Microsoft PowerPoint - Chapter 6 Photovoltaic DevicesChapter 6 Photovoltaic Devices
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Honda’s two seated Dream car is powered by photovoltaics. The Honda Dream was first to finish 3,010 km in four days in the 1996 World Solar Challenge. (Courtesy of Photovoltaics Special Research Centre, University of New South Wales,Sydney, Australia)
Photovoltaic devices (or solar cells) Convert the incident solar radiation energy into electrical energy.
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6.1 Solar Energy Spectrum
0
AM0
AM1.5
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0
0.5
1.0
1.5
2.0
2.5
or kW m-2 (μm)-1
The spectrum of the solar energy represented as spectral intensity (Iλ) vs wavelength above the earth's atmosphere (AM0 radiation) and at the earth's surface (AM1.5 radiation). Black body radiation at 6000 K is shown for comparison (After H.J. Möller, Semiconductors for Solar Cells, Artech House Press, Boston, 1993, p.10) From S.O. Kasap, Optoelectronics and Photonics: Principles and Practices (Prentice
Fraunhofer absorpt (absorbed by hydro
Absorption: by various molecules ozone, air, and water vapor molecules
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Spectral intensity Iλ
Solar constant or air-mass zero (AM0) radiation: 1.353 kW m-2
The integrated intensity above Earth’s atmosphere gives total power flow through a unit area perpendicular to the direction of the sun
: Intensity per unit wavelength
`
the shortest path Air mass
o
o
Direct Diffuse
(a) Illustration of the effect of the angle of incidence θ on the ray path length and the
definitions of AM0, AM1 and AM(sec θ). The angle α between the sun beam and the horizon
is the solar latitude (b) Scattering reduces the intensity and gives rise to a diffused radiation
Atmosphere
AM0
AM1
θ
When , the actual radiation path m (AMm) =
the shortest path Air mass
o
o
>
Diffuse: scattering reducing the intensity and and random angle Shorter λ is stronger ~20% on a clear day Significantly higher on cloudy days
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6.2 Photovoltaic Device Principles
Neutral n-region
Neutral p-region
Le
The principle of operation of the solar cell (exaggerated features highlight principles)
Lh
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Very narrow and more heavily doped n-side is very narrow Most of the photons will be absorbed within the depletion region(W) and within the neutral p-side( p ) and photogenerate electron – hole pairs (EHPs) in these regions.
E0: internal built in electrical field ⇒ electron to n-side ⇒ hole to p-side ⇒ open circuit voltage
2 : recombination life time : diffusion coefficient
e e e
Finger electrodes
Bus electrode for current collection
Finger electrodes on the surface of a solar cell reduce the series resistance
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
A thin antireflection coating on the surface (not shown in the figure) reduces reflections and allows more light to enter the device.
Anti-reflection coating
Source: Richard M. Swanson, SunPower Corporation, 2005.
SunPowerSunPower
0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8
Wavelength (μm)
Absorption coefficient (α) vs. wavelength (λ) for various semiconductors (Data selectively collected and combined from various sources.)
α (m-1)
Indirect bandgap: Less photon absorp. less sharp λg
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L e
exp(−αx)
Photogenerated carriers within the volume Lh + W + Le give rise to a photocurrent Iph. The variation in the photegenerated EHP concentration with distance is also shown where α is th absorption coefficient at the wavelength of interest.
: (due to the flow of the photogenerated carriers)phI photocurrent
2 may be smaller than
e e e
Typically n <= 0.2 μm
e ~ 200-500 μm
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Crystalline silicon bangap 1.1 eV => threshold wavelength 1.1 μm Light of longer wavelength is wasted (~25%)
high concentration of recombination centers at crystal surfaces and interfaces High energy photons become absorbed near the crystal surface and are lost by recombination in the surface region. (~40%)
Combined effect (100-25-40)% = 45%
Imperfect antireflection coating => times 0.8~0.9
=> Upper limit of the efficiency of single crystal Si: 24~26% at room temp.
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6.3 pn Junction Photovoltaic I-V Characteristics
Iph
R
I
R (a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions of positive voltage and positive current. (b) The solar cell in short circuit. The current is the photocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current I in the circuit.
Light
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where is the light intensity
K is a constant
Iph
R
I
R (a) (b) (c)
(a) The solar cell connected to an external load R and the convention for the definitions of positive voltage and positive current. (b) The solar cell in short circuit. The current is the photocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current I in the circuit.
Light
Short circuit current in light (dep. on the # of EHPs)
Photocurrent Iph
Photocurrent does not dep. on the V if second effect (channel width modulation) is neglected. R
⇒ reduce built-in V ⇒ diode current
p n
V
0.60.40.2
20
?0
0
Iph
Voc
Typical I-V characteristics of a Si solar cell. The short circuit current is Iph
and the open circuit voltage is Voc. The I-V curves for positive current requires an external bias voltage. Photovoltaic operation is always in the negative current region.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
exp 1ph o B
= − + −

