Outdoor applications of solar panels are ubiquitous and its lifetime can last up to 30 years or more, especially solar systems installed on the rooftop or on the ground. The obverse side of the solar panel has to expose to rainy, wet or any other heavy weathers, so they should not break, short circuit, etc.

Since we use the solar panels outdoors, then everyone will have such a question, are solar panels waterproof?

First, we need to understand the structure of the solar panel. It is mainly composed of solar cells and related laminated materials. The solar cell is not waterproof, because it has circuits and wires on its surface. If some water is on the cell, short circuits will occur. The cells will be damaged, etc. But do not worry, the solar cells are encapsulated in laminated materials or in some special resins.

Therefore, the waterproofing function is mainly reflected in the outer laminated technology and edge treatment. Generally, we use the solar panels in the large on-grid system are basically covered by tempered glass, and the glass material is waterproof. And the back sheet is usually made from a durable polymer-based material. That means the solar panel has to be waterproof in order to keep producing power for more than 30 years. There are also some other materials besides tempered glass, such as PET, ETFE, which are also waterproof.

So, do not worry, your solar panels are waterproof.

If your solar panel is still under warranty, and if rare water damage occurs, the manufacturer or the brand company will take it. And sometimes, your solar panel manufacturer will not only pay for the new panel but also will pay for shipping and reinstallation. Most equipment is warranted for more than 10 years.

There are some best sellers on Amazon and eBay.

12V Flexible Water proof Solar Panel

This 20 Watt 12 Volt semi-flexible solar panel is made from the highest efficiency solar cells that can up to 23.7%. Lightweight and bendable.

Waterproof Solar Panel Car Battery Charger

I must say it is the best solar car battery charger for drivers. Comes with alligator clip and Car mounted charger makes you can go anywhere you do!

Waterproof Solar Backpack

Built-in 7 W solar panel with USB port, a must have for any outdoor enthusiast and business traveler.

Solar panels are not only waterproof – but also protect against most weather-related stresses. There is no need to worry about snow in cold climates, and solar panels can withstand considerable weight without collapsing. The colder weather can even increase the production of solar energy. Even extreme weather such as hurricanes and hail storms will not have much impact on solar panels – because of the proper shelf systems prevent strong wind damage, and durable glass makes it can be used in most hailstorms.

If you have any questions or anything concerns about this topic, please send an email to us, we are all ears!

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What are the 24V Solar Panels?

The 24V solar panel means that the optimum output voltage of the solar panel is 24V at STC. Some manufacturers name the optimum voltage as Vmax. Here 24v is different from open circuit voltage (Voc). The optimum working voltage can be limited with a resistor that takes the absolute Voc edge off. Hovall can customize its power according to the specific application. The power can be large and small, there are not only many 24V solar panels below 1W on the market but also many panels larger than 100 Watt.

Charging 12V Battery with 24V Solar Panel?

Yes, the 24 Volt Solar panels can charge and maintain all of the 12V batteries, but you should have the solar controllers between the solar panels and the batteries.

Why do we use a solar panel higher than 12V to charge a 12V battery? Because of under lower sunlight conditions you will still get the minimum voltage required for your load. As you know sunlight comes and go, birds, clouds, shadings, buildings etc cause the panel’s output to fluctuate and thus your loads would likely stop and start repeatedly. So, the rule of thumb, 12V Solar System has an EQ cycle at about 15 Volts, that means the 12v battery can hold up to about 15 Volt, so, you need a solar controller to keep the battery at about 15 Volts all times. If without a solar controller, it will shorten the battery lifespan, that is the reason why we use solar panels between 18v and 24 volts to charge 12V batteries.

The same thing, 24V Solar System has an EQ at about 30 Volt, you should have at least 30V to power the solar controller for a 24 Volt system. In this paragraph, I believe you already learned about why a 24V Solar Panel CANNOT charge the 24 Volt Battery. And you need more voltages than the battery to pump a charge into them.

What is the open circuit voltage (Voc) of the 24V solar panel? Actually, this needs to be detected by professional testing equipment. And the open circuit voltage of the same power or the same size solar panel can be different because of the efficiency of the solar cells and loads.

Here are some popular 24V solar panels for your reference.

1, 300 Watt 24V Solar Panel

The maximum power is 300W, people use the 300 Watt mono solar panels in multiple panel solar arrays for home or commercial buildings. Complete a large solar system with this 300W 24V solar panel, perfect for all on or off-grid projects.

