Photovoltaic Physics and Materials

Lecture Note2009/2010 Semester 2

WANG, QINGDepartment of Materials Science and Engineering

National University of Singapore

1

About this module:1. Main Reference: The Physics of Solar Cells. J. Nelson. Imperial

College Press, 2003.

2. Assignments (10%) 3. Tutorials: recitation/discussion/Q&A (10%)4. Project Report (30%)5. Final Exam: open book (50%)6. I assume you have the basic knowledge of thermodynamics,

semiconductor physics. 7. A web blog will be built for this module (http://blog.nus.edu.sg/

msewq/). You are welcome to write to me for any questions you have for this module. (EMAIL: msewq@nus.edu.sg)

2

The term "photovoltaic" comes from the Greek φώς (phos) meaning "light", and "voltaic", meaning electric, from the name of the Italian physicist Volta, after whom a unit of electrical potential, the volt, is named. The term "photo-voltaic" has been in use in English since 1849.

http://en.wikipedia.org/wiki/Solar_cell

3

4

1. Introduction and Characteristics of Solar Cells2. Photon in, Electron out: Basic Principle of PV3. Electrons and Holes in Semiconductors4. Generation and Recombination5. Junctions and Analysis of p-n Junction6. Monocrystalline Solar Cells7. Thin Film Solar Cells (I)8. Thin Film Solar Cells (II)9. Photoelectrochemical Cells (I)10.Photoelectrochemical Cells (II)11.Over the Limit: Third Generation Photovoltaics (I)12.Over the Limit: Third Generation Photovoltaics (II)

What we will discuss...

5

Introduction and Characteristics of Solar Cells

Lecture 1

References:1. The Physics of Solar Cells. Jenny Nelson. Imperial College Press,

2003.2. Photovoltaic Materials, Series on Properties of Semiconductor

Materials, Vol.1, Richard H. Bube, Imperial College Press, 1998.3. Wikipedia (http://en.wikipedia.org/wiki/Main_Page).

6

Our Energy ChallengesEnergy Sources: Yesterday, Today and Tomorrow

Schollnberger 1998 after Shell International Ltd 1996

• At MINIMUM we need 10 Terawatts from some new clean energy source by 2050. For worldwide peace and prosperity we need it to be cheap. • We simply can not do this with current technology. • Our energy challenges are real and here!

7

Solar energy supply to the earth: ca 3 million exajoules per year. Current energy demand of the world is 474 exajoules. This could be fully met by covering 0.15 % of the earth’s surface with PV panels having 10% efficiency.

World Solar Energy Map

http://cleantechlawandbusiness.com/

http://en.wikipedia.org/wiki/World_energy_resources_and_consumption

8

Utilization of Solar Energy: Ancient Day

Archimedes Burning Mirror (Death Ray), 514 A.D.

Power of Light-The Epic Story of Man’s Quest to Harness the Sun, Frank T. Kryza, McGraw-Hill, 2003.

A Mouchot solar concentrator, Paris, 1880

9

The Photovoltaic Effect

Metal UV light

e-

Photoelectric effect

p light

e-

n

Current

Load

Photovoltaic effect

• Light is made up of packets of energy, called photons.• Energy of light depends only upon its frequency or color.• Blue or ultraviolet light provides energy for electron to escape from

the surface of a metal---Photoelectric effect.• Built-in asymmetry in photovoltaic device pulls the excited electrons

to the external circuit before they can relax. €

E =hcλ

10

Brief History of Solar Cells

1. Photovoltaic effect was first reported by Alexandre-Edmond Becquerel in 1839. The first photo-electrochemical cell.

2. The first solid state photovoltaic devices were constructed by William Adams and Richard Day in 1876. Pt/Se/Pt

3. The first large area solar cell was prepared by Charles Fritts in 1894. Au/Se/M

The early cells were thin film Schottky barrier devices.First revealed by Goldman and Brodsky in 1914, and later during the 1930s, the theory of Metal/Semiconductor barrier layers was developed by Walter Schottky, Neville Mott and others.

