Comparison between Semiconductor- Metal junction and Semiconductor- Liquid Electrolyte junction- Dye sensitized solar cell

Comparison between Semiconductor-Metal junction and Semiconductor-Liquid Electrolyte junction.

 

Semiconductor-Liquid Electrolyte junction.
A contact between a semiconductor and a liquid electrolyte represents an electrochemical junction. The transport of charge to and from the semiconductor surface is performed by a redox couple, which is included in the electrolyte. The redox couple is two ionic species of different state of charge: oxidized species and reduced species. Like any other material, a redox couple has a Fermi level. The energy levels of the oxidized and reduced species can be associated with the conduction and valence band energies respectively. The current density in the electrolyte provides a current directed from the semiconductor to the electrolyte.
Junction formation and light effect on junction.
When an n-type semiconductor is immersed in a solution that contains a supporting electrolyte (for example, a salt) and a redox couple, charge (electrons) can move to and from the solid semiconductor to the liquid. For specification, we will consider the properties of a liquid junction with an n-type semiconductor. Due to the excess of electrons in the n-type material, electrons will generally flow from the semiconductor to the liquid. Because electrons are removed from the semiconductor, a region near its surface is formed that has net positive charge. This fixed positive charge creates an electric field within the semiconductor and is responsible for the photo activity of the semiconductor photo electrochemical system.
When light of energy greater than the semiconductor band gap energy is incident on the semiconductor, an excess electron and an excess hole will be created within the solid. Because of the electric field near the interface with the solution, the electron is driven away from the interface and the hole is driven toward the interface. When the hole reaches the interface, the hole can oxidize a species in solution. The electron is collected at the electrode’s metal contact, which is connected to external electronics or a load (for example, a light bulb) and ultimately to a counter electrode, where a reduction reaction occurs. The semiconductor working electrode is also connected to the metal counter electrode through the solution completing the circuit. The opposite argument holds for a p-type semiconductor, in which case electrons are driven to the semiconductor surface causing reduction reactions.

a)   Energy level in semiconductor and redox electrolyte shown on a common vacuum level.
b)  Semiconductor (n-type) electrolyte interface before and after equilibrium.
c)   Semiconductor (p-type) electrolyte interface before and after equilibrium.

 Semiconductor-Metal junction:-

Metal-semiconductor junction is the simplest type of charge separating junction. If we have an n-type semiconductor of work function Φ_n and metal of work function Φ_m, such that Φ_m > Φ_n, it is called a Schottky barrier.
 
 

 (a): Band profiles of n-type semiconductor and metal in isolation 

 

 (b): Band profiles of semiconductor-metal junction in equilibrium

When metal and semiconductor are separate from each other, the Fermi levels will look like in fig. (a). When they are in electronic contact, the Fermi levels will line up. This is achieved by the exchange of charge carriers across the junction, with the consequence that the layers approach the thermal equilibrium. The energy at the conduction band edge at the interface between semiconductor and metal is higher than in the bulk of the semiconductor. The electrostatic potential energy is shown in fig (b) by the change in E_vac.

The space charge region or depletion region is the region where there is a net charge.

As E_vac changes by a certain amount, so must the conduction and valence band energies, and by the same amount. This happens because the electron affinity and band gap are invariant in the semiconductor, and is called band bending.

 

ENTROPY AND THE SECOND LAW OF THERMODYNAMICS

 

ENTROPY AND THE SECOND LAW OF THERMODYNAMICS:

............How the universe works

The first law of thermodynamics is simply a statement of energy conservation. That is, it states that energy can always be accounted for, that the energy of the universe is a constant - it can be transferred between objects and can change form, but the total doesn't change. But the first law does not preclude things occuring that we know do not occur: A glass of water does not spontaneously separate into ice cubes and warm water even though the energy balance equations used in calorimetry problems would allow it. That is, energy conservation - the first law of thermodynamics - would allow for the possibility that a system in thermal equilibrium could separate into two systems - one at a higher temperature than the other - and that temperature difference could then be used to drive a heat engine to do work. The second law of thermodynamics explains why the universe does not work that way. It articulates the underlying principle that gives the direction of heat flow in any thermal process. The result, of course, fits our everyday experience. The second law states the reason why it is true.

