type and N-type interface are called P-N junctions by some processes. P-N junction with unidirectional conductivity, is the basic structure of the crystal diode, but also the core of many semiconductor devices. According to the process points, there are growth knot, alloy junction, diffusion junction, epitaxial junction and injection junction, etc. According to the material, there are many types of PN junctions: homojunction and heterojunction; .
(5) composite process
The recombination process in semiconductors can be divided into direct recombination, indirect recombination and Auger recombination. Direct recombination, that is, conduction band electron transition to the valence band and hole direct recombination. Indirect recombination refers to recombination of excess carriers through recombination centers formed by impurities and defects. There are four processes: electron capture, electron emission, hole trapping, and hole emission. Auger recombination refers to the recombination of electrons and holes, in addition to the energy released in the form of photons, but also energy transfer to the conduction band of another electron.
The following further on the monocrystalline silicon solar cells to illustrate the working principle, of course, will also involve a simple polycrystalline silicon solar cells.
As shown in Figure 5-4, to the surface solar spectrum, for example, the solar spectrum from ultraviolet to infrared, its wavelength is very wide. But with the ozone, water vapor, carbon dioxide and other absorption layer changes, about 5700K blackbody radiation continuous spectrum gradually become overlapping type. The solar spectrum is affected by the atmosphere and can be expressed by the atmospheric mass m. Where m = 1 as defined in Chapter 1, that is, m = 0 in the universe, m == l is equivalent to the normal incidence of sunlight to the surface, generally when the incident angle of sunlight on the surface as, then M == secas. The solid line in Figure 5-4 represents the spectrum of atmospheric mass 2.
Solar spectrum As shown in Figure 5-4, the solar cell absorbs the solar spectrum and generates an electron-hole pair (excess carrier), which is drawn from the outside to generate electricity. For the solar cell material, its light absorption spectrum can be used to express its main features. Fig. 5-5 shows the light absorption spectrum a (?) Of the single-crystal silicon. Silicon is an indirect transport semiconductor, through the absorption of light from the valence band to the conduction band excited electrons, the electron-hole pairs in the optical migration process in the presence of phonons (phonons that crystal lattice vibrational energy quantum), so the light The absorption coefficient is small, and the absorption spectrum rises steadily. As the light absorption coefficient is small, so to get a larger photocurrent, fully absorb the incident light of the silicon wafer thickness must reach 200μm. However, if the use of trapping technology, even if the wafer thickness as thin as 50μm can get a larger photocurrent.
When the incident light of the general wavelength λ enters the semiconductor, the formula of the carrier incidence G (λ, x) from the entrance plane to the depth x is:
G (λ, x) = a (λ) F (λ, x)

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