Voc: typ. 0.4~0.6 V
Total current
V
V′
The Load Line for R = 30 (I-V for the load)
I-V for a solar cell under an illumination of 600 Wm-2.
Operating Point Slope = – 1/R
P I′
(a) When a solar cell drives a load R, R has the same voltage as the solar cell but the current through it is in the opposite direction to the convention that current flows from high to low potential. (b) The current I′ and voltage V′ in the circuit of (a) can be found from a load line construction. Point P is the operating point (I′, V′). The load line is for R = 30 .
Light I
VI R
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maximun power outputfill factor ( FF ) desirable goal output
m m
sc oc
6.4 Series Resistance and Equivalent Circuit
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Neutral n-region
Neutral p-region
Finger electrode
Back electrode
Depletion region
Series and shunt resistances and various fates of photegenerated EHPs.
?1999 S O Kasap Optoelectronics (Prentice Hall)
A
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
series resistance sR
shunt resistance pR
(due to all these electron paths in the n-layer surface region to finger electrodes)
(due to a fraction of the photogenerated carriers which can flow through the crystal surfaces(edge of the device) or through grain boundaries in polycrystalline devices instead of flowing in the external load RL ,
smaller than Rs, usu. negligible, unless highly polycrystalline)
exp 1d o B
eVI I nk T
I (mA)
V 0
Iph
The series resistance broadens the I-V curve and reduces the maximum available power and hence the overall efficiency of the solar cell. The example is a Si solar cell with n ≈ 1.5 and Io ≈ 3 × 10-6 mA. Illumination is such that the photocurrent Iph = 10 mA.
?1999 S O Kasap Optoelectronics (Prentice Hall)
exp 1ph o B
= − + −
Rs reduces max. power High Rs reduces Isc Rs does not affect Voc Rp reduces Voc
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0.60.40.20246
5
15
2 cells in parallel
Current vs. Voltage and Power vs. Current characteristics of one cell and two cells in parallel. The two parallel devices have Rs/2 and 2Iph.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall) NTU EO/EE
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A
Iph
V
Iph
Id
B
Rs
RL
I/2
Id
Iph
I
RsI/2
Two identical solar cells in parallel under the same illumination and driving a load RL.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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6.5 Temperature Effects
The output voltage and the efficiency of a solar cell increases with decreasing temperature; solar cells operate best at lower temperatures.
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exp 1d o B
eVI I nk T
B
exp 1ph o B
= − + −

when ln( ) , B oc
eVnk T K KV or e I nk T I
I I
2 2 1 2 1 2 2
ln( ) ln( ) ln( ) ln( )− = − = ≈oc oc o i
B B o o o i
eV eV I nK K k T k T I I I n
I I
2 1
− = − 2 2 2 1
= + −
= − + −

e.g. Voc1 = 0.55 V at 20 deg. (293K) => Voc2 = 0.475 V at 60 deg. (333K) 2
2 1 1
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6.6 Solar Cells Materials, Devices and Efficiencies
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Solar cell efficiency: fraction of incident light energy converted to electrical energy
Efficiency depends on semiconductor material properties, device structure Ambient conditions: temperature, high radiation damage by high energy particles Locations: different spectrum, diffuse
Solar cell concentrators: focus solar light to increase the overall efficiency
Most solar cells are silicon based. Typ. efficiency: ~18% for polycrystalline,
22-24% for high efficiency single crystal devices
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Light Oxide
n p
Le
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Inverted pyramids a second or even a third change for absorption entering at a oblique angle => absorbed in photogeneration volume
High efficiency (expensive) single crystal PERL (Passivated emitter Rear Locally Diffused) cell
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//
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100% Incident radiation
× 0.59
× 0.95 Collection efficiency of photons
× 0.6 Voc ≈ (0.6Eg)/(ekB)
η ≈ 21%
FF ≈ 0.85
Overall efficiency
Accounting for various losses of energy in a high efficiency Si solar cell
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p-AlGaAs window (< 0.02 μ m)
p-GaAs
n-GaAs
Passivated GaAs surface
AlGaAs window layer on GaAs passivates the surface states and thereby increases the low wavelength photogeneration efficiency ?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
An AlGaAs window layer overcomes the surface recombination limitation => improve the cell efficiency (efficiency ~24%)
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2 eV
1.4 eV
A heterojunction solar cell between two different bandgap semiconductors (GaAs and AlGaAs) ?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Energetic photons (hv > 2 eV) are absorbed in AlGaAs layer Photons with energy less (1.4< hv < 2.0 eV) are absorbed in GaAs layer (Graded changing bangap can further increase the efficiency)
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np
n p
Connecting region.
A tandem cell. Cell 1 has a wider bandgap and absorbs energetic photons with hυ > Eg1. Cell 2 absorbs photons that pass cell 1 and have hυ > Eg2.
?1999 S O Kasap Optoelectronics (Prentice Hall)
Tandem or cascaded cells Cell 1: hv > Eg1 Cell 2: Eg2< hv < Eg1
Tandem cell + concentrator e.g. 34%, GaAs-GaSb at 100-sun condition (100 times that of ordinary sunlight)
E.g. amorphous Si: ~12%, a-Si:H + a-Si:Ge:H (can be fabricated in larger areas)
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0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8
Wavelength (μm)
Absorption coefficient (α) vs. wavelength (λ) for various semiconductors (Data selectively collected and combined from various sources.)
α (m-1)
Indirect bandgap: Less photon absorp. less sharp λg
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PVPV
www.epia.org
http:// www.jpea.gr.jp
http://www.nrel.gov
IEA Photovoltaic Power Systems IEA Photovoltaic Power Systems ProgrammeProgramme
http://www.oja-services.nl/iea-pvps/links/