L-27L-27Photoconductivity and Photoconductivity and

PhotovoltaicsPhotovoltaics The phenomenon of an increase in the electrical The phenomenon of an increase in the electrical

conductivity of insulating crystal when the light conductivity of insulating crystal when the light radiation falls on the crystal is calledradiation falls on the crystal is called Photoconductivity. Photoconductivity.

Photoconductivity has practical application in Photoconductivity has practical application in television cameras, infrared detectors, light metres television cameras, infrared detectors, light metres and indireatly photographic process. Photoconductivity and indireatly photographic process. Photoconductivity arises because the incident photons excile the arises because the incident photons excile the electrons from valance band into conduction band, electrons from valance band into conduction band, while they become mobile, creation of holes in the while they become mobile, creation of holes in the valance band,where they are also mobile.valance band,where they are also mobile.

Inperfections introduce discrete energy Inperfections introduce discrete energy levels in the forbidden energy gap. Which levels in the forbidden energy gap. Which are often called are often called traps .traps .

Photoconductivity is the Photoconductivity is the process where electrons (hole) are freed process where electrons (hole) are freed from bound state by photons , pass from bound state by photons , pass sometime in the conduction band (valance sometime in the conduction band (valance band) in which they act as current carriers band) in which they act as current carriers and are finally recaptured by traps.and are finally recaptured by traps.

L-28L-28Simple model of Simple model of photoconductorsphotoconductors

(Variation of Photoconductivity (Variation of Photoconductivity with illumination)with illumination)

Supposed that electron hole pairs Supposed that electron hole pairs are produced uniformly are produced uniformly

throughout the volume of the crystal throughout the volume of the crystal by light source.by light source.

It is convenient to suppose that It is convenient to suppose that the mobility of the holes may be the mobility of the holes may be neglected in comparision with the t neglected in comparision with the t mobility of the of the electrons mobility of the of the electrons

This model is highly hypothetical and rarely This model is highly hypothetical and rarely exist in reality.exist in reality.

Based on this model , the rate of change of Based on this model , the rate of change of electron concentration n is given byelectron concentration n is given by

dndn = L – Anp = L- An = L – Anp = L- An22 using n = p -------- using n = p --------(1)(1)

dt dt Similarly The hole concentration is Similarly The hole concentration is dp dp = L- Anp ---------(2) = L- Anp ---------(2) dt dt

L→ is number of photons absorbed L→ is number of photons absorbed per unit volume of the specimen per per unit volume of the specimen per unit time.unit time.

Anp→ gives the recombination Anp→ gives the recombination rate .It is proportional to the product rate .It is proportional to the product of hole and electron concentrationof hole and electron concentration

A→ proportionally constant A→ proportionally constant

In steady stateIn steady state dn dn = 0 = 0 dt dt Therefore Eqn.(1) Therefore Eqn.(1) L- AnL- An22 = 0 = 0 i.e i.e AnAn22 = L = L nn00 =( L/A) =( L/A) 1/ 21/ 2

Therefore in steady state the Therefore in steady state the electron Conductivityelectron Conductivity

σ = nσ = n00 e μ = ( L/A) e μ = ( L/A) 1/ 21/ 2 e e μ --------(4)μ --------(4)

Where μ is electron Where μ is electron mobility.mobility.

Therefore photocurrent will vary Therefore photocurrent will vary with light intensity as L with light intensity as L 1/ 21/ 2..

If the light is switched off , the decay of If the light is switched off , the decay of dn dn = - An = - An2 2 since L =0 -------(5) since L =0 -------(5) dt dt Its solution is Its solution is dn/ ndn/ n22 =-Adt =-Adt Integrating 1/n - At + cIntegrating 1/n - At + c At t =0 , n = nAt t =0 , n = n00 therefore c= 1/ n therefore c= 1/ n00

Thus 1/n = At + 1/ nThus 1/n = At + 1/ n00 1/n = (1+ At n1/n = (1+ At n00)/ n)/ n00

n = nn = n00 / (1+ At n / (1+ At n00) ) n = n = n n00 1+ Atn1+ Atn00 where nwhere n00 conc. At t = 0 i.e when light was turned off . conc. At t = 0 i.e when light was turned off .

