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Dec 29 2017

Working principle and Development of Solar Cell

Warm hints: The word in this article is about 2600 and reading time is about 15 minutes.

Summary

Due to the continuous demand of mankind for renewable energy, people are devoted to developing new sources. The energy that the sun shines on the Earth's surface in 40 minutes can be used for one year at a speed of the current global energy consumption.Reasonable utilization of solar energy will be a long-term development strategy for mankind to solve energy problems and it is alsoone of the most studied research hot spots. This article will introduce various types of new solar cells, as well as the principle and development of solar cells. At the same time, we will compare the conversion efficiency and development prospects of them. 


Article Core

Working principle and Development of Solar Cell

Material type

Silicon, compound semiconductors, organic materials and etc.

English name

Solar Cell

Category

Power,Energy

Principle

Photovoltaic Effect

Classify

Silicon based semiconductor batteries, dye sensitized batteries, and so on



Catalogs

Catalogs

I.Solar Cell Backgroud

4. Nanocrystalline solar cells

3. SunCats

II.Types of Solar Cells

5. Organic solar cells

4. sunLight 

1. Silicon solar cell

III.Some fancy designs of Solar Cells

IV.Working Principle of Solar Cells

2. Multicompound thin film solar cells

1. E-Saving Battery

V.Photovoltaic effect

3. Polymer multilayer modified electrode type solar cell

2. Foldable solar cell





Introduction

I.Solar Cell Backgroud

Energy industry is not only the basic industry of national economy, but also a technology-intensive industry. "Safe, efficient and low-carbon" embodies the characteristics of modern energy technology and is also the main direction to seize the commanding height of future energy technologies.

 At present, the development of new energy sources mainly concentrates on renewable energies such as solar energy, hydrogen energy, wind energy and geothermal energy, among which solar energy resources are abundant and widely distributed, and are the most promising renewable energy sources. With the global energy shortage and environmental pollution issues such as increasingly prominent, solar photovoltaic power generation has drawn the world's attention and focus on the development of new industries because of its clean, safe, convenient, efficient and other characteristics.

 From the discovery by the French scientist E. Becquerel in 1839 of the photovoltaic effect of the liquid (referred to as photovoltaic phenomenon), the solar cell has undergone a long development history of more than 160 years. In terms of the overall development, both basic research and technological advancement have played a positive role in promoting them. The practical application of solar cells played a decisive role since the successful development of monocrystalline silicon solar cells made by three scientists from the United States Bell Laboratories, which is a milestone of the development history of solar cells. So far, the basic structure and mechanism of solar cells has not changed.

 Due to the continuous demand of mankind for renewable energy, people are devoted to developing new sources. The energy that the sun shines on the Earth's surface in 40 minutes can be used for one year at a speed of the current global energy consumption. Reasonable utilization of solar energy will be a long-term development strategy for mankind to solve energy problems and it is also one of the most studied research hot spots. This article will introduce various types of new solar cells, as well as the principle and development of solar cells. At the same time, we will compare the conversion efficiency and development prospects of them.

II.Types of Solar Cells

1. Silicon solar cell

Silicon solar cells are divided into mono-crystalline silicon solar cells, poly-crystalline silicon thin film solar cells and amorphous silicon thin film solar cells.

 Mono-crystalline silicon solar cells has the cell conversion efficiency, whose technology is also the most mature. The highest conversion efficiency in the lab is 24.7% and the productivity at the scale of production is 15%. It is still dominant in large-scale applications and industrial production. However, due to the high cost of mono-crystalline silicon, it is very difficult to sharply reduce the cost. In order to save silicon materials, the development of poly-crystalline silicon and amorphous silicon film is appeared as substitutes of mono-crystalline silicon solar cell.

solar panels--

Compared with mono-crystalline silicon, poly-crystalline silicon thin film solar cell has lower cost .At the meanwhile, it has higher efficiency than amorphous silicon thin film cell. Its highest conversion efficiency is 18% in lab and 10% in industrial scale. As a result, poly-crystalline silicon thin-film batteries will soon dominate the market for solar power.

 Amorphous silicon thin film solar cells has great potential with its advantages of low cost, high conversion efficiency and ease of mass production. However, because of its material-induced photoelectric efficiency decay effect, the stability is not high, a direct impact on its practical application. If we can further solve the stability problem and improve the conversion rate, then, amorphous silicon solar cell is undoubtedly one of the main development of solar cells.

