Photovoltaic cell Abstract Background Working principle Fabrication Arrays and Systems Potential

Few application of photo cell

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.

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 other energy 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

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

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

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.

Performance of some types of PV cell

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.

Illumination of pn junction

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

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

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

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

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 hight efficiency 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

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

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 contact potential

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

Type of solar cell Single Crystal solar cells in panel Silicon solar cells are made using either single crystal wafers, polycrystalline wafers or thin films approx. 1/3 to 1/2 of a millimeter thick The silicon must be of a very high purity and have a near perfect crystal structure Polycrystalline solar panel Polycrystalline wafers are made by a casting process Amorphous-Si solar panel Amorphous silicon, one of the thin film technologies

fabrication Single Crystal solar cells Polycrystalline solar 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 monocrystalline cells 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) 

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

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

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