Heat naturally flows from higher temperatures to lower temperatures.

No natural process has as its sole result the transfer of heat from a cooler to a warmer object.

No process can convert heat absorbed from a reservoir at one temperature directly into work without also rejecting heat to a cooler reservoir. That is, no heat engine is 100% efficient.

Carnot Cycle - Maximum Thermodynamic Efficiency in a Cyclic Process

It was observed by Sadi Carnot - a French scientist and engineer trying to improve the efficiency of steam engines in the mid-1800s - that there is always waste heat rejected by a heat engine. And that waste heat limits the efficiency of the engine since energy has to be conserved or accounted for. In trying to understand the limits of efficiency, he stated that in any heat engine in principle there would always be rejected heat (even in an ideal engine) - and the net work done would be the difference between the heat absorbed and that rejected. He then set out to determine the principles that would affect that efficiency. He stated that the most efficient heat engine possible would be one that worked reversibly - an ideal that could never be attained. This would mean that heat transferred into or out of the system (the heat engine) would only occur at constant temperatures - the high or the low temperatures between which the heat engine operated. That is, the system would stay at the temperatures of the reservoirs during those heat transfers - necessary for the process to be reversible since the heat flow could not be reversed to go from the lower to the higher temperature. And furthermore, said Carnot, the maximum conceivable efficiency would be limited by those two temperatures. The most efficient thermodynamic cycle operated between any two temperatures is therefore called a Carnot cycle.

The Carnot cycle is a four step process involving two isothermal processes (which are said to be ideal reversible processes) at the temperatures Th and Tc and two adiabatic processes (ie, without heat transfer) which operate between those two temperatures. In the isothermal steps, there is no change in internal energy and the heat exchanged is equal to the work done. In the two adiabatic processes, there is no heat exchanged. No such system can ever be built - since it is an idealized process (the two isothermal steps being reversible and quasistatic which means, in effect, they occur infinitely slowly). The importance of the process is that it gives an upper limit to the efficiency of any cyclic process between the same two temperatures.

Entropy and the Second Law of Thermodynamics

In trying to synthesize the ideas of Kelvin, Joule, and Carnot - that is, that energy is conserved in thermodynamic processes and that heat always "flows downhill" in temperature - Rudolf Clausius invented the idea of entropy in such a way that the change in entropy is the ratio of the heat exchanged in any process and the absolute temperature at which that heat is exchanged. That is, he defined the change in entropyDS of an object which either absorbs or gives off heat Q at some temperature T as simply the ratio Q/T.

With this new concept, he was able to put the idea that heat will always flow from the higher to the lower temperature into a mathematical framework. If a quantity of heat Q flows naturally from a higher temperature object to a lower temperature object - something that we always observe, the entropy gained by the cooler object during the transfer is greater than the entropy lost by the warmer one since Q/Tc.>|Q|/Th. So he could state that the principle that drives all natural thermodynamic processes is that the effect of any heat transfer is a net increase in the combined entropy of the two objects. And that new principle establishes the direction that natural processes proceed. All natural processes occur in such a way that the total entropy of the universe increases. The only heat transfer that could occur and leave the entropy of the universe unchanged is one that occurs between two objects which are at the same temperature - but that is not possible, since no heat would transfer. So a reversible isothermal heat transfer that would leave the entropy of the universe constant is just an idealization - and hence could not occur. All other processes - meaning, all real processes - have the effect of increasing the entropy of the universe. That is the second law of thermodynamics.

Entropy is a measure of the disorder of a system. That disorder can be represented in terms of energy that is not available to be used. Natural processes will always proceed in the direction that increases the disorder of a system. When two objects are at different temperatures, the combined systems represent a higher sense of order than when they are in equilibrium with each other. The sense of order is associated with the atoms of system A and the atoms of system B being separated by average energy per atom - those of A being the higher energy atoms if system A is at a higher temperature. When they are put in thermal contact, energy flows from the higher average energy system to the lower average energy system to make the energy of the combined system more uniformly distributed - ie, less ordered. So the disorder of the system has increased - and we say the entropy has increased. But the process of increasing the disorder has removed the possibility that the energy that was transferred from A to B can be used for any other purpose - for example, work cannot be extracted from the energy by operating a heat engine between the two reservoirs of different temperatures. So although energy was conserved in the transfer (the first law), the entropy of the universe has increased in becoming more disordered (the second law) and consequently the availability of energy for doing work has decreased.

