physics Archives - Page 4 of 4

How do solar cells work?

By Pratik Barve | October 31, 2017 | Comments 0 Comment

The solar cell is an important candidate for an alternative terrestrial energy source because it can convert sunlight directly to electricity with good conversion efficiency, can provide nearly permanent power at low operating cost, and is virtually non-polluting. Solar cell also called as photovoltaic cell and are building blocks of solar panels. You must have seen these panels (large collection of solar cells) in green energy campaigns and also in developed cities in large arrays. It is used as a primary source of energy in space applications. ISS has large solar panels! Let’s understand the basics of solar cells.

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. p-n junctions of silicon solar cells are made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). To get an idea about p-n junction click here. Now when light or photon hits the p-n junction it dislodges an electron and creates a hole in its place. Now the dislodged electron and hole are free to move in silicon crystal. Due to the electric field present, the electron moves to the n-type region and hole moves to the p-type region. The mobile electrons created in n-type are collected by thin metal fingers on the top of n-type region. Photons having energy equal to the band gap of silicon crystals are the only ones contributing in cells electrical output. Energy greater than band gap is lost as heat. We might also lose energy if electrons and holes recombine as soon as they are formed. So basically a solar cell works by knocking off electron of same energy as band gap of crystal material used and essentially converting light energy into electrical energy.

Schematic representation of silicon p-n junction Solar cell.

The radiative energy output from the sun derives from a nuclear fusion reaction. In every second, about 6 x 10^11 kg hydrogen is converted to helium, with a net mass loss of about 4 x 10^3 kg. We get a lot of energy from sun and if there is a way to harness this energy what’s stopping us from being completely reliant on solar power? There are various factors at play.

Terrestrially the sunlight is attenuated by clouds and by atmospheric scattering and absorption. Also we don’t receive sunlight during night time and during bad weather conditions. This is something we can’t do anything about. As mentioned above, only the photon having energy equal to band gap contributes in electrical output. Photons having energies other than band gap of semiconductor crystals either reflects back or goes through or the energy is just lost as heat energy. Antireflective coating is done to avoid reflection of photons. To deal with photons of energies other than band gap of that semiconductor crystal there is a different approach.

Spectrum Splitting :

Spectrum splitting is a very good way of increasing efficiency of solar cells by splitting sunlight into narrow wavelength bands and directing each band to a cell that has a band gap optimally chosen to convert just this wavelength band of light. There is one more way, by simply stacking cell on top of one another with the highest band gap cell at the top which automatically achieves an identical spectral-splitting effect, making this “tandem” cell approach a reasonably practical way of increasing cell efficiency.

This is an area of active research where scientists all over the world are trying to increase the efficiency of solar cells. The most efficient solar cell yet still converts only 46% of solar energy into electricity. And most commercial systems convert only 15-20% of solar energy. Solar energy is a source of green and sustainable energy and probably a candidate for future source of energy.

What is p-n junction?

By Pratik Barve | October 30, 2017 | Comments 0 Comment

The p-n junctions are elementary building blocks of semiconductor electronic devices. As the name suggest it is a junction between two types of semiconductor materials the p-type and the n-type. The p-type is the positive side of junction and n-type is the negative side. The p-type consists of excess holes (missing electrons) and n-type consists of excess electrons. The junction is the boundary or interface between these 2 types inside a single crystal of semiconductor. When a p-n junction is formed i.e when it is fabricated there are excess electrons on n side and holes on p sides so electrons and holes combine at the junction. Departure of electrons from n side to p side leaves with a positive donor ion on n side and likewise hole leaves negative acceptor ion on p side. The uncompensated ions are positive on n side and negative on p side which creates an electric field which provides a force opposing the continued exchange of charge carriers.

When the electric field is sufficient to stop further transfer of charge carriers the depletion layers reaches an equilibrium. When a positive voltage is applied to the p-side with respect to the n-side, a large current will flow through the junction. However, when a negative voltage is applied, virtually no current flows. This “rectifying” behaviour is the most important characteristic of p-n junction. This is a very short description of p-n junction and how it works.

The p-n junction was invented by Russell Ohl an American physicist of Bell Laboratories in 1939. The p-n junction is created by doping (adding impurities) by various methods on any one type of base semiconductor material. Various methods like ion implantation, diffusion, etc. Semiconductors for instance silicon, is doped with boron, phosphorus, or arsenic by ion implantation method. In diffusion method the impurity moves into the semiconductor crystal by solid-state diffusion. In the ion-implantation method, the intended impurity is introduced into the semiconductor by accelerating the impurity ions to a high-energy level and then implanting the ions in the semiconductor.

The p-n junction can be used in electronics circuits in two different configurations, forward bias and reverse bias.

