SUBDISCIPLINAS Y CORRIENTES DE INVESTIGACIÓN ORIENTADAS AL

Solar Cells are devices, which convert solar energy directly into electricity.

The most common form of solar cells is based on the photovoltaic (PV) effect in which light falling on a two-layer semi-conductor device produces a photo voltage or potential difference between the layers. This voltage is capable of driving a current through an external circuit and thereby producing useful work.

To have a deeper understanding of PV effect, it is essential to become familiar with the principles of construction and operation of a two-layer semiconductor device popularly known as PN junction.

It is well known from the first course of physics that all mater is made of atoms which consist of a small dense nucleus containing positive and neutral particles (protons and neutrons) a surrounding “cloud” of fast moving negatively charged particles (electrons).

The outer most electrons (valence electrons) seem to be arranged in symmetrical elongated shells or orbitals, like stretched out clouds. Neighboring atoms share outer electrons, forming “bonds”. These bonds where electrons are shared between atoms is what holds all mater together. The valence electrons play very important role in defining the electricity conducting capacity of a material.

As defined in earlier chapter, the electric current is the flow of free (un-bonded) charged particles (electrons) in a matter. An electron can take part in conduction of electric current if it is loosely bonded with the atoms. In all metals, the valence electrons are loosely bonded with the atom and with some minimal external energy applied (in the formal thermal energy) they become free and ready to take part in conduction of electric current. In metals each atom can release one electron to become free. Therefore the number of free electrons available in metals is very high (in one cubic meter of matter there are about 10²⁹ atoms; each atom releasing one electron to become free results in about 10²⁹ free electrons in metals) resulting very good conduction capacity (very low resistivity) by the metals. On the other hand, materials classified as insulators have valence electrons tightly bonded with atoms. Great deal of external energy is required to let these electrons free. At normal temperature, the insulators have virtually no free electrons to contribute for electricity conduction. That is why the conduction capacity of insulating materials is extremely low (very high resistivity).

There is another group of material whose conductivity (or say resistivity) lies between that of conductors and insulators. This group of materials are called semiconductor.

These semiconductors are basic building blocks of all the electronic components and the solar cells. Silicon and Germanium are the examples of semiconductor materials. A silicon atom has 4 outer electrons. Crystalline silicon consists of orderly bonding of each silicon atom with 4 neighboring silicon atoms. Such a highly ordered structure of atoms is also called a crystal lattice. Each of the four outer electrons of one atom is shared by surrounding four atoms to form an effect of 8 outer electrons (the most stable condition)

for each atom. The bond that binds each outer most electrons together is called covalent bond (fig. 5.1.1).

At the atomic level, light acts as a flux of discrete particles called photons. Photons carry momentum and energy but are electrically neutral. When semiconductor material is illuminated by light, photons of light actually penetrate into the material, traversing deep into the solid. Photons with enough energy can collide with bonded electrons and knock them out of their original position. During the collision the photon disappears and its energy is transferred to the dislodged electron. The newly dislodged electron now becomes free and can wander around the semiconductor material as conduction electron.

This free electron carries a negative charge and usable energy. It is at this moment of releasing the electron that sunlight energy has been converted into electrical energy. And this effect of converting light energy into electrical energy is called photovoltaic effect.

Whenever an electron is freed, it leaves a vacant position in its original position in the covalent bond. Such an incomplete bond (with missing electron) is called a "hole". A nearby electron with higher energy level can jump from its bond into the hole and fill it, but this leaves a hole where the electron came from. In this way the hole moves in the material. But wherever the hole is, an electron is missing, so there is a localized net positive electrical imbalance there. The atom with a hole is referred to as positive ion.

Therefore the hole appears to be a positive charge moving in the solid, although it is really an absence of an electron moving about. Overall, the net charge of the material is neutral.

