- Structure of solar cell
Taking crystalline silicon solar cells as an example, crystalline silicon solar cells work with large-area p-n junctions made of silicon materials. Generally, the structure of N + / P homojunction is adopted, that is, a very thin heavily doped n-type layer is made on p-type silicon wafer by diffusion method, and then a metal grid line is made on the n-type layer as the front contact electrode. A metal film is also made on the whole back as a back contact electrode. In order to reduce the reflection loss of light, the upper surface is generally covered with an antireflection film.
- Working principle of solar cell
When light shines on a semiconductor, photons provide energy to electrons, which will transition to a higher energy state. Among these electrons, the available electrons in practical photoelectric devices are valence band electrons, free electrons or holes, and electrons existing in impurity energy levels.
The electrons available to solar cells are mainly valence band electrons. The absorption of light determined by the energy transition of valence band electrons absorbing photons to the conduction band is called intrinsic absorption or intrinsic absorption.
The photovoltaic effect of semiconductor p-n junction is the basis of solar cell energy conversion. When the light shines on the semiconductor photovoltaic device, electron hole pairs are generated in the device. The carriers generated near the p-n junction in the semiconductor are not combined and can reach the space charge region. Attracted by the built-in electric field, the electrons flow into the N region and the holes flow into the p region. As a result, the N region stores excess electrons and the p region has excess holes. They form a photogenerated electric field opposite to the built-in electric field near the p-n junction. In addition to partially counteracting the effect of the barrier electric field, the photogenerated electric field also makes the p region positively charged and the N region negatively charged. The electromotive force is generated in the thin layer between the N region and the p region, which is the photovoltaic effect. At this time, if the external circuit is short circuited, there will be a photocurrent in the external circuit proportional to the incident light energy. This current is called short-circuit current (ISC). On the other hand, if the two ends of the p-n junction are open circuited, the Fermi level in the N region is higher than that in the p region because electrons and holes flow into the N region and P region respectively, and a potential difference VOC is generated between the two Fermi levels. This value can be measured and called open circuit voltage. Since the p-n junction is in the forward bias at this time, the above short-circuit photocurrent is equal to the forward current of the diode, and thus the value of VOC can be determined.
- Equivalent circuit
In order to describe the working state of the battery, the battery and load system are often simulated by an equivalent circuit. Under constant illumination, the photocurrent of a working solar cell does not change with the working state. It can be regarded as a constant current source in the equivalent circuit. Part of the photocurrent flows through the load RL and establishes the terminal voltage V at both ends of the load. In turn, it is positively biased to the p-n junction diode, causing a dark current IBK opposite to the photocurrent direction. However, due to the contact between the front and back electrodes and the certain resistivity of the material itself, additional resistance will inevitably be introduced into the base region and the top layer. When the current flowing through the load passes through them, it will inevitably cause loss. In the equivalent circuit, their total effect can be expressed by a series resistance rs. Due to the leakage at the edge of the battery and the leakage of the metal bridge formed at the microcracks and scratches of the battery when making the metallized electrode, part of the current that should have passed through the load is short circuited. This effect can be equivalent to a parallel resistance RSH, in which the dark current is equal to the product of the total area at and JBK, and the photocurrent IL is the product of the effective light receiving area AE and JL of the battery. At this time, the junction voltage is not equal to the terminal voltage of the load.
- Basic characteristics of solar cells
4.1 short circuit current
The short-circuit current of a solar cell is equal to its photogenerated current. The most convenient way to analyze the short-circuit current is to divide the solar spectrum into many small spectral regions. Each small region has only a narrow wavelength range. Calculate the current corresponding to the spectrum of each small region. The total short-circuit current of the battery is the sum of the full spectrum contribution.
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4.2 open circuit voltage
In weak sunlight, the open circuit voltage of silicon solar cells changes approximately linearly with the intensity of light. When there is strong sunlight, VOC is directly proportional to the logarithm of the intensity of the incident light. Figure 3-11 shows the relationship between ISC and VOC of representative silicon and GaAs solar cells. Compared with Si and GaAs, because the band gap of GaAs is wide, the I0 value is several orders of magnitude smaller than that of Si, and the VOC value of GaAs is about 0.45V higher than that of Si. If the quality of semiconductor p-n junction is very good, the wider the band gap, the greater the open circuit voltage VOC.
4.3 . conversion efficiency
The conversion efficiency represents the maximum energy conversion efficiency obtained when the external circuit is connected with the optimal load resistance R, that is, the ratio of the maximum power output of the battery to the incident power.