1 1 = = At nAt n00 +1 +1 = At + = At + 11 ------(6) ------(6)

n nn n00 n0 n0

let carrier concentration drop to nlet carrier concentration drop to n00/2 in time t/2 in time t00 tt00 = 1/ A n = 1/ A n00 = 1 = 1 A (L/ A)A (L/ A)1/21/2 [n [n00 from eqn. (3)] from eqn. (3)]

tt00 = (LA) = (LA) -1/2-1/2 [From eqn. L- A n [From eqn. L- A n0022 = 0 ,A = L/ n = 0 ,A = L/ n00

22] ]

tt00 = (L. L/ n = (L. L/ n0022 ) ) -1/2-1/2 = (L = (L22/ n/ n00

22 ) ) -1/2-1/2 = n = n00 / L--------(7) / L--------(7)

Therefore combining with eqn. (4) we get, nTherefore combining with eqn. (4) we get, n00 = σ / e μ = σ / e μ Eqn (7) gets the form Eqn (7) gets the form tt00 = σ / e μ L -------- (8) = σ / e μ L -------- (8) Where tWhere t00 = response time. = response time. Therefore response time should be directly Therefore response time should be directly

proportional to Photoconductivity at a given proportional to Photoconductivity at a given illumination level sensitive photoconductors have illumination level sensitive photoconductors have long response timelong response time . .

Sensitivity or Gain FactorSensitivity or Gain Factor G = number of carrier crossing the specimenG = number of carrier crossing the specimen number of photons absorbed in the specimennumber of photons absorbed in the specimen

Let d = thickness of specimen.Let d = thickness of specimen. Crosssection area = unityCrosssection area = unity The voltage v produces the The voltage v produces the

particle current (flux) particle current (flux) JJnn = I / e = n = I / e = n00 μv / d ----------(1) μv / d ----------(1) By eqn. (3) By eqn. (3)

nn00 = (L/ A) = (L/ A)1/21/2

Eqn. (1) gets the form Eqn. (1) gets the form JJnn = Particle flux J = Particle flux Jnn = I / e = v μ (L/ A) = I / e = v μ (L/ A)1/21/2 . .

LdLd d . D L d . D L = v μ. Ld -------(2)= v μ. Ld -------(2)

dd22 .( AL) .( AL)1/21/2

As G = As G = Particle fluxParticle flux = J = Jnn / Ld / Ld LdLd

G = G = v μ. Ldv μ. Ld = v μ ------(3) = v μ ------(3) dd22 .( AL) .( AL)1/21/2Ld dLd d22.( AL).( AL)1/21/2 The transit time TThe transit time Tdd of a carrier between the of a carrier between the

electrode is given by.electrode is given by. TTdd = d = = d = dd22 -----------(4) -----------(4) (v μ /d) v μ(v μ /d) v μ

TTee = time of the electron before recombination is = time of the electron before recombination is

TTee = (AL) = (AL) -1/2-1/2 ---------(5) ---------(5)

TTee = (AL) = (AL) -1/2-1/2 = v = vμμ -------- --------(6)(6)

TTdd d d22 / vμ d / vμ d22.( AL).( AL)1/21/2 From eqn. (6) and (3) From eqn. (6) and (3) G = TG = Tee / T / Tdd -----------(7) -----------(7) i.e gain is equal to the ratio of i.e gain is equal to the ratio of

the carrier life time to the transit time.the carrier life time to the transit time.

LimitationLimitationss

The relation (7) shows that this expression for The relation (7) shows that this expression for gain is quite general and is not limited to the gain is quite general and is not limited to the specific model.specific model.

If TIf Tcc = is taken as observed time = is taken as observed time Then value of G from eqn. (7) are very Then value of G from eqn. (7) are very

much longer than those observed much longer than those observed experimentally experimentally

This shows the failure of the model and This shows the failure of the model and suggests that a new element must be added suggests that a new element must be added to the present picture of the photoconductivity to the present picture of the photoconductivity process . This new element is process . This new element is effect of traps.effect of traps.

