h.hÜseyİn erkaya technology for solar cells 2015

Current Technologies for

Solar Cells Hasan Hüseyin Erkaya

Eskişehir Osmangazi University

Afyon Kocatepe University January 15-18, 2015

Renewable Energy Systems Winter School

2

Efficiency of Solar Cells

• Historical Developments

• Fundamentals

• Operation Principle

• Improvements

• Commercial Cells

• Summary

3

Historical Developments

• 1839: Alexandre-Edmond Becquerel

– Discoverry of photoelectric effect

• 1883: Charles Fritts

– Gold-Selenium Contact (1% efficiency)

• 1946: Russel Ohl

– Modern solar cell patent

• 1954: Bell Labs

– Silicon solar cell production • (source: Wikipedia)

4


• 1958: Peter Iles

– First solar cell for a satellite

– Solar cell research international colloboration

– Silicon, 6% efficiency

• 1970: Zhores Alferov

– GaAs Hetero-junction solar cells in sputniks

5


• 1973-74: Petroleum Crisis

– Search for alternative energy sources

– Increasing interest towards solar cells

– Petroleum companies do R&D work and

production

• Diminishing interest in solar cell R&D after

the crisis. (High petroleum prices in recent years

renewed the interest in renewables.)

6


• 1988: Applied Solar Energy Corp. – GaAs production (17% efficiency)

• 1989: Applied Solar Energy Corp. – GaAs on Ge substrate (19% eff)

• 1993: Applied Solar Energy Corp. – Double-junction mass production (20% eff)

• 2000 triple-junction cell (24% eff)



7


• 2007: Two companies producing high efficiency cells:

– Emcore Photovoltaics

8

• 2007: Two companies producing high efficiency cells:

– Spectrolab

9 9


• 2015: Two companies producing high efficiency cells :

– SolAero Technologies Corp. (Previously Emcore Photovoltaics)

(%29.5 space, %39 terrestrial)

10 10

• 2015: Two companies producing high efficiency cells :

– Spectrolab

Concentrator Cells (CPV)

C4MJ 40% Point Focus Solar Cells

C3P5 39.5% Point Focus Solar Cells

C3MJ 38.5% Point Focus Products

Space Cells

29.5% NeXt Triple Junction (XTJ) Solar Cells

28.3% Ultra Triple Junction (UTJ) Solar Cells

26.8% Improved Triple Junction (ITJ) Solar Cells

11


• First Generation Solar Cells

– Silicon substrate (single crystal)

– One junction

– Large area

– Efficiency less than 20%

– 86% of the market

12


• Second Generation Solar Cells

– Thin-film technology

– Lattice matched to the substrate

– Common in space/satellite applications

• AM0 conditions 28-30 % efficiency

• Expensive

– Terrestrial applications

• AM0 conditions 7-10 % efficiency

• Inexpensive

13


• Second generation cells (cont.)

– Silicon

• Amorphous silicon

• Polycrystalline silicon

• Micro crystalline silicon

– Cadmium Telluride

– Copper Indium Selenide

– GaAs based (37% efficiency targeted)

– Thin film on flexible substrates

14


• Third Generation Solar Cells

– Multi-junction cells

– Quantum dot

– Carbon nano tube

– Nanocrystal structure

– Electrochemical structure

– Organic structure

– 45 % efficiency (AM0) targeted

15

Source: Martin Green

16

December

2014

17 17

Highest Efficiencies

(in the lab: 2014)

18 18


(in the lab: 2014)

19 19


(in the lab: 2014)

20 20


(in the lab: 2014)

21 21


(in the lab: 2014)

22 22


(in the lab: 2014)

23

Operation Principle

Solar Cell

(Semiconductor)

Light

(Photons)

Electrical

Energy

24

Fundamentals

These words are from the short story “La Bamba” by Gary Soto– the drawing is mine–HHE

25

Fundamentals

The words and the drawing are mine–HHE

26

Semiconductor Fundamentals

conductivity

insulators conductors semiconductors

GaAs Si

Ge

- Conductivity of a semiconductor is “adjusted” by doping.

- Doping can be n-type or p-type.

- One side of the semiconductor can be doped p-type while the

rest is n-type: a pn-junction.

27


• Pure silicon

– 1 cm3 silicon has 1022 atoms

– Each silicon atom forms covalent bonds with its 4

neighbors.

– @ room temp (300˚K) 1.5x1010 valance electrons

break their bonds and become “free” in silicon

– Broken bond is called a “hole” that can move around.

– These electrons and holes can carry current.

– @ 0˚K all bonds complete

28


• Pure silicon

– Min. Energy to break a covalent bond = Eg

– Eg for silicon: 1.12 eV (electron volt).