Cu/CuO, Pb/PbS, Tl/TlS

http://www.swiscontrol.com/SWISCONTROL/Mapas%20de%20Bocas%20del%20Toro/PAGINAS%20HTML/HYSTORIA%20PV.html

11

Brief History of Solar Cells4. The first silicon p-n junction solar cell was reported by Bell Lab

researchers D.M. Chapin, C.S. Fuller and G.L. Pearson in 1954. 5. In the 1950s, CdS, GaSe, InP, CdTe p-n junction photovoltaic

devices were studied experimentally and theoretically.6. In 1958, the first PV-powered satellite (Vanguard I) was launched,

despite the production cost of ~$200 per Watt.7. In the 1970s, the “energy crisis” led to intense interest for R&D of

photovoltaics. Various strategies and routes for cheaper and more efficient photovoltaics were explored. Our understanding of the science of photovoltaics is mainly rooted in this period.

8. In the late 1990s, by expanding the production scale photovoltaics first became competitive in contexts where conventional electricity supply is most expensive.

9. Benefitted from the advances of silicon technology for the micro-electronics industry, silicon remained and remains the foremost photovoltaic material.

http://www.swiscontrol.com/SWISCONTROL/Mapas%20de%20Bocas%20del%20Toro/PAGINAS%20HTML/HYSTORIA%20PV.html

12

41.6% @364X

13

Global Solar Cells Production

Cost and production of PV modules

Source: JRC PV Status Report 2009

14

Global Solar Cells Production

15

16

Types of Solar Energy Converter

Solar thermal converter: • The radiant energy absorbed is converted

mainly into internal energy and raised the temperature of the cell.

• It operates as a heat engine and does work.• It utilizes the full range of solar wavelengths.• It is thermally insulated from the ambient.

Photovoltaic converter: • Convert the incident radiant energy

mainly into electrochemical potential energy.

• Absorption of photon promotes electron to higher energy (excited state), which should be separated from the ground state by an energy gap (e.g. band gap in semiconductors).

• Charges are separated, collected and extracted to external circuit and do electrical work.

• It extracts solar energy only from photons with energy sufficient to bridge the band gap.

• It is designed to be in good thermal contact with the ambient.

Photochemical converter: • The radiant energy absorbed results in a

permanent increase in chemical potential. e.g. photosynthesis.

• The excited electron population drives a chemical reaction.

Eg

excited state

ground state

Means of charge separation

hν

17

Current

Volta

ge

Increasing light intensity

battery e.m.f.

BATTERY

RL

A Comparison with BatteryBattery:• EMF: Permanent electrochemical

potential difference between two phases in the cell.

• Power delivered to a constant load is relatively constant.

• Exhausted as it is completely discharged. It can be recharged for rechargeable batteries.

• Voltage generator

Solar Cell:• EMF: Temporary change in

electrochemical potential caused by the light.

• Power delivered depends on the incident light intensity.

• Never exhausted since it can be continually recharged with light.

• Current generator

RL

SOLAR CELL hν

18

Some Important DefinitionsOpen Circuit Voltage (VOC): The voltage developed as the terminals are isolated (or with infinite load resistance).Short Circuit Current (ISC): The current drawn as the terminals are connected (or with zero load resistance).

Bias Voltage, V

Cur

rent

Den

sity

, J

VOCVm

Jm

JSC

0

For any intermediate load resistance RL, the cell develops a voltage V between 0 and VOC, and delivers a current I with

RL

SOLAR CELL hν

€

V = IRLBoth I (photocurrent) and V (photovoltage) are determined by the illumination and the load.

19

Characteristics of Solar Cells

€

JSC = q bs E( )QE E( )dE∫

Relation between photocurrent density and quantum efficiency:

bs(E): the incident spectral photon flux density. The number of photons of energy in the range E to E+dE which are incident on unit area in unit time.QE(E): the quantum efficiency. The probability that an incident photon of energy E will deliver one electron to the external circuit.

Photocurrent and Quantum Efficiency

QE depends on the absorption coefficient of the solar cell material, the efficiency of charge separation and efficiency of charge collection. It is a key quantity in describing solar cell performance.

20

Incident Photo-to-Electron Conversion Efficiency (IPCE) Spectrum/Action Spectrum

€

E =1240 nm

λeV( )

Characteristics of Solar CellsPhotocurrent and Quantum Efficiency

€

JSC = q bs E( )QE E( )dE∫

21

-

+

V

JSC Jdark

hν

Characteristics of Solar CellsDark Current and Open Circuit Voltage

€

Jdark V( ) = JO expqVkBT

−1

Dark current: in the opposite direction to the photocurrent, generated by a potential difference developed between the terminals of the cell in the presence of a load. Analogue to the current Idark (V) which flows across the device under an applied voltage V in the dark.