L-29L-29Effect of trapsEffect of traps

A trap is an A trap is an impurity atomimpurity atom or other or other imperfection in crystal capable of imperfection in crystal capable of capturing capturing an electron or holean electron or hole, the captured carriers , the captured carriers may be re emitted at a subsequent time , and may be re emitted at a subsequent time , and may may move to another traps move to another traps

There are two types of trapsThere are two types of traps One type helps electrons and hole to One type helps electrons and hole to

recombine. This type of trap is called recombine. This type of trap is called recombination centrerecombination centre . .

Second type of trap Second type of trap does not contributedoes not contribute directly in an important way to recombination directly in an important way to recombination but but effect the freedomeffect the freedom of motion of charge. of motion of charge.

A trap is an impurity atom or other A trap is an impurity atom or other imperfection in crystal capable of capturing an imperfection in crystal capable of capturing an electron or hole, the captured carriers may be electron or hole, the captured carriers may be re emitted at a subsequent time , and may re emitted at a subsequent time , and may move to another traps move to another traps

There are two types of trapesThere are two types of trapes One type helps electrons and hole to One type helps electrons and hole to

recombine. This type of trap is called recombine. This type of trap is called recombination centre .recombination centre .

Second type of trap does not contribute Second type of trap does not contribute directly in an important way to recombination directly in an important way to recombination but effect the freedom of motion of charge.but effect the freedom of motion of charge.

A trap is an impurity atom or other A trap is an impurity atom or other imperfection in crystal capable of capturing an imperfection in crystal capable of capturing an electron or hole, the captured carriers may be electron or hole, the captured carriers may be re emitted at a subsequent time , and may re emitted at a subsequent time , and may move to another traps move to another traps

There are two types of trapesThere are two types of trapes One type helps electrons and hole to One type helps electrons and hole to

recombine. This type of trap is called recombine. This type of trap is called recombination centre .recombination centre .

Second type of trap does not contribute Second type of trap does not contribute directly in an important way to recombination directly in an important way to recombination but effect the freedom of motion of charge.but effect the freedom of motion of charge.

Photon Full Valence Band

Trap

Trap

Trap

Recombination centre

Empty Conduction Band

Trap N

E1

E2

Consider a crystal with N electron Consider a crystal with N electron trap level per unit volume . Let temp. trap level per unit volume . Let temp. is low .is low .

Let us assume that Let us assume that recombination coefficient a is same recombination coefficient a is same for electron hole recombination as for electron hole recombination as for electron trap capture for electron trap capture

Then dn / dt = L - AnThen dn / dt = L - An22 becomes becomes dn / dt = L - An(N+n) + Bndn / dt = L - An(N+n) + Bntt ----------(1) ----------(1) Where n = connc. Of electrons in Where n = connc. Of electrons in

conduction band.conduction band. BnBntt= rate of thermal evaporation of = rate of thermal evaporation of

trapped carrier back into the conduction band trapped carrier back into the conduction band This term to be neglectedThis term to be neglected

In steady state In steady state

nn00(n(n00 + N) = L /A ---------------(2) + N) = L /A ---------------(2) it is difficult to grow crystals with trap it is difficult to grow crystals with trap

concentration N much less than 10concentration N much less than 101414 cm cm-3-3 . . In the limit nIn the limit n00 << N , we have. << N , we have. nn00 = L/ AN ------------(3) = L/ AN ------------(3) At high level of illumination if nAt high level of illumination if n0 0 >> N >> N nn0 0 = (L/A)= (L/A)1/21/2 -------------(4) -------------(4)

The decay of the carrier (photoelectrons) on switching The decay of the carrier (photoelectrons) on switching off the light is given byoff the light is given by

log n+N - log nlog n+N - log n00 + N = NAt + N = NAt n nn n00 In the limit N >> nIn the limit N >> n00 , the above solution reduce , the above solution reduce

to to n = nn = n00ee-NAt -NAt So the time for the signal (photocurrent) to fall to e-So the time for the signal (photocurrent) to fall to e-

1 of initial value is1 of initial value is tt00 = 1 / NA = 1 / NA comparing this result with eqn. tcomparing this result with eqn. t00 = n = n00/ L ( in the / L ( in the

absence of trap) absence of trap)

The presence of traps reduces the conductivity and The presence of traps reduces the conductivity and also reduces the response time . also reduces the response time .

