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Buonassisi (MIT) 2011
Charge Extraction
Lecture 9 – 10/06/2011 MIT Fundamentals of Photovoltaics
2.626/2.627 – Fall 2011 Prof. Tonio Buonassisi
Buonassisi (MIT) 2011
2.626/2.627 Roadmap
You Are Here
Buonassisi (MIT) 2011
2.626/2.627: Fundamentals
Charge Excitation
Charge Drift/Diff
usion
Charge Separation
Light Absorption
Charge Collection
Outputs
Solar Spectrum
Inputs
Conversion Efficiency Output Energy
Input Energy
Every photovoltaic device must obey:
For most solar cells, this breaks down into:
total absorptionexcitation drift/diffusion separation collection
Buonassisi (MIT) 2011
Liebig’s Law of the Minimum
total absorptionexcitation drift/diffusion separation collection
S. Glunz, Advances in Optoelectronics 97370 (2007)
Image by S. W. Glunz. License: CC-BY. Source: “High-Efficiency CrystallineSilicon Solar Cells.” Advances in OptoElectronics (2007).
Buonassisi (MIT) 2011
1. Describe the purpose of contacts, and their most common types.
2. Describe the impact of good and poor contacts on IV characteristics.
3. Sketch the IV characteristics of Schottky and Ohmic contacts.
4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction.
5. Sketch common band alignments (Types 1, 2, 3 junctions).
6. Sketch common solar cell device architectures.
Learning Objectives: Charge Extraction
Buonassisi (MIT) 2011
• …extract carriers from device.
• …prevent back-diffusion of carriers into device.
• …are studied extensively in the semiconductor industry (several good review papers) for “common” semiconductors.
• …are semiconductor-specific: While fundamentals generally apply universally, the devil is in the details, and each material system requires individual optimization.
• … are influenced heavily by surface states (i.e., repeatable surface preparation is a must!)
Contacts
Buonassisi (MIT) 2011
Materials Commonly Used for Contacts
• Metals
– Optically opaque.
– Electrically conductive.
• Transparent Conducting Oxides (TCOs)
– Optically transparent.
– Electrically conductive.
Buonassisi (MIT) 2011
Tra
nsp
aren
cy
1 3 2 0
0
1
Energy of light (eV)
Vis
ible
n - carrier conc. (cm-3)
- mobility (cm2/Vs)
e - charge per carrier
= n e
6 2 -2 -6 -10 -14 -18
Insulator Semi
conductor Metal
log (S/cm)
Quartz Glass Si Ge ITO Ag
Transparency
Transmittance: > 80% (Films)
Range: 400 ~ 700 nm
Band gap > 3.1eV
Conductivity ()
Properties of TCOs
Buonassisi (MIT) 2011
E3 = very small EF
CB
VB
E1 = Large
E2 = Large
How TCOs Work
E
x
Buonassisi (MIT) 2011
1. Describe the purpose of contacts, and their most common types.
2. Describe the impact of good and poor contacts on IV characteristics.
3. Sketch the IV characteristics of Schottky and Ohmic contacts.
4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction.
5. Sketch common band alignments (Types 1, 2, 3 junctions).
6. Sketch common solar cell device architectures.
Learning Objectives: Charge Extraction
Buonassisi (MIT) 2011
Vja V
J0
Equivalent Circuit: Simple Case
J J0 expqV
kT
1
JL
1.E-10
1.E-08
1.E-06
1.E-04
1.E-02
1.E+00
0 0.2 0.4 0.6 0.8
Cu
rren
t D
ensi
ty (
mA
/cm
2)
Voltage (V)
I-V Curve
0.E+00
2.E-01
4.E-01
6.E-01
8.E-01
1.E+00
0 0.2 0.4 0.6 0.8
Cu
rre
nt
De
nsi
ty (
mA
/cm
2)
Voltage (V)
I-V Curve
Lin Scale
Log Scale
Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case
Vja V
Rs J0
J J0 expq V JRs kT
1
JL
1.E-10
1.E-08
1.E-06
1.E-04
1.E-02
1.E+00
0 0.2 0.4 0.6 0.8
Cu
rren
t D
ensi
ty (
mA
/cm
2)
Voltage (V)
I-V Curve
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
0 0.2 0.4 0.6 0.8
Cu
rre
nt
De
nsi
ty (
mA
/cm
2)
Voltage (V)
I-V Curve
Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case
J J0 expq V JRs kT
1
V JRs
Rsh
JL
1.E-10
1.E-08
1.E-06
1.E-04
1.E-02
1.E+00
0 0.5 1
Cu
rren
t D
ensi
ty (
mA
/cm
2)
Voltage (V)
I-V Curve
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
0 0.5 1
Cu
rre
nt
De
nsi
ty (
mA
/cm
2)
Voltage (V)
I-V Curve
Vja V
Rs
Rsh
J0
Buonassisi (MIT) 2011
Equivalent Circuit: Simple Case
J J0 expq V JRs kT
1
V JRs
Rsh
JL
Vja V
Rs
Rsh
J0
Firing contacts? Three possibilities: 1. Contact just right: low Rs, large Rsh. 2. “Underfired” contact: Poor contact with Si, large Rs. 3. “Overfired” contact: Metal drives too deep into Si, low Rsh.
Courtesy of PVCDROM. Used with permission.
Buonassisi (MIT) 2011
1. Describe the purpose of contacts, and their most common types.
2. Describe the impact of good and poor contacts on IV characteristics.
3. Sketch the IV characteristics of Schottky and Ohmic contacts.
4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction.
