Fabrication of All-Aluminum p-Type Silicon Solar Cells

Fabrication of All-Aluminum p-Type Silicon Solar Cells

Raul FloresPrincipal Investigator: Dr. Meng Tao

Arizona State University

Research Motivations and Objectives:

What are we trying to do and why?

Research Motivations and Objectives:

Need for an environmentally responsible energy source• 80% of energy produced from fossil fuels

Need for an environmentally responsible energy source

Solar cells are a promising energy technology• 165,000 terawatts of sunlight

Research Motivations and Objectives:

Need for an environmentally responsible energy source

Solar cells are a promising energy technology

Challenge of global implementation• 500,000 km2 (~ size of Spain)

Research Motivations and Objectives:

Need for an environmentally responsible energy source

Solar cells are a promising energy technology

Challenge of global implementationUse cheap and abundant materials

• Cost prohibiting and Resource prohibiting• Silver currently used as front contact material

Research Motivations and Objectives:

Need for an environmentally responsible energy source

Solar cells are a promising energy technology

Challenge of global implementationUse cheap and abundant materials

• Cost prohibiting• Resource prohibiting

Research Motivations and Objectives:

Our lab’s objective• Replace silver with aluminum

Solar Cell Basics:

Basic solar cell physics and structure

P-type Si

N-type Si

Rear Contact (Aluminum)

Solar Cell Basics: Basic Solar Cell Structure

• Simplest functional silicon (Si) solar cell• 4 Parts: n-type Si, p-type Si, 2 electrodes

Front Contact (Silver)

P-type Si

N-type Si

Rear Contact (Aluminum)

Solar Cell Basics: Basic Solar Cell Structure

• Simplest functional silicon (Si) solar cell• 4 Parts: n-type Si, p-type Si, 2 electrodes

Front Contact (Silver)

P-type Si

N-type Si

Rear Contact (Aluminum)

Solar Cell Basics: Basic Solar Cell Structure

• p-n junction formed at boundary• Permanent electric field formed

Front Contact (Silver)

Electric Field

P-type Si

N-type Si

Solar Cell Basics: Interaction With Sunlight

• Photons absorbed—electron-hole pairs created• Charges separated by