Presented By: -Abd El- Rahman Mohamed. -Ramy Magdy. -Aly Ahmed. -Mohamed Shehata.

Semiconductors: They are materials that have electric conductivity less that the conductors and greater than insulators Atomic structure of semiconductors: Every atom is held with other atoms by bonding actions due to the tendency to complete it’s last orbit with 8 electrons In semiconductors, bond between atoms are co-valent bonds Co-valent bonds are formed by sharing electrons.

For Example: Each germanium atom positions itself between other four germanium atoms Each neighbouring atom shares one electron with the central atom, so that the central atoms completes its last orbit with 8 electrons revolving around the nucleus Each valence electron of an atom forms bond with an electron of adjacent electron, that’s why the valence electrons are not free

Crystals: They are materials in which the atoms are arranged in orderly pattern, All semi conductors are crystals Commonly used semiconductors: the two most used semiconductors are germanium (Ge) and silicon (Si), that because the energy required to break down their co-valent bonds and release free electrons is small ((0.7 ev) for germanium and (1.1 ev) for silicon )

Germanium : Germanium can be purified relatively well and crystrallized easily Germanium has 32 electrons ( 2 in the 1 st orbit, 8 in the 2 nd, 18 in the 3 rd, 4 in the valence orbit ) Germanium atoms are arranged in orderly pattern, therefore it has crystalline structure

Silicon : It’s in the most common rocks, the silicon compounds can be chemically reduced to silicon which is suitable to be used as semiconductor Silicon has 14 electrons ( 2 electrons in the 1 st orbit, 8 electrons in the 2 nd, 4 in the valence orbit) Silicon atoms are arranged in orderly pattern, therefore it has crystalline structure

Energy band description of semiconductors: Semiconductors have almost filled valence band and nearly empty conduction band with very small energy gap ( almost = 1 ev ) Therefore, a relatively small amount of energy is needed to move the electron from the valence band to the conduction band At room temperature, some electrons acquire sufficient energy to move to the conduction band, but the amount of this electrons can produce very small current

Effects of Temperature on a semiconductor : The electric conductivity of the Semiconductors changes apparently with the change of the temperature i) At absolute zero temperature, all the electrons are tightly held by the semiconductor atoms. the valence electrons are engaged in covalent bonding. At this temperature, the covalent bonds are very strong and there are no energy to be acquired by electrons,so there is no free electrons. Therefore, the semiconductor crystal behaves as a perfect insulator.

(ii) When the temperature is raised, some of the covalent bonds in the semiconductor break due to the thermal energy supplied. The breaking of bonds sets those electrons free which are engaged in the formation of these bonds. The result is that a few free electrons exist in the semiconductor. These free electrons can constitute a tiny electric current if potential difference is applied across the semiconductor crystal This shows that the resistance of a semiconductor decreases as the temperature increases, it has negative temperature coefficient of resistance Each time an electron enters the conduction band, a hole created in the valence band

Hole Current : When a potential difference is applied on the semi conductor at the room temperature, the free electrons can produce a current At the same time, another current is produced in the semi conductor called the hole current When a covalent bond is broken due to the thermal energy, an electron leaves vacancy (missing electron in the covalent band ), this missing electron is called a hole and it acts as a positive charge For one electron leaving a bond, a hole is created, therefore thermal energy creates hole – electron pair

Suppose an electron at L has become free, this creates an hole in the covalent bond at L An electron from nearly bond at M comes to the fill the hole at L, this creates a hole at M Another valence electron from nearly bond at N may comes to fill the vacancy at M, but also this leaves a hole at N Therefore we can consider that the hole is a +ve charge that has moved from L to N towards the –ve terminal of the semiconductor

Intrinsic semiconductors: A semi conductor that is extremely pure is called intrinsic semiconductor (let IS be an abbreviation for Intrinsic semiconductor) When an potential difference is applied to (IS), there is two current takes place in (IS),the electron current due to free electrons that left its bonds and the hole current due to created holes in the left bonds. The total current in the semi conductor is the sum of the two currents

As the holes reaches the - ve terminal of (IS), electrons enters the semiconductor near the terminal and combine with the hole At the same time, the electrons that leave their atoms and move to the +ve terminal, create holes again near the +ve terminal and move towards the –ve terminal That’s why there is no hole current passing through the external wire

Extrinsic Semiconductors: The Intrinsic semiconductor has a little current conduction capability at room temperature. The pure semiconductor must be modified to increase its conducting properties to be useful in our applications, then its called extrinsic semiconductor. The process of adding impurities to a semiconductor is called doping. The purpose of adding impurity is to increase either the number of free electrons or holes, and according to that the extrinsic semiconductor is classified, and it’s classified into : N-type semiconductor P-type semiconductor

N-type semiconductor: it’s the semiconductor that a pentavalent impurity is added to it. The addition of pentavalent impurities provides a large number of free electrons in the semiconductor crystal. This impurities are called donor impurities because they donate free electrons to the semi conductors. When a pentavalent impurity added to semiconductor, four of its atoms form bonds with the semiconductor, and the fifth electron will be a free electron So an added small amount of pentavalent impurity will provide millions free electrons

N-type Conductivity : The current conduction in n-type semiconductor is by free electrons When a p.d is applied across the n-type, the donated free electrons will move towards the +ve terminal, producing electric current As the current flow through the semiconductor is by free electrons which are carriers of –ve charges It may noted that the conduction is just as in ordinary metals