4.4. Effects of temperature and irradiance on output characteristics of solar cells
The diffusion length of minority carriers increases slightly with the increase of temperature. Therefore, the photogenerated current JL also increases with the increase of temperature, and the VOC decreases sharply with the increase of temperature. The filling factor decreases, so the conversion efficiency decreases with the increase of temperature. With the increase of irradiance, the short-circuit current increases linearly and the maximum power increases continuously. Therefore, the appropriate type of solar cell module should be selected according to the environment of different regions.
5 . Factors affecting the conversion efficiency of solar cells
(1) On the other hand, the short-circuit current density JSC decreases with the increase of energy band width eg. As a result, a peak in solar cell efficiency can be expected at a certain eg.
(2) The efficiency increases with the increase of temperature η Drop. ISC is very sensitive to temperature T, and temperature also plays a major role in VOC. For Si, for every 1 ℃ increase in temperature, VOC decreases by 0.4% of the room temperature value, η Thus, it is reduced by about the same percentage. For example, the efficiency of a silicon cell is 20% at 20 ℃ and only 12% when the temperature rises to 120 ℃. Another example is GaAs battery. When the temperature increases by 1 ℃, VOC decreases by 1.7mv or 0.2%
(3) Recombination lifetime of photogenerated carriers for semiconductors of solar cells, the longer the recombination lifetime of photogenerated carriers, the greater the short-circuit current ISC. In indirect band gap semiconductor materials such as Si, the distance from the p-n junction is 100 μ A considerable number of carriers can also be generated at m if the photogenerated carrier lifetime at these locations can be greater than 1 μ s. It can be collected by the p-n junction and transmitted to the external circuit. In direct band gap materials such as GaAs or guzs, the composite life as long as 10ns is long enough. The long lifetime of carriers will also reduce the dark current and increase VOC. The key to achieve long life is to avoid the formation of composite centers in the process of material preparation and battery production. In the processing process, appropriate and frequent related process treatment can remove the composite center and prolong the service life.
(4) Light intensity focuses sunlight on the solar cell, which can make a small solar cell produce a large amount of electric energy. Assuming that the light intensity is concentrated by x times, the input power and JSC per unit battery area will increase by x times, and the VOC will also increase by (kt / Q) INX times. Therefore, the increase of output power will greatly exceed x times, and the result of focusing also improves the conversion efficiency.
(5) Another factor that doping concentration and profile distribution have a significant impact on VOC is semiconductor doping concentration. The higher the doping concentration, the higher the VOC. A phenomenon called heavy doping effect has attracted more attention in recent years. At high doping concentration, due to the deformation of energy band structure and the change of electronic statistical law, Nd and Na in all equations should be replaced by (nd) eff and (NA) eff.
At present, the doping concentration is about 1016cm-3 in silicon solar cells and 1017cm-3 in solar cells made of direct band gap materials. In order to reduce the series resistance, the doping concentration in the front diffusion region is often higher than 1019cm-3, so the heavy doping effect is more important in the diffusion region.
(6) The surface recombination rate with low surface recombination rate helps to improve ISC and VOC due to the decrease of I0. The recombination rate of the front surface is difficult to measure and is often assumed to be infinite. A battery design called back surface field (BSF) is to diffuse a p + additional layer on the back of the battery before depositing metal contact.
(7) The series resistance exists in any actual solar cell, and its source can be the lead, metal contact grid or battery resistance. However, in general, the series resistance mainly comes from the thin diffusion layer. The current collected by p-n junction must pass through the surface thin layer and then flow into the nearest metal wire. This is a route with resistance. Obviously, the series resistance can be reduced through the dense distribution of metal wires. The effect of a certain series resistance Rs is to change the position of the I-V curve.
(8) Metal grid lines and metal grid lines with light reflected on the front surface cannot pass through sunlight. In order to maximize ISC, the area occupied by metal grid lines should be minimized. In order to reduce RS, the metal grid line is generally made into a dense and thin shape. Because of the reflection of sunlight, not all light can enter silicon. The reflectivity of bare silicon surface is about 40%. The use of antireflection film can effectively reduce the reflectivity.
The conversion efficiency of a solar cell is the ratio of its output power to its input power. In order to obtain high efficiency, it is desirable to have large short-circuit current, high open circuit voltage and large filling factor. If the solar cell is made of materials with small band gap (eg), the short-circuit current is large. Good manufacturing technology and good battery design can also increase the short-circuit current due to the minimum carrier recombination. If the solar cell is made of a material with large eg, it has a high open circuit voltage. The filling factor is a measure of the steepness at the inflection point of the I-V curve, which can be reduced by series resistance. Generally, when the open circuit voltage is high, FF is also large. The conversion efficiency increases with the increase of light intensity and with the decrease of temperature. Solar cells made of materials with an eg value of 1.2 ~ 1.6ev are expected to achieve the highest efficiency. Direct band gap semiconductors are preferred for thin-film batteries because they can absorb photons near the surface.