The second law of thermodynamics can be summarized in many different statements - and has been by many thermodynamicists in the last century and a half. All of the statements are an attempt to put a reason to the things all of us have observed - that when two objects are in thermal contact, heat always goes from the warmer to the cooler and never the other way. This universal result has probably as many explanations as there are physicists trying to explain it - and is still the subject of serious consideration by some of the best theorists. The difficulty does not lie in what the second law says - or how it should be interpreted - but rather in what the fundamental, underlying reason is for why nature behaves in that way.

Any process either increases the entropy of the universe - or leaves it unchanged. Entropy is constant only in reversible processes which occur in equilibrium. All natural processes are irreversible.

All natural processes tend toward increasing disorder. And although energy is conserved, its availability is decreased.

Nature proceeds from the simple to the complex, from the orderly to the disorderly, from low entropy to high entropy.

The entropy of a system is proportional to the logarithm of the probability of that particular configuration of the system occuring. The more highly ordered the configuration of a system, the less likely it is to occur naturally - hence the lower its entropy.

In the language of entropy, the Carnot cycle still represents the theoretical maximum efficiency in any cyclic process. That is, maximum efficiency would occur if the entropy of the universe did not increase, hence there would be no loss of availability of doing work. But entropy can only remain constant in a reversible isothermal process. So, again, any heat transfer would have to occur isothermally. Therefore the most efficient cyclic process possible involves only reversible isothermal steps and steps in which no heat is transferred - ie, adiabatic. And even in this idealized reversible process in which the entropy of the universe was left unchanged, the efficiency of conversion of heat to work is limited by the two temperatures involved in the isothermal steps.

Based on the ideas of Lord Kelvin, Joule, Boltzmann, Carnot, and Clausius, the first and second laws of thermodynamics can now be restated in two profound sentences:

The total energy of the universe is a constant.

The total entropy of the universe always increases.

And these two fundamental principles of nature describe how the universe works.

Solar cell design by using Photonic Simulation software

 

Basic solar cell design includes P-N junction with thin N layer being at top and bulk P layer at bottom. It is designed without the ARC coating. But since bare silicon is oxidised due to reaction of oxygen so top and bottom side of solar cell is thin silica oxide layer.

Dimensions of solar cell- 

Overall dimension of cell=14 x 10 x 6 um (considered for simulation)

N – type layer at top of cell diffused doping surface concentration = 10^19 /cm3 and reference concentration = 10^10/cm3, With thickness of 1 um and junction width of 0.4 um.

Remaining material is p-type doped with density 10^16/cm3.

At upper and lower surfaces 1 um thick silicon oxide layer.

At top emitter side the silver current collector wire of  2 um breadth and 0.5um thickness.

At bottom base side aluminium current collector of 1 um thickness.

The picture of lumerical simulation model is given below-

To do the simulation in device first of all we need generation curve(wavelength range 0.3 – 1.1 um) which is obtained from the FDTD. Generation curve gives the variotion of electon hole pair generation with the solar cell depth.

After that we imported the generation rate data in the device applied  biasing across the solar cell to get the following I-V curve.

This curve shows the variation of current density of solar cell with respect to the voltage.

The short circuit current is maximum current of solar cell obtained at zero voltage, and open circuit voltage is maximum voltage obtained at open circuit i.e. zero current. It is showing that the current is nearly constant up to certain voltage after that it falls very fast.

The above curve shows the power density variation with respect to voltage. We can get the maximum value of power from the above curve so that we can find the efficiency.

 Efficiency η % =( Pmax / solar irradiance at 1.5  AM) * 100.

Results.=

Voc = 562.222 mV

Jsc = 12.37 mA/cm^2

Pmax = 5.76976 mW/cm^2

η % = 5.76976%

Voltage and current waveform of various type of loads- Simulations

In this post, I have plotted volatage and current wavefrom under various loads such as resistive, capacitive, inductive and thier combination under sinusoidal input through simulation softwares.