P-type semiconductor: it’s the semiconductor that a trivalent impurity is added to it. The addition of trivalent impurities provides a large number of holes in the semiconductor crystal. This impurities are called acceptor impurities because they create holes to the semi conductors that can accept electrons. When a trivalent impurity is added to a semiconductor, it will form bonds with its nearly semiconductor atoms by all it’s three valence electrons, but in the fourth co-valent bond of the semiconductor will be containing a hole because the all impurity’s three electrons are engaged with other semiconductors So an added small amount of trivalent impurities will provide millions holes

P-type Conductivity : The current conduction in p-type semiconductor is by holes When a p.d is applied across the p-type, the donated holes will move from co- valent bond to another, as they are +ve charged they will move towards the -ve terminal, producing the hole current

Charge on n-type and P-type semiconductors: The n-type semiconductor has many electrons but the extra electrons are donated by the neutral donor impurity ( i.e when a pentavalent atom denote an electron it becomes charged ion (+1), so the total charge become zero ), the same with the p-type semiconductor, so n-type and p-type are electrically neutral Majority and minority carriers: The n-type is a semiconductor in the first place, at room temperature some of it’s co-valent bonds are broken up forming free electrons and holes, but with impurity addition the number of free electrons become greater than that of holes, Then we called the free electron (majority carriers ) and the holes (minority carriers) The same with the p-type, but the holes is the majority carriers instead of free electrons

Carrier Transport Diffusion Drift Refers to the motion of electrons and holes due to the presence of a Concentration gradient. A gradient in concentration is the driving force for the transport of particles. is the charged particle motion in response to Electric field.

E

Formation of pn-junction

Forward-Bias

Reverse-Bias

IV-Characteristics

* Breakdown Voltage: is the minimum reverse voltage at which the pn-junction breaks down with sudden rise in reverse current. * Knee Voltage: it is the forward voltage at which the current through the junction starts to increase rapidly.

Generation & Recombination When an electron in a bond absorbs a photon, the electron is promoted to the conduction band leaving behind a hole. this is so-called Generation Direct Band gapIndirect Band gap

Recombination 1.Direct Recombination. 2.Shockley-Read-Hall (SRH) Recombination. 3.Auger Recombination. When an electron annihilate with a hole and release energy.

Direct Recombination

SRH Recombination

Auger Recombination

Solar cells Solar cell operators Solar cell parameters

What is a solar cell? The requirements for the prosses A material absorb light The movement of electrons in an external circuit The dissipation of the electrons energy The basic steps in the operation of a solar cell The generation of carriers The collection of carriers The generation of voltage across solar cell The dissipation of power in a load

Absorption of light All photons that falls on solar cell is either reflected, transmitted absorbed The key in determining if the photon is going to be transmitted or absorbed by material is the energy of the photon it self

Absorption coefficient & absorption depth Absorption coefficient: It is a measure of the decrease of intensity of light as it passes through material I= I 0 exp(- αx) where αis the absorption coefficient typically in cm -1 ; x is the distance into the material at which the light intensity is being calculated; and I 0 is the light intensity at the top surface. The absorption depth is the reciprocal of the absorption coefficient The absorption coefficient in inversely proportional to the wavelength of the incident photon

Light generated current It consists of two process Absorption of light The absorption of photons The instability of electron hole pair Collection of light using P- Njunction

Parameters that affect collection probability and it’s definition Diffusion length: the average distance that a carrier travel from the point of generation to the point of recombination As diffusion length increases the collection probability increase Surface recombination It happens due to defects in the periodicity of crystal lattice leading to the formation of dangling bond Defects increases recombination & depletes minority carriers Surface recombination velocity: the rate of recombination at surface is limited by the rate of movement of minority carriers from bulk to surface We use surface passivation method to reduce dandling bonds

Collection probability What is the meaning of collection probability? It depends on Distance that the carrier is created compared to diffusion length Surface properties of device

Quantum efficiency What is meant by quantum efficiency? Max quantum efficiency Limits of quantum efficiency Parameters that affect quantum efficiency Difference between external quantum efficiency curve & internal quantum efficiency curve

Spectral response What is meant by spectral response ?and it’s dimensions? What is the difference between spectral response curve and quantum efficiency curve? We can measure spectral response from solar cell & from it we can get quantum efficiency SR=(qʎ/hc)(QE)

Photovoltaic effect The collection of light- generated carriers does not by itself give rise to power generation. In order to generate power, a voltage must be generated as well as a current. Voltage is generated in a solar cell by a process known as the "photovoltaic effect“

Solar cell parameters And other external effects.

Solar cell parameters :

At equilibrium, the voltage that causes the balance between the light generated current and the diffusion current is called “the open circuit voltage”.

Solar cell parameters :

The I-v characteristic curve : O p Maximum power point

The I-v characteristic curve :

3-The fill factor FF :

4- the efficiency of solar cell : The efficiency of a solar cell is defined as the fraction of incident power which is converted to electricity and is defined as: Where is the maximum power. The input power for efficiency calculations is 1 kW/m2 or 100 mW/cm2. Thus the input power for a 100 × 100 mm2 cell is 10 W and for a 156 × 156 mm2 cell is 24.3 W.

Temperature effects on the solar cell parameters :

The effect of temperature T