2, 200W 24V Solar Panel

The maximum power is 200 Watt, the optimum working voltage is 24 Volts at STC, and the Voc is 30V. It is a great idea to build a large scare of on-grid or off-grid solar projects with 200 Watt solar panels.

3, 24V Solar Panel Battery Charger

This solar charger comes with Plug & Play adapters, it is a great device to charge your trolling motor for your Boat, Marine, and RVs.

4, 24V Solar Panel Kit

It is pre-wired solar kit comes with Solar Panels, Solar Controller, Cables, Connectors. This solar kit is great for solar newbies, easy to use and install. Your 24V off-grid best choice to CHARGE 24 V batteries.

Hovall 24V Monocrystalline Solar Panels generate highest efficiency conversations and offer great performance to meet your solar energy needs, Hovall designs solar panels for all applications and always come consumer ready to use or install, Hovall is your excellent choice.

If you have any questions about this topic or anything we can do, please do not hesitate to let us know, we are all ears.

The post Something you should know about 24V Solar Panel appeared first on Hovall.

Why we make it folding? Everyone knows that most of the existing solar panels are coming with the fixed-planar structures, we usually install them out of doors or on the roof of the house. After installing the solar panels, they are basically fixed in position and cannot be disassembled, especially in the case of severe weather or relocating, the solar panels cannot be recycled in time, because it easily damages the solar panels and causes huge economic losses. So for the convenience of use, there are many foldable solar panels that are used in different applications on the market.

How to choose the right power Foldable Solar Panels?

Let us try to choose a solar panel to power a 6 watt device for 5 hours a day, we first do a math that the 6W device will consume 5 x 6 = 30 watts each day, so the solar panel will need to produce 30 watts per day. Let’s assume six hours of sunlight so the solar panel needs to generate 5W per hour, consider the loss of the solar power, so a 6 watt solar panel will do the job. There are many devices and many watts to choose from, here are some best sellers on the market.

200 Watt Foldable Solar Panel

The power of this solar panel is 200W, unfolded size is very large, it looks like a blanket, but it can be put into the backpack after folded.

100 Watt foldable Solar Panel

From the personal aspect, the 100W solar panel is the most practical and suitable for most devices, it is good enough for outdoor use. It comes with dual outputs and it is also the best seller on the market.

60 Watt Foldable Solar Panel

With a 60W power, it can charge most of the 12v lithium and lead acid batteries, such as laptops, tablets, lights, etc.

5 Watt Foldable Solar Panel Charger

This is a very small solar panel, usually it comes with USB 5V output to charge the SMALL devices that voltage less than 4v, such as the Android smartphones, USB lamps, flashlights, etc. But 5V solar charger is not good for iPhone devices.

300 Watt Foldable Solar Panel

300W is too large but it is good to have it as a part of solar systems for home use. It is suitable for TV, Small Air Conditioner, Refrigerator, Electric Cooker, etc.

How to choose the right voltage Portable Foldable Solar Panels?

To select a solar panel to power a 3.7v mobile phone, first of all, you need to know the battery specifications of your mobile phone, the usual mobile phone voltage is 3.6v-4v so we choose the 5V output to charge this mobile phone, in the same way, we use 18v solar panels to charge a 12v battery. For larger solar panels, you need a solar regulator or solar controller to control the current, or it will cause damage to your battery, the solar regulator will ensure the battery is not overcharged all you need is the sun to produce clean and free solar energy.

Foldable Solar Panel 12V

12 Volt solar panel is good for charging 6-9V devices, such as Lamps, LED Lights, etc.

18 Volt Foldable Solar Panel

18v solar panel is good for charging 12v lithium and lead-acid battery, such as laptops, tablets, etc.

5v Foldable Solar Panel

5 Volt solar panel is good for charging the device voltage less than 5v, such as smartphones, USB fans, flashlights, etc.

What is the Best Foldable Solar Panel for your application?

The best foldable solar panel must have the right power and right voltage that suitable for your application, voltages, and power are the most important features. For example, you cannot use a 5 watt 5 volt solar panel to charge 12v 10Ah battery. There are some applications as below:

Military Foldable Solar Panel

Hovall has some military solar panels for Germany and Japan Army, it must be durable, lightweight and military color. These foldable solar panels are the ultimate portable folding solar panels for performance and mobility.

Foldable Solar Panel for Camping, Hiking, Fishing, etc.