1. With Resistive Load- RESISTOR 10 ohm

2. With Inductive load- 10mH

3. Capacitive load- 10 mF (10 mili Farad)

4. With Resistive and Capacitive ( RC ) Load R- 10ohm & C- 10 mF 

5. With Resistive and Capacitive ( RC ) Load R- 10ohm & C- 1mF 

Comparison with C= 10mF & 1 mF

6. With Resistive and Inductive ( RL ) Load R- 10ohm & L- 10mH

7. With Resistive and Inductive ( RL ) Load R- 10ohm & L- 1mH

Comparison between with 10 mH & 1 mH

8. With Resistive, Capacitive and Inductive ( RL ) Load R- 10ohm, C- 10mF & L- 1mH

There are many simulation software such as Multisim, Matlab, proteus available for simulation trial version may be downloaded for beginning. It will help you in understand the theory in good way.

Calculation of the theoretical heat generated in the Proton Exchange Membrane fuel cell & Electrical potential Difference

Calculation of the theoretical heat generated in the fuel cell.

Basic reaction at fuel cell electrodes-

                 At the anode:       H₂ →  2H⁺ + 2e^-                                                                                                   

                 At the cathode:    ½ O₂  + 2H+ + 2e- →  H₂O                                            

                 Overall:               H₂ + ½ O₂ →  H₂O                                                                      

The overall reaction is same as the hydrogen combustion. Since combustion is exothermic process so heat will be released -

                                  H₂ + ½ O₂ →  H₂O + heat

Standard enthalpy of reaction can be calculated by difference of heat of formation of products and reactant.

       ΔH= (hf)_H2O(l)- (hf)_H2- (hf)_0.5O2

Heat of formation of liquid water is -

(hf)_H2O = -286 kJmol^-1 (at 25°C)

Heat of formation of reactant gases 

(hf)H₂₌ (hf)0.5O₂ =0 kJ/mol

So,                                                  ΔH= -286-0-0= -286 kJ/mol.

-ve sign indicates that reaction is exothermic.

But if products H2O is not cooled enough to become liquid, and it remains as vapour then the enthalpy of reaction reduced by enthalpy of vaporisation of water ie. 44 kJ/mol

     ΔH= (hf)_H2O(g)- (hf)_H2- (hf)_0.5O2

              = -286+44 -0-0= -242 kJ/mol.

All the generated energy cannot be converted into electricity due the reaction entropy. The portion of the reaction enthalpy (or hydrogen's higher heating value) that can be converted to electricity in a fuel cell corresponds to Gibbs free energy and is given by the following equation:

                                                       DG = DH- TDS    .

So Gibbs free energy in the liquid water conversion at 25⁰ C

                                                      DG= -286-(25+273)*(-0.1663)= -237 kJ/mol.

Heat produced theoretically in the reaction is = -237-(-286)= 49 kJ/mol

And Gibbs free energy in the vapour water conversion at 25⁰ C

                                                     DG= -242- (25+273)*(-0.0444) = -229 kJ/mol.

Heat produced theoretically in the reaction is = -229- (-242)= 13kJ/mol.

Electrical work:                           W _el= nFE,       

 n= no of electrons per mole. E= electric potential

Maximum amount of electrical energy generated in a fuel cell corresponds to Gibbs free energy, DG:                                              

W_el = - DG =nFE

So theoretical electrical potential   E= = - DG/ nF

                                                            =- (-237*1000)/ (2*96500) = 1.23 V 

Fore more details on fuel cell, please commment, I can share more details of fuel cell. 

School project for Kids- Solar powered fan in a Cap

 

Aim- To make familiar with solar panel and its electrical output by running a small DC fan with the solar panel.

Components required-

1.       Solar panel (6V, 200mA)

2.       Diode (1N4007)

3.       Small DC Fan (5V, 200mA)

Theory-

The solar panel generates electrical power when it is placed under the sun light. The output depends on the solar panel dimensions and solar radiation available to solar panel. The diode is unidirectional device. It conducts the current in one direction and blocks in the opposite directions. Due to this property diode is used to stop the reverse power flow toward the solar panel. The fan will work as load and will take electrical power generated from the solar panel to blow the air.