In the absence of any external electrical field, newly freed electrons wander for a short time and then recombine with a wandering hole. During recombination, the energy

Co-valent bond Photon

Fig. 5.1.1 Crystalline structure of semiconductor material

gained by the freed electron is released and converted into heat. The key idea of producing usable output current is to sweep the freed electrons out of the material before they recombine with the holes. This task of sweeping the free charge carriers is accomplished by creating internal electric field in a junction of two different types of semiconductors.

In pure silicon, the number of freed electrons is always equal to holes. Adding impurities in it can increase the conductivity of pure or intrinsic silicon. The impurity is referred to as dopant and the process of adding dopant is called doping. Depending upon the type of dopant used, the impure or extrinsic semiconductor is called P type or N type semiconductor. By joining these two types of semiconductors, it is possible to create internal electric field to sweep freed electrons out of the material and force them to produce usable current.

P Type Semiconductor

Boron is a type of semiconductor material having only three valence electrons. If we add boron to intrinsic semiconductor, then each boron atom will bond with three atoms of silicon leaving one covalent bond of silicon half complete (fig.5.1.2).

The half complete bond represents a hole. The nearby electron can vibrate and jump into this hole leaving a hole in its original position. So there exists in the semiconductor structure a wandering absence of an electron. In other words, each doped boron atom will

Deficit of one electron leaves

"Hole"

Fig.5.1.2 P-type semiconductor

Silicon atom Boron atom

Nearby electron can move in and fill the hole

why the extrinsic semiconductor doped with trivalent impurity is called P type or positive type semiconductor. The concentration of boron is quite low, usually around one boron atom to every 10⁶ silicon atoms. The overall net charge in the semiconductor is neutral.

But in the small regions, the boron atom has net negative charge because one extra electron has fallen in the empty bond. And the silicon atom from where the electron ran away remains positively charged because one electron is missed from the bond.

N Type Semiconductor

Now if penta-valent impurity (e.g. phosphorous) is added to intrinsic semiconductor, the four outer electrons of dopant make covalent bond with four silicon atoms (fig. 5.1.3).

The fifth electron of dopant atom breaks away easily as there is no bond to hold it. This free electron moves around the material carrying negative charge. Since there exists localized excess of negative charge, the extrinsic semiconductor is called N type semiconductor.

The concentration of phosphorous atoms is again quite low, but typically greater than the boron concentration, usually around one impurity atom for every 10³ silicon atoms.

The PN Junction or Internal Electric Field

Regions of P type and N type semiconductors are created adjacent to another to form a PN junction (fig.5.1.4). Immediately after creation of the adjacent regions, free electrons from N type semiconductor cross the junction and permanently fall into the holes of P

Free electron Phosphorous atom

Silicon atom

Fig. 5.1.3 N-type semiconductor

region. As this cross over continues, every boron site that contributed a hole becomes permanently negatively charged.

And every phosphorous atom that gave up an electron becomes permanently positively charged. Two equivalent but oppositely charged regions grow on the either side of the PN interface or junction, creating an electric field. This internal electric field, also called, potential barrier, is oriented to push electrons in one direction, towards the N type region.

Any holes are swept by this field toward the P type region. Any stray charges that enter the zone of influence of the electric filed are immediately swept out of that zone, so the zone is also called depletion region.

The Solar Cell

The solar cell is nothing but a large area PN interface or junction. It is the internal electric field of the PN junction that sweeps electrons out of the cell. When light penetrates into the semiconductor material, knocking free electrons and giving them potential energy, the freed electrons wander until they are pushed by the electric field across the PN junction.

They are forced out of the cell, and are available for useful work.

The electrons with higher energy level flow out of the cell through the wire to the load.

After releasing the excess energy into the load these electrons return back to the cell and -

- -

+ + +

Permanent internal electric field and lines of force

Fig. 5.1.4 PN Junction and internal electric field

P-type N-type

fall into the holes. So as soon as an electron leaves the cell from one side and enters the wire, an electron at the other end of the wire moves into the cell. So the solar cell cannot

“run down” like a battery, nor can it “run out of electrons”. It produces output (electrical energy) in response to the input “fuel” (light energy). A solar cell thus cannot store electrical energy; it can only convert light energy into electrical energy.