PV Cell – Basics & Technologies - globaldres -...

THE PV CELLHISTORY, BASICS & TECHNOLOGIES

Presented By,Sudarshan B SAssistant ProfessorDept. of EEERVCE, Bangalore

BASICS OF SOLAR CELLSA solar cell or a PV cell is a semiconductor device that converts sunlight into direct current (DC) or electricity.Groups of PV cells are configured into modules and arrays, which can then be used to charge batteries, operate motors and to power any number of electrical loads.By using appropriate power conversion equipment, alternating current can be produced to power AC loads (P.S. All loads used in our home are AC loads)In addition, the system (after conversion to AC) can operate in parallel with, and interconnected to, the utility grid.

HISTORY OF PHOTOVOLTAICSThe central component of the solar photovoltaic system is the PV cell. It can be visualized as a two-terminal device.In 1839, Edmond Becquerel discovered the photovoltaic effect which became known as the ‘Becquerel Effect’. Young Edmond was the son of great discoverer Henry Becquerel, who discovered piezoelectric effect and radioactivity with Curie. Edmond discovered that when light was thrown on an electrode, it started to produce current. Hence Edmond Becquerel became known as the Father of Photovoltaics.

HISTORY OF PHOTOVOLTAICS

The earliest PV cell as developed by Becquerel was a black coated container with an acidic solution (electrolyte) housing two electrodes separated by a thin membrane and connected to an external circuit.The black coating was done to trap light and the two electrodes where coated AgCl and AgBr respectively.When light was incident on the electrodes, an external circuit current was observed.He incident blue light, UV light and sunlight and recorded the observations.This is considered as the earliest PV cell.


In 1877, W. G. Adams and R. E. Day published an article entitled ‘The action of light on Selenium’ in the Philosophical Transactions of the Royal Society of London.In this, the authors describe a selenium PV cell based on the discovery of Willoughby Smith that, ‘when an electrical current was passing through a bar of crystalline Selenium, its resistance was less when the bar was exposed to the action of light than it was when the bar was kept in the dark.’The authors used a vitreous selenium bar connected to a platinum electrode. This setup was enclosed in a glass tube and light was incident on the glass tube.A current was observed to flow in the external circuit.

The Selenium PV cell


In 1883, C. E. Fritts published a research entitled ‘On A new form of Selenium photocell’ in the American Journal of Science.

This was the first thin-film selenium solar cell. It consisted of a metal substrate (e.g. Brass) on which a 25 micron selenium layer was pressed and a gold leaf (bar) was pressed on this.When light was incident on the selenium layer, a current was observed to flow in the external circuit.

First Thin-Film Selenium PV cell

The first solar array — 1884 — installed on a New York City rooftop by Charles Fritts.

HISTORY OF PHOTOVOLTAICSClassical physics was unable to explain

the concept and working of solar cell.In 1900, Max Planck introduced Quantum Physics where energy was conceptualized to be in discrete packets or ‘quanta’, governed by the equation

where h is the Planck’s constantIn 1905, Albert Einstein proposed the concept of light quanta or photon. Through this principle, he explained the concept of photovoltaic effect and photovoltaic cell.

HISTORY OF PHOTOVOLTAICSIn 1933, L. O. Grondahl published his work

on the Coper-Cuprous Oxide solar cell.In his paper entitled ‘The copper-cuprous-oxide rectifier and photoelectric effect’, he detailed the manufacturing and characteristics of the aforementioned solar cell.

Grondahl, L.O., "The Copper-Cuprous-Oxide Rectifier and Photoelectric Cell", Review of Modern Physics, Vol. 5, p. 141, 1933.

This solar cell was popular due to its low production cost. Grondahl documents 38 publications dealing with copper-cuprous oxide photovoltaic cells over the period 1930-32.