2. Multicompound thin film solar cells

The material of multicompound thin film solar cells is inorganic salts, including gallium arsenide III-V compounds, cadmium sulfide, cadmium sulfide and copper occluded selenium thin film cell.

 Cadmium sulfide, cadmium telluride poly-crystalline thin film cells provide a higher efficiency than amorphous silicon thin film solar cell , lower cost than mono-crystalline silicon cells, and also are easy to mass production. However, cadmium is highly toxic, which will cause serious environmental pollution; therefore, it is not the ideal substitute to crystalline silicon solar cells.


Multicompound thin film solar cells

 

The conversion efficiency of GaAs III-V compound cells can reach up to 28%. GaAs compounds have a very good optical band-gap and high absorption efficiency. They have strong anti irradiation ability and are insensitive to heat, which are suitable for manufacturing high efficiency single junction cell. However, the price of GaAs materials is high, which limits the popularity of GaAs cells to a large extent.

Copper indium selenide thin film cells (referred to as CIS) are suitable for photoelectric conversion. There is no photodegradation problem. They have the same conversion efficiency as polysilicon. With low prices, good performance and simple process, etc., CIS will be an important direction of the future development of solar cells. The only problem is the source of the material, as indium and selenium are relatively rare elements, therefore, the development of such batteries must be limited.

3. Polymer multilayer modified electrode type solar cell

 The replacement of inorganic materials with organic polymers is a research direction of a newly started solar cell manufacturing. With advantages of good flexibility , easy fabrication, wide range of materials sources and low cost , organic materials are of great significance for large-scale use of solar energy and provision of low-cost electric energy. However, the study of producing solar cells with organic materials has just begun, both their lifespan and battery efficiency can not compared with inorganic materials, especially silicon cells. Whether it can be developed into a practical product still needs a further explored.


Polymer multilayer modified electrode type solar cell

 

4. Nanocrystalline solar cells

Nanocrystalline TiO 2 Chemistry Solar cell is a newly developed product. Its advantages include low cost, simple process and stable performance. At the same time, its photoelectric efficiency is stable at above 10%, and the production cost is only 1/5 to 1/10 of the silicon solar cell while its lifespan can reach over 20 years.

However, due to the research and development of such cells has just started, it is estimated that Nanocrystalline solar cells will gradually enter the market in the near future .

Nanocrystalline solar cells

5. Organic solar cells

Organic solar cells, as its name implies, are solar cells that form of organic materials. We are not familiar with organic solar cells, which is a reasonable thing. More than 95% of today's solar cells are silicon-based, while less than 5% of the remaining solar cells are made from other inorganic materials.

Organic solar cellsOrganic solar cells

Here is a table of the conversion efficiency of different types of solar cells:

Types of Solar Cells

Crystalline Silicon Solar Cells

Thin film solar cell


monocrystalline silicon

polycrystalline silicon

CdTe

CIS

A-Si

MC-Si

Industrial production  efficiency

19.6%

18.5%

11.1%

12%

7%

9%

Achievable efficiency goals

>20%

20%

18%

18%

10%

15%



Detail

III.Some fancy designs of Solar Cells

1. E-Saving Battery

E-saving battery

The E-Saving Battery has the perfect balance of solar cell area (power generation efficiency) and portability. This product looks no much difference to the usual portable power bank. It still has columned shape and output through the USB port - but it has built-in flexible solar cells, when needed, holding the back of the rod, and you can pull out the solar cell like a reel to get the maximum light area, thus improve the power generation efficiency. In peacetime, you can also put the panel up, which is both convenient and not occupied.

2. Foldable solar cell

In E-saving battery, we mentioned a flexible solar cell that can be rolled up. Then, can the solar cell be rolled up or not? As early as 2009, an American named Frederik Krebs created a solar film that is able to be curled or straightened, which even had an ultra-thin lithium battery and an LED attached to it. During the days you can straighten it and stick it to the wall and it will be able to convert solar energy into electricity and store up. in the evening, you can put it in the house as room lighting. If you want, it can also be rolled into tube as a flashlight. According to Krebs's vision, the cost of each will be less than 7 US dollars in the final mass production of such a solar LED film.

Foldable solar cell

3. SunCats

SunCats is a design of Knut Karlsen. Actually, it's more like a solar sticker than a solar cell, which is equivalent to the solar cells that attached to the ordinary rechargeable battery surface. So when it is power off, throw it on the windowsill and let it catch some sunlight will be OK.

