A Review: Solar Cells and Comparative Studies about Different Generations
Renewable Energy Sources
World’s population is increasing day by day very rapidly, this growing humanity demands the more energy resources. Currently, most of the energy requirements are fulfill using fossil fuel but there are some issues in use of fossil fuel: the reserves of fossil fuel are getting decreased; combustion of fossil fuel is causing air pollution by the emission of CO2; causing global warming; deforestation etc.; these reserves are non-renewable and are also limited. In 21st century, the major concern of developed countries is to find the sustainable, environmental friendly and long lasting energy resources such energy resources are termed as renewable energy resources. List of major renewable energy sources comprises wind, biomass, sun, tides, waves, and heat of earth (geothermal). Solar energy is called renewable energy because it is directly obtained from sunlight, as long as sun will shine it will be available for us.
Energy comes from the Sun on Earth nearly 1366 W/m2 (watt per square meter). The idea of first photovoltaic effect was given by Alexander Edmond Becquerel in 1839. But in 1946, first modern cell was developed by Russel Ohl. These cells are thin silicon wafer cell. In modern cells, electron and holes are the cause of electricity between two layers (P-type and N-type). When many solar cells are assembled that used to make solar modulus. Cells are used as photovoltaic cells when no source of sunlight is available. Various types of materials can be used in photovoltaic cell (single crystalline, multi crystalline and amorphous crystalline). So, these are categorized into various classes.
First Generation Solar Cell (Wafer Based)
These are produced by using silicon wafer. When silicon is doped with other materials then according to this, positive and negative charges are flow on a film. These solar cells are mostly used in commercial application. It covers 86% solar cells in market. These are mostly used because of their high efficiency. This technique is oldest but powerful because of high efficiency.
These cells have its two further categories:i) Single/Mono Crystalline Silicon Solar Cell
As the name indicated, it is made by one silicon cell by a process of Czochralski Process. These crystals are sliced when manufacturing is done. It requires precise crystallization process. It is very expensive and multi process. The efficiency of silicon crystalline cell lies between 17-18%.ii) Polycrystalline Silicon Solar Cell (Poly-Si)
Polycrystalline are formed by number of different crystals that are arranged in one cell. This cell is more economical. It is produced by cooling a molten metal in graphite mold. These are more used solar cell, currently. It can be occupied 48% of solar energy in worldwide during 2008. During cooling, various structures are formed. They are less costly as compare monocrystalline but also less efficient 12-14%.
Second Generation of Solar Cell (Thin Film Based)
Mostly thin films and silicon are second generation solar cells. In these cells, photons are absorbed on a thin film and thin layers are formed by one another. Each layer absorbs different wavelength of light. But they have lower efficiency as compered silicon solar cells. These cells are more economical. Silicon wafer cell can absorb the light up to 350μm thick while thin films absorbed less light 1μm. These are classified as:i) Amorphous Silicon (a-Si) Thin Films Solar Cell
a-Si is the first solar cell made in industry. These are processing at low temperature but flexible substrate at low cost. Cells use smaller amount of energy for processing also easily available. Amorphous means not a definite arrangement of crystals of silicon material. So, it shows non-crystalline arrangement and are not highly structured. Silicon is doped on the backside of substrate glass plate. Reflecting side is dark brown while conducting side is silver.
The main problem of these Si-cells that they have unstable efficiency, the commercial efficiency range is 4%-8%. It can be used at high temperatures and stable for climatic changes.ii) Cadmium Telluride (CdTe) Thin Film Solar Cell
CdTe cells are one of the cost effective and economically better candidates and it is first photovoltaic devices in low cost. Band gap is 1.5V with better chemical stability. Due to these attractive properties, they are more useful in solar thin films.
It is good crystalline semiconductor have better efficiency that makes the absorption easier. That is formed by sandwiching the cadmium sulfide as p-n junction. There are three steps as follows:
1) Glass is chosen as substrate and poly crystalline material is synthesized.
2) Next is deposition, in which layers of CdTe are deposited on the substrate by different methods. It has high absorption coefficient 5x1015 /cm so its efficiency ranges 9%-11% . It can also be coated in polymer substrate and flexible. But there are some environmental issues with CdTe. Cadmium is heavy metal and toxic in nature so it can accumulate in the body of human, plants and animals. So, it is necessary to dispose the cadmium because recycling is very expensive in our society. So, limited supply of cadmium and its potential hazard in society its major concern less use in solar cells.iii) Copper Indium Gallium Di – Selenide (CIGS) Solar Cell
It consists of four elements: copper, indium, gallium and selenide also quaternary direct band gap semiconductor compound. CIGS have a higher efficiency 10%-12% as compare to CdTe. Due to its higher efficiency and economy, these are most widely used thin solar cells. Following techniques are used for processing:
- Electrochemical coating
- Electrochemical Deposition
Sputtering is performed in two or multiple steps in which deposition and interaction with selenium is performed. It can be done in one step. Substrate for CIGS can be glass, polymer, steel and aluminum etc. The main advantage is that it has a prolonged life without degradation. The properties show the indication of better properties with enhanced efficiency.