Early Grondahl-Geigercopper-cuprous oxide photovoltaic cell


In 1941, R. S. Ohl applied for a patent on the first silicon Photovoltaic cell (‘Light-sensitive electric device including silicon’).This silicon cell had an efficiency less than 1% and hence did not have any commercial value.


In 1954, D. M. Chaplin, C. S. Fuller and G. L Pearson published their research on a silicon solar cell with 6% efficiency.This was published n the Journal of Applied Physics.

HISTORY OF PHOTOVOLTAICSThe earliest solar cells were used in telephone repeaters.The Vanguard-1 (1958) satellite used 6 Si-based panels of power 5mW. The cells allowed to operate the transmitters for over six years after the batteries died.

HOW DOES A PV CELL WORK ?

A typical PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorous-doped (N-type) silicon on top of a thicker layer of boron-doped (P-Type) silicon.An electrical field is created near the top surface of the cell when these two materials are in contact and is called the P-N Junction.When sunlight strikes the surface of a PV cell, the valance electrons (i.e., electrons in the outermost orbit) of the N-type layer absorb the energy, break the bonds and come out of the parent atom.The energy absorbed, along with the momentum provided by the electric field stimulates the electrons and they flow into the external circuit (load).When electron comes out of the parent atom, a hole is created in that place. In other words, when sunlight (photons) strike, an electron-hole pair is created in the solar cell.

HOW DOES A PV CELL WORK ?

TYPICAL SOLAR CELLRegardless of its size, a typical silicon solar cell produces about 0.5-0.6 V DC under open-circuit conditions (without any load connected).The current and power output of a cell depends on its efficiency and size (surface area), and is proportional to the intensity of sunlight striking the surface of the cell.Example:Under peak sunlight conditions, a typical solar cell with a surface area of 160cm2 will produce about 2W peak power.If the sunlight were 40% of peak, then the cell would produce about 0.8W

SOLAR ENERGY - COSTMain parameter is the energy cost per unit (Rs. Per kWh) deliveredDepends on

PV Conversion EfficiencyCapital cost per watt capacity

These two parameter indicate how competitive (economically) PV electricity is.Conversion Efficiency is PV Cell is,

COMPONENTS OF SOLAR PHOTOVOLTAIC (SPV) SYSTEM

COMPONENTS OF SOLAR PHOTOVOLTAIC (SPV) SYSTEM - 1

The PV array is the central component of the SPV system.

It is made of several interconnected solar cells to form a panel, interconnected panels to form modules and interconnected modules to form an array.

The number of cells in an array is decided based on the output voltage, current and power required.

The solar array converts the incident solar radiation from the sun (insolation or irradiance) directly into electricity.

The solar cell has low efficiency. Hence it is necessary to ensure its operation at maximum power point (i.e., we should ensure it generates as much power as possible at the specific irradiance and temperature conditions).


The DC – DC converter is usually employed in all SPV systems.

A DC-DC converter converts input DC voltage at one level to output DC voltage at a different level to supply various DC loads.

For example, If input is 100V DC, the converter may be used to increase the voltage to 150V (step-up/boost DC-DC converter), or reduce it to 50V (step-down/buck DC-DC converter).

A single DC-DC converter could also be designed to do both buck and boost operation (buck-boost converter and its derivatives) depending on the control signal.

The ‘MPPT’ stands for maximum power point tracking. It is done to ensure the PV array generates maximum possible power at any given time. It can be analog MPPT or digital MPPT control.


The DC – AC converter (or inverter) is usually employed in SPV systems that have to supply AC (Alternating Current) loads.

A DC-AC converter converts input DC voltage to output AC voltage at a different level to supply various AC loads and to connect to the grid.

If used in homes, a single-phase inverter is sufficient as all our residential loads work on single phase.

However, for industrial users a three-phase inverter is preferred.

Various topologies on inverter exist and can be used based on the required accuracy of output, efficiency, cost constraints, space constraints, etc.