Now improve the model , working in the Now improve the model , working in the approximation n<<N .approximation n<<N .

We new assume EWe new assume E11, the heightof traps above , the heightof traps above valance band , >> kTvalance band , >> kT

And EAnd E22, the depth of the traps below the , the depth of the traps below the conduction band but not much larger kTconduction band but not much larger kT

We must then consider the thermal We must then consider the thermal excitation the trapped electrons back to the excitation the trapped electrons back to the valence band. & ultimately the trap population valence band. & ultimately the trap population decays .decays .

If N>> nt E1 >>n where nt = is concentration of e- in traps .

Then rate eqn. are

dn = L + Bnt – ANn - Cnnt

dt ----------(5)

dnt = ANn - Bnt

dt Here Bnt → thermal ionisation rate of trapped carriers.

Cnnt → recombination of electrons with trapped holes

The conc. Of trapped holes is The conc. Of trapped holes is n + nn + ntt = n = ntt as long n as long ntt >>n >>n in steady state under illumination, we have in steady state under illumination, we have dn/dt = 0, Bnt = 0,ANn = 0 then L = C ndn/dt = 0, Bnt = 0,ANn = 0 then L = C n00 nt then nt then

nn00 = L/ Cn = L/ Cntt dndntt/dt =0 ANn/dt =0 ANn00 = B n = B ntt then then nn00 = =

BnBntt / AN / AN

so that on multiplying nso that on multiplying noo22 = L/ Cn = L/ Cntt x Bn x Bntt / AN / AN

nn00= ( BL/ ANC)= ( BL/ ANC)1/21/2 ------------(6)------------(6)

tt00 = 1/ Cn = 1/ Cn00 = = nn00 AN AN ------- -------(7) (7)

L BL B Thus the response time is Thus the response time is

increased by factor AN/ B >>1 increased by factor AN/ B >>1

This feature is in agreement This feature is in agreement with the experimental result.with the experimental result.

Application of Application of photoconductivityphotoconductivity

Photoconductivity effects find Photoconductivity effects find applications in T.V cameras light applications in T.V cameras light meter infrared detectors and meter infrared detectors and indorsed in photographic process indorsed in photographic process

L-30L-30 Photoelectric cellPhotoelectric cell

The device with the help of with light The device with the help of with light energy is converted to electrical energy is converted to electrical energy are called energy are called Photoelectric cellPhotoelectric cell

Photoelectric cell are three typesPhotoelectric cell are three types Photoconductive Cell Photoconductive Cell Photovoltaic CellPhotovoltaic Cell Photo emissive CellPhoto emissive Cell

Photoconductive CellPhotoconductive Cell

PrinciplePrinciple ----- Photoconductive Cell ----- Photoconductive Cell are based on the principle that the are based on the principle that the electrical resistant of semiconductor like electrical resistant of semiconductor like bad sulphide, selenium etc., decreases bad sulphide, selenium etc., decreases when they are exposed to radiation . The when they are exposed to radiation . The decrease in resistance( or increase in decrease in resistance( or increase in conductivity) of semiconductors on conductivity) of semiconductors on exposing to light may be explained as exposing to light may be explained as follows follows

If a photon striking the surface of such If a photon striking the surface of such photosensitive material has energyphotosensitive material has energy

E = hν greater than the energy gap E = hν greater than the energy gap between the valance band and conduction band between the valance band and conduction band of the material .The sufficient energy will be of the material .The sufficient energy will be imparted to an electron to raise into conduction imparted to an electron to raise into conduction band . Therefore , a hole is left in valance band. band . Therefore , a hole is left in valance band. This electron hole pair is free to serve as current This electron hole pair is free to serve as current carriers and hence the conductivity of materials carriers and hence the conductivity of materials increase or electrical resistance is reduced increase or electrical resistance is reduced

Construction and Construction and workingworking

Commonly material used in photoconductive cell Commonly material used in photoconductive cell is the cadmium sulphide (Cds)is the cadmium sulphide (Cds)

