Dr. Halldor G. Svavarsson

Halldor G. Svavarsson

Associate professor

School of Science and Engineering

Reykjavik University

Materials for Sustainable Energy Conversion Reykjavik University 02 May 2016

Photovoltaics Solar Energy Converted to Electricity

Halldor G. Svavarsson Associate professor

School of Science and Engineering Reykjavik University

Outline

Solar cells – main types

Principles of solar cells

- p-n junction

Factors affecting the Efficiency of solar cells

Global prospect for PV’s

OUTLINE

First generation: wafer-based cells — crystalline

silicon, predominant PV technology – high purity Si:

• polysilicon

• monocrystalline silicon

http://sustainable-nano.com/2013/08/13/liquor-aging-tiny-barrels-and-next-generation-solar-cells/

Typically 15-20% efficiency

~ 80-90% of the world market

Solar cell technologies are traditionally divided into

three generations

Solar cells – main types

Second generation: thin film solar cells –

mainly:

• amorphous silicon

• CdTe

• CIGS

Significant for stand-alone power system

Typically 5-15% efficiency

~ 15% of the world market

http://fabricalo.net/index.php/how-are-

thin-film-solar-cells-cigs-made/

Solar cells – main types

CIGS solar cell

Third generation: aim to produce low-cost, high-

efficiency solar cells - emerging technology—still

in R&D phase.

• copper zinc tin sulfide (CZTS)

• dye-sensitized (aka Grätzel cell)

• organic

• quantum dot

• perovskite

Suffer from low efficiency and instability issues

(degradation)

https://reginnovations.org/key-energy-

storage-system/efficiently-photo-charging-

lithium-ion-batteries-by-perovskite-solar-cell/

Solar cells – main types

Perovskite solar cell

Solar cells types

Principles of solar cells

- p-n junctions

Efficiency of solar cells

Global prospect for PV’s

OUTLINE

The operation of a photovoltaic cell requires 3 basic steps:

1) absorption of light - generating electron-hole pairs

or excitons. The energy of the light (hc/λ) must be

above certain threshold level (Eg)

2) separation of charge carriers of opposite types

3) separate extraction of carriers to an external circuit

Principles of solar cells - p-n junctions

Most easily understood in terms of semiconductors

Si Si Si Si Si Si Si Si Si

:

:

:

: :

:

:

: :

:

:

:

:

: :

: : : :

: : : :

: .

Principles of solar cells - p-n junctions

.

+ . +

+

Light of energy higher than Eg creates electron-hole pairs

+ + + +

EF

EC

EV

Eg

- - - -

E = h > Eg

Heat

Schematic of a bandstructure of undoped Si

How can we harness the movement of the electrons to produce electricity?

Principles of solar cells - p-n junctions

Light of energy higher than Eg creates electron-hole pairs

A p-type region is created by replacing several Si-atoms with atoms having fewer valence electrons

The potential level of the crystal can be affected by doping

An n-type region is created by replacing several Si-atoms with atoms having higher number of valence electrons

A built-up potential formed across a p-n junction

Principles of solar cells - p-n junctions

n-type (excess e-)

p-type (excess h+)

Electrons are being attracted towards the holes, leaving behind positively charged donors

Holes are being attracted towards the electrons, leaving behind negatively charged acceptor

+ _ D+ A-

+ _ D+ A-

Principles of solar cells - p-n junctions

p-n junction

+

+ n-type

p-type

n-type

p-type Eg

E = h > Eg E = h > Eg

+ + + + + + + +

EF

E

Energy band-structure at p-n junctions

The Fermi-levels of both sides aligns

Creates potential difference across the p-n junction

EF

EF

p-type n-type

EC

EV

p-type n-type

Potential difference created across adjacent layer of p-type Si

(shortage of electrons) and n-type Si (excess of electrons)

+ + + +

qVo

Physics of p-n junctions in solar cells

p-type

n-type .

.

Principles of solar cells - p-n junctions

Light must be able to create e-h pairs on both sides of the p-n junction

p-type

n-type .

.

.

.

Principles of solar cells - p-n junctions

Light must be able to create e-h pairs on both sides of the p-n junction

Free electrons on p-side lower its potential energy by moving to n-side

Free holes on n-side lower its potential energy by moving to p-side

Outline

Solar cells types

Principles of solar cells

- p-n junction

Factors affecting the Efficiency of solar cells

Global prospect for PV’s

OUTLINE

Ephoton = hv = hc/λ

Light (photons) with less energy than Eg doesn't excite

electrons from their position

Light of less energy than Eg passes through the material

Energy in excess of Eg is lost as heat in the material

EC

EV

Eg

E = h > Eg

Heat

d

+

Factors affecting the Efficiency of solar cells

Integrated power = 137 mW/cm2 at AM 0 and 100 at AM 1.5G

Eg for Si equals to 1100 nm (close to optimum for single junction cell)

Ephoton 1

λ

Factors affecting the Efficiency of solar cells

http://www.nrel.gov/continuum/spectrum/awards.html

Multi-junctions can increase efficency

SJ3: 43% efficiency at 418 suns

1.9 eV ≡ 650 nm

1.4 eV ≡ 885 nm

1 eV ≡ 1250 nm

Factors affecting the Efficiency of solar cells

Stacked layer of semiconductors with different bandgap

Allows the absorbance of a broader range of wavelengths, improving the cell's sunlight to electrical energy conversion efficiency.

Adsorption coefficient plays important role

Si has relatively low absorption coefficient, partly due to indirect energy-gap

Factors affecting the Efficiency of solar cells

Must be relatively thick to absorb sufficient light or have very good light

trapping mechanism

Light trapping

Textured surface

Incoming light

Nanowires

Factors affecting the Efficiency of solar cells

Outline

Solar cells types

Principles of solar cells

- p-n junction

Factors affecting the Efficiency of solar cells

Global prospect for PV’s

OUTLINE

EVOLUTION OF GLOBAL SOLAR PV ANNUAL INSTALLED CAPACITY 2000-2014

Source: European Photovoltaic Industry Association (EPIA), Global Market Outlook for Photovoltaics 2015-2019

.

Pow

er [

MW

]

Year

Global prospect for PV’s

Solar PV is covering more than 7 % of the electricity demand in 3 countries in Europe: Italy, Germany and Greece Solar Power could grow in Europe by 80 % by 2019 the market growth experienced in 2013 and 2014

Solar Power covers more than 1% of the world electricity demand

Total module costs of leading Chinese solar companies are claimed to be below $0.50/W - close to grid parity in many areas

Source: European Photovoltaic Industry Association (EPIA), Global Market Outlook for Photovoltaics 2015-2019

.

Global prospect for PV’s

Thank you for your attention