Charge Extraction - MIT OpenCourseWare · charge separation is not a goal for metallization. • Schottky: – Exponential I-V curve. – Used when charge separation is desired. Current

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

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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

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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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