The best stuff for camping, hiking, and fishing, you can use the solar panels to run all electric devices without battery or electricity power.

Top 3 Foldable solar panel reviews

1, 42 W Folding Solar Panel

This Hovall 42 Watt folding solar panel is a plug and play charger that comes with all in one design, it has USB5V and DC 18V outputs and can compatible with all devices such as the cellphones, Laptops, tablets, TVs, Fans, etc. Fast charging intelligent technology. Easy to use and carry. It made from waterproof fabrics and high efficiency of 23.7% Solar cells, Ultra-thin and Ultra-light, durable.

Specifications:

• Solar Cell: 23.7% efficiency solar cells
• Working Power: 42 Watt
• Output: DC 18V 2.4A and USB 5V 2A
• Unfolded Dimensions: 35.5 x 11.8 x 0.12 inch
• Folded Dimensions: 13 x 11.8 x 0.6 inch
• Net Weight: 29.5 Oz
• Package Include: 42 Watt folding solar panel x 1pc, Suction Cups x 4pcs, Alligator Clip x 1pc

2, 100W Folding Solar Panel

This 100W foldable solar panel is the most popular monocrystalline foldable solar panel, it has the high efficiency of 23.7% solar cell, it is a very good device for hiking, camping, fishing, etc. Dual Outputs and overcharge protection.

Specifications:

• Maximum Power: 100 Watt
• Working Temperature: -4 ~ 158 ℉
• Outputs: DC18 V 5.6A & USB5V 2.1A
• Unfolded Dimensions: 60 x 35.5 x 0.12 inch
• Foldable Dimensions: 17.5 x 11.8 x 1.58 inch
• Net Weight: 78 Oz
• Package: 100W Foldable Solar Panel x 1pc, Alligator Clip x 1pc, Car Mounted Charger x 1pc.

3, 20W Folding Solar Panel

Features:

• Charge smart and fast.
• Design to be Carry Around
• Renewable energy source anywhere whenever.
• The paper thickness design makes it perfect for outdoor use, such as trekking, traveling, fishing, camping, etc..
• Tied of packing bags of batteries on your backpack? Bored with the heavy external battery which needs the recharge in advance? Absolutely unfold a Hovall solar panel and feature peace of mind for enjoying the clean and unlimited energy from the direct daylight.
• A smart chip inside will routinely detect the quickest feasible currency for your device and the monocrystalline solar panel will provide up to twenty-five percent high conversion efficiency to achieve fast speed charging. What’s more, you could charge 2 cell phones on the identical time through two USB ports.

Specifications:

• Optimum Working Power: 20 Watt
• Outputs: USB 5V 2A & DC 12V 1.7A
• Unfolded Dimensions: 21.7 x 10.24 x 0.12 inch
• Folded Dimensions: 10.24 x 6.3 x 1.38 inch
• Net Weight: 28.2 Oz
• Package Include: 20W Foldable Solar Charger x 1pc, Suction Cups x 8pcs

If you have any questions or anything concerns about this article or our products, please feel free to contact us at info@hovall.com

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What is more about the best flexible solar panels?

In addition to the above two features, another two features that are also very important, there are Service Life and Cost performance. In general, you should know the following four features about the best flexible solar panels.

1, Conversion Efficiency

Generally speaking, flexible solar panels are made from monocrystalline silicon wafers and laminated materials. The conversion efficiency of monocrystalline silicon wafers is uneven on the market, basically between 15% and 25% efficiency, usually the conversation efficiency less than 19% silicon wafers are not suitable for making the flexible solar panels.

Currently, the most efficient monocrystalline silicon wafers are produced by American brands, some solar cells have a conversion efficiency of  24% or more, but it is slightly more expensive than others. Usually, we purchase high conversion efficiency solar panels because of insufficient space, beautiful and practical performance. In addition, there are many Chinese monocrystalline silicon wafers have the efficiency of  19% or more, it is good enough to use it to build flexible solar panels. Although the conversion efficiency is low, it is also welcomed by users.

2, Flexible

Base on this flexible solar panel that needs to be mounted to irregular surfaces that are on Vans, Cars, Home roofs, so it must be a little flexible and bendable. However, in fact, there are many mono-crystal flexible solar panels cannot be bent to a large arc, especially the mono flexible solar panels come from China, there are preferably not bent more than 20 degrees arc, otherwise, it is very easy to damage the monocrystal silicon inside, and it will affect its conversion efficiency, current and voltage outputs. However, some brand monocrystalline wafers are capable of bending at 30-degree arc without breaking. So, you should select the flexible solar panels for your applications very carefully.

