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Outdoor solar lights' photovoltaic effect and characterization parameters

2021-08-04 19:02:23

Outdoor solar light's photovoltaic effect

According to the conductivity, objects can be roughly divided into conductors, semiconductors and insulators. The effect of a beam of solar light on semiconductors is very different from that on other objects. There are many free electrons in metals, and the change of electrical conductivity caused by solar light can be completely ignored; The insulator can not excite more electrons to participate in conduction at very high temperature; The binding force of semiconductor with conductivity between metal and insulator on electrons in the body is far less than that of insulator. The photon energy of visible solar light can excite it from binding to free conductive state, which is the photoelectric effect of semiconductor. When there is an electric field in the local area of the semiconductor, the photogenerated carriers will accumulate, which is very different from that without an electric field. The photoelectric voltage will be generated on both sides of the electric field due to charge accumulation, which is the photogenerated volt effect, referred to as photovoltaic effect. Let's talk about semiconductors in detail.

Pure semiconductor materials are called intrinsic semiconductors. When v-group impurity elements (phosphorus, arsenic, etc.) are doped into the intrinsic semiconductor material, and the impurity provides electrons so that the electron concentration is greater than the hole concentration, n-type semiconductor material is formed, and the impurity is called donor; At this time, the electron concentration is greater than the hole concentration, which is the majority carrier, while the hole concentration is lower, which is the minority carrier. Similarly, group III impurity elements (boron, etc.) are doped into the semiconductor material, so that the hole concentration is greater than the electron concentration, and the crystalline silicon becomes a p-type semiconductor. For example, taking silicon as an example, adding a little boron, aluminum, gallium and other impurities into high-purity silicon is a p-type semiconductor; Adding a little phosphorus, arsenic, antimony and other impurities is n-type semiconductor. In n-type semiconductors, nonequilibrium electrons are called nonequilibrium majority carriers, and nonequilibrium holes are called nonequilibrium minority carriers. The opposite is true for p-type semiconductors. In semiconductor devices, nonequilibrium minority carriers often play an important role.

Outdoor solar lights' photovoltaic effect and characterization parameters 1

Both n-type semiconductor materials and p-type semiconductor materials are electrically neutral when they exist independently. The charge of ionized impurities is equal to the total charge of carriers. When two kinds of semiconductor materials are connected together, for n-type semiconductor materials, electrons are most carriers with high concentration; In p-type semiconductors, electrons are minority carriers with low concentration. Due to the existence of concentration gradient, electrical diffusion is bound to occur, that is, electrons diffuse from high concentration n-type semiconductor material to low concentration p-type semiconductor material, and a PN junction is formed at the interface between n-type semiconductor and p-type semiconductor. Near the PN junction interface, the electron concentration in the n-type semiconductor gradually decreases, while the electrons diffused into the p-type semiconductor compound with most carrier holes and disappear. Therefore, near the interface of the n-type semiconductor, due to the decrease of most carrier electron concentration, the number of positive charges of ionized impurities is higher than the remaining electron concentration, and a positive charge region appears. Similarly, in p-type semiconductors, due to the diffusion of holes from p-type semiconductors to n-type semiconductors, the number of negative charges of ionized impurities near the interface is higher than the remaining hole concentration, and a negative charge region appears. This positive and negative charge region is called the space charge region of PN junction, forming an electric field from n-type semiconductor to p-type semiconductor, which is called built-in electric field, also known as barrier electric field. Because the resistance here is particularly high, it is also called barrier layer. This electric field resists the diffusion of multipons in the two regions and helps the drift of minority electrons until the diffusion current reaches equilibrium when it is equal to the drift current, and a stable built-in electric field is established on both sides of the interface. The so-called diffusion means that under the influence of an external electric field, a randomly moving free electron has an accelerated motion in the direction opposite to the electric field, and its velocity increases continuously with time. In addition to drift motion, carriers in semiconductors can also flow due to diffusion. When any particles, such as gas molecules, are too concentrated, they will disperse themselves if they are not limited. The basic reason for this phenomenon is the irregular thermal motion of these particles. With the progress of diffusion, the space charge region is widened and the internal electric field is enhanced. Because the role of the internal electric field is to hinder the multi son diffusion and promote the minority son drift, when the diffusion motion and drift motion reach a dynamic balance, a stable PN junction will be formed. The PN junction is very thin with few electrons and holes, but there are positively charged ions near the n-type side and negatively charged ions near the p-type side. Due to the lack of carriers in the space charge region, PN junction is also called depletion layer region.

When the semiconductor with PN junction is illuminated, the number of electrons and holes increases. Under the action of the local electric field of the junction, the electrons in P region move to n region and the holes in N region move to p region. In this way, there is charge accumulation at both ends of the junction and a potential difference is formed.