Third Generation Solar Cell
This is new technology but not commercially used and investigated. It enhances the technology that is poor in second generation solar cells as wells as low production cost. They do not need any p-n junction in solar cells. In 3rd generation solar are as follows:
i) Nano Crystal Based Solar Cell
ii) Polymer Based Solar Cell
iii) Dye Sensitized Solar Cell
iv) Concentrated Solar Cell
v) Perovskite Based Solar Cell
vi) Quantum Dot Solar Cell
Nano Crystal Based Solar Cell
They can also be called as Quantum Dots (QD) semiconductor solar cell. In these solar cell, nano range transition metals are used. Material mostly porous silicon or TiO2 are used. These are used to replace bulk use material such as CdTe or CIGS. Basically, in QD theoretical method is used to design p-i-n solar cells. Nano crystals are mixed and coated on Si substrate. By using centrifugal force, they rotate very fast and flow away. But if we are moving to past, then photon is excited to create an electron-hole pair. So, in these cases numerous electron hole pairs such as 2, 3 or 7 are generated when these are striking to each other.
Polymer Based Solar Cell
PSC are flexible because polymer substrate is used. First Polymer solar cell was made in a research group of Tang et. al. at Kodak Research. Thin functional multiple layers are coated on polymer foils. It works on donor (polymer) and acceptor (fullerene). Different materials are used for sun absorption including conducting material. In 2000, MacDairmid, Shirikawa and Heeger got a noble prize for developing a new sheet of conducting polymer. These solar cells have same principle as PV solar cells. Energy is transformed in the form of electromagnetic radiation to electrical current. These solar cells have high conversion efficiency. This innovation helped to derive more polymer materials. The efficiency can be improved 3% by controlling another parameter. This opens the innovation of new flexible solar cells in field of textile and fabrics. Polarizing Organic Photovoltaic (ZOPVs) is a modern technique, in which liquid crystals are used.
Dye Sensitized Solar Cell (DSSC)
Now solar efficiency is improved by using molecular manipulation. Michel Gratzel was the first scientist who introduced DSSC. It is dye molecule between different electrodes. It has four components as follows:
- Semiconductor electrode (p-type NiO and n-type TiO2)
- Redox mediator
- A dye sensitizer
- Counter electrode (carbon or Pt)
It has simple processing methods like printing. DSSC are flexible, transparent as well as cost effective. Nano grained coating of TiO2 is used in these solar cells with visible optical dyes. It increases efficiency up to 10%. But there are some problems like degradation of dyes and stability problems. When dyes are expose to ultraviolet and infrared then they decomposed and lifetime is decreased. When barrier layer is coated then process become expensive also efficiency is decreased.
Concentrated Solar Cell
Concentrating Photovoltaic (CPV) has been processed since 1970. This is newly developing technology. This is performed by using large mirrors and lens to focus light on a small area. Due to convergence of heat a large amount of energy can be obtained. CPV are most used in world as better way. These may be in low, medium and high according to the efficiency of solar cell. Solar efficiency can be greater than 40%, no thermal mass and greater response.
Perovskite Based Solar Cell
Compounds have a formula ABX3 in this X represents the halogens such as Br-, Cl-, I- while A and B are cations. These are newly developed research that has better properties as compare to thin solar cells and Silicon. Because Silicon based solar cells require high cost, multiple steps and more processing. Perovskite solar cells have efficiency greater than 31%. It can be used in future electric batteries according to Volkswagen. But there is a problem of stability and durability in perovskite. Material is also degradable and efficiency is reduced with time.
Quantum Dot Solar Cell
Quantum dots are used as a photovoltaic material these quantum dots have a band gaps that has a capability to tuned large energy as compare to bulk material so these solar cells used semiconductor quantum dots as light absorber. It has various advantages; tunable absorption greater optical extinction coefficient QD sensitized 3.6mm thick. Quantum dot solar cell have maximum quantum efficiency nearly 78.6% at 530nm while conversion efficiency from solar to electrical is 6.54%.
Doping is important in quantum dots. Mainly two methods are used for a selective doping:
- Direct doping
It gives a comprehensive assessment of doping. If there is a negative impact on p it gives high crystals. If doping is done of n type, then a photovoltaic characteristic can be observed.
There are many categorization of quantum dot solar cell, following groups II-IV, III-V, IV-VI, and IV. One way to increase the efficiency of QD solar cell is to create multiple junctions. Mainly there are following three types of solar cells:
1) Photo electrodes, composed of quantum dots, make a schottky junction with metallic layer, p-n junction as second semiconductor in matrices or i in p-i-n device.
2) QD sensitized nano crystalline have TiO2 films
3) Dispersed on a multiphase structure, have a hole and electron conducting matrices. For example, Co with hole conducting polymers like polythiophene respectively.