3, Service Life.

If the flexible solar panels are installed and maintained properly, the service life can last more than 10 years. There are two types due to the production process, one is PET laminated and another one is ETFE laminated solar panel. ETFE solar panel is better than PET’s because it has better corrosion resistance and makes it more suitable for applications at sea, such as RVs, ships, sailboats, marines, boats, etc, and ETFE service life is longer than PET, but the price is much more expensive than PET.

4, Cost-Effective

The price/performance ratio of the flexible solar panel is mainly reflected in the wholesale price, and the wholesale price is actually mainly reflected in the cost of raw materials. PET laminated solar panel is much cheaper than ETFE flexible solar panel, but we strongly recommend the ETFE panel to use in the applications at the sea. Compared with the US solar cell, the most cost-effective is the monocrystalline silicon wafer of no less than 19% efficiency comes from China. Although the conversion efficiency is low, the price/performance ratio is the best.

Well, above are the most important four features to judge the best flexible solar panels. If you have any questions, feel free to contact us. We offer OEM and ODM service to a lot of brands and make our own brand. Which is the best flexible solar panel for your own systems? Give us an email, Hovall is your excellent choice.

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What is the working principle of solar cells? Solar cells are devices that directly convert sunlight energy into electricity through photoelectric or photochemical effects. When sunlight shines on a semiconductor, some of it is reflected off the surface and the rest is absorbed or transmitted by the semiconductor. Some of the absorbed light, of course, becomes hot, while others collide with the valence electrons of the atoms that make up the semiconductor, producing electron-hole pairs. In this way, light energy is converted into electric energy by the form of electron hole pairs.

1, The physical basis of solar cells

When the sun illuminates the p-n junction, the electrons in the semiconductor emit electrons because they have obtained the light energy, and the electron-hole pairs are produced accordingly. Under the action of the barrier electric field, the electrons are driven to the type region, and the holes are driven to the P type region, so that there are surplus electrons in the area, and there are surplus holes in the P region. Thus, the photoelectric field opposite to the potential electric field is formed near the p-n junction.

If a P-N junction exists in a semiconductor, a barrier electric field will be formed at both sides of the P-N junction, which can drive electrons to the N region and holes to the P region. As a result, there are surplus electrons in the N region and holes in the P region.

There are more than ten kinds of semiconductor materials that make solar cells, so there are many kinds of solar cells. At present, the most mature and commercially valuable solar cell is silicon solar cell. Next, we take silicon solar cell as an example to introduce in detail the working principle of solar cells.

1). Intrinsic semiconductor

The conductivity of matter depends on the atomic structure. Conductors are generally low-valent elements, and their outermost electrons easily break away from the nucleus and become free electrons. Under the action of an external electric field, they produce directional movement, forming an electric current. High-valent elements (such as inert gases) or polymers (such as rubber), their outermost electrons are strongly bound by the nucleus, it is difficult to become free electrons, so conductivity is very poor, become insulators. Silicon (Si) and germanium (Ge) are tetravalent elements, and their outermost electrons are neither as easily free from the bonds of nuclei as conductors, nor as tightly bound by nuclei as insulators, so their conductivity is between them.

A pure semiconductor is made into a single crystal through a certain process, namely an intrinsic semiconductor. Atoms in crystals form orderly arrays in space, and adjacent atoms form covalent bonds.

Covalent bonds in crystals have strong binding force, so at room temperature, only a few valence electrons get enough energy due to thermal motion (thermal excitation), thus breaking the bond into free electrons. At the same time, a hole is left in the covalent bond. The atom is positively charged by losing a valence electron, or the hole is positively charged. In intrinsic semiconductors, free electrons and holes appear in pairs, that is, the number of free electrons and holes is equal.

If free electrons meet holes in the process of motion, they will fill holes and disappear at the same time. This phenomenon is called recombination. At a certain temperature, the number of pairs of free electrons and holes produced by the intrinsic excitation is equal to the number of pairs of compound free electrons and holes, so the dynamic equilibrium is achieved.

