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


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


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).

http://dx.doi.org/10.1155/2007/97370

http://dx.doi.org/10.1155/2007/97370


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


• …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


Materials Commonly Used for Contacts

• Metals

– Optically opaque.

– Electrically conductive.

• Transparent Conducting Oxides (TCOs)

– Optically transparent.

– Electrically conductive.


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


E3 = very small EF

CB

VB

E1 = Large

E2 = Large

How TCOs Work

E

x










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



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



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



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.

http://www.pveducation.org/pvcdrom






5. Sketch common band alignments (Type 1, 2, 3, and 4 junctions).




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

+

-

- +










Step #1: Schottky Theory (the ideal case)


Contacts: Schottky Model

q fM

q c

EC

EF

EV

Vacuum

Semiconductor Metal

E

x



q fM q c

EC EF

EV

Vacuum

Semiconductor

Metal

E

x



• 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.



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

+

-

- +


Evaluating Metals for Contacts - Schottky Model





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…





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.



• For Ohmic contact: fm > fs

• Barrier Height: fb = fm - c

• Contact Potential: Vbi = fm - fs

• Space-charge region width:


W 2 s

qND

Vo




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.


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












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?



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



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