OPTOELECTRONICS 1 Prof. Wei-I Lee

OPTOELECTRONICS
2 Prof. Wei-I Lee

QUIZ 姓名 學號 母親生日
OPTOELECTRONICS 3 Prof. Wei-I Lee

QUIZ 姓名 學號 母親生日
A solar cell under an illumination of 600 W/m
2 has a short circuit current
I sc of 16 mA and an open circuit output voltage V oc of 0.5 V. What is the short circuit current when the light intensity is doubled?
OPTOELECTRONICS 4 Prof. Wei-I Lee

Solar Energy
Solar Energy at earth’s surface, average solar energy ~ 4 x 10 24 J/year or ~ 5 x 10 20 J/hr global energy consumption in 2001 ~ 4 x 10 20 J/year
OPTOELECTRONICS 5 Prof. Wei-I Lee

Solar Energy Spectrum
Solar Energy Spectrum Above Earth’s Atmoaphere radiation intensity form sun resembles a black body radiation @ 6000K
I λ , spectral intensity : intensity per unit wavelength
I λ
δλ
: intensity in a small wavelength interval
δλ air-mass zero, AM0 (solar constant) : total power flow through a unit area above Earth’s atmosphere perpendicular to the direction of the sun
( ~1.353 kW/m 2 )
OPTOELECTRONICS 6 Prof. Wei-I Lee

Solar Energy Spectrum
Solar Energy Spectrum Above Earth’s Atmoaphere radiation intensity form sun resembles a black body radiation @ 6000K
I λ , spectral intensity : intensity per unit wavelength
I λ
δλ
: intensity in a small wavelength interval
δλ air-mass zero, AM0 (solar constant) : total power flow through a unit area above Earth’s atmosphere perpendicular to the direction of the sun
( ~1.353 kW/m 2 )
OPTOELECTRONICS 7 Prof. Wei-I Lee

Solar Energy Spectrum
Solar Energy Spectrum at Earth’s Surface actual intensity spectrum on Earth’s surface depends on the absorption and scattering effects of the atmosphere and hence on the atmospheric composition and the radiation path length through the atmosphere air-mass m, AMm : (the actual radiation path) / (shortest path)
m = h / h
0
= sec
θ
AM1.5 Î incident energy on a unit area normal to sun rays which travel the atmospheric length of 1.5 h
0
OPTOELECTRONICS 8 Prof. Wei-I Lee

Solar Energy Spectrum
Solar Energy Spectrum at Earth’s Surface ozone, air and water vapor molecules can cause sharp absorption peaks atmospheric molecules and dust particles scatter the sun Î terrestrial light has a diffuse component in addition to the direct component shorter
λ experience more scattering than longer
λ on a cloudy day, diffuse component ~ 20% of total radiation (higher on cloudy days)
OPTOELECTRONICS 9 Prof. Wei-I Lee

Photovoltaic Device Principles
Schematic of a Typical Single Junction Si Solar Cell consider a Si pn junction with a very thin and more heavily doped n region with finger electrodes and thin antireflection (AR) coating on the surface
S
OPTOELECTRONICS 10 Prof. Wei-I Lee

Photovoltaic Device Principles
Generation of Electron-Hole Pairs (EHP) prefer to have most photons absorbed in the depletion region
Î photogenerated EHP immediately separate by built-in E
0 field longer-
λ photons absorbed in
S neutral p-region Î only photogenerated e
- within minority carrier diffusion length
L e can reach depletion region and contribute to photovoltaic effect short-
λ photons absorbed in neutral n-region Î only photogenerated hole within minority carrier diffusion length
L h can reach depletion region and contribute to photovoltaic effect
OPTOELECTRONICS 11
1/
μ m
Prof. Wei-I Lee

Photovoltaic Device Principles
Open Circuit Voltage and Photocurrent photogenerated EHP within
(L e
+ W + L h
) contribute to photovoltaic effect
L e
> L h
Î choose n on p structure photogenerated e
- drift to n region
S and phogenerated holes drift to p region
Î open circuit voltage (V oc
) developed
( p-side positive w.r.t. n-side ) with externally connected load
Î excess e
on n-side flow through load to recombine with excess holes on p-side
Î photocurrent
OPTOELECTRONICS 12 Prof. Wei-I Lee

Photovoltaic Device Principles
Solar Cell Structure Design photogenerated EHP near device surface disappear by recombination due to surface defects
E g of Si ~ 1.1 eV Î threshold absorption
λ
~ 1.1
μ m
@
λ
~ 1 – 1.2
μ m,
α of Si is small
Î absorption depth (1/
α
) > 100
μ m
Î need thick p-side ( 200 ~ 500
μ m )
S and large L e to have most light absorbed in depletion region Î n region must be thin ( < 0.2
μ m)
L h doesn’t have to longer than n-side l n
Î n can be doped high to reduce series resistance and provide good metal contact
OPTOELECTRONICS 13
1/
μ m
Prof. Wei-I Lee

Photovoltaic I-V Characteristics
Solar Cell I-V Characteristics load R = 0 , V = 0 Î I = I sc or – I ph
(
|
I sc
|
: short circuit current )
I sc
= - I ph
= - K I , I : illumination light intensity
I sc
( I ph
) does not depend on the voltage across the pn junction
R ≠ 0 Î V ≠ 0 Î a forward diode current I d arises total current :
+ – + –
+ –
OPTOELECTRONICS 14 Prof. Wei-I Lee