Outdoor solar lights' photovoltaic effect and characterization parameters 2

The cell that directly converts light energy into electric energy by using photovoltaic effect is called solar cell (solar cell for short). The so-called photovoltaic effect is the phenomenon that electromotive force is generated at both ends after the system absorbs light energy when light of appropriate wavelength is irradiated on the semiconductor.

When the PN junction is illuminated, both the intrinsic and extrinsic absorption of photons will produce photogenerated carriers, but only a few carriers excited by the intrinsic absorption can cause the photovoltaic effect. Because the photogenerated holes in the p region and the photogenerated electrons in the N region belong to multipons, they are blocked by the potential barrier and cannot cross the junction. Only the photogenerated electrons in the p region and the photogenerated holes in the N region and the electron hole pair (minority) in the junction region can drift through the junction under the action of the built-in electric field when they diffuse near the junction electric field. Photogenerated electrons are pulled to n region and photogenerated holes are pulled to p region, that is, electron hole pairs are separated by built-in electric field. This leads to the accumulation of photogenerated electrons near the boundary of N region and photogenerated holes near the boundary of P region. They generate a photogenerated electric field opposite to the built-in electric field of the thermal equilibrium PN junction, and its direction is from P region to n region. This electric field reduces the potential barrier, that is, the photogenerated potential difference, p-terminal positive and N-terminal negative. Therefore, the junction current flows from P region to n region, and its direction is opposite to the photogenerated current.

Outdoor solar lights' photovoltaic effect and characterization parameters 3

In fact, not all the generated photogenerated carriers contribute to the photogenerated current. Set n-zone hollow hole in service life τp The time diffusion distance of P is Lp, and the lifetime of electrons in P region is τp The time diffusion distance of n is Ln. Ln+Lp=L is much larger than the width of the PN junction itself, so it can be considered that the photogenerated carriers generated within the average diffusion distance l near the junction contribute to the photogenerated current, while the electron hole pairs whose positions are more than l away from the junction region will all compound in the diffusion process and have no contribution to the photoelectric effect of the PN junction.

In order to understand the above process, the following briefly introduces the concepts of download stream lifetime, mobility and diffusion length.

Carrier lifetime refers to the average lifetime of nonequilibrium carriers before recombination, which is the abbreviation of nonequilibrium carrier lifetime. In the case of thermal equilibrium, the generation rate of electrons and holes is equal to the recombination rate, and their concentrations maintain equilibrium. Under the action of external conditions (such as solar light), additional non-equilibrium carriers, namely electron hole pairs, will be generated; After the external conditions are cancelled, because the recombination rate is greater than the generation rate, the non-equilibrium carriers will gradually disappear and return to the thermal equilibrium state. The decay law of nonequilibrium carrier concentration with time generally obeys the exponential relationship. In semiconductor devices, the nonequilibrium minority carrier lifetime is called minority carrier lifetime for short.

The recombination process can be roughly divided into two types: the direct transition of electrons between the conduction band and the valence band, resulting in the disappearance of a pair of electron holes, which is called direct recombination; Electron hole pairs may also be combined through the energy level in the forbidden band (in recombination), which is called indirect recombination. The minority carrier lifetime of each semiconductor is not a fixed value, it will vary greatly with the chemical composition and crystal structure. Mobility refers to the average drift velocity of carriers (electrons and holes) under the action of unit electric field, that is, a measure of the velocity of carriers under the action of electric field. The faster they move, the greater the mobility; Slow movement and low mobility. In the same semiconductor material, the mobility of different carrier types is also different. Generally, the mobility of electrons is higher than that of holes. Under the action of a constant electric field, the average drift velocity of carriers can only take a certain value, which means that carriers in semiconductors are not accelerated without any resistance. In fact, in the process of its thermal movement, carriers constantly collide with lattice, impurities and defects, and change their movement direction irregularly, that is, scattering occurs. Inorganic crystals are not ideal crystals, while organic semiconductors are essentially amorphous, so there are lattice scattering and ionized impurity scattering, so the carrier mobility can only have a certain value.

Because minority carriers have a certain lifetime, that is, minority carrier lifetime. Therefore, in the process of diffusion, the minority carriers will diffuse and compound at the same time. After a certain distance, the minority carriers will disappear, which is the so-called diffusion length.

Solar light absorption of semiconductors. The absorption of solar light by semiconductors is mainly determined by the band gap of semiconductor materials. For semiconductors with a certain band gap, low-energy photons with low frequency have a small degree of light absorption, and most of the light can penetrate; As the frequency increases, the ability to absorb light increases sharply. In fact, the light absorption of semiconductors is determined by various factors. Here, only the transition between electron energy bands used in solar cells is considered. Generally, the wider the band gap, the smaller the absorption coefficient of a certain wavelength. In addition, the absorption of light also depends on the density of states of conduction band and valence band.

