Photovoltaic cell

1. Photovoltaic cell Abstract Background Working principle Fabrication Arrays and Systems Potential

2. Few application of photo cell

3. Abstract • Solar photovoltaic energy conversion is a one-step conversion process which • generates electrical energy from light energy. • Light is made up of packets of energy called Photons. When they hit a solid • surface they excite the electrons, bound into solid, up to a higher energy level in which • they are more free to move. But these electrons relax and come back to the ground • state within no time. • In a photovoltaic device, however, there is some built-in asymmetry which pulls the • excited electrons away before they can relax, and feeds them to an external circuit.

4. Background • most commonly manufactured PV cells are made of crystalline silicon and have energy conversion efficiency of 12%. • The cost of these cells is $3 per Watt of power generated under solar AM 1.5G conditions • these costs need to be reduced by an order of magnitude to around $0.3 per Watt for PV cells to be competitive with otherenergy generation system • reducing the costs of PV cells may be achieved if the semiconductor • were deposited from solution onto large flexible substrates in reel-to-reel coating • reducing the costs of PV cells may be achieved if the semiconductor • were deposited from solution onto large flexible substrates in reel-to-reel coating

5. Working principle • PHOTOCURRENT • The photo current generated by a solar cell under illumination at short circuit • is dependent on the incident light. • The photocurrent density Jsc is • QE(E) is the probability that an incident photon of energy ‘E’ will deliver one electron • to the external circuit. • bs(E) is the incident spectral photon flux density, the number of photons of energy in • the range E. • QE and spectrum can be given as functions of either photon energy or • wavelength, λ

6. DARK CURRENT AND OPEN CIRCUIT VOLTAGE • When a load is present, a potential difference develops between the terminals of the • cell. This potential difference generates a current which acts in the opposite direction • to the photocurrent, and the net current is reduced from its short circuit value. This • reverse current is usually called the dark current. • Where Jo is a constant, kB is Boltzmann's constant and T is temperature in degrees • Kelvin. • When the contacts are isolated, the potential difference has its maximum • value, the open circuit voltage Voc. This is equivalent to the condition when • the dark current and short circuit photocurrent exactly cancel out. For the • ideal diode, from ideal diode equation

7. EFFICIENCY • The cell power density is given by P=JV • P reaches a maximum at the cell's operating point or maximum power point. This • occurs at some voltage Vm with a corresponding current density Jm. • The fill factor is defined as the ratio FF = (JmVm) / (JscVoc) • The efficiency of the cell is the power density delivered at operating point as a • fraction of the incident light power density, Ps • Efficiency is related to Jsc and Voc using FF. • These four quantities: Jsc, Voc, FF and η are the key performance characteristics • of a solar cell.

8. Performance of some types of PV cell

9. Non-ideal diode behaviour • The ideal diode behaviour is seldom seen. It is common for the dark current to • depend more weakly on bias. The actual dependence on V is quantified by an ideality • factor, m and the current-voltage characteristic given by the non-ideal diode equation, • m typically lies between 1 and 2.

10. Illumination of pn junction

11. Some characteristic of diode Due to doped element gradient electron and hole get drifted to other side that cause built in potential at junction

12. For positive voltage current will exponential and for negative voltage it will constant negative exponential

13. When circuit is working in 4’th quadrant power driven to circuit will be positive and in there two case it will be negative so photovoltaic cell do work in fourth quadrant

14. Design parameter of photo voltaic cell • Material • Impurity as adaptor and donor • Impurity quantity • Impurity injection method • Resistance of cell • Efficiency • Band gap

15. Efficiency • To increase efficiency we use material which has proper band gap • To ensure full absorption of photo we use anti reflective material on cell • large mirrors or lenses to concentrate and focus the sunlight onto a string of cell can be used to improve efficiency by reduction in no. of cell • Efficiency is inversely proportional to temperature so hightefficiency can be achieved by keep cooling the panel • To get maximum photon flux panel should facing to sun • Efficiency can be maximize by multiple carrier generation by single photon

16. Resistance of cell • Series resistance of only few ohm can seriously cause In reduction in power loss • Resistance could be minimize by increasing cell aria • Resistance can be further minimize by distributing the contact over n region so current would distributed over the surface

17. Dimension of cell • Dimension of cell should be such that generated electron-hole pair could reach the surface before recombination take place • So there should be proper match between diffusion length and thickness of p region and penetration depth 1/diff. coff. • life time of carrier is inversely proportional to concentration of doping • Contact potential is directly propositional to doping • So there is trade-off between lifetime of Carrier and contactpotential

18. Working principle • Solar cell is simple diode with special desgin • Enough energetic photon cause generation of electron-hole pair • Excited electron and hole get drifted by built-in potential in depletion region • The drift current cause current in circuit. • Voltage across individual cell is equal to built in potential

19. Type of solar cell

20. fabrication • Single Crystal solar cells • Single crystal wafers are sliced from a large single crystal ingot • It is a very expensive process • The silicon must be of a very high purity and have a near perfect crystal structure • Polycrystalline solar • Polycrystalline wafers are made by a casting process • molten silicon is poured into a mould and allowed to set • Then it is sliced into wafers  • it is not as efficient as monocrystallinecells • The lower efficiency is due to imperfections in the crystal structure resulting from the casting process • Amorphous-Si solar • Amorphous silicon is one of the thin film technologies • It is made by depositing silicon onto a glass substrate from a reactive gas such as silane (SiH4) 

21. Pn junction formation • dopant atoms introduced to create a p-type and an n-type region • doping can be done by high temperature diffusion • where the wafers are placed in a furnace with the dopant introduced as a vapour • Once a p-n junction is created, electrical contacts are made to the front and the back of the cell • evaporating or screen printing metal on to the wafer to form contact

22. Arrays and Systems •  PV cells have a working voltage of about 0.5  • they are usually connected together in series (positive to negative) to provide larger voltages • low power panels are made by connecting between 3 and 12 small segments of amorphous silicon PV • larger systems can be made by linking a number of panels together • PV panel array, ranging from two to many hundreds of panels • the output voltage is limited to between 12 and 50 volts, but with higher amperage  • This is both for safety and to minimize power losses • Arrays of panels are being increasingly used in building construction

24. Potential • The photovoltaic industry is growing rapidly as concern increases about global warming • For most of the eighties and early nineties the major markets for solar panels were remote area power supplies and consumer products • However in the mid nineties a major effort was launched to develop building integrated solar panels for grid connected applications • energy output from PV panels will vary depending on the orientation, location, daily weather and season • On a clear sunny day, the power density of is approximately 1kW/m2 • The solar energy received by Earth is more than 10,000 times the current use of fossil fuels and nuclear energy combined • harnessing such a large potential energy source has the potential to replace a significant amount of carbon based fuels