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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
• PowerPoint (or equivalent) based presentation • Two possibilities
With a partner – 25 min Independent – 15 min
• Select one of the given publications• Presentation scope
Background for the work • Introduce the field• Define the motivation for the work
Describe the concept/methodology/results in detail Critically comment on the work
• Significance of the result and impact on the field• Other similar approaches?
Follow-up work needed (or already performed) to fulfill the promise of the concept
Shockley–Queisser limit
Tp µ Eg
V
Net electron flux through device:
𝐽𝐽 = −𝑞𝑞 �𝐸𝐸𝑔𝑔
∞
𝐵𝐵 𝐸𝐸 𝑑𝑑𝐸𝐸
𝐽𝐽 = −𝑞𝑞 �𝐸𝐸𝑔𝑔
∞2𝜋𝜋𝑐𝑐2ℎ3
𝐸𝐸2𝑑𝑑𝐸𝐸
𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸 − 𝜇𝜇𝑘𝑘𝐵𝐵𝑇𝑇
− 1
𝐽𝐽 = −𝑞𝑞Φ
0
2
4
6
8
10
12
14
0.95 1 1.05 1.1Eg 1.05Eg
B(E)
µ = 99.9 % Eg
µ = 99.0 % Eg
µ = 90.0 % Eg
Shockley–Queisser limit
T1
Tp
Q1
Q2
W
Tp, µ, Eg
k1
Maximum solar energy conversion efficiency for planet earthwith a semiconductor of band gap Eg
Tp µ Eg
V
Ts
Then:
𝐽𝐽 = −𝑞𝑞𝑘𝑘 𝐶𝐶𝐶𝐶 �𝐸𝐸𝑔𝑔
∞𝐸𝐸2𝑑𝑑𝐸𝐸
𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸𝑘𝑘𝐵𝐵𝑇𝑇𝑠𝑠
− 1− 1 − 𝐶𝐶𝐶𝐶 �
𝐸𝐸𝑔𝑔
∞𝐸𝐸2𝑑𝑑𝐸𝐸
𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸𝑘𝑘𝐵𝐵𝑇𝑇𝑝𝑝
− 1− �
𝐸𝐸𝑔𝑔
∞𝐸𝐸2𝑑𝑑𝐸𝐸
𝑒𝑒𝑒𝑒𝑒𝑒 𝐸𝐸 − 𝑞𝑞𝑞𝑞𝑘𝑘𝐵𝐵𝑇𝑇𝑝𝑝
− 1
Losses in semiconductor solar cells
Useful photons
ℎ𝜈𝜈 < 𝐸𝐸𝑔𝑔
Electron hole relaxation
Radiative recombination
𝑞𝑞𝑞𝑞 < 𝐸𝐸𝑔𝑔
The position of the Fermi level in a semiconductor
The number of electrons in the conduction band:
𝑛𝑛𝑒𝑒 = 𝑁𝑁𝐶𝐶 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝐶𝐶 − 𝐸𝐸𝑓𝑓𝑘𝑘𝐵𝐵𝑇𝑇
Similarly for holes:
𝑛𝑛ℎ = 𝑁𝑁𝑉𝑉 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝑓𝑓 − 𝐸𝐸𝑉𝑉𝑘𝑘𝐵𝐵𝑇𝑇
Then we can define :
𝑛𝑛ℎ𝑛𝑛𝑒𝑒 = 𝑛𝑛𝑖𝑖2 = 𝑁𝑁𝐶𝐶𝑁𝑁𝑉𝑉 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝐶𝐶 − 𝐸𝐸𝑉𝑉𝑘𝑘𝐵𝐵𝑇𝑇
𝑛𝑛𝑖𝑖2 = 𝑁𝑁𝐶𝐶𝑁𝑁𝑉𝑉 𝑒𝑒𝑒𝑒𝑒𝑒 −𝐸𝐸𝐺𝐺𝑘𝑘𝐵𝐵𝑇𝑇
Under global equilibrium the number of carriers in a semiconductor always equals 𝑛𝑛𝑖𝑖2
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+
𝑛𝑛𝑒𝑒 𝑛𝑛ℎ 𝐸𝐸𝐹𝐹
n-type 𝑛𝑛𝑒𝑒 ≈ 𝑛𝑛𝐷𝐷 𝑛𝑛ℎ =
𝑛𝑛𝑖𝑖2
𝑛𝑛𝑒𝑒≈𝑛𝑛𝑖𝑖2
𝑛𝑛𝐷𝐷𝐸𝐸𝐹𝐹 = 𝐸𝐸𝐶𝐶 − 𝑘𝑘𝐵𝐵𝑇𝑇𝑇𝑇𝑛𝑛
𝑁𝑁𝐶𝐶𝑛𝑛𝐷𝐷
p-type 𝑛𝑛𝑒𝑒 =
𝑛𝑛𝑖𝑖2
𝑛𝑛ℎ≈𝑛𝑛𝑖𝑖2
𝑛𝑛𝐴𝐴𝑛𝑛ℎ ≈ 𝑛𝑛𝐴𝐴 𝐸𝐸𝐹𝐹 = 𝐸𝐸𝑉𝑉 − 𝑘𝑘𝐵𝐵𝑇𝑇𝑇𝑇𝑛𝑛
𝑁𝑁𝑉𝑉𝑛𝑛𝐴𝐴
Metal contact
φM
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
φM
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)
-
-
-
-
-
-
-
𝜂𝜂𝑠𝑠𝑠𝑠𝑇𝑇𝑠𝑠𝑠𝑠 =𝑃𝑃𝑠𝑠𝑃𝑃𝑒𝑒𝑠𝑠 𝑠𝑠𝑜𝑜𝑜𝑜𝑃𝑃𝑠𝑠𝑃𝑃𝑒𝑒𝑠𝑠 𝑖𝑖𝑛𝑛 =
𝐽𝐽𝑞𝑞𝐼𝐼𝑇𝑇𝑜𝑜𝐼𝐼𝑖𝑖𝑛𝑛𝑠𝑠𝑜𝑜𝑖𝑖𝑠𝑠𝑛𝑛 𝑖𝑖𝑛𝑛𝑜𝑜𝑒𝑒𝑛𝑛𝑠𝑠𝑖𝑖𝑜𝑜𝑖𝑖
𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 =( 𝐽𝐽𝑞𝑞 )𝑀𝑀𝑀𝑀𝑀𝑀
𝑃𝑃𝑖𝑖𝑖𝑖=𝐽𝐽𝑆𝑆𝐶𝐶 𝑞𝑞𝑂𝑂𝐶𝐶 𝐹𝐹𝐹𝐹
𝑃𝑃𝑖𝑖𝑖𝑖𝐹𝐹𝐹𝐹 =
( 𝐽𝐽𝑞𝑞 )𝑀𝑀𝑀𝑀𝑀𝑀𝐽𝐽𝑆𝑆𝐶𝐶 𝑞𝑞𝑂𝑂𝐶𝐶
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
• Thin p-type layer (2.5 μm) formed over a n-type base.• Assuming Eg = 1.02 eV and μ = 0.5 eV, 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 = 22% was
considered possible• 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 → Si + 2 HCl + SiCl4
Si growth methods
Single crystalline silicon by the Czochralski process
Si growth methodsDopant incorporation during crystal growth
• Dopants are added to the melt to provide a controlled N or P doping level in the wafers.
• However, the dopant incorporation process is complicated by dopant segregation.
• Generally, impurities “prefer to stay in the liquid” as opposed to being incorporated into the solid.
