Lecture 8 solar cell led metal semiconductor junction and heterojunction
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Lecture 8: Solar Cell, LED, Metal/Semiconductor Junction and Heterojunction. Requirement: understand and explain in word. * Some of the content from C. Hu : “Modern Semiconductor devices for Integrated Circuits”. 5.7 Solar Cells. Solar Cells is also known as photovoltaic cells .

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Lecture 8: Solar Cell, LED, Metal/Semiconductor Junction and Heterojunction

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Lecture 8 solar cell led metal semiconductor junction and heterojunction

Lecture 8: Solar Cell, LED, Metal/Semiconductor Junction and Heterojunction

Requirement: understand and explain in word.

* Some of the content from C. Hu : “Modern Semiconductor devices for Integrated Circuits”


Lecture 8 solar cell led metal semiconductor junction and heterojunction

5.7 Solar Cells

  • Solar Cells is also known asphotovoltaic cells.

  • Converts sunlight to electricity with 10-30% conversion efficiency.

  • 1 m2 solar cell generate about 150 W peak or 25 W continuous power.

  • Low cost and high efficiency are needed for wide deployment.


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Short Circuit

I

Dark IV

light

N

Eq.(4.9.4)

P

I

0.7 V

sc

-

0

V

E

Solar Cell

c

IV

Eq.(4.12.1)

Maximum

E

I

power-output

v

sc

+

(a)

Solar Cell Basics


Direct gap and indirect gap semiconductors

Direct-Gap and Indirect-Gap Semiconductors

direct-gap semiconductor

indirect-gap semiconductor

  • Electrons have both particle and wave properties.

  • An electron has energy E and wave vector k.

Direct-gap semiconductor: Absorption coefficient is larger .

Si is most prevalent for solar cell because of low cost.


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Light Absorption

α(1/cm): absorption coefficient

1/α: light penetration depth

A thinner layer of direct-gap semiconductor can absorb most of solar radiation than indirect-gap semiconductor.

Si solar cell > 50 um in thickness to absorb most of the photons because of low α


Short circuit current and open circuit voltage

Short-Circuit Current and Open-Circuit Voltage

If light shines on the N-type semiconductor and generates holes (and electrons) at the rate of G s-1cm-3 ,

If the sample is uniform (no PN junction), d2p’/dx2 = 0  p’ = GLp2/Dp= Gtp


Solar cell short circuit current i sc

Solar Cell Short-Circuit Current, Isc

Assume very thin P+ layer and carrier generation in N region only.

G is really not uniform. Lp needs be larger than the light penetration depth to collect most of the generated carriers.


Open circuit voltage

Open-Circuit Voltage

  • Total current is ISC plus the PV diode (dark) current:

  • Solve for the open-circuit voltage (Voc) bysetting I=0

How to raise Voc ?


Lecture 8 solar cell led metal semiconductor junction and heterojunction

A particular operating point on the solar cell I-V curve maximizes the output power (I V).

Output Power

  • Si solar cell with 15-20% efficiency dominates the market now

  • Theoretically, the highest efficiency (~24%) can be obtained with 1.9eV >Eg>1.2eV. Larger Eg lead to too low Isc (low light absorption); smaller Eg leads to too low Voc.

  • Tandem solar cells gets 35% efficiency using large and small Eg materials tailored to the short and long wavelength solar light.


Lecture 8 solar cell led metal semiconductor junction and heterojunction

NRL’s new triple-junction solar cells could achieve 50 percent efficiency


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Ec

Non-radiative recombination through traps

Radiative recombination

Ev

5.8 Light Emitting Diodes and Solid-State Lighting

  • Light emitting diodes (LEDs)

  • LEDs are made of compound semiconductors such as InP and GaN.

  • Light is emitted when electron and hole undergoradiative recombination.


Direct and indirect band gap

Direct and Indirect Band Gap

Trap

Indirect band gap

Example: Si

Direct recombination is rare as k conservation is not satisfied

Direct band gap

Example: GaAs

Direct recombination is efficient as k conservation is satisfied.


