Photovoltaics Cells

We are on the cusp of a new era of Energy Independence

Name-Ruchit Bubna Roll no-UE-114050

Topics to be covered in Presentation1.1. What are PhotoVoltaic cell? 2. How are PhotoVoltaic cell fabricated? 3. What are type of PhotoVoltaic cell? 4. What are the parameters?

Photovoltaics (PV) is a method of converting solar energy into direct current electricity using semiconducting materials that exhibit the photovoltaic effect. The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light.

n-type semiconductorp-type semiconductor + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - Physics of Photovoltaic Generation

Depletion Zone

Photovoltaic SystemTypical output of a module (~30 cells) is 15 V, with 1.5 A current

PV CELL FABRICATIONThe process of fabricating conventional single- and polycrystalline silicon PV cells begins with very pure semiconductor-grade polysilicon - a material processed from quartz and used extensively throughout the electronics industry. The polysilicon is then heated to melting temperature, and trace amounts of boron are added to the melt to create a P-type semiconductor material. Next, an ingot, or block of silicon is formed, commonly using one of two methods:

by growing a pure crystalline silicon ingot from a seed crystal drawn from the molten polysilicon or 2) by casting the molten polysilicon in a block, creating a polycrystalline silicon material.

Individual wafers are then sliced from the ingots using wire saws and then subjected to a surface etching process. After the wafers are cleaned, they are placed in a phosphorus diffusion furnace, creating a thin N-type semiconductor layer around the entire outer surface of the cell.

Next, an anti-reflective coating is applied to the top surface of the cell, and electrical contacts are imprinted on the top (negative) surface of the cell. An aluminized conductive material is deposited on the back (positive) surface of each cell, restoring the P-type properties of the back surface by displacing the diffused phosphorus layer. Each cell is then electrically tested, sorted based on current output, and electrically connected to other cells to form cell circuits for assembly in PV modules.

Photovoltaic Cell

TYPE OF PHOTOVOLTAIC CELL

Silicon Crystalline Technology Thin Film Technology

Mono Crystalline PV Cells Amorphous Silicon PV Cells Multi Crystalline PV Cells Poly Crystalline PV Cells ( Non-Silicon based)

Silicon Crystalline TechnologyCurrently makes up 86% of PV market Very stable with module efficiencies 10-16%

Mono crystalline PV CellsMade using saw-cut from single cylindrical crystal of SiOperating efficiency up to 15%

Multi Crystalline PV CellsCaste from ingot of melted and recrystallised siliconCell efficiency ~12%Accounts for 90% of crystalline Si market

Thin Film Technology Silicon deposited in a continuous on a base material such as glass, metal or polymersThin-film crystalline solar cell consists of layers about 10m thick compared with 200-300m layers for crystalline silicon cells

PROS Low cost substrate and fabrication process

CONS Not very stable

Amorphous Silicon PV Cells The most advanced of thin film technologies Operating efficiency ~6% Makes up about 13% of PV market

PROS Mature manufacturing technologies available

CONS Initial 20-40% loss in efficiency

Poly Crystalline PV CellsCopper Indium Diselinide CIS with band gap 1eV, high absorption coefficient 105cm-1 High efficiency levels

PROS 18% laboratory efficiency >11% module efficiencyCONS Immature manufacturing process Slow vacuum process

Non Silicon Based Technology

Poly Crystalline PV CellsCadmium Telluride ( CdTe)Unlike most other II/IV material CdTe exhibits direct band gap of 1.4eV and high absorption coefficient

PROS 16% laboratory efficiency6-9% module efficiency CONSImmature manufacturing process

Non Silicon Based Technology

Semiconductor Material Efficiencies

Parameters of P.V cells

Parameters Substrate Material (usually silicon) :-Bulk crystalline silicon dominates the current photovoltaic market, in part due to the prominence of silicon in the integrated circuit market. As is also the case for transistors, silicon does not have optimum material parameters.

Cell Thickness (100-500 m) :-An optimum silicon solar cell with light trapping and very good surface passivation is about 100 m thick. However, thickness between 200 and 500m are typically used, partly for practical issues such as making and handling thin wafers, and partly for surface passivation reasons.

Doping of Base (1 cm)A higher base doping leads to a higher Voc and lower resistance, but higher levels of doping result in damage to the crystal.

Doping of Base ( 1 cm)A higher base doping leads to a higher Voc and lower resistance, but higher levels of doping result in damage to the crystal.

Reflection Control (front surface typically textured):-The front surface is textured to increase the amount of light coupled into the cell.

Emitter Dopant (n-type):-N-type silicon has a higher surface quality than p-type silicon so it is placed at the front of the cell where most of the light is absorbed. Thus the top of the cell is the negative terminal and the rear of the cell is the positive terminal.

Emitter Thickness (

Doping Level of Emitter (100 /)The front junction is doped to a level sufficient to conduct away the generated electricity without resistive loses. However, excessive levels of doping reduces thematerial's quality to the extent that carriers recombine before reaching the junction.

Grid Pattern. (fingers 20 to 200 m width, placed 1 - 5 mm apart)The resistivity of silicon is too high to conduct away all the current generated, so a lower resistivity metal grid is placed on the surface to conduct away the current. The metal grid shades the cell from the incoming light so there is a compromise between light collection and resistance of the metal grid. Rear Contact.The rear contact is much less important than the front contact since it is much further away from the junction and does not need to be transparent. The design of the rear contact is becoming increasingly important as overall efficiency increasesand the cells become thinner.

Concluding RemarksThe key to successful solar energy installation is to use quality components that have long lifetimes and require minimal maintenance.

The future is bright for continued PV technology dissemination. PV technology fills a significant need in supplying electricity, creating local jobs and promoting economic development in rural areas, avoiding the external environmental costs associated with traditional electrical generation technologies.

Major power policy reforms and tax incentives will play a major role if all the above said is to be effectively realized.

The Light at the end of the Tunnel By 2020 global solar output could be 276 Terawatt hours, which would equal 30% of Africa's energy needs or 1% of global demand. This would replace the output of 75 new coal fired power stations. The global solar infrastructure would have an investment value of US$75 billion a year. By 2040 global solar output could be more than 9000 Terawatt hours, or 26% of the expected global demand

Report European Photovoltaic Industry Association (EPIA) and Greenpeace

Can technological developments and the transition to a culture that is more aware of the need to safeguard the environment help create a world powered by the Suns Energy ?