In this cell , thin film of cds is deposited In this cell , thin film of cds is deposited on one side of an iron plate and placed below a on one side of an iron plate and placed below a transparent metal film.transparent metal film. When light radiation of When light radiation of sufficient energy falls onsufficient energy falls on

Transparent foil , the electrical resistance of cds Transparent foil , the electrical resistance of cds larger gets reduced and electrical conductance is larger gets reduced and electrical conductance is increased . Consequentely , a current starts increased . Consequentely , a current starts flowing in the battery circuit connected between flowing in the battery circuit connected between the iron plate and transparent metal foil the iron plate and transparent metal foil

Purpose of batteryPurpose of battery:---- It generates a direction :---- It generates a direction and provide a path for a current to flow.and provide a path for a current to flow.

CharacteristicsCharacteristics:------ Graph shows the :------ Graph shows the relationship between illumination and resistance. relationship between illumination and resistance. When there is no illumination the cell has a When there is no illumination the cell has a range of 100kΩ which is called dark resistance. range of 100kΩ which is called dark resistance. When illumination with strong light, the cell When illumination with strong light, the cell resistance falls to few hundred ohms (light resistance falls to few hundred ohms (light resistance).The ratio of dark to light resistance of resistance).The ratio of dark to light resistance of cell is 1000:1cell is 1000:1

•Transparent metal sheet Light

CdS

•Iron plate

•+ -

R

Spectral responseSpectral response

The response is sensitive to visible The response is sensitive to visible light . It is maximum over the visible light . It is maximum over the visible region and traps off towards the region and traps off towards the ultraviolet and infrared. It is closely ultraviolet and infrared. It is closely matched the response of human eye.matched the response of human eye.

Use of photoconductive Use of photoconductive cellcell

-- To measure the intensity of illumination -- To measure the intensity of illumination --- To work as ON- OFF switch. --- To work as ON- OFF switch. In street lighting control. In street lighting control.

l l In camera exposure setting. In camera exposure setting. In counting application.In counting application. In aircraft and missile tracking system In aircraft and missile tracking system In burglar alarm.In burglar alarm. As voltage regulator As voltage regulator

AdvantagesAdvantages

1—High sensitivity 1—High sensitivity

2-- Low cost2-- Low cost 3 – Long life3 – Long life

4--- High voltage capability.4--- High voltage capability. 5—Hight dark to light resistance ratio 5—Hight dark to light resistance ratio

(1000:1)(1000:1)

DrawbacksDrawbacks

1 --- The current changes with 1 --- The current changes with change in light intensity with a time change in light intensity with a time lag.lag.

2--- A relatively narrow spectral 2--- A relatively narrow spectral response.response.

Photo DiodePhoto Diode

A photo diode is a reverse biased P-A photo diode is a reverse biased P-N junction diode which is designed to N junction diode which is designed to respond to photon absorption.respond to photon absorption.

PrinciplePrinciple

A reverse biased P-N junction diode has a A reverse biased P-N junction diode has a reverse saturation current which is mainly reverse saturation current which is mainly due to flow of the minority carriers. If light due to flow of the minority carriers. If light is allowed to fall on such a reverse biased is allowed to fall on such a reverse biased P-N junction diode , addition electron –P-N junction diode , addition electron –hole pairs are generated in both P and N hole pairs are generated in both P and N region . It produced a very large change region . It produced a very large change in minority carriers conc. And hence in minority carriers conc. And hence increases the reverse current through the increases the reverse current through the diode. diode.

Reverse Voltage V

Dark current

1000 Lm/m2

1500 Lm/m2

2000 Lm/m2

Diode

Current (mA)

Working and Working and CharacteristicsCharacteristics

When photodiode is kept under dark condition When photodiode is kept under dark condition and sufficient reverse voltage is applied, then and sufficient reverse voltage is applied, then almost constant current is obtained , This almost constant current is obtained , This current is called dark current because it flows current is called dark current because it flows when no light is incident . It is proportional to when no light is incident . It is proportional to the concentration of minority carriers ( hole in the concentration of minority carriers ( hole in N and electrons in P region) and is denoted by IN and electrons in P region) and is denoted by Idd

Under large reverse bias condition , the total Under large reverse bias condition , the total current is given bycurrent is given by

I = II = Iss + I + Idd Where IWhere Iss = short circuit current and is = short circuit current and is

proportional to light intensity.proportional to light intensity.