Most solar cells behave like a diode in the dark and show rectifying behavior since an asymmetric junction is needed to achieve charge separation. For an ideal diode,

JO is reverse saturation current, kB is Boltzmann’s constant (1.38×10-23 JK-1).

22

Characteristics of Solar Cells

Dark Current and Open Circuit Voltage

Superposition approximation:

€

J = JSC − JO expqVkBT

−1

At open circuit voltage VOC, no current flows across the cell, for an ideal diode,

€

VOC =kBTqln JSC

JO+1

-

+

V

JSC Jdark

hν

Here voltage is defined so that the photovoltage occurs in forward bias, where V>0.At V<0, the device acts as a photodetector, consuming power to generate a photocurrent. Light dependent/bias independent.At V>VOC, the device acts as light emitting diode, again consuming power.

Light Current

VOC

ISC

Bias Voltage

Cur

rent

Dark Current0

€

Jdark V( ) = JO expqVkBT

−1

23

Jm

Power Density

Characteristics of Solar CellsEfficiency of Solar Cells

Maximum Power Point

Bias Voltage, V

Cur

rent

Den

sity

, J

VOC

JSC

0 Vm

The cell power density is given by

P reaches a maximum at the cell’s operating point or maximum power point at Vm and Jm. The optimum load has resistance of Vm/Jm.€

P = JV

Efficiency η of the cell: the power density delivered at operating point as a fraction of the incident light power density Ps.

€

η =JmVm

PSRelating to JSC and VOC,

€

η =JSCVOCFF

PSFF is the fill factor describing the “squareness” of the J-V curve, defined as

€

FF =JmVm

JSCVOC

24

Performance of the Mainstream Solar Cells

Cell type Area (cm2) Voc (V) Jsc (mA/cm2) FF Efficiency (%)

crystalline Si 4.0 0.706 42.2 82.8 24.7

crystalline GaAs 3.9 1.022 28.2 87.1 25.1

poly-Si 1.1 0.654 38.1 79.5 19.8

a-Si 1.0 0.887 19.4 74.1 12.7

CuInGaSe2 1.0 0.669 35.7 77.0 18.4

CdTe 1.1 0.848 25.9 74.5 16.4

Characteristics of Solar Cells These four quantities: JSC, VOC, FF and η are the key performance characteristics of a solar cell. All of these should be defined for particular illumination condition.

25

-

+

V

JSC Jdark

hνRsh

Rs

Characteristics of Solar Cells

Parasitic ResistanceFor a real cell, two parasitic resistances used to account for the power dissipation through the resistance of the contacts and through leakage currents around the sides of the device. One is in series (RS) and one is in parallel (Rsh) with the cell.

€

J = JSC − J0 expq V + JARS( )

kBT

−1

−

V + JARS

ARsh

26

Characteristics of Solar Cells

Bias

Cur

rent

Rs increasing

BiasC

urre

nt

Rsh decreasing

Effect of Parasitic Resistance The series resistance arises from the resistance of the cell material to current flow, a particular problem at high current densities. The parallel or shunt resistance arises from the leakage of current through the cell, a problem in poor rectifying devices.

Both RS and Rsh reduce the fill factor of the cell. For an efficient cell, RS should be as small and Rsh should be as large as possible.

27

Characteristics of Solar Cells

Non-ideal Diode Behavior

€

J = JSC − JO expqVmkBT

−1

In real device, it is common for the dark current to depend more weakly on bias. The actual dependence on V is quantified by an ideality factor, m.

The ideal diode equation assumes that all the recombination occurs via band to band or recombination via traps in the bulk areas from the device (i.e. not in the junction). The presence of other recombination mechanisms renders the deviation of m from 1. Typically, 1<m<2.

28

Solar Cells, Modules and Systems

0V

+12V

+VSt

ring

0

• The solar cell is the basic building block of solar photovoltaics.

• To produce useful dc voltage, the cells are connected in series and encapsulated into modules.

29

Solar Cells, Modules and Systems

Residential grid-connected PV system Grid Tie with Backup Power (battery based)

PV generator

Power conditioning Load

Storage Battery-dc/Grid-ac

30

Summary

• History and current status of solar cells.• Equivalent circuit of solar cells.• Some important parameters for solar cells: dark current,

short circuit current, open circuit voltage, maximum power point, fill factor, efficiency, quantum yield, parasitic resistance, PV module.

• Diode equation.

31