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Solar Photovoltaics & Energy Systems - EPFL
Transcript of Solar Photovoltaics & Energy Systems - EPFL
Solar Photovoltaics & Energy Systems
Lecture 4. Structure and performance of crystalline semiconductor solar cells
ChE-600
Kevin Sivula, Spring 2014
Presentation schedule
22nd 27thAndrea Pisoni Aiman RahmanudinXiaoyun Yu Bornoz PaulineCarlos Morales Martin MuellerJelena Vukajlovic Plestina Paulo and ZhiXavier Jeanbourquin Sadig and Kasparas
โข PowerPoint (or equivalent) based presentation โข Two possibilities
With a partner โ 25 min Independent โ 15 min
โข Select one of the given publicationsโข Presentation scope
Background for the work โข Introduce the fieldโข Define the motivation for the work
Describe the concept/methodology/results in detail Critically comment on the work
โข Significance of the result and impact on the fieldโข Other similar approaches?
Follow-up work needed (or already performed) to fulfill the promise of the concept
ShockleyโQueisser limit
Tp ยต Eg
V
Net electron flux through device:
๐ฝ๐ฝ = โ๐๐ ๏ฟฝ๐ธ๐ธ๐๐
โ
๐ต๐ต ๐ธ๐ธ ๐๐๐ธ๐ธ
๐ฝ๐ฝ = โ๐๐ ๏ฟฝ๐ธ๐ธ๐๐
โ2๐๐๐๐2โ3
๐ธ๐ธ2๐๐๐ธ๐ธ
๐๐๐๐๐๐ ๐ธ๐ธ โ ๐๐๐๐๐ต๐ต๐๐
โ 1
๐ฝ๐ฝ = โ๐๐ฮฆ
0
2
4
6
8
10
12
14
0.95 1 1.05 1.1Eg 1.05Eg
B(E)
ยต = 99.9 % Eg
ยต = 99.0 % Eg
ยต = 90.0 % Eg
ShockleyโQueisser limit
T1
Tp
Q1
Q2
W
Tp, ยต, Eg
k1
Maximum solar energy conversion efficiency for planet earthwith a semiconductor of band gap Eg
Tp ยต Eg
V
Ts
Then:
๐ฝ๐ฝ = โ๐๐๐๐ ๐ถ๐ถ๐ถ๐ถ ๏ฟฝ๐ธ๐ธ๐๐
โ๐ธ๐ธ2๐๐๐ธ๐ธ
๐๐๐๐๐๐ ๐ธ๐ธ๐๐๐ต๐ต๐๐๐ ๐
โ 1โ 1 โ ๐ถ๐ถ๐ถ๐ถ ๏ฟฝ
๐ธ๐ธ๐๐
โ๐ธ๐ธ2๐๐๐ธ๐ธ
๐๐๐๐๐๐ ๐ธ๐ธ๐๐๐ต๐ต๐๐๐๐
โ 1โ ๏ฟฝ
๐ธ๐ธ๐๐
โ๐ธ๐ธ2๐๐๐ธ๐ธ
๐๐๐๐๐๐ ๐ธ๐ธ โ ๐๐๐๐๐๐๐ต๐ต๐๐๐๐
โ 1
Losses in semiconductor solar cells
Useful photons
โ๐๐ < ๐ธ๐ธ๐๐
Electron hole relaxation
Radiative recombination
๐๐๐๐ < ๐ธ๐ธ๐๐
The position of the Fermi level in a semiconductor
The number of electrons in the conduction band:
๐๐๐๐ = ๐๐๐ถ๐ถ ๐๐๐๐๐๐ โ๐ธ๐ธ๐ถ๐ถ โ ๐ธ๐ธ๐๐๐๐๐ต๐ต๐๐
Similarly for holes:
๐๐โ = ๐๐๐๐ ๐๐๐๐๐๐ โ๐ธ๐ธ๐๐ โ ๐ธ๐ธ๐๐๐๐๐ต๐ต๐๐
Then we can define :
๐๐โ๐๐๐๐ = ๐๐๐๐2 = ๐๐๐ถ๐ถ๐๐๐๐ ๐๐๐๐๐๐ โ๐ธ๐ธ๐ถ๐ถ โ ๐ธ๐ธ๐๐๐๐๐ต๐ต๐๐
๐๐๐๐2 = ๐๐๐ถ๐ถ๐๐๐๐ ๐๐๐๐๐๐ โ๐ธ๐ธ๐บ๐บ๐๐๐ต๐ต๐๐
