Photovoltaic Devices - ocw.nctu.edu.tw
Transcript of Photovoltaic Devices - ocw.nctu.edu.tw
OPTOELECTRONICS Prof. Wei-I Lee 4
QUIZ 姓名
學號
母親生日
A solar cell under an illumination of 600 W/m2 has a short circuit current
Isc of 16 mA and an open circuit output voltage Voc of 0.5 V. What is the
short circuit current when the light intensity is doubled?
OPTOELECTRONICS Prof. Wei-I Lee 5
at earth’s surface, average solar energy ~ 4 x 1024 J/year or ~ 5 x 1020 J/hr global energy consumption in 2001 ~ 4 x 1020 J/year
Solar Energy
Solar Energy
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radiation intensity form sun resembles a black body radiation @ 6000KIλ, spectral intensity : intensity per unit wavelength Iλ δλ : intensity in a small wavelength interval δλair-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/m2 )
Solar Energy Spectrum Above Earth’s Atmoaphere
Solar Energy Spectrum
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radiation intensity form sun resembles a black body radiation @ 6000KIλ, spectral intensity : intensity per unit wavelength Iλ δλ : intensity in a small wavelength interval δλair-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/m2 )
Solar Energy Spectrum Above Earth’s Atmoaphere
Solar Energy Spectrum
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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 / h0 = secθAM1.5 incident energy on a unit area normal to sun rays which travelthe atmospheric length of 1.5 h0
Solar Energy Spectrum at Earth’s Surface
Solar Energy Spectrum
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ozone, air and water vapor molecules can cause sharp absorption peaksatmospheric molecules and dust particles scatter the sun 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)
Solar Energy Spectrum at Earth’s Surface
Solar Energy Spectrum
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consider a Si pn junction with a very thin and more heavily doped n regionwith finger electrodes and thin antireflection (AR) coating on the surface
Schematic of a Typical Single Junction Si Solar Cell
Photovoltaic Device Principles
S
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prefer to have most photons absorbed in the depletion region
photogenerated EHP immediatelyseparate by built-in E0 field longer-λ
photons absorbed in
neutral p-region only photogeneratede- within minority carrier diffusion length Le can reach depletion region and contribute to photovoltaic effect short-λ
photons absorbed in neutral
n-region only photogenerated hole within minority carrier diffusion length Lh can reach depletion region and contribute to photovoltaic effect
Generation of Electron-Hole Pairs (EHP)
Photovoltaic Device Principles
1/μm
S
OPTOELECTRONICS Prof. Wei-I Lee 12
S
photogenerated EHP within (Le + W + Lh) contribute to photovoltaic effectLe > Lh choose n on p structure photogenerated e- drift to n region and phogenerated holes drift to p region
open circuit voltage (Voc) developed( p-side positive w.r.t. n-side )
with externally connected load excess e- on n-side flow through load
to recombine with excess holes on p-sidephotocurrent
Open Circuit Voltage and Photocurrent
Photovoltaic Device Principles
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S
photogenerated EHP near device surface disappear by recombinationdue to surface defectsEg of Si ~ 1.1 eV threshold absorption λ
~ 1.1 μm
@λ
~ 1 – 1.2 μm, α
of Si is small
absorption depth (1/α) > 100 μmneed thick p-side ( 200 ~ 500 μm )and large Le
to have most light absorbed in depletionregion n region must be thin ( < 0.2 μm)
Lh doesn’t have to longer than n-side lnn can be doped high to reduce series
resistance and provide good metal contact
Solar Cell Structure Design
Photovoltaic Device Principles
1/μm
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load R = 0 , V = 0 I = Isc or –Iph (|Isc| : short circuit current ) Isc = - Iph = - K I , I : illumination light intensity
Isc ( Iph ) does not depend on the voltage across the pn junctionR ≠ 0 V ≠ 0 a forward diode current Id arises
total current :
Solar Cell I-V Characteristics
Photovoltaic I-V Characteristics
+ – + – + –
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total current :
I-V characteristics of a typical Si solar cell I-V under dark, or dark I-V, shifted by Isc ( −Iph )
Voc’s dependence by illumination light intensity is weak across the load : I R = –V I = – V / R
load line w. the slope of (–1/R)operation point crossing point of the diode I-V curve and the load line
Determination of Operation Point
Photovoltaic I-V Characteristics
+ –
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power delivered to the load, Pout = I’V’
fill factor FF = (ImVm) / (Isc Voc )
(ImVm) : maximized delivered power, i.e. the largest
(I’V’ ) rectangular area obtainable ( by changing R or
illumination intensity )
typical FF : 70 ~ 85%
Delivered Power and Fill Factor
Photovoltaic I-V Characteristics
+ –
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To maximize solar cell energy conversion efficiency
maximize |Isc | and VocIsc depends mainly on solar cell material’s band-gap : Eg ↘ |Isc|↗
from
( assuming Voc >> nkBT/e )
Io
Eg ↘ Io ↗ Voc ↘
Eg ↘ |Isc|↗ , Voc ↘
there is a theoretically optimized band-gap for highest solar cell efficiency
Solar Cell Efficiency and Band-gap
Photovoltaic I-V Characteristics
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Despite the low maximum-efficiency values, solar cells remain the most efficient way yet demonstrated converting sunlight to electricity.