With any bias V, the reverse current due to thermal With any bias V, the reverse current due to thermal electron hole pairs i.e.electron hole pairs i.e.

Dark current is given by.Dark current is given by. IIdd = I = I00 (1- e (1- e ve/ ηkTve/ ηkT ) ) Hence the V-I Characteristics photo diode is Hence the V-I Characteristics photo diode is

given by given by I = II = Iss+ I+ I00 (1- e (1- e ve/ ηkTve/ ηkT ) ) Where η = 1 for Ge and 2 for Si.Where η = 1 for Ge and 2 for Si. Only the curve representing the dark current passes Only the curve representing the dark current passes

through the origin.through the origin. The current increases in the level of illumination.The current increases in the level of illumination.

UsesUses

Photodiode can turn its current ON and Photodiode can turn its current ON and OFF in nanosecond therefore , it is used OFF in nanosecond therefore , it is used where light is required to be switched OFF where light is required to be switched OFF and ON at a very fast rateand ON at a very fast rate

A photodiode is used in light detection in A photodiode is used in light detection in light operated switches, reaching of light operated switches, reaching of computer punched cards and type etc.computer punched cards and type etc.

In optical communication system.In optical communication system. Used in instrumentation, control Used in instrumentation, control

automation and communication automation and communication

Photo-Voltaic cellPhoto-Voltaic cell Becquarel in 1839 discovered that when a Becquarel in 1839 discovered that when a

pair of electrodes is immersed in an pair of electrodes is immersed in an electrolyte and light is allowed to incident on electrolyte and light is allowed to incident on one of them , a potential difference is created one of them , a potential difference is created between electrodes . This phenomenon is between electrodes . This phenomenon is called photovoltaic effect. Device based on called photovoltaic effect. Device based on this effect are known as this effect are known as Photo-Voltaic cell . Photo-Voltaic cell . ThisThis photovoltaic cell are the divices in which light photovoltaic cell are the divices in which light energy is use to creat a potential difference energy is use to creat a potential difference so developed is directly proportional to the so developed is directly proportional to the frequency and intensity of incident light.frequency and intensity of incident light.

Metal

Semiconductor

Light

+

-

R

Construction and Construction and workingworking

A basic photovoltaic cell consist of a piece of A basic photovoltaic cell consist of a piece of semiconducting material bonded to a metal plate semiconducting material bonded to a metal plate used for preparing photovoltaic cell . When light is used for preparing photovoltaic cell . When light is made to fall on semiconducting material , valance made to fall on semiconducting material , valance electrons and hols are liberated from its crystal electrons and hols are liberated from its crystal structure. The electrons so librated move towards structure. The electrons so librated move towards the metal plate and holes flowin opposite direction . the metal plate and holes flowin opposite direction . Thus a potential difference is created between the Thus a potential difference is created between the semiconducting material and metal plate . Then the semiconducting material and metal plate . Then the conventional current (due to holes) flows in the conventional current (due to holes) flows in the external circuit through a load resistor r.external circuit through a load resistor r.

Acttually photo-Voltaic cell a thin Acttually photo-Voltaic cell a thin metallic film of silver, gold or metallic film of silver, gold or platinum is deposited on a platinum is deposited on a semiconducting layer like cuprous semiconducting layer like cuprous oxide (Cuoxide (Cu22o) or iron selenide . The o) or iron selenide . The whole arrangement is then attached whole arrangement is then attached to a metal plate (say copper)to a metal plate (say copper)

When external light is allowed to fall on metallic When external light is allowed to fall on metallic film F, it penetrates easily and the barrier layer film F, it penetrates easily and the barrier layer between metallic film and the semiconductor , between metallic film and the semiconductor , photoelectrons emission occurs, The , photoelectrons emission occurs, The , photoelectrons so emitted from the layer , move photoelectrons so emitted from the layer , move towards the metallic film , consequently , the towards the metallic film , consequently , the metallic film F becomes negatively charged and metallic film F becomes negatively charged and copper base plate positively charged . Hence a copper base plate positively charged . Hence a potential difference is developed between two potential difference is developed between two end a current flows in the external circuit.end a current flows in the external circuit.