COMPONENTS OF SOLAR PHOTOVOLTAIC (SPV) SYSTEM - 3The output of the inverter should ideally be at a

frequency standard of 50Hz for India. This frequency is called the fundamental frequency.

However, output of any inverter always contains waves at fundamental frequency and also waveforms at frequencies that are integral multiples of fundamental frequencies. Such waves are called harmonics. Many times, non-integral multiple frequency waves called inter-harmonics may also exist. The harmonics and inter-harmonics degrade the performance of the connected loads, since every AC load is designed to operate at the highest efficiency at fundamental frequency only.

Therefore, the output of the inverter needs to be filtered to obtain voltage and current at fundamental frequency to supply the AC loads and connect to the grid.

GOALS OF PV RESEARCHPrimary Goals

Improve the conversion efficiencyImproving the performance parametersTo reduce the cost of solar cell and module

Secondary GoalsTo improve manufacturing yieldsReducing energy consumption, manufacturing costReducing impurities and defects

This has lead to development of various PV technologies

PV CELL TECHNOLOGIESThe following are the major technologies available in solar cells:1. Single-Crystalline (Mono-crystalline) Silicon2. Poly-crystalline (Multi-crystalline) and Semi-crystalline Silicon3. Thin Film Cells4. Amorphous Silicon5. Spheral Cells6. Concentrator Cell7. Multi-junction Cell

MONO-CRYSTALLINE SOLAR CELLMonocrystalline silicon is the most widely used cell material since a long time.Typically, the energy conversion efficiency ranges from 14-18%, but new silicon cells have efficiencies up to 23%.Method of production (Czochralski process)

Silicon raw material is first melted and purified in a crucible.A seed crystal is then placed in the liquid silicon and drawn at a constant slow rate.

This results in a solid, single-crystal cylindrical ingot.The ingot is sliced using a diamond saw into 200 to 400 micro-meter(0.005 – 0.01

inch) thick wafers.The wafers are further cut into rectangular cells to maximize the number of cells that can be mounted together on a rectangular panel.

The manufacturing process is slow and intensive. Hence the raw material cost is high, at $20 to $25 per pound of solar cell.

SEED CRYSTAL AND PULLER ROD

Seed Crystal

Puller Rod with Seed Crystal

MONO-CRYSTALLINE SOLAR CELL

POLY-CRYSTALLINE SOLAR CELLThis is a relatively fast and low-cost process to manufacture crystalline cellsInstead of drawing single crystals using seeds, the molten silicon is cast into ingots. In the process, it forms multiple crystals.The conversion efficiency is lower, but the cost is much lower, giving a low cost per watt of power.Because the crystal structure is somewhat random (imperfect) to begin with, it cannot degrade further with imperfections in the manufacturing process or in operation.It comes in both thick- and thin-film cells and is overtaking the cell market in commercial applications.

POLY-CRYSTALLINE SOLAR CELL – GRAIN BOUNDARYThe material quality of multi-crystalline material is lower than that of single crystalline material due to the presence of grain boundaries.Grain boundaries introduce high localised regions of recombination due to the introduction of extra defect energy levels into the band gap, thus reducing the overall minority carrier lifetime from the material. In addition, grain boundaries reduce solar cell performance by blocking carrier flows and providing shunting paths for current flow across the p-n junction.

POLY-CRYSTALLINE SOLAR CELL

Slab of multi-crystalline silicon after growth. The slab is further cut up into bricks and then the bricks are sliced into wafers

POLY-CRYSTALLINE SILICON WAFER

A 10 x 10 cm2 multi-crystalline wafer. The wafer has been textured so that grains of different orientation show up as light and dark.

POLY-CRYSTALLINE SOLAR CELL

THIN-FILM CELLThese are new types of PV entering the market. Copper indium diselenide (CuInSe2 or CIS), cadmium telluride (CdTe), and gallium arsenide (GaAs) are all thin-film materials, typically a few micrometers or less in thickness, directly deposited on a glass, plastic, stainless steel, ceramic, or other compatible substrate material.In this manufacturing process, layers of different PV materials are applied sequentially to a substrate. This technology uses much less material per square area of the cell, and hence, is less expensive per watt of power generated. Cadmium telluride appears to be a promising thin-film technology for low-cost-per-watt capacity.

THIN-FILM CELL

MONO, POLY AND THIN FILM SOLAR CELLS

AMORPHOUS SILICON SOLAR CELL

In this technology, a 1-2-μm-thick amorphous silicon vapour film is deposited on a glass or stainless steel roll.

Compared to crystalline silicon, this technology uses only about 1% of the material. Its efficiency is about half that of crystalline silicon technology at present, but the cost per watt is significantly lower.

AMORPHOUS SILICON SOLAR CELL “Amorphous” refers to objects having no definite shape and is defined as a

non-crystal material. Unlike crystal silicon, in which atomic arrangements are regular, amorphous silicon features irregular atomic arrangements.

As a result, the reciprocal action between photon and silicon atom occurs more frequently in amorphous silicon than in crystal silicon, allowing much more light to be absorbed.

Thus, an ultra-thin amorphous silicon film of less than 1 micro meter can be produced and used for power generation.

Also, by utilizing metal or plastics for the substrates, flexible solar cells can be produced.

AMORPHOUS SILICON SOLAR CELL

SPHERAL CELLS This is yet another technology that is being explored in the laboratories. The raw material is low-grade silicon crystalline beads, presently costing about $1 per pound. The beads are applied on typically 4-in squares of thin perforated aluminium foil.

In the process, the impurities are pushed to the surface, where they are etched away. Because each sphere works independently, the individual sphere failure has a negligible impact on the average performance of the bulk surface.

The Southern California Edison Company estimates that a 100-ft 2 spheral panel can generate 2000 kWh per year in an average Southern California climate.

CONCENTRATOR CELLIn an attempt to improve conversion efficiency, sunlight is concentrated tens or hundreds of times the normal intensity by focusing on a small area using low-cost lenses.A primary advantage of this is that such a cell requires a small fraction of area compared to the standard cells, thus significantly reducing the PV material requirement.However, the total sunlight collection area remains approximately the same for a given power output.Advantages:

increased power and reduced size or number of cellsthe cell efficiency increases under concentrated light up to a pointsmall active cell area (It is easier to produce a high-efficiency cell of small area than to produce large-area cells with comparable efficiency)

CONCENTRATOR CELLAn efficiency of 37% has been achieved in a cell designed for terrestrial applications, which is a modified version of the triple-junction cell that Spectrolab developed for space applications.The major disadvantage of the concentrator cell is that it requires focusing optics, which adds to the costConcentrator PV cells have seen a recent resurgence of interest in Australia and Spain.

CONCENTRATOR CELL

MULTI-JUNCTION CELLThe single-junction n-on-p silicon cell converts only red and infrared light into electricity, but not blue and ultraviolet.The PV cell converts light into electricity most efficiently when the light’s energy matches the semiconductor’s energy level, known as its band gap.The layering of multiple semiconductors with a wide range of band gaps converts more energy levels (wavelengths) of light into electricity. Such a cell is called a multi-junction cell.The multijunction cell uses multiple layers of semiconductor materials to convert a broader spectrum of sunlight into electricity, thus improving the efficiency.

MULTI-JUNCTION CELLWith the gallium indium phosphide/gallium arsenide/germanium (GaInP/GaAs/Ge) triplejunction cell, NREL and Spectrolab have reported a record 34% efficiency under concentrated sunlight.The cell captures infrared photons as well. Efficiencies up to 40% in triple-junction space-qualified cells have been measured by Spectrolab, Inc.

MULTI-JUNCTION CELL Currently, the GaInP/GaAs/Ge cell is being widely used in satellite power systems but may soon trickle down to terrestrial applications.

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