Theoretical Single Junction Solar Cell Efficiencies
Photovoltaic I-V Characteristics
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Series Resistance and Parallel Resistance
Series Resistance and Equivalent Circuit
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, Rp less important than Rs
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Effects of Series and Parallel Resistance
Series Resistance and Equivalent Circuit
Ideal
Rs ↗ FF ↘
solar cell efficiency ↘
when Rs is sufficiently
large | Isc | ↘
Rs does not affect Voc
Rp leads to a reduced Vocsolar cell efficiency ↘
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Temperature Effects
Temperature Effects
T ↗ solar cell output voltage ↘, cell efficiency ↘
from
( assuming Voc >> nkBT/e )
and Io
, since Voc < Eg /e : when T ↗ Voc ↘
( above is a first order estimation, a complete calculation is more complicated, e.g. Nc and Nv are T-depend. , and T ↗ Eg ↘ Voc ↘ , but |Isc| ↗ )
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Si Cells With Textured Surface
Solar Cell Materials, Devices and Efficiencies
textured surface improves light absorption
after fraction, photons would enter at oblique angles and absorbed within
Le more effective electron-hole pair generation
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Window Layer on GaAs Cells
Solar Cell Materials, Devices and Efficiencies
AlGaAs window layer passivates GaAs surface defects
reduce surface recombination and improve cell efficiency
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Heterojunction Solar Cells
Solar Cell Materials, Devices and Efficiencies
use lattice matched III-V semiconductors of different band-gaps
hυ
> 2 eV photons absorbed by wide band-gap AlGaAs
1.4 eV < hυ
< 2 eV photons absorbed in GaAs
reduce energy loss by lattice thermalization
energy loss by lattice thermalizaton
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Tandem or Multi-Junction Solar Cells
Solar Cell Materials, Devices and Efficiencies
state of the art is 3-junction cells
typical 3J cell contains 20 layers or more
Source : Spectrolab
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Concentrator Solar Cells
Solar Cell Materials, Devices and Efficiencies
3J cell can reach peak efficiency at 500 suns, making big difference in
system economics
Source : Spectrolab
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Compound Solar Cell Development
Solar Cell Materials, Devices and Efficiencies
Source : Spectrolab
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Best 3J Cell in 2006
Solar Cell Materials, Devices and Efficiencies
Source : Spectrolab
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Concentrator Photovoltaic System – I
Solar Cell Materials, Devices and Efficiencies
Source : Spectrolab
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Concentrator Photovoltaic System – II
Solar Cell Materials, Devices and Efficiencies
Source : Spectrolab
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Solar Cell Efficiency Development Milestones
Solar Cell Materials, Devices and Efficiencies
Source : National Renewable Energy Lab.
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Four and Five Junction Solar Cells
Solar Cell Materials, Devices and Efficiencies
Source : Purdue University Energy Center
High-EfficiencyMulti-junction Photovoltaics
Infrared cell
GaP high band gap topcell in multi-junctionstack needed to achieve50% (theDARPA targetefficiency)current 4 junction
stack yields an efficiencyof 43% with optical concentration