• Doped silicon

– Periodical table col. 5 dopants: n-type • (more free electrons than holes: n>p)

– Periodical table col. 3 dopants : p-type • (more holes than free electrons: p>n)

– Easy levels of doping: 1015-1019 cm-3

29


p n

A pn-junction

30


p n - +

- +

- +

- +

- +

Electric field

When a pn-junction is formed, some carriers move to the other side leaving

behind the dopant ions; hence, an E-field is formed.

31

Solar Cell Operation Principle

• If Ephoton ≥ Eg photon is absorbed

• An electron-hole-pair is generated

• Such electrons and holes are separated

and collected → electric current

• An E-field is needed for this.

• PN junction has the built-in E-field.

32


p n

Active region

- +

- +

- +

- +

- +

Electric field Depletion region

Electron-hole pairs

that are generated in

the active region have

a chance to enter the

depletion region and

get collected.

33

A more Realistic Solar Cell

34

Solar Cell Energy Band Diagram

Light

35

Solar Cell Equivalent Circuit

36


• Current vs voltage

P

N

+

-

v

i

i

v

in the dark

with light

37


• Efficiency =

• Why not 100%? – Some photons are reflected back

– Some photons are absorbed outside the active region

– Some photons have insufficient energy

– Excess energy turns to heat

– Contact series resistance and leakage currents

Optical power from sun

Electrical power

38 38

Solar Irradiance

http://wikipedia.org Si GaAs Ge

39 39

Theoretical Limit for Efficiency

(Shockley–Queisser limit) One-Junction Cells

http://wikipedia.org

40

Improvements

• Efficiency =







Electrical power

41 41

Improvement

• Preventing reflection

– Thin clear layer

“anti reflection coating”

http://pveducation.org

42 42

Improvement

• Preventing Reflection

– Thin clear layer

• Titanium Dioxide

• Silicon Dioxide

• Silicon Nitrate

• Spin-on solutions


3SiH4 + 4NH3 Si3N4 + 12H2

http://pveducation.org/

43 43

Improvement

• Preventing reflection

– Rough surface


Shiny surface Rough Surface


44 44

Improvement

• Preventing Reflection

– Rough Surface

• Preferential etching



45

Improvements

• Efficiency =







Electrical power

46 46

Improvement

p n

Active region

- +

- +

- +

- +

- +

Electric field Depletion region

If the top layer is very

thin, there will be series

resistance

Use high quality

semiconductor to make

the active region wide..

• Form the active layer close to the surface

Light

Absorption

47 47

Improvement

Active region width = w + Ln + Lp

Single crystal silicon 50 – 100 μm

Single crystal GaAs 4 – 5 μm

Polycrystal silicon 3 – 4 μm

Other thin films 2 – 3 μm

Two thin layers suffice thin film technology

48

Improvements

• Efficiency =







Electrical power

49 49

Improvement

Solution: Use a semiconductor with a low energy gap

New problem: Efficiency falls for other reasons (Shockley–Queisser limit)

Wavelength

Den

sit

y

Eg

• Low energy photons pass through

Eg

η

50

Improvements

• Efficiency =







Electrical power

51 51

Improvement

Solution: Use a high-gap semiconductor

New problem: Efficiency falls for other reasons (Shockley–Queisser limit)

Eg

Photon

excess energy turns into heat

P N

Eg

η

52 52

Improvement

A better solution: Use high and low gap

semiconductors together

•Let the high-energy photons be

absorbed first

•Then, let the other ones be absorbed

Eg1 > Eg2 > Eg3

P

N

P

N

P

N Eg1

Eg2

Eg3

53 53

Improvement

New Problem: How do we connect these

layers?

•Epitaxial PNPNPN structure will have

some junctions reverse biased which

would not let the current pass.

New Solution: Insert heavily doped thin

layers and form tunnel diodes. P

N

P

N

P

N Eg1

Eg2

Eg3

54 54

Improvement

Inserting heavily doped thin layers to

form tunnel dides:

P

N

P

N

P

N Eg1

Eg2

Eg3

N++

P++

N++

P++

P++ N++ junction E

v

i

PN diode

EF

EC

EV

55 55

Multi-Junction Structure

56

Improvements

• Efficiency =







Electrical power

57 57

Improvement

Solution: the series resistance cen be reduced by

increasing the contact area

New Problem: Increased contact area would

block some light.

58 58

Improvement

Solution: passify the surface to reduce the

leakage currents

(a good friend when in need: Hydrogen)

While anti reflection layer is deposited, this issue

is taken care of. (SixNy:H)

Improvement

Solution: use concentrators to reduce the ratio of

leakage currents

cell

sunlight

60 60

The Concept

Imagine some liquid flowing in a leaky pipe:

input output

leak

input output

leak

pipe

input output

leak

While the leak

remains

almost

constant, the

ratio of output

flow rate to

input flow rate

increases with

increasing

input flow rate

Concentrators

Lens:

Concentrators

Parabolic mirrors:

Concentrators

Reflectors:

Concentrators

Luminescent concentrators:

Concentrators

New Problem: Solar cell gets extremely hot!

