In the ntype layer, there is an excess of electrons, and in the ptype layer, there is an excess of positively charged holes which are vacancies due to the lack of valence. Solar cells used are smaller, but highly efficient, multijunction mj solar cells. We give the upper limit to the solar cell e ciency. Single junction solar cells the plain and simple solar cell.
They were originally developed for special applications such as satellites and space investigation. Feb 07, 2018 there is no difference in a pn junction and solar cell infact solar cell is a type of pn junction diode the only special character of solar cell it that one of the region p or n is made very thin so that the light reaches the depletion regi. Cells are typically made using a crystalline silicon wafer. A photovoltaic device produces a current or a voltage at its output in the presence of light. Pn junction diode is one of the fundamental elements in electronics.
Each materials pn junction will produce electric current in. The arrow direction in the diode symbol indicates a. Free electrons in nregion attracted to positive charge in pregion, drift on over 2. Since the ntype region has a high electron concentration and the ptype a high hole concentration, electrons diffuse from the ntype side to the ptype side. The energy conversion consists of absorption of light photon energy producing electronhole pairs in a semiconductor and charge carrier separation. Questions you should be able to answer by the end of todays lecture. Jan 29, 2019 pn junction diode is one of the fundamental elements in electronics. Junction nasacr149669 sodlatells studies f iw of techfical silicon. Lindholm, principal investigator department of electrical engineering university of florida gainesville, florida 32611. Experimental in order to realize a radial pn junction nanorod solar cell experimentally. Shockley model from sze, 1981 vv at 1 j j e tot s n n p p p n s l d n q l d p j 0 0 j j x j x tot n p p n a i n n n d n q 2 w p n. Multi junction mj solar cells are solar cells with multiple pn junctions made of different semiconductor materials. A siliconwafer based pn junction solar cell by aluminuminduced recrystallization and doping.
The theory of solar cells explains the process by which light energy in photons is converted into. In practice, pn junctions of silicon solar cells are not made in this way, but rather by diffusing an ntype dopant into one. It consist of silicon, germanium pn junction with a glass windows on the top surface layer of ptype, the ptype material is made very thin and wide so that the incident light photon may easily reach to pn junction. Dec 30, 2017 the depletion region is the heart of s solar cell. Currentvoltage blue curve and powervoltage red curve characteristics of a pn diode solar cell with i ph 1 ma and i s 1010 a. The highest theoretical efficiency possible with a. A typical pnjunction singlecrystal silicon solar cell. Silicon a single silicon atom consists of fourteen negatively. It is used for many forms of rectification for current levels both large and small, as well as high and low voltage levels, and it finds many uses in all manner of electronic circuits. In practice, pn junctions of silicon solar cells are not made in this way, but rather by. When photons hit the solar cell, free electrons attempt to unite with holes in the ptype layer. Multijunction mj solar cells are solar cells with multiple pn junctions made of different semiconductor materials. Yes, it is possible to have more than one junction in a solar cell.
Multijunction solar cell an overview sciencedirect topics. It consist of silicon, germanium pn junction with a glass windows on the top surface layer of ptype, the ptype material is made very thin and wide so that the. Demonstrate a 40%efficient, 1cm2 solar cell optimized for terrestrial application using a nonlatticematched tandem heterojunction fabricated by wafer bonding and layer transfer. The pn junction diode is the basic semiconductor diode format. The crosshatched area indicates the power generated by the solar cell. Leaves nregion with net positive charge and pregion with net negative charge. Due to the pn junction, a built in electric field is always present across the solar cell. The pn junction is the fundamental building block of the electronic age. Reverse saturation current in a silicon pn junction diode nearly doubles for very a. A pn diode can be used to realize a photodetector of the photovoltaic type. Pnjunction diode when the junction is illuminated, a net current flow takes place in an external lead connecting the ptype and ntype.
A pn junction diode is a twoterminal electronics element, which can be classified as either step graded or linear graded. Fourjunction solar cell with 40% target efficiency fabricated. This is a good description of the book, which begins with the physics of conductors, semiconductors, and the junctions on which many devices depend. Solar cells can be spaced farther apart since light can be focused on each cell. Remove the quartz boat with the solid, white, phosplus tp. Bias and currents in pn junction bias potential barrier at the junction energy band equilibrium v0.
Such junctions show a pronounced rectifying behavior. Principles and varieties of solar energy phys 4400 and. Most common approaches to model a pn junction solar cell are detailed balance model dbm and driftdiffusion model ddm. Solar cell or photovoltaic cell is nothing but pn junction when the photons with energy greater than or equal to the band gap of pabsorber impenge on solar cells, the electrons excite from valence band to conduction band in the absorber. Their high efficiency comes from the possibility to grow three or more junctions for the same cell. A solar cell is an electronic device which directly converts sunlight into electricity. The cell can be considered as a two terminal device which conducts like a diode in the dark and generates a photovoltage when charged by the sun. Junction solar cell an overview sciencedirect topics. In the ntype layer, there is an excess of electrons, and in the ptype layer, there is an excess of positively charged holes which are vacancies due to the lack of valence electrons. Resistive losses constitute one of the main underlying. What is the difference between the pn junction and a. The boron doped ptype monocrystalline silicon wafer is sawn from a boule of silicon grown using the czochralski method and the pn junction ptype silicon. Pn junction pn junction solar cell is a large area pn junction or a diode. The generation of electric current happens inside the depletion.