Under global equilibrium the number of carriers in a semiconductor always equals ๐๐๐๐2
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ P5+ Si4+
Si4+ Si4+ Si4+ Si4+
e-
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ B3+ Si4+
Si4+ Si4+ Si4+ Si4+
h+
n-type
p-type
Controlling the Fermi level in a semiconductor
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ P5+ Si4+
Si4+ Si4+ Si4+ Si4+
e-
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ Si4+ Si4+
Si4+ Si4+ B3+ Si4+
Si4+ Si4+ Si4+ Si4+
h+
๐๐๐๐ ๐๐โ ๐ธ๐ธ๐น๐น
n-type ๐๐๐๐ โ ๐๐๐ท๐ท ๐๐โ =
๐๐๐๐2
๐๐๐๐โ๐๐๐๐2
๐๐๐ท๐ท๐ธ๐ธ๐น๐น = ๐ธ๐ธ๐ถ๐ถ โ ๐๐๐ต๐ต๐๐๐๐๐๐
๐๐๐ถ๐ถ๐๐๐ท๐ท
p-type ๐๐๐๐ =
๐๐๐๐2
๐๐โโ๐๐๐๐2
๐๐๐ด๐ด๐๐โ โ ๐๐๐ด๐ด ๐ธ๐ธ๐น๐น = ๐ธ๐ธ๐๐ โ ๐๐๐ต๐ต๐๐๐๐๐๐
๐๐๐๐๐๐๐ด๐ด
Metal contact
ฯM
Ohmic junction
n-type Semiconductor
+ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +
Elec
tron
ene
rgy
Ef
Elec
tron
ene
rgy Ef
Elec
tron
ene
rgy Ef
Elec
tron
ene
rgy Ef
Elec
tron
ene
rgy Ef
Conduction band
Valence band
Eg
Metal contact
ฯM
Schottky junction
n-type Semiconductor
+ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +
Elec
tron
ene
rgy
Conduction band
Ef
Valence band
Eg
Elec
tron
ene
rgy
Conduction band
Ef
Valence band
Eg
Elec
tron
ene
rgy
Conduction band
Ef
Valence band
Eg
Elec
tron
ene
rgy
Conduction band
Ef
Valence band
Eg
Elec
tron
ene
rgy
Conduction band
Ef
Valence band
Eg
n-type Schottky junction diode
V
J
B
D
C
n-type Schottky junction solar cell
V
J
pn-junction diode
pn-junction diode solar cell
V
J
Forward biasReverse bias
Forward bias
Reverse bias
Equilibrium
The J-V curve and power conversion
Potential (volts)
Curr
ent d
ensit
y (m
A) J
J Power production (m
W)
-
-
-
-
-
-
-
๐๐๐ ๐ ๐ ๐ ๐๐๐ ๐ ๐ ๐ =๐๐๐ ๐ ๐๐๐๐๐ ๐ ๐ ๐ ๐๐๐๐๐๐๐ ๐ ๐๐๐๐๐ ๐ ๐๐๐๐ =
๐ฝ๐ฝ๐๐๐ผ๐ผ๐๐๐๐๐ผ๐ผ๐๐๐๐๐ ๐ ๐๐๐๐๐ ๐ ๐๐ ๐๐๐๐๐๐๐๐๐๐๐ ๐ ๐๐๐๐๐๐
๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐๐๐ ๐ ๐๐ =( ๐ฝ๐ฝ๐๐ )๐๐๐๐๐๐
๐๐๐๐๐๐=๐ฝ๐ฝ๐๐๐ถ๐ถ ๐๐๐๐๐ถ๐ถ ๐น๐น๐น๐น
๐๐๐๐๐๐๐น๐น๐น๐น =
( ๐ฝ๐ฝ๐๐ )๐๐๐๐๐๐๐ฝ๐ฝ๐๐๐ถ๐ถ ๐๐๐๐๐ถ๐ถ
Timeline of photovolatic development
1883 - Charles Fritts develops a solar cell using selenium on a thin layer of gold to form a device giving less than 1% efficiency.
