Photovoltaic Devices - ocw.nctu.edu.tw

OPTOELECTRONICS Prof. Wei-I Lee 1

Photovoltaic Devices


哪一天期末考 ? 6/20 (五)

LED, LD, Solar Cells

EF 252, EF 254


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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?


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


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


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



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



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



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


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)


1/μm

S


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



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


1/μm


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

+ – + – + –


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


+ –


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


+ –


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



Solar Cell Structure Design



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



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


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 ↘


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| ↗ )


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


Window Layer on GaAs Cells


AlGaAs window layer passivates GaAs surface defects

reduce surface recombination and improve cell efficiency


Heterojunction Solar Cells


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


Tandem or Multi-Junction Solar Cells


state of the art is 3-junction cells

typical 3J cell contains 20 layers or more

Source : Spectrolab


Concentrator Solar Cells


3J cell can reach peak efficiency at 500 suns, making big difference in

system economics

Source : Spectrolab


Compound Solar Cell Development


Source : Spectrolab


Best 3J Cell in 2006


Source : Spectrolab


Concentrator Photovoltaic System – I


Source : Spectrolab


Concentrator Photovoltaic System – II


Source : Spectrolab


Solar Cell Efficiency Development Milestones


Source : National Renewable Energy Lab.


Four and Five Junction Solar Cells


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

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Photovoltaic Devices - ocw.nctu.edu.tw