Band theory:

a,When the electrons in a single atom move around the nucleus, the electrons in each orbit have their own specific energy.

b, The closer to the nuclear orbit, the lower the electron energy.

c, According to the principle of minimum energy, electrons always occupy the lowest level of energy.

d, The energy band occupied by valence electrons is called the valence band.

e, There is a forbidden band above the valence band. There is no energy level occupied by electrons in the forbidden band.

f, Above the forbidden band is the conduction band. The energy level in the conduction band is the energy level that the valence electron can occupy when it breaks away from the covalent bond.

g, The forbidden band width is expressed by Eg, and its value is related to the material and temperature of semiconductor. T=300K, Eg=1.1eV of silicon, Eg=0.72eV of germanium.

2). Impurity semiconductor

Impurity Semiconductor: Impurity Semiconductor can be obtained by adding a small amount of impurity elements into the intrinsic semiconductor through diffusion process.

N-type and P-type semiconductors can be formed according to the impurity elements doped, and the conductivity of the impurity semiconductors can be controlled by controlling the concentration of the impurity elements doped.

N-type semiconductors: N-type semiconductors are formed by adding pentavalent elements (such as phosphorus) to pure silicon crystals to replace silicon atoms in the lattice.

Because the impurity atom has five valence electrons in its outermost layer, it has one more electron besides forming a covalent bond with the silicon atom around it. The electrons are free from covalent bonds and become free electrons. In N-type semiconductors, the concentration of free electrons is higher than that of holes, so the free electrons are called majority carriers and the holes are minority carriers. Because impurity atoms can provide electrons, they are called donor atoms.

P-type semiconductors: pure silicon crystals doped with trivalent elements (such as boron), so that it replaces the position of silicon atoms in the lattice, the formation of a P-type semiconductor.

Because the outermost layer of the impurity atom has three valence electrons, when they form a covalent bond with the surrounding silicon atom, a “vacancy” is created, and when the outermost layer of the silicon atom fills the vacancy, a hole is created in the covalent bond. Therefore, in P semiconductor, holes are many and free electrons are few. Because the vacancies in impurity atoms absorb electrons, they are called acceptor atoms.

3). PN junction

PN junction: PN junction is formed at the interface of P-type semiconductor and N-type semiconductor fabricated on the same silicon wafer by different doping process.

Diffusion motion: A substance always moves from a place of high concentration to a place of low concentration. This motion due to the difference in concentration is called diffusion motion.

When a P-type semiconductor and an N-type semiconductor are fabricated together, the concentration of the two carriers varies greatly at their interfaces, so the hole in the P-region must diffuse to the N-region. Meanwhile, the free electrons in the N-region must diffuse to the P-region, as shown in the figure.

Because the free electrons diffused into the P region recombine with the holes, and the holes diffused into the N region recombine with the free electrons, the concentration of many electrons decreases near the interface, the negative ion region appears in the P region, and the positive ion region appears in the N region. They can’t move, which is called the space charge region, thus forming the built-in electric field-epSILon;.

As the diffusion proceeds, the space charge region widens and the built-in electric field strengthens. The direction of the built-in electric field points from the N region to the P region, which prevents the diffusion from proceeding.

Drift motion: under the action of electric field force, the motion of carriers is called drift motion.

When the space charge region is formed, the minority carrier drifts, the hole moves from the N region to the P region, and the free electron moves from the P region to the N region. In the absence of an external electric field and other excitations, the number of multiple particles participating in the diffusion motion is equal to the number of few particles participating in the drift motion, thus achieving a dynamic equilibrium and forming a PN junction, as shown in the figure. At this time, the space charge region has a certain width, the potential difference is =Uho, and the current is zero.

2, the principle of solar cells

1). Photovoltaic effect:

The photovoltaic energy conversion of solar cells is based on the photovoltaic effect of semiconductor pn junction. As mentioned earlier, photons with energy greater than the silicon forbidden band pass through the antireflective film into silicon when illuminated by a semiconductor photovoltaic device, and photogenerated electron-hole pairs are excited in N, depletion and P regions.

Depletion Zone: The photogenerated electron-hole pairs are separated by the built-in electric field immediately after they are generated in the depletion zone. The photogenerated electrons are fed into the N zone and the photogenerated holes are pushed into the P zone. According to the depletion approximation condition, the carrier concentration at the boundary of the depletion region is approximately 0, that is, p=n=0.

In the N region, the photogenerated holes diffuse to the P-N junction boundary after the photogenerated electron-hole pairs are formed. Once they reach the P-N junction boundary, they are immediately affected by the built-in electric field. They are pulled by the electric field force and drift across the depletion region into the P region. The photogenerated electrons (polyelectrons) are left in the N region.