1904 - Wilhelm Hallwachs makes a semiconductor-junction solar cell (copper and copper oxide).
1932 - Audobert and Stora discover the photovoltaic effect in Cadmium selenide (CdSe).
1954 - Bell Labs announces the invention of the first modern silicon pn junction solar cell with about 6% efficiency.
Modern Si p-n Junction PVs
โข Thin p-type layer (2.5 ฮผm) formed over a n-type base.โข Assuming Eg = 1.02 eV and ฮผ = 0.5 eV, ๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐๐๐ ๐ ๐๐ = 22% was
considered possibleโข Losses were reported as reflection (50%), e-h recombination,
and resistance in the surface layer and the contacts.
Modern Si p-n Junction PVs
Si growth methods
Polycrystalline silicon made from the Siemens process
2 HSiCl3 โ Si + 2 HCl + SiCl4
Si growth methods
Single crystalline silicon by the Czochralski process
Si growth methodsDopant incorporation during crystal growth
โข Dopants are added to the melt to provide a controlled N or P doping level in the wafers.
โข However, the dopant incorporation process is complicated by dopant segregation.
โข Generally, impurities โprefer to stay in the liquidโ as opposed to being incorporated into the solid.
โข This process is known as segregation. The degree of segregation is characterized by the segregation coefficient, ko, for the impurity.
CS
CL
kO = CS
CL
CS and CL are the impurity concentration just on the either side of the solid/liquid interface.
Si growth methods
Dopant incorporation during crystal growth
kO = CS
CL
Most k0 values are <1 which means the impurity prefers to stay in the liquid.Thus as the crystal is pulled, dopant concentration will increase.In other words, the distribution of dopant along the ingot will be graded.
Si growth methods
Dopant incorporation during crystal growthNote the relatively flat profile produced by boron with a ko close to 1. Dopants with ko << 1 produce much more doping variation along the crystal.
Si growth methods
Si processing
Structure of a modern Si pn junction PV
alkaline etching
diffusion furnace
Structure of a modern Si pn junction PV
Timeline of Si pn junction development
Ribbon silicon reduces waste
Structure of a modern Si pn junction PV
Modern Si PV installations
7.7MW in Rion-des-Landes
Proliferation and price of c-Si PVs
Data: Navigant Consulting Graph. PSE AG 2013
Proliferation and price of c-Si PVs
Data: Navigant Consulting Graph. PSE AG 2013
Global Cumulative PV Installation until 2012
Data: from 2000 to 2011: EPIA; for 2012: IHS. Graph: PSE AG 2013
All percentages are related to the total global installation
Global Cumulative Silicon PV Installation
Data: Navigant. Graph: PSE AG 2013
About 40 GWp of Silicon PV installed until 2012
Global Cumulative Silicon PV Installation
Data: Navigant. Graph: PSE AG 2013
Electrical Capacity of Renewable Energy Sources in Germany
In 2012 about 23% of the electricity in Germany has been generated by renewable energy (RE) sources according to BDEW
Proliferation and price of c-Si PVs
Future convergence with Grid?
C. Wolden et al. J. Vac. Sci. Technol. A 29, 030801 (2011)
Crystalline silicon PV overview
โข Modern silicon pn junction invented in 1954 by Bell labsโข Achieved ๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐๐๐ ๐ ๐๐ = 25%โข Standard commercial cells are at ๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ =20 โ 23% with
module efficiency of 15 โ 20% โข Total installed capacity is around 40 GW and c-Si
accounts for about 90% of all installed PV.*โข Price is now as low as 0.20$/kWh (or 1.30 $/Wp)โข Energy/CO2 payback time is now about 2.5 years**โข Current trends will see this technology reach 1 TW
installed capacity by 2020
*Renewables 2011 GLOBAL STATUS REPORT http://www.ren21.net/Portals/97/documents/GSR/GSR2011_Master18.pdf
**Solar Energy Volume 85, Issue 8, 2011, Pages 1609โ1628
๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐๐๐ ๐ ๐๐
๐ถ๐ถ๐๐๐ ๐ ๐๐
๐ถ๐ถ = 1
Multiโcolorโ conversion
i = 1 Eg = Eg1
i = 2 Eg = Eg2 < Eg1
i = 3 Eg = Eg3 < Eg2
.