Photovoltaic I-V Characteristics
Determination of Operation Point total current :
I-V characteristics of a typical Si solar cell
Î I-V under dark, or dark I-V, shifted by I sc
(
−
I ph
)
V oc
’s dependence by illumination light intensity is weak across the load : I R = – V Î I = – V / R
Î load line w. the slope of ( – 1/R) operation point Î crossing point of the diode I-V curve and the load line
+ –
OPTOELECTRONICS 15 Prof. Wei-I Lee

Photovoltaic I-V Characteristics
Delivered Power and Fill Factor power delivered to the load, P out
= I’V’ fill factor FF = (I m
V m
) / (I sc
V oc
)
(I m
V m
) : maximized delivered power, i.e. the largest
(I’V’ ) rectangular area obtainable ( by changing R or illumination intensity ) typical FF : 70 ~ 85%
+ –
OPTOELECTRONICS 16 Prof. Wei-I Lee

Photovoltaic I-V Characteristics
Solar Cell Efficiency and Band-gap
To maximize solar cell energy conversion efficiency
Î maximize
|
I sc
| and V oc
I sc depends mainly on solar cell material’s band-gap : E g
↘ Î |
I sc
| ↗ from Î
Î ( assuming V oc
>> nk
B
T/e )
I o
E g
↘ Î I o
↗ Î V oc
↘
E g
↘ Î |
I sc
| ↗ ,   V oc
↘
Î there is a theoretically optimized band-gap for highest solar cell efficiency
OPTOELECTRONICS 17 Prof. Wei-I Lee

Photovoltaic I-V Characteristics
Solar Cell Structure Design
OPTOELECTRONICS 18 Prof. Wei-I Lee

Photovoltaic I-V Characteristics
Theoretical Single Junction Solar Cell Efficiencies
Despite the low maximum-efficiency values, solar cells remain the most efficient way yet demonstrated converting sunlight to electricity.
OPTOELECTRONICS 19 Prof. Wei-I Lee

Series Resistance and Equivalent Circuit
Series Resistance and Parallel Resistance sources of series resistance in the diode :
- conduction of electrons in thin n region toward the finger electrode
- conduction resistance of the electrodes ( when electrodes are thin )
- resistance due the neutral p region ( usually small ) sources of shunt (or parallel) resistance in the diode : photo-generated carriers flow through crystal surface ( edges of the device ) or through grain boundaries in polycrystalline devices typically, R p less important than R s
OPTOELECTRONICS 20 Prof. Wei-I Lee

Series Resistance and Equivalent Circuit
Effects of Series and Parallel Resistance
R s
↗ Î FF ↘
Î solar cell efficiency ↘ when R s is sufficiently large Î |
I sc
| ↘
R s does not affect V oc
R p leads to a reduced V oc
Î solar cell efficiency ↘
Ideal
OPTOELECTRONICS 21 Prof. Wei-I Lee

Temperature Effects
Temperature Effects
T ↗ Î solar cell output voltage ↘ ,   cell efficiency ↘ from Î
Î
Î
( assuming V oc
>> nk
B
T/e ) and I o
Î
Î , since V oc
< E g
/e : when T ↗ Î V oc
↘
( above is a first order estimation, a complete calculation is more complicated, e.g. N c and N v are T-depend. , and T ↗ Î E g
↘ Î V oc
↘ , but
|
I sc
| ↗ )
OPTOELECTRONICS 22 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Si Cells With Textured Surface textured surface improves light absorption after fraction, photons would enter at oblique angles and absorbed within
L e
Î more effective electron-hole pair generation
OPTOELECTRONICS 23 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Window Layer on GaAs Cells
AlGaAs window layer passivates GaAs surface defects
Î reduce surface recombination and improve cell efficiency
OPTOELECTRONICS 24 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Heterojunction Solar Cells use lattice matched III-V semiconductors of different band-gaps h
υ
> 2 eV photons absorbed by wide band-gap AlGaAs
1.4 eV < h
υ
< 2 eV photons absorbed in GaAs
Î reduce energy loss by lattice thermalization
OPTOELECTRONICS 25 energy loss by lattice thermalizaton
Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Tandem or Multi-Junction Solar Cells state of the art is 3-junction cells typical 3J cell contains 20 layers or more
OPTOELECTRONICS
Source : Spectrolab
26 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Concentrator Solar Cells
3J cell can reach peak efficiency at 500 suns, making big difference in system economics
OPTOELECTRONICS
Source : Spectrolab
27 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Compound Solar Cell Development
OPTOELECTRONICS
Source : Spectrolab
28 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Best 3J Cell in 2006
OPTOELECTRONICS 29
Source : Spectrolab
Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Concentrator Photovoltaic System – I
OPTOELECTRONICS
Source : Spectrolab
30 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Concentrator Photovoltaic System – II
OPTOELECTRONICS
Source : Spectrolab
31 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Solar Cell Efficiency Development Milestones
OPTOELECTRONICS
Source : National Renewable Energy Lab.
32 Prof. Wei-I Lee

Solar Cell Materials, Devices and Efficiencies
Four and Five Junction Solar Cells
current 4 junction stack yields an efficiency of 43% with optical concentration
GaP high band gap top cell in multi-junction stack needed to achieve
50% (theDARPA target efficiency)
Infrared cell
OPTOELECTRONICS
Source : Purdue University Energy Center
33 Prof. Wei-I Lee