Steps-

1.      Take the solar panel and put in the opposite way.

2.    Find the + (plus) and – (minus) symbol on back side.

3.       To the +side connect the black side of diode.

4.       To the silver side of diode connect the fan + terminal (red wire).

5.       Connect the –ve terminal (black wire) of Fan to the –ve side of solar panel.

6.       Now take the solar panel in the sun light and put front side up. The fan will start running.

7.       Now just put solar panel in different sun lights intensity and see the variation in the fan speed.

Connection Diagram-

Follwing type product can be made using above-

sample design.

More innovative design can be made. Your ideas may please  be shared in the comment section. 

Design aspect of Solar panel operated ventilation system or Airconditioner for avoiding Heat-stroke in Car using

 

Heatstroke-

 Heatstroke (or sunstroke) is a heat illness defined as a body temperature of greater than 40.6 °c (105.1 °F) due to environmental heat exposure with lack of thermoregulation.

It happens due to lack of water in body so that body temperature becomes more than the 40.6⁰c.

Fact and Causes of Heatstroke-

When car is parked in 25°C ambient condition under normal sunlight and car inside temperature is 22°C, within 15 minute the inside temperature rises to 48°C even when window is partially open.

This is due to greenhouse effect of interior of Car. The glasses passes the visible light(sun light) but does not allow to IR radiation(heat waves) to come out. Due to which temperature increases very fast inside the car.

The human body wants to stay at 98.6 degrees F(37⁰ C). The only way to stay at 98.6 is to sweat. By putting moisture on the skin and letting it evaporate, our body can cool itself very effectively and keep its temperature in the proper range.

At 48-50 ⁰C the rate of sweating becomes very much so human body needs lots of water. Due to unavailability of water body temperature increases very fast and leads to heat stroke.

Effect of heatstroke-

Sometimes parents leaves their child in the car and goes for shopping or other purposes. The heatstroke in car leads to death of the child inside. Many cases have been reported in this regard.

Due to high temperature of the car seat and inside air, if the persons comes to drive the car, he has to run the A. C. at very high cooling rate for few minute which leads to more fuel consumption.

Also sometimes, Car driver stays inside the car and run the car engine to cool it, which leads to much more fuel consumption.

Following are few designs-

Design-1

Targeted for lower end customer and less stylish design-

·         A solar vehicle ventilator also called solar car vent or solar car fan

·         Sun's energy converted  into the low-voltage electricity required to drive a small fan

·         The fan blows stale, hot air out of your vehicle and draws fresher air in.

Features -

1) Auto cool solar powered car fan

2) Auto cool is the revolutionary solar powered ventilation system to keep it up to 30 C cooler than it would normally be

(3) Being compact, it fits in any car window and does not need batteries

Specification -

 Colour: Same as car

 Item size: 14.5x 11 x 6cm

 Solar panel size: 10x7 cm

 Cost: $ 10

 Package item: 342 g

Design-2

Design for upper end customer and good looking design

•      Solar panel on the top of CAR will be placed.

•      Can give enough power to run a fan for good Ventilation

•      With use of high efficiency panel Air-condition Also can be run.

·         Area available= 3m²

·         Power available by panel= 3*150= 450 W_p

·         Panel cost(for m-c Si )= 23,000 INR

·         Ventilation system cost= 5,000 INR

·         Installation cost= 4,000 INR

·         Total cost= 32,000 INR

·         Can maintain temperature near to the ambient condition

Design-3

Design for long cars and more cooling requirement design

• Foldable panel design
• Can give large area to absorb irradiance
• Air-conditioning system can be used  cool the car
• Panel will give continuously energy at low power, it will be stored in the battery, battery will give power to charge super-capacitor at low power for more time. To run AC Super-capacitor will give high power for short time.

• On long Car available total area available is= 7 m²
• So panel can be give power= 7x150= 1050 W_p

•      This Panel will cost= 50,000 INR

•      Super capacitor cost= 10,000 INR

•      BLDC motor cost= 3,000 INR

•      Installation cost= 5,000 INR

• Total cost= 68,000INR
• Can maintain temperature ~ 22⁰C

Prototype design result-

we designed a solar panel driven Thermo- electric based cooler in a closed box of around 1ft x1ft x 0.5 ft and measured the test data as follwing. 

Conclusion-

•      This is good idea to solve current problems of sunstroke in Cars by using solar power.

•      By using this idea we can able to reduce AC operating cost. 

•      It will reduces fuel consumption so it will cut carbon footprint.