.
.
Eg = Egn < Egn-1i = n
IV
IV
IV
IV
๐๐โโ68.2 %
๐๐โโ86.8 %
III-V semiconductors
III-V semiconductors
A modern III-V multijunction cell
Advances in OptoElectronicsVolume 2007 (2007), Article ID 29523
Band diagram of a multijunction solar cell
Proc. of SPIE Vol. 6339 633909-1
Multi junction cells for high ๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐
Aerospace Industry
Spectolab 15.5 mGa,InP/GaAs/Ge
Multi junctionSolar panels๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ = 30%
Concentrated PV power plants
Concentrated PV power plants
x-Si approaches grid parity
Continuing trends with C-Silicon
โข Low efficiency Standard commercial cells are at 15-18% Need at least 22.5% cell efficiency
โข Poor use of material Current cells use 3-9 g Si/Wp Should be under 0.6 g/Wp
โข Current manufacturing processes are too complex and expensive
x-Si approaches grid parity
Without a significant paradigm shift in fabrication
โGenerationsโ of solar cells
Amorphous silicon
DOI: 10.1143/JJAP.50.030001
H
H H
H
H H
HH
H
StaeblerโWronski (SW) effect
a-Si devices and modules 2MW Solar Model Power (Amorphous Silicon)Location: Bangbu City, Anhui Province
a-Si based tandem cells
DOI: 10.1143/JJAP.50.030001
a-Si PV costs and outlookโข Price is currently about 20-30% less
than x-Si in $/Wpโข Relatively low efficienciesโข Slow deposition rates
High capital costsโข Only 4.2% of global sales
Switzerland-based Oerlikon Solar $0.47/Wp in 2014
The search for a new material
The search for a new material
Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072.
The search for a new material
Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072.
CdTe photovoltaics
โข CdTe Eg = 1.5 eVโข Absorption coefficient
greater than 105 cm-1
โข Thickness of only 2 ยตm is required for 99% absorption
โข Zincblende crystal structure
โข PVD or CVD methods used for thin film formation
Glass
p-CdTe
n-CdS
CdTe photovoltaics
โข Best cell ๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ =17 % (Normally modules are around 10%) โ 0.9$/Wp(first cell below 1$/W)
โข High-Rate Vapor Transport Deposition
Waldpolenz Solar Park (40MW)550โ000 First solar panelsLocation:GermanyCompleted in 2008
Drawbacks:โข CdTe is toxicโข Tellurium is an
extremely rare element
CuInxGa(1-x)Se2 (CIGS)
โข I-III-VI2 semiconductor material
โข General material class of Chalcopyrites
โข Eg = 1.0 eV โ 1.7 eVโข Deposition methods:
Selenization of metal precursors that are in the form of either stacked layers (Cu-Ga/In), nanoparticles
or inks consisting of Cu, In,andGa
Sputtering from metal selenide targets (CuโSe, InโSeโ, (In,Ga)โSeโ);
thermal evaporation from pure
CIGS device structure
CIGS device performance
โข ๐๐๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ ๐ = 15-16 % typical (20.3 % champion cells)โข Solar Frontier and Miasolรฉ using PVD techniques 0.75 $/Wp
โข Nanosolar actively producing modules using solution based approaches.
Thin Film performance overview
Thin film PV installations overview
Thin film PV installations overview
Thin film PV installations overview
Data: Navigant. Graph: PSE AG 2013
About 50 GWp of Silicon + thin film PV installed until 2012
Energy Payback time
Data: Mono- and multi- Silicon data: ISE 2011; CPV data: โEnvironmental Sustainability of Concentrator PV Systems: Preliminary LCA Results of the Apollon Projectโ 5th World Conference on PV Energy Conversion. Valencia, Spain, 6-10 September 2010; all other data: Wild-Scholten(ECN),Sustainability Dec. 2009. Graph:PSE AG 2013
Energy Payback time
Data: ISE 2011 (for mono, multi); de Wild-Scholten (ECN),Sustainability Dec. 2009. Graph: PSE AG 2013
Energy Payback time