Lecture 8 solar cell led metal semiconductor junction and heterojunction

4.13.1 LED Materials and Structure


Lecture 8 solar cell led metal semiconductor junction and heterojunction

LED Materials and Structure

compound semiconductors

binary semiconductors:

- Ex: GaAs, efficient emitter

ternary semiconductor :

- Ex: GaAs1-xPx , tunable Eg (to vary the color)

quaternary semiconductors:

- Ex: AlInGaP , tunable Eg and lattice constant (for growing high quality epitaxial films on inexpensive substrates)

Eg(eV)

red

yellow

blue

Red

Yellow

Green

Blue

Light-emitting diode materials


Lecture 8 solar cell led metal semiconductor junction and heterojunction

AlInGaP Quantun Well

Common LEDs


Lecture 8 solar cell led metal semiconductor junction and heterojunction

5.9 Metal-Semiconductor Junction

  • Two kinds of metal-semiconductor contacts:

  • Rectifying Schottky diodes:metal on lightly doped silicon

  • Low-resistance ohmic contacts: metal on heavily doped silicon


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Vacuum level,

E

0

= 4.05 eV

c

c

Si

Si

q

y

y

M

M

q

f

E

Bn

c

E

f

E

v

fBn Increases with Increasing Metal Work Function

: Work Function of metal

: Electron Affinity of Si

Theoretically,

fBn= yM – cSi


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Depletion

Metal

Neutral region

layer

qfBn

Ec

Ef

N-Si

Ev

Ec

P-Si

Ef

Ev

qfBp

SchottkyBarriers

Energy Band Diagram of Schottky Contact

  • Schottky barrier height, fB, is a function of the metal material.

  • fB is the most important parameter. The sum of qfBn and qfBp is equal to Eg .


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Schottky barrier heights for electrons and holes

fBn + fBp Eg

fBnincreases with increasing metal work function


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Vacuum level,

E

0

c

= 4.05 eV

Si

q

y

M

q

f

E

Bn

c

+

-

E

f

E

v

Fermi Level Pinning (Schottky barrier lowering)

  • A high density of energy states in the bandgap at the metal-semiconductor interface pins Ef to a narrow range and fBn is typically 0.4 to 0.9 V

  • Question: What is the typical range of fBp?


Lecture 8 solar cell led metal semiconductor junction and heterojunction

qfbi

qfBn

Ec

Ef

Ev

qfBn

q(fbi + V)

qV

Ec

Ef

Ev

Using C-V Data to Determine fB

Question:

How should we plot the CV data to extract fbi?


Lecture 8 solar cell led metal semiconductor junction and heterojunction

qfbi

2

qfBn

1

/C

Ec

Ef

Ev

V

-

f

bi

Using CV Data to Determine fB

Oncefbi is known, fB can be determined using


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Thermionic Emission Theory

v

thx

-

E

q(

f

-

V)

B

c

q

f

N-type

B

E

qV

E

fn

V

Metal

Silicon

fm

E

v

x


Lecture 8 solar cell led metal semiconductor junction and heterojunction

Forward biased

V = 0

I

V

Reverse bias

Forward bias

Reverse biased

SchottkyDiodes


Lecture 8 solar cell led metal semiconductor junction and heterojunction

I

Schottky

Schottky diode

I

f

f

PN junction

PN junction

B

B

diode

V

V

Applications of Schottly Diodes

  • I0 of a Schottky diode is 103 to 108 times larger than a PN junction diode, depending on fB .A larger I0 means a smaller forward drop V.

  • A Schottky diode is the preferred rectifier in low voltage, high current applications.


Lecture 8 solar cell led metal semiconductor junction and heterojunction

5.10 Heterojunction

Heterojunction gives us additional parameters to manipulate the ratio of electron/hole current

More will be discussed in ECE684: HEMT


Lecture 8 solar cell led metal semiconductor junction and heterojunction

What is the energy band diagram at thermal equilibrium?

What is Vbi


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