In the P region, photogenerated electrons (minority electrons) enter the N region first because of diffusion and then because of drift, and photogenerated holes (polyelectrons) remain in the P region. The accumulation of positive and negative charges on both sides of the P-N junction results in the storage of excess electrons in the N region and the remaining holes in the P region. Thus forming a photoelectric field opposite to the built-in electric field.

a, In addition to partially counteracting the effect of the barrier electric field, the photovoltaic field also makes the P region positive and the N region negative, and the electromotive force is generated in the thin layer between the N region and the P region, which is called photovoltaic effect. When the battery is connected to a load, the photocurrent flows from the P region to the N region through the load, and the power output is obtained from the load.

b, If the ends of P-N junctions are opened, the electromotive force can be measured, which is called open circuit voltage Uoc. For crystalline silicon batteries, the typical value of open circuit voltage is 0.5 ~ 0.6V.

c, If the external circuit is short-circuited, there is a current in the external circuit which is proportional to the incident light energy. This current is called short-circuit current Isc.

Factors affecting photocurrent:

a, The more electron hole pairs produced by light in the interface layer, the greater the current.

b, The more light energy the interfacial layer absorbs, the larger the area of the interfacial layer, i.e. the cell, and the larger the current formed in the solar cell.

c, Photoinduced carriers can be generated in the N region, depletion region and P region of solar cells.

d, The photogenerated carriers in each region must pass through the depletion region before recombination to contribute to the optical current. Therefore, the generation and recombination, diffusion and drift in each region must be taken into account in solving the actual photogenerated current.

Equivalent circuit, output power and fill factor of solar cells

A，Equivalent circuit

In order to describe the working state of the battery, the battery and load system are often simulated by an equivalent circuit.

a, Constant Current Source: Under constant illumination, a working solar cell, its photocurrent does not change with the working state, in the equivalent circuit can be regarded as a constant current source.

b, Dark current Ibk: A part of the photocurrent flows through the load RL, and a terminal voltage U is established at both ends of the load. In turn, it is positively biased in the PN junction, causing a dark current Ibk opposite to the optical current.

c, In this way, the equivalent circuit of an ideal PN homogenous junction solar cell is plotted as shown in the figure.

d, Series resistance RS: Because the front and back electrodes contact, and the material itself has a certain resistivity, base and top layer are inevitably introduced additional resistance. The current flowing through the load will inevitably cause loss when passing through them. In the equivalent circuit, the total effect can be represented by a series resistance RS.

e, Parallel resistance RSh: Because of the leakage of the battery edge and the metal bridge formed at the cracks and scratches when making the metallized electrode, the part of the current should be short-circuited through the load. This effect can be equivalent to a parallel resistance RSh.

When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it can be obtained:

The P in the formula is the output power of the solar cell irradiated on the load RL.

B, Output power

When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it can be obtained:

The P in the formula is the output power of the solar cell irradiated on the load RL.

When the load RL changes from 0 to infinity, the output voltage U changes from 0 to U0C, and the output current changes from ISC to 0. Thus the load characteristic curve of solar cells can be drawn. Any point on the curve is called the working point. The connection between the working point and the origin is called the load line. The reciprocal of the slope of the load line is equal to RL. The horizontal and vertical coordinates corresponding to the working point are the working voltage and the working current.

When the load resistance RL is adjusted to a certain value Rm, a point M is obtained on the curve. The product of the corresponding working current Im and the working voltage Um is the largest, that is, Pm = ImUm.

Generally speaking, M point is the best working point (or maximum power point), Im is the best working current, Um is the best working voltage, Rm is the best load resistance, and Pm is the maximum output power.

C，Fill factor

a, The ratio of maximum output power to (Uoc *Isc) is called fill factor (FF), which is one of the important indicators to measure the output characteristics of solar cells.

b, Filling factor characterizes the quality of solar cells. Under certain spectral irradiance, the larger the FF, the more square the curve, and the higher the output power.

D, solar cell efficiency, factors affecting efficiency

(1). Solar cell efficiency:

When a solar cell is irradiated, the ratio of the output power to the incident light power is called the efficiency of the solar cell, also known as the photoelectric conversion efficiency. Generally refers to the maximum energy conversion efficiency when the external circuit is connected with the best load resistance RL.

