Charge Extraction Lecture 9 10/06/2011 MIT Fundamentals of - - PowerPoint PPT Presentation

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 absorptionexcitation drift/diffusion separation collection

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Liebig’s Law of the Minimum

฀ total absorptionexcitation drift/diffusion separation collection

• S. Glunz, Advances in

Optoelectronics 97370 (2007)

Image by S. W. Glunz. License: CC-BY. Source: “High-Efficiency Crystalline Silicon 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

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• …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.

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Transparency

1 3 2 1

Energy of light (eV)

Visible

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

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Vja V J0

Equivalent Circuit: Simple Case

฀ J  J0 exp qV kT      1       JL

1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 0.2 0.4 0.6 0.8 Current Density (mA/cm2) Voltage (V)

I-V Curve

0.E+00 2.E-01 4.E-01 6.E-01 8.E-01 1.E+00 0.2 0.4 0.6 0.8 Current Density (mA/cm2) Voltage (V)

I-V Curve Lin Scale Log Scale

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Equivalent Circuit: Simple Case

Vja V Rs J0

฀ J  J0 exp q V  JRs

 

kT        1         JL

1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 0.2 0.4 0.6 0.8 Current Density (mA/cm2) Voltage (V)

I-V Curve

0.E+00 1.E-02 2.E-02 3.E-02 4.E-02 5.E-02 0.2 0.4 0.6 0.8 Current Density (mA/cm2) Voltage (V)

I-V Curve

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Equivalent Circuit: Simple Case

฀ J  J0 exp q 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 0.5 1 Current Density (mA/cm2) Voltage (V)

I-V Curve

0.E+00 1.E-02 2.E-02 3.E-02 4.E-02 5.E-02 0.5 1 Current Density (mA/cm2) Voltage (V)

I-V Curve Vja V Rs Rsh J0

Buonassisi (MIT) 2011

Equivalent Circuit: Simple Case

฀ J  J0 exp q 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.

Current (a.u.) Voltage (a.u.)

Ohmic and Schottky Contacts

Schottky Ohmic

+

• +

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

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Step #1: Schottky Theory (the ideal case)

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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.

Current (a.u.) Voltage (a.u.)

Ohmic and Schottky Contacts

Schottky Ohmic

+

• +

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Evaluating Metals for Contacts - Schottky Model

http://www.iue.tuwien.ac.at/phd/ayalew/node56.html

Courtesy of Tesfaye Ayalew. Used with permission.

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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

• r B element?).

– Interface dipoles. – and more…

http://www.iue.tuwien.ac.at/phd/ayalew/node56.html

Courtesy of Tesfaye Ayalew. Used with permission.

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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.

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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?

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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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