Suncats

4. sunLight 

The sunLight is designed by the German designer Hermann Eske. The main body of sunLight is a solar panel that can be rolled together. Apart from charging the electronics directly as most solar devices do,it still has other special function. If you have a close-up view of it, you will find that looks a little different. There are six hollow small cylinders on the back. All of the mysteries lie within these cylinders, any of which can be thought as a small LED flashlight powered by built-in two AAAA rechargeable batteries and rolled up as a powerful flashlight with six LEDs.

sunLight

IV.Working Principle of Solar Cells

How-Solar-Cell-Works

Solar cells, a type of semiconductor device that efficiently absorbs solar radiation and converts it into electrical energy, are also known as photovoltaic cells because of their photo-voltaic effect using various potential barriers. The core of these devices is the electron-releasable semiconductor. The most commonly used semiconductor material is silicon. As crustal reserves of silicon is rich, it can be said to be inexhaustible. When the sunlight shines on the semiconductor surface, the valence electrons of the atoms in the N and P regions of the semiconductor are excited by solar photons, and the energy beyond the forbidden band width Eg is obtained by optical irradiation. The conduction band thus produces many electron-hole pairs that are in an unbalanced state within the semiconductor material. These photoexcited electrons and holes collide freely or recombine in the semiconductor to an equilibrium state. Composite process does not show the external conductive effect. It is a part of the automatic loss of solar cell energy. A small number of carriers in the photoexcited carriers can move to the P-N junction region and drifts to the opposite region through the P-N junction minority carrier pulling effect, and the opposite direction is formed opposite to the electric field of the P-N junction barrier Photoelectric field. Once connected to the external circuit, you can have power output. When a large number of such small solar photovoltaic cells are combined in a series and parallel manner to form a photovoltaic cell module, a large enough electric power is output under the action of solar energy. It is important that semiconductor materials for solar cells have a suitable forbidden band width. Semiconductor with different bandgap absorbs only part of the solar radiation energy to generate electron-hole pairs. The smaller the forbidden band width, the larger the available part of the solar spectrum to be absorbed, and at the same time the amount of wasted energy will be greater near the solar spectrum peaks. It can be seen that the solar spectrum can be more effectively used only by choosing semiconductor materials with suitable forbidden band widths. Since the direct-transfer semiconductor has a higher light absorption efficiency than the indirect transfer type, it is preferable that it be a direct-transfer type semiconductor.



Analysis

V.Photovoltaic effect

As it is mentioned above, we need to explain photovoltaic effect here.

Here is a video about Photovoltaic effect: 

Associate Professor of Materials Science and Engineering Jeff Grossman explains photovoltaics/solar cells

The so-called photo-voltaic effect is when the object is illuminated, the charge distribution within the object state changes have an effect of electromotive force and current. When sunlight or other light shines on the PN junction of a semiconductor, a voltage appears on either side of the PN junction, which is called photo-induced voltage.

When light strikes the PN junction, an electron-hole pair is generated. The carriers generated near the PN junction in the semiconductor are not recombined to reach the space charge region. Due to the attraction of the internal electric field, the electron flows into the N region and the hole flows into P region. As a result, excess electrons are stored in the N region and excess holes are present in the P region. They form a photogenerated electric field opposite to the barrier in the vicinity of the p-n junction. In addition to partially counteracting the role of the potential barrier electric field, the photogenerated electric field also makes the P region positive, and the N region negative. Then produces electromotive force between the thin layer in the N and P region , which is the photovoltaic effect.



Book Recommendation


  • The Physics of Solar Cells (Properties of Semiconductor Materials)

This book provides a comprehensive introduction to the physics of the photovoltaic cell. It is suitable for undergraduates, graduate students, and researchers new to the field. It covers: basic physics of semiconductors in photovoltaic devices; physical models of solar cell operation; characteristics and design of common types of solar cell; and approaches to increasing solar cell efficiency. The text explains the terms and concepts of solar cell device physics and shows the reader how to formulate and solve relevant physical problems. Exercises and worked solutions are included. Contents: Photons In, Electrons Out: Basic Principles of PV; Electrons and Holes in Semiconductors; Generation and Recombination; Junctions; Analysis of the p n Junction; Monocrystalline Solar Cells; Thin Film Solar Cells; Managing Light; Over the Limit: Strategies for Higher Efficiency.


--Jenny Nelson  (Author)

  • Solar Cells : Operating Principles, Technology and System Applications

The primarily focus bookes on single junction silicon devices, but some of the III-V semiconductors are also described. Mostly the physics of solar cells is covered, but there is some info on practical installation issues.


--Martin A. Green  (Author)



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