• 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
• Modern silicon pn junction invented in 1954 by Bell labs• Achieved 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 = 25%• Standard commercial cells are at 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 =20 – 23% with
module efficiency of 15 – 20% • Total installed capacity is around 40 GW and c-Si
accounts for about 90% of all installed PV.*• Price is now as low as 0.20$/kWh (or 1.30 $/Wp)• Energy/CO2 payback time is now about 2.5 years**• 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 1609–1628
𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚
𝐶𝐶𝑚𝑚𝑠𝑠𝑚𝑚
𝐶𝐶 = 1
Multi“color” conversion
i = 1 Eg = Eg1
i = 2 Eg = Eg2 < Eg1
i = 3 Eg = Eg3 < Eg2
.
.
.
Eg = Egn < Egn-1i = n
IV
IV
IV
IV
𝑖𝑖→∞68.2 %
𝑖𝑖→∞86.8 %
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 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
Aerospace Industry
Spectolab 15.5 mGa,InP/GaAs/Ge
Multi junctionSolar panels𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 30%
Concentrated PV power plants
Concentrated PV power plants
x-Si approaches grid parity
Continuing trends with C-Silicon
• Low efficiency Standard commercial cells are at 15-18% Need at least 22.5% cell efficiency
• Poor use of material Current cells use 3-9 g Si/Wp Should be under 0.6 g/Wp
• Current manufacturing processes are too complex and expensive
x-Si approaches grid parity
Without a significant paradigm shift in fabrication
“Generations” of solar cells
Amorphous silicon
DOI: 10.1143/JJAP.50.030001
H
H H
H
H H
HH
H
Staebler–Wronski (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 outlook• Price is currently about 20-30% less
than x-Si in $/Wp• Relatively low efficiencies• Slow deposition rates
High capital costs• Only 4.2% of global sales
Switzerland-based Oerlikon Solar $0.47/Wp in 2014
The search for a new material
The search for a new material
Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072.
The search for a new material
Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ. Sci. Technol. 2009, 43, 2072.
CdTe photovoltaics
• CdTe Eg = 1.5 eV• Absorption coefficient
greater than 105 cm-1
• Thickness of only 2 µm is required for 99% absorption
• Zincblende crystal structure
• PVD or CVD methods used for thin film formation
Glass
p-CdTe
n-CdS
CdTe photovoltaics
• Best cell 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 =17 % (Normally modules are around 10%) → 0.9$/Wp(first cell below 1$/W)
• High-Rate Vapor Transport Deposition
Waldpolenz Solar Park (40MW)550’000 First solar panelsLocation:GermanyCompleted in 2008
Drawbacks:• CdTe is toxic• Tellurium is an
extremely rare element
CuInxGa(1-x)Se2 (CIGS)
• I-III-VI2 semiconductor material
• General material class of Chalcopyrites
• Eg = 1.0 eV – 1.7 eV• 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 (Cu₂Se, In₂Se₃, (In,Ga)₂Se₃);
thermal evaporation from pure
CIGS device structure
CIGS device performance
• 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 15-16 % typical (20.3 % champion cells)• Solar Frontier and Miasolé using PVD techniques 0.75 $/Wp
• 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: “Environmental Sustainability of Concentrator PV Systems: Preliminary LCA Results of the Apollon Project“ 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