In the above formula, if Ats are replaced by Aa (also called active area), that is, the area of grid lines is deducted from the total area, so the calculation efficiency is higher. This should be noted when reading the literature at home and abroad.

Prince of the United States first calculated the theoretical efficiency of silicon solar cells by 21.7%. In the 1970s, M. Wolf discussed it in detail. He also found that the theoretical efficiency of silicon solar cells was 20% ~ 22% under the AM0 spectral condition, and then modified it to 25% (AM1.0 spectral condition).

To estimate the theoretical efficiency of a solar cell, all possible losses from incident light to output power must be taken into account. Some of these losses are related to materials and processes, while others are determined by basic physical principles.

(2), Factors affecting efficiency

To sum up, the open-circuit voltage Uoc, short-circuit current ISC and filling factor FF must be increased to improve the efficiency of solar cells. These three parameters are often mutually constrained, if unilaterally increased one of them, may therefore reduce the other, so that the total efficiency not only did not improve but also decreased. Therefore, the material and design process must be considered in a comprehensive way so as to maximize the product of the 3 parameters.

1). Material band width:

The open-circuit voltage UOC increases with the increase of bandwidth Eg, but the short-circuit current density decreases with the increase of bandwidth Eg. As a result, the peak of solar cell efficiency can be expected at a certain Eg. The solar cells with Eg value of 1.2 ~ 1.6eV can be expected to achieve the highest efficiency. Direct bandgap semiconductors for thin film batteries are preferable because they can absorb photons near the surface.

2). Temperature:

The diffusion length of minority carrier increases slightly with the increase of temperature, so the photocurrent increases with the increase of temperature, but the UOC decreases sharply with the increase of temperature. The filling factor decreases, so the conversion efficiency decreases with the increase of temperature.

3). Irradiance:

With the increase of irradiance, the short-circuit current increases linearly and the maximum power increases. Focusing sunlight on solar cells can make a small solar cell generate a lot of electricity.

4). Doping concentration:

Another factor that has a significant effect on UOC is the doping concentration of semiconductors. The higher the doping concentration is, the higher the UOC is. However, when the concentration of impurities in silicon is higher than 1018/cm3, it is called high doping effect. The phenomenon of band gap contraction, impurity ionization and minority carrier lifetime decrease caused by high doping should also be avoided.

5). Photoinduced carrier recombination lifetime:

For semiconductor solar cells, the longer the recombination life of photogenerated carriers, the greater the short-circuit current. The key to long life is to avoid the formation of composite centers in the process of material preparation and battery production. The composite center can be removed and the service life can be prolonged by proper and frequent processing.

6). Surface recombination rate:

Low surface recombination rate helps to increase Isc. It is difficult to measure the recombination rate of the front surface and is often assumed to be infinite. A battery called the Back Electric Field (BSF) is designed to diffuse a P + layer on the back of the battery before depositing metal contact.

7). Series resistance and metal grating:

Series resistance comes from the resistance of lead wire, metal contact grid or battery body, and metal grid can not pass through the sun. In order to maximize the Isc, the area occupied by the metal grid should be the smallest. Generally, the metal grid is made into a dense and thin shape, which can reduce the series resistance and increase the light transmission area of the battery.

8). Use suede batteries to design and select high quality antireflection coatings.

Relying on the pyramid-shaped square cone structure of the surface, the multiple reflection of light not only reduces the loss of reflection, but also changes the direction of light in silicon and prolongs the light path, thus increasing the yield of photogenerated carriers; the zigzag surface also increases the area of PN junction, thus increasing the collection rate of photogenerated carriers and short-circuit current. Increase 5% to 10% and improve the red light response of the battery.

9). Effect of shadow on solar cells:

Solar cells will cause uneven illumination due to shadow occlusion and so on, and the output power will be greatly reduced.

At present, the application of solar cells has entered industries, commerce, agriculture, communications, household appliances and public utilities from the military and aerospace fields, especially in remote areas, mountains, deserts, islands and rural areas, to save expensive transmission lines. But at this stage, the cost is still high, and it costs tens of thousands of dollars to send out a kilowatt of electricity, so large-scale use is still economically constrained.

However, in the long run, with the improvement of solar cell manufacturing technology and the invention of new photoelectric conversion devices, the protection of the environment and the huge demand for renewable and clean energy in various countries, solar cells will still be a more practical way to utilize solar radiation energy, and can be used in large-scale for human beings in the future. It can open up broad prospects.

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