Basics of Third Generation Solar Cells and Their Types

Third Generation Solar Cells :

They are proposed to be very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers.

For space applications quantum well devices (quantum dots, quantum ropes etc.) and devices incorporating carbon nanotubes are being studied with a potential for up to $45 %$ production efficiency.

For terrestrial applications, these new devices include photoelectrochemical cells, polymer solar cells, nanocrystal solar cells,dye sensitized solar cells and are still in the research phase.

Types of Third Generation Solar Cells :

A.) Organic Photovoltaic Cell :

1. The solar cells based on organic semiconductor can provide a low cost alternative for photovoltaic solar.

2. The thickness of the active layer of organic solar cells is only $100 nm$ thin, which is about $1000$ times thinner than the crystalline silicon solar cells, and it is about 10 times thinner than the current inorganic thin film solar cells.

3. In the low material consumption per solar cell and the relatively simpler cell processing of organic semiconductors, there is a large potential for low cost large area solar cells.

4. Due to this reason, there is a considerable interest in organic photovoltaic devices.

5. Their principal advantage is that they are flexible and can bend without breaking, unlike $Si$, which is brittle.

6. They are also very light and cheap.

7. They may folded or cut into required size and can still be used.

B.) Dye Sensitized Solar Cell (DSSC):

1. Dye Sensitized Solar Cell converts any visible light into electrical energy.

2. The dye sensitized solar cells can be considered as a thin film solar cell device. This technology is not yet commercialized but is on the verge of commercialization.

3. The dye sensitized solar cells can be made flexible. It has a very good potential for being a low cost effect solar cell technology.

4. This is mainly possible because of the large availability and low cost of the ingredient material as well as due to the low processing temperatures.

5. The dye sensitized solar cells is a photo-electro-chemical device. In its operation it involves a photon, an electron and a chemical reaction.

6. The operation of dye sensitized solar cell is considered similar to that of a photosynthesis process.

7. The DSSC has a number of attractive features; it is simple to make using conventional roll-printing techniques, is semi-flexible and semi- transparent which offers different type of uses not applicable to glass-based systems, and cost of most of the materials used in DSSC are very low.

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Basics of Semiconductor Materials

Semiconductor materials:

Those materials with conductivity greater than insulators and less than conductors are known as semiconductor materials.

According to band gap theory:

Those materials that have a band gap between the conduction band and the valence band is approximately one electron volt are called semiconductor materials.

Description of semiconductor material based on Bandgap theory:

a.) At Room temperature:

The conduction band and valence band are partially filled at room temperature.

b.) At very high temperature:

At very high temperatures, the conduction band is completely filled and the valence completely empty due to this, the semiconductor behaves like a conductor.

c.) At very low temperature:

At very low temperatures, the conduction band is completely empty and the valence band is completely filled due to this the semiconductor behaves like an insulator.

Types of semiconductor material:

There are two types of semiconductor materials:

1.) Intrinsic semiconductor materials
2.) Extrinsic semiconductor materials

1.) Intrinsic semiconductor materials:

The pure form of the semiconductor materials is known as intrinsic semiconductor materials.

Examples: Carbon, Germanium, Silicon, etc.

General description:

Intrinsic semiconductor material is the pure form of the semiconductor material like carbon Germanium silicon. The atoms of the semiconductor material has four valence electrons and tightly bound to the nucleus. all the atoms are bound with covalent bonds.

At very low temperatures or Absolute temperatures, all valence electrons are tightly bound to the core of the atom and no free electrons are available to conduct electricity through the semiconductor crystal.

At room temperature, a few valence electrons are thermally excited into the conduction band and free to move about. These few thermally excited electrons leave holes in the valence band. the conductivity of an intrinsic semiconductor is very poor that is only one covalent bond breaks in $10^{9}$ atoms of a semiconductor like Germanium. It means that only one atom in $10^{9}$ atoms is available for conduction. The concentration of free electrons and holes in intrinsic semiconductors are equal.

At very high temperatures, a large number of electrons and holes are produced. When an electric field is applied to the semiconductor crystal the free electrons in the conduction band move in the opposite direction of the applied field and holes in the valence band move in the direction of the applied field, both give rise to electric current. The motion of holes is apparent.

2.) Extrinsic semiconductor materials:

When a small amount of impurity (i.e. external material atoms) is added to intrinsic semiconductor materials then these materials are known as extrinsic semiconductor materials.

Note:

a.) What is doping?

Answer: The process of adding a small amount of impurity atoms in intrinsic semiconductor materials is known as doping.

b.) What is impurity or doped semiconductor?

Answer: The impurity or doped semiconductor is the atoms of external material with a valency of pentavalent or trivalent.

The pentavalent impurity atom (i.e. outer shell has $+5$ electrons) is also called the donor atom. Because it donates conducting electrons to the atom of a semiconductor crystal. The trivalent impurity atom (i.e. outer shell has $+3$ electrons) is also called the acceptor atom. Because it accepts the conducting electron from the neighbor atom of the semiconductor crystal.

Example: $1$ impurity atom added in $108$ semiconductors atoms of Germanium increases the conductivity of $16$ times

Example:
Pentavalent: Antimony, Phosphorus or arsenic etc.
Trivalent: Boron, Aluminium, Gallium or Indium etc

Types of extrinsic semiconductor materials:

The extrinsic semiconductor materials are two types-

i.) N-Type Semiconductor Materials
ii.) P-Type Semiconductor Materials

i.) N-Type Semiconductor Materials:

When a very small amount of pentavalent impurity atoms are added to the intrinsic semiconductor material, this semiconductor material is known as an n-type semiconductor. The electrons in n-type semiconductors are majority charge carriers.

General description:

When the pentavalent impurity atoms (like Phosphorus) are added to the semiconductor materials (like $Ge$), they replace the semiconductor atoms and take place in between them. Now the four electrons of the pentavalent atom make the covalent bond with neighbor semiconductor atoms. Still, the fifth electron does not make the covalent bond. It becomes free (a very small amount of energy is required to free i.e. $0.01 eV $ in $Ge$ and $0.05 eV $ for $Si$ lattice) at room temperature and moves in semiconductor crystal as charge-carrier. The electrons are charge carriers because of that it is called negative-type semiconductors or n-type semiconductors.

ii.) P- Type Semiconductor Materials:

When a very small amount of trivalent impurity atoms are added to the intrinsic semiconductor material, this semiconductor material is known as a p-type semiconductor. The holes in p-type semiconductors are majority charge carriers.

General description:

When the trivalent impurity atoms (like Boron) are added to the semiconductor materials (like $Ge$), it replace the semiconductor atoms and take place them. Now the three electrons of the pentavalent atom make the covalent bond with neighbor semiconductor atoms but the fourth electron of the neighbor semiconductor atom does not make the covalent bond with the trivalent impurity atom because of that and empty space is created near the trivalent atom. This empty space is called a hole. A hole moves in the semiconductor crystal as a charge carrier in the opposite direction of the flow of electrons (or in the direction of an external electric field) in the presence of an external electric field. These charge-carries act as positive charge-carries because of that it is called positive-type semiconductors or p-type semiconductors.

Note:

Explanation of flow of hole in semiconductor crystal:

When an external field is applied, an electron of a semiconductor atom bound near a trivalent impurity atom moves toward a hole (near to impurity atom) and leaves the new hole behind. This process is continuous and the hole moves in the semiconductor crystal in the direction of an external electric field or in the opposite direction of electron flow ( i.e. aThe electric potential of electron is negative and the hole is zero so electron move from lower potential to higher potential.).

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