What is solar cell?
A structure that converts solar energy directly to DC electric energy.
It is like a battery because it supplies DC power
It is different from the battery in the sense that the voltage supplied by the cell changes with changes in the resistance of load.
COMPONENTS OF SOLAR CELLS
These include solar cell panels, one or more batteries, a charge regulator or controller for a standalone system, an inverter for a utility grid-connected system, requirement of alternating current(AC) rather than direct current(DC), wiring, and mounting hardware or a framework.
It is an unpolished p-type wafer referred to as p-region base material.
The important parameters to be kept in mind while choosing a wafer for solar cells are its orientation, resistivity, thickness and doping.
Typical thickness of wafers used for solar cells is 180-300Β΅m.
The typical resistivity values are in 1-2Ξ©cm.
The doping should be close to 5×10^15/〖ππ〗^3 to 1×10^16 /〖ππ〗^3.
The wafer can be single crystalline or multi crystalline.
EMITTER
The emitter formation involves the doping of silicon with pentavalent impurities such has Ph, Ar and antimony.
However, for solar applications Ph is widely used has impurity.
Doping is done by process of diffusion.
The junction depths are in the range of 0.2- 1Β΅m.
This is also commonly known as n- region diffused layers.
ELECTRICAL CONTACT
These are essential to a photovoltaic cell since they bridge the connection between semiconductor material and external electrical load.
It includes:
Back contact: It is metallic conductor completely covering back. The back contact of a cell is located on the said away from incoming sun light and is relatively simple. It usually consists of a layer of aluminium.
Front contact: Current collection grid of metallic finger type is arranged in such a way that photon energy fall on N-region diffused layers. The front contact is located on the side facing the light source and is more complicated. When light falls on solar cell a current of electrons flow over the surface.
Anti- reflective coatings: They are applied to reduce surface reflection and maximize cell efficiency in solar glass and silicon solar cell manufacturing. When a thin-film nano-coating anti- reflecting coating of silicon dioxide and titanium dioxide is applied, there seems to be an increase in cell efficiency by 3-4%.
SOLAR CELL MATERIALS
Many combinations of materials and methods of fabrication of photovoltaic cells are now either in practical use or in various developmental stage.
Silicon is the most widely used basic material because of its suitability and its availability in abundance.
More than 80% are crystalline silicon solar cells.
Other 20% are developed as amorphous silicon solar cells.
The band gap of a semi-conductor material is the minimum energy needed to move an electron from its bound state within an atom to a free state.
The lower energy state is ‘valence band’ and higher is ‘conduction band’ .
SILICON
This remains the most popular material for solar cells, including these types;
Mono crystalline or single crystal silicon
Polycrystalline silicon
Amorphous silicon
Polycrystalline wafers are made by a casting process in which molten silicon is poured into a mould and allowed to set.
They are sliced into wafers.
They are significantly cheaper but not efficient.
Amorphous silicon, one of the thin film technologies, is made by depositing silico onto a glass substrate from a reactive gas like silane.
There are many advantages of thin film cells
Easier deposition and assembly
Ability to deposited on inexpensive substrates
Ease of mass production
Suitability to large applications.
PRACTICAL SOLAR CELLS
Solar cells are now manufactured from a number of different semiconductors that are summarized in the following points.
Crystalline silicon cells: They dominate the photovoltaic market. To reduce the cost, they are often made from multi-crystalline material, rather then from the more expensive single crystals. The technology is well established. The modules have long life time (20years or more).
Amorphous silicon solar cells: they are cheaper type of silicon cells made in the form of amorphous thin films that are used to power a variety of consumer products; but they are less efficient.
Cadmium telluride and copper indium diselenide: Thin-film modules are now beginning to appear on the market and hold the promise of combining low cost with acceptable conversion efficiencies.
High efficiency solar cells: From gallium arsenide, indium phosphide or their derivatives are used in specialized applications, for example, to power satellites or in systems that operates under high-intensity concentrated sunlight.
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