5. Sketch common band alignments (Type 1, 2, 3, and 4 junctions).
6. Sketch common solar cell device architectures.
Learning Objectives: Charge Extraction
Buonassisi (MIT) 2011
Classes of Contacts
• Ohmic: – Linear I-V curve.
– Typically used when charge separation is not a goal for metallization.
• Schottky: – Exponential I-V curve.
– Used when charge separation is desired.
Cu
rre
nt
(a.u
.)
Voltage (a.u.)
Ohmic and Schottky Contacts
Schottky
Ohmic
0
0
+
-
- +
Buonassisi (MIT) 2011
1. Describe the purpose of contacts, and their most common types.
2. Describe the impact of good and poor contacts on IV characteristics.
3. Sketch the IV characteristics of Schottky and Ohmic contacts.
4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction.
5. Sketch common band alignments (Types 1, 2, 3 junctions).
6. Sketch common solar cell device architectures.
Learning Objectives: Charge Extraction
Buonassisi (MIT) 2011
Step #1: Schottky Theory (the ideal case)
Buonassisi (MIT) 2011
Contacts: Schottky Model
q fM
q c
EC
EF
EV
Vacuum
Semiconductor Metal
E
x
Buonassisi (MIT) 2011
Contacts: Schottky Model
q fM q c
EC EF
EV
Vacuum
Semiconductor
Metal
E
x
Buonassisi (MIT) 2011
Contacts: Schottky Model
• For Ohmic contact: fm > fs
• Barrier Height: fb = fm - c
• Contact Potential: Vbi = fm - fs
• Space-charge region width:
http://www.iue.tuwien.ac.at/phd/ayalew/node56.html
W 2 s
qND
Vo
Courtesy of Tesfaye Ayalew. Used with permission.
Buonassisi (MIT) 2011
Classes of Contacts
• Ohmic: – Electron barrier
height ≤ 0 (for n-type) – Linear I-V curve. – Typically used when
charge separation is not a goal for metallization.
• Schottky:
– Electron barrier height > 0 (for p-type)
– Exponential I-V curve. – Used when charge
separation is desired.
Cu
rre
nt
(a.u
.)
Voltage (a.u.)
Ohmic and Schottky Contacts
Schottky
Ohmic
0
0
+
-
- +
Buonassisi (MIT) 2011
Evaluating Metals for Contacts - Schottky Model
http://www.iue.tuwien.ac.at/phd/ayalew/node56.html
Courtesy of Tesfaye Ayalew. Used with permission.
Buonassisi (MIT) 2011
Reality: Deviations from Schottky theory
• Substantial deviations from Schottky theory are possible, due to interface effects including: – Orientation-dependent surface states.
– Elemental nature of surface termination in binary compounds (e.g., A or B element?).
– Interface dipoles.
– and more…
http://www.iue.tuwien.ac.at/phd/ayalew/node56.html
Courtesy of Tesfaye Ayalew. Used with permission.
Buonassisi (MIT) 2011
Role of Surface States
D.K. Schroder, IEEE Trans. Electron Dev. 31, 637 (1984)
For related visuals, please see the lecture 9 video or the reference below.
Buonassisi (MIT) 2011
Contacts: Schottky Model
• For Ohmic contact: fm > fs
• Barrier Height: fb = fm - c
• Contact Potential: Vbi = fm - fs
• Space-charge region width:
http://www.iue.tuwien.ac.at/phd/ayalew/node56.html
W 2 s
qND
Vo
Courtesy of Tesfaye Ayalew. Used with permission.
Buonassisi (MIT) 2011
Thermionic Emission & Field Emission Effects
D.K. Schroder, IEEE Trans. Electron Dev. 31, 637 (1984)
For related visuals, please see the lecture 9 video or the reference below.
Buonassisi (MIT) 2011
Evaluating Metals for Contacts - Practical
• Sources:
– Reference books
– Review articles
– Scientific articles
– Trusted websites
• NB:
– Surface states matter!! Be sure you have repeatable surface preparation.
https://web.archive.org/web/20130818214213/http://www.siliconfareast.com/ohmic_table.htm
Buonassisi (MIT) 2011
1. Describe the purpose of contacts, and their most common types.
2. Describe the impact of good and poor contacts on IV characteristics.
3. Sketch the IV characteristics of Schottky and Ohmic contacts.
4. Describe what fundamental material parameters determine the IV characteristics of a contact/semiconductor junction.
5. Sketch common band alignments (Types 1, 2, 3 junctions).
6. Sketch common solar cell device architectures.
Learning Objectives: Charge Extraction
Buonassisi (MIT) 2011
Evaluating Heterojunctions
Not always possible to dope a material both n- and p-type. Not always possible to find the perfect contact material. Need: heterojunction.
(At least) three types of heterojunction:
What junction will separate charge?
Buonassisi (MIT) 2011
Evaluating Heterojunctions
Simplest case (analogy to Schottky band alignment for metal-
semiconductor contacts): 1- Set chemical potential equal across entire device. 2- Then, align vacuum levels. 3- Note that VB and CB must follow vacuum levels.
E
x
Buonassisi (MIT) 2011
Evaluating Heterojunctions
Simplest case (analogy to Schottky band alignment for metal-
semiconductor contacts): 1- Set chemical potential equal across entire device. 2- Then, align vacuum levels. 3- Note that VB and CB must follow vacuum levels.
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2.627 / 2.626 Fundamentals of PhotovoltaicsFall 2013
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