+

-

Mettalic film Cu2O

Cu plate

Light

R

Uses ofUses of Photo-Voltaic cellPhoto-Voltaic cell

1 – Operation of relays 1 – Operation of relays 2 -- Photographic exposure2 -- Photographic exposure 3 -- Direct reading illumination 3 -- Direct reading illumination

metro.metro.

Solar cellSolar cell

A Solar cell or solar battery is A Solar cell or solar battery is basically a P-N junction diode which basically a P-N junction diode which converts solar energy into electrica converts solar energy into electrica energy . It is also called a solar energy . It is also called a solar energy converter and simply a energy converter and simply a photodiode operator at zero bias photodiode operator at zero bias voltage .voltage .

Sun Light Sun Light

N

Metal contact -

+ Glass

Metal rings

P

I

RL

ConstructionConstruction

a solar cell consist of a P-N junction diode a solar cell consist of a P-N junction diode generally made of Ge or Si . It can also be generally made of Ge or Si . It can also be gallium arsenide (GaAs) The P-N junction gallium arsenide (GaAs) The P-N junction diode placed in a can with window on top diode placed in a can with window on top so that light may fall upon Pand N type so that light may fall upon Pand N type materials . The thickness of both P- region materials . The thickness of both P- region and N-region is kept small . so that in P- and N-region is kept small . so that in P- region electrons generated can diffuses to region electrons generated can diffuses to junction before recombination and similarly junction before recombination and similarly in N- region hole generated diffuse to in N- region hole generated diffuse to junction before they recombine.junction before they recombine.

A heavy doping of P and N region A heavy doping of P and N region recommended to obtain a large recommended to obtain a large photovoltage . a nickel plate ring is photovoltage . a nickel plate ring is provided around the P-layer which provided around the P-layer which acts as +ve output terminal . A metal acts as +ve output terminal . A metal contact at the bottom serves as –ve contact at the bottom serves as –ve output terminal . output terminal .

WorkingWorking

When light is allowed to fall on P-N When light is allowed to fall on P-N junction diode, photons collides with junction diode, photons collides with valance electrons and impart them valance electrons and impart them sufficient energy enabling them to leave sufficient energy enabling them to leave their parent atoms . Thus electron hole their parent atoms . Thus electron hole pairs are generated in both the P and N pairs are generated in both the P and N side of the junction . These electron and side of the junction . These electron and holes reach the depletion layer W by holes reach the depletion layer W by diffusion then separated by the strong diffusion then separated by the strong barrier field.barrier field.

P

N

W

+

+

-

-

However the minority carrier, electrons in p-However the minority carrier, electrons in p-side drawn the potential barrier to reach the side drawn the potential barrier to reach the N side and holes in N side moves to P- side . N side and holes in N side moves to P- side . There flow constitute the minority current There flow constitute the minority current which is directly proportional to illumination .which is directly proportional to illumination .

The accumulation of electrons and holes on The accumulation of electrons and holes on the two sides of the junction gives rise to an the two sides of the junction gives rise to an open circuit voltage( vopen circuit voltage( vococ) which is function of ) which is function of illumination.illumination.

CharacteristicsCharacteristics

--- --- It may be seen that 100mW/cmIt may be seen that 100mW/cm22 illumination.the open circuit voltage illumination.the open circuit voltage is obtain 0.57V and short circuit is obtain 0.57V and short circuit current is 50Acurrent is 50A

Maximum power output is obtained Maximum power output is obtained when the cell is operated at the knee when the cell is operated at the knee of the curveof the curve

UsesUses

Solar cell are used in satellites and Solar cell are used in satellites and space vechicles to supply power to space vechicles to supply power to electronic and other equipments or electronic and other equipments or to charge storage batteries.to charge storage batteries.