Concentrators

New Problem: Solar cell efficiency gets smaller with increasing temperature (It is not as bad as it sounds )

67 67

Panels

• Series and parallel connection of cells

• Isolation of cells

• Protection of cells from

Mechanical effects

Atmospherical effects

Chemical effects

array

panel

68 68

Panels

Low reflective,

transparent,

Low iron,

Self cleaning glass

Ethyl Vinyl Acetate

150 °C heat process

69 69

Concentrator Panels

70 70

Concentrator Dish Arrays

71 71

Materials in Solar Cell Production

72 72

Materials in Solar Cell Production

73 73

Price history chart of crystalline silicon solar cells (wikipedia)

74 http://pv.energytrend.com/pricequotes.html

75 http://pv.energytrend.com/pricequotes.html

76 76

Silicon

Type Symbol Grain

size

Growth Technique

Single crystal sc-Si >10cm Czochralski (CZ) float

zone (FZ)

Multicrystalline mc-Si 1mm–

10cm

Cast, sheet, ribbon

Polycrystalline pc-Si 1µm–

1mm

CVD

Microcrystalline µc-Si <1µm Plasma

77 77

Silicon Production

Sand + Coal Silicon + Smoke

1500-2000 °C : SiO2 + C Si + CO2 (98% pure Si)

Siemens Process:

300 °C : Si + 3HCl SiHCl3 + H2

SiHCl3 is purified.

1100 °C : SiHCl3 + H2 Si + 3HCl

78 78

Czochralski Method

Seed

Molten silicon

20 cm diameter

150 kg weight

79 79

Silicon Production: Floating Zone Method

20 cm diameter

600 kg weight

Moving

hot zone

20 cm diameter

150 kg weight

80 80

Silicon Production: Squaring

Green MA. 2013 Silicon solar cells: state of the art. Phil Trans R Soc A 371:

20110413. http://dx.doi.org/10.1098/rsta.2011.0413

81 81

Silicon Production: Casting

molten silicon is cast into molds of 50 cm x 50 cm x 25 cm

82 82

Silicon Production: Casting

Cast silicon is cut into smaller blocks (bricks) of 10 cm x 10 cm x 25 cm

Generation 5: 5x5 = 25 bricks (common)

Generation 8: 8x8 = 64 bricks (expected)

83 83

Silicon Production: Slicing

Cast silicon is sliced with wires

84 84

Silicon processing

P-type phosphorus diffusion

Formation of contacts

Anti-reflective coating

Electrical connection

Panel formation

85 85

Silicon Production:

Cell having mono and multi crystalline regions

Green MA. 2013 Silicon solar cells: state of the art. Phil Trans R Soc A 371:

20110413. http://dx.doi.org/10.1098/rsta.2011.0413

86 86

Silicon Production:

Panel

Green MA. 2013 Silicon solar cells: state of the

art. Phil Trans R Soc A 371: 20110413.

http://dx.doi.org/10.1098/rsta.2011.0413

87 Green MA. 2013 Silicon solar cells: state of the art. Phil Trans R Soc A 371: 20110413.


Standard screen-printed silicon solar cell, where the front and

rear metal contacts are applied by screen printing (silicon nitride antireflection coating not shown)



The ‘black’ cell, the first silicon cell to exceed 17% energy

conversion efficiency



SunPower rear junction cell schematic



25% efficient PERL cell.



Laser doped, selective emitter solar cell



Sanyo’s heterojunction with intrinsic thin layer (HIT) cell.



(a) Emitter Wrap Through (EWT) cell

(b) Metal Wrap Through (MWT) cell

New Structures

94 94

Other Structures

95 95

Other structures

96 96

Other structures

97 97

Other structures

98 98

Other structures

99 99

Other structures

100 100

Other structures

101 101

Other structures

102 102

Other structures

103

How Much Power

• Solar energy reaching the face of the earth

≈1000 W/m2

• Assume 10% efficiency

• 1m2 panel output 100 W

• 100 W light-bulb: 1 m2 panel area

• 2400 W oven: 24 m2 panel area

104

Photovoltaic System

Solar cell

panels

batteries DC loads

AC loads

charging

system inverter

Panel . . . . . . $$$

Charger . . . . $$

Inverter . . . . . $$

Batteries . . .$$$

Support struc $

Labor . . . . . $$

_____________

Total . . . $$$$

105 105

Summary

• 100% efficiency is not possible.

• One-junction cells: low efficiency.

• Better quality material, higher efficiency

• Multi-junctions: higher efficiency

• Concentrators: higher efficiency

106 106

Thanks for your attention.

Hasan Hüseyin Erkaya

Eskişehir Osmangazi University Electrical-Electronics Engineering Department

January 18, 2015

h.hÜseyİn erkaya technology for solar cells 2015

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