A fresnel lens is placed on top of every solar cell to focus sun light on the solar cell. The process of conversion first requires a material which absorbs the solar energy. A photodiode is a light detector that operates in reverse bias. It supplies a voltage and a current to a resistive load light, battery, motor. They are also an essential part of metaloxidesilicon fieldeffects. What is the difference between the pn junction and a solar cell. In this type of diode, we dope one side of a semiconductor piece with an acceptor impurity and another side with a donor impurity. Solar cell is a key device that converts the light energy into the electrical energy in photovoltaic energy conversion. Ch4 solar cell operational principles tu delft ocw.
Multijunction photovoltaic technologies for high performance. Physically, the main property of a mj solar cell is having more than one pn junction in order to catch a larger photon energy spectrum while the main property of the thin film solar cell is to use thin films instead of thick layers in order to decrease the cost efficiency ratio. They are also called pn diodes in analogy with vacuum diodes. Consists of a large area, single layer pn junction diode. In most cases, semiconductor is used for solar cell material. A crosssectional view of the conventionalsilicon solar cell structure that has dominated production up to the present is given in fig. This chapter begins with the basic semiconductor physics, which is necessary to understand the operation of pn junction solar cell, and then describes the basic. A solar cell is a solidstate electrical device pn junction that converts the energy of light directly into electricity dc using the photovoltaic effect. The term pn junction refers to the joint interface and the immediate surrounding area of the two semiconductors. This will decrease the number of solar cells needed. Denis ar con ljubljana, may 2011 abstract this seminar is a short overview of current photovoltaics. The generation of electric current happens inside the depletion zone of the pn junction.
If the electrons and holes were not charged, this diffusion process. Application of pn junctions within solar cells is discussed. A solar cell is essential a pn junction with a large surface area. The process of conversion first requires a material which absorbs the solar energy photon, and then raises an electron to a higher energy state, and then the flow of this. You can see that the n type material is kept thin to allow the light to pass through the pn junction and reach the depletion region. The markers indicate the voltage and current, v m and i m, for which the maximum power, p m is generated. You would have likely heard about the pn junction of the solar cell, which is in fact the junction that generates the photovoltaic current. This question is part of the super big solar panel faq from solar mango, where expert answers to over 100 important questions on solar panels are provided.
What happens when we join ptype and ntype semiconductors. Semiconductor devices have made a major impact on the way we work and live. Direction of hole flow direction of conventional current c. Pn junctions consist of two semiconductor regions of opposite type. In addition, they can be used as solar cells, photodiodes, light emitting diodes and even laser diodes. Multijunction mj solar cells are one of the most promising technologies achieving high sunlight to electricity conversion efficiency. A pn junction diode is a twoterminal electronics element, which can be classified as either. Each materials pn junction will produce electric current in response to different wavelengths of light.
What is the highest efficiency possible theoretical with single junction solar cells. The most commonly known solar cell is configured as a largearea pn junction made from silicon. The highest theoretical efficiency possible with a single junction solar cells is about 34%. A pn junction is formed when two types of semiconductors, n type excess electrons and p type excess holes, come into contact. Today semiconductor pn junction diode devices are experiencing substantial growth.
Free holes in pregion attracted to negative charge in nregion, drift on over 3. The term band gap refers to the energy difference between the top of the valence outer electron band and the bottom of the conduction free. Aug 11, 2011 this textbook introduces the physical concepts required for a comprehensive understanding of pn junction devices, light emitting diodes and solar cells. The pn junctionwhich contains a strong electric field and a fixed space charge separates the minority carriers that are collected from the emitter and the base. V principle of solar cell v current generation v iv characteristic of an illuminated pn junction v physical process of solar cell v iv characteristic of solar cell v solar cell parameter v applications v reference v device that converts optical energy into electrical energy. A typical csi solar cell structure is shown in figure 3. Light shining on the solar cell produces both a current and a voltage to generate electric power. However the electric field, a oneway downhill road superimposed across the cell, allows the electrons to flow only in one direction. Effects of pn junction position depth on output current of. A pn junction is the metallurgical boundary between the n and pregions of a semiconductor crystal. In typical solar cell applications there is about 1 dopant atom for every 5,000,000 silicon atoms. A textured solar cell with 40 m depth of pn junction has the efficiency 3 times more than same nontextured solar cell.
The ntype material is kept thin to allow light to pass through to the pn junction. The n region is heavily doped and thin so that the light can penetrate. Similarly, holes flow by diffusion from the ptype side to the ntype side. Four junction solar cell with 40% target efficiency fabricated. The solar cell is the basic building block of solar photovoltaics.
The pn junction is a versatile element, which can be used as a. Gallium arsenidebased multijunction solar cells are the most efficient solar cells to date, reaching the record efficiency of 42. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction. Pdf a number of solid state organic solar cells have been prepared and tested in simulated sunlight. This is a good description of the book, which begins with the. Pn junction diode and characteristics of pn junction diode. The pn junction has the very useful property that electrons are only able to flow in one direction. As shown in the given diagram the solar cell is like an ordinary diode. A solar cell consists of a layer of ptype silicon placed next to a layer of ntype silicon fig. As a simplification, one can imagine bringing a layer of ntype silicon into direct contact with a layer of ptype silicon. Pn junction diode when the junction is illuminated, a net current flow takes place in an external lead connecting the ptype and ntype. A siliconwafer based pn junction solar cell by aluminuminduced recrystallization and doping appl.