When different types of semiconductors are in contact (forming PN junctions) or semiconductors are in contact with metals, diffusion occurs due to the concentration difference of electrons (or holes) and a potential barrier is formed at the contact. Therefore, this kind of contact has single conductivity. Using the unidirectional conductivity of PN junction, semiconductor devices with different functions can be made, such as diode, triode, thyristor and so on. PN junction also has many other important basic properties, including current voltage characteristics, capacitance effect, tunnel effect, avalanche effect, switching characteristics and photovoltaic effect. Current voltage characteristics, also known as rectifier characteristics or volt ampere characteristics, are the most basic characteristics of PN junction, while solar photoelectric conversion is the photovoltaic effect generated by the built-in electric field of PN junction.

Characterization parameters of solar cells

The working principle of solar cells is based on photovoltaic effect. When light irradiates the solar cell, a photogenerated current IPH from n region to p region will be generated. At the same time, due to the characteristics of PN junction diode, there is a forward diode current ID, which is opposite to the photogenerated current from P region to n region. Therefore, the actually obtained current I is

I = Iph - ID = Iph - I0 [exp(qUD/nkBT)-1]

Where, UD is the junction voltage; I0 is the reverse saturation current of the diode; Iph is a photogenerated current proportional to the intensity of incident light, and its proportional coefficient is determined by the structure and material characteristics of solar cells; N is the ideal coefficient (n value), which is a parameter representing the characteristics of PN junction, usually between 1 and 2; Q is the electron charge; kB is Boltzmann constant; T is the temperature.

If the series resistance RS of the solar cell is ignored, UD is the terminal voltage U of the solar cell, then

I = Iph - I0 [exp(qU/nkBT)-1]

When the output end of the solar cell is short circuited, U= 0 (UD ≈ 0), the short-circuit current can be obtained from the formula

Isc=Iph

In short, the short-circuit current is the maximum current measured when the solar cell is short circuited from the outside, expressed in Isc. It is the maximum current that the photocell can get in the external circuit under a certain light intensity. Without considering other losses, the short-circuit current of the solar cell is equal to the photogenerated current Iph, which is directly proportional to the intensity of the incident light.

When the output terminal of the solar cell is open circuit, I = 0, and the open circuit voltage can be obtained from the formula

Uoc=nkBT/q*In(Isc/I0+1)

Simply put, the open circuit voltage means that the illuminated solar cell is in the open circuit state, and the photogenerated carriers can only accumulate at both ends of the PN junction to generate the photogenerated electromotive force. At this time, the potential difference measured at both ends of the solar cell is represented by the symbol Uoc.

When the solar cell is connected to load R, load R can range from zero to infinity. When the load Rm maximizes the power output of the solar cell, its corresponding maximum power Pm is

Pm=ImUm

Where Im and Um are the optimum working current and the optimum working voltage respectively.

When the solar cell is connected to the load, a current flows through the load, which is called the working current of the solar cell, also known as load current or output current. The voltage at both ends of the load is called the working voltage of the solar cell. The working voltage and current of the solar cell change with the load resistance. The volt ampere characteristic curve of the solar cell can be obtained by making a curve of the working voltage and current corresponding to different resistance values.

Outdoor solar lights' photovoltaic effect and characterization parameters 4

If the selected load resistance value can maximize the product of output voltage and current, the maximum output power is obtained, which is represented by the symbol Pmax. The working voltage and current at this time are called the optimal working voltage and optimal working current, which are represented by symbols Ump and Imp respectively.

The ratio of the maximum power Pm to the product of UOC and ISC is defined as the filling factor FF, then

FF=Pm/UOCISC=UmIm/UOCISC

FF is an important characterization parameter of solar cell. The larger the FF, the higher the output power. FF depends on the incident light intensity, the band gap width of the material, the ideal coefficient, series resistance and parallel resistance.

Fill factor FF is an important parameter to measure the output characteristics of solar cells. It is the ratio of the maximum output power to the product of open circuit voltage and short circuit current. It represents the maximum power output of the solar cell with the best load. The greater its value, the greater the output power of the solar cell. The value of FF is always less than 1, which can be given by the following empirical formula

FF=UOC-In(UOC+0.72)/UOC+1

Where UOC is the normalized open circuit voltage.

The photoelectric conversion efficiency of a solar cell refers to the maximum energy conversion efficiency when the optimal load resistance is connected to the external circuit, which is equal to the ratio of the output power of the solar cell to the energy incident on the surface of the solar cell. The conversion efficiency of photocell to convert light energy directly into useful electric energy is an important parameter to judge the battery quality η express

η=Pmax/Pm=ImpUmp/Pm=ImpUmp/FFUOCISC

That is, the ratio of the maximum output power of the battery to the incident light power.

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