In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. With a battery connected this way, the holes in the p-type region and the electrons in the n-type region are pushed toward the junction and start to neutralize the depletion zone, reducing its width. And thus current flows through the junction.Connecting p-type region to the negative terminal of the battery and the n-type region to the positive terminal corresponds to reverse bias. Very little current will flow until the diode breaks down in reverse bias configuration.

The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. When the p–n junction is forward-biased, electric charge flows freely due to reduced resistance of the p–n junction. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal. p-n junction is used in many semiconductor devices such as diodes, transistors, solar cells, LED’s, integrated circuits, etc.

Why do we need to redefine mass?

By Pratik Barve | August 28, 2017 | Comments 0 Comment

The current SI unit of mass i.e. Kilogram is defined as mass equal to the mass of 1 dm³ of water at its maximum density, approximately 4 °C. It is also defined as being equal to the mass of the International Prototype of the Kilogram (IPK). The IPK is made of a platinum alloy known as “Pt‑10Ir”, which is 90% platinum and 10% iridium (by mass) and is machined into a right-circular cylinder (height = diameter) of about 39 millimetres to minimize its surface area.

Figure 1: K20 IPK at National Institute of Standards and Technology.

The alloy is chosen due to its properties like extreme resistance to oxidation, extremely high density (almost twice as dense as lead and more than 21 times as dense as water). Satisfactory electrical and thermal conductivities, and low magnetic susceptibility. The IPK is just a cylinder which is used to define one of the most widely used units of measurement. Unlike definitions of other physical quantities such as meter defined as distance light travels in a vacuum during a time interval of ¹⁄_299,792,458 of a second Kilogram is susceptible to variations due to physical interactions like scratches on prototype or oxidation etc. Even if the change is very small in the order of micrograms it matters over long duration as other physical quantities depend on this value. This is the main reason scientists are working on redefining the unit of mass in terms of numerical constants in order to make it universal and in accordance with other constants.

The stability of the IPK is crucial because the kilogram underpins much of the SI system of measurement as it is currently defined and structured. For instance, the newton is defined as the force necessary to accelerate one kilogram at one metre per second squared. If the mass of the IPK were to change slightly, so too must the newton by a proportional degree. In turn, the pascal, the SI unit of pressure, is defined in terms of the newton. This chain of dependency follows to many other SI units of measure. For instance, the joule, the SI unit of energy, is defined as that expended when a force of one newton acts through one metre. Next to be affected is the SI unit of power, the watt, which is one joule per second. The ampere too is defined relative to the newton, and ultimately, the kilogram. Redefinition will not make the kilogram more precise, but it will make it more stable. A physical object can lose or gain atoms over time, or be destroyed, but constants remain the same. And a definition based on constants would, at least in theory, allow the exact kilogram measure to be available to someone anywhere on the planet, rather than just those who can access the safe in France.

Now how did the researchers go about redefining the term? The CIPM’s (International Committee for Weights and Measures) committee on mass recommends that three independent measurements of Planck’s constant agree, and that two of them use different methods. First method is counting the atoms in two silicon-28 spheres that each weigh the same as the reference kilogram. This allows them to calculate a value for Avogadro’s constant, which the researchers convert into a value for Planck’s constant. Kilogram is expressed in terms of Planck’s constant, which relates a particle’s energy to its frequency, and, through E = mc², to its mass. This means first setting the Planck value using experiments based on the current reference kilogram, and then using that value to define the kilogram. Another method uses a device called a watt balance also known as Kibble balance to produce a value for Planck’s constant by weighing a test mass calibrated according to the reference kilogram against an electromagnetic force.

Figure 2: Silicon Sphere at NIST

Why is Planck’s constant of such importance in defining mass? The Planck constant is a fundamental constant of nature which sets a limit on the accuracy with which we can measure both the position and momentum of a physical system. It depends on the SI units of length, mass and time: the metre, kilogram and second, respectively. As the second and metre are defined by universal constants such as the speed of light, they can be used in conjunction with a fixed value of the Planck constant to redefine the kilogram. This would remove the need for the International Prototype Kilogram.

Ian Robinson with the Kibble Balance at NPL.

Figure 3: The Kibble balance or watt balance at NPL ( National Physical Laboratory).

Figure 4: Bryan Kibble and Ian Robinson working together
on the improved Kibble balance.

The proposed definition of Kilogram is: “The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626070040×10⁻³⁴when expressed in the unit J⋅s, which is equal to kg·m²·s⁻¹, where the metre and the second are defined in terms of c and Δν_Cs.”

Where,

c = Numerical value of speed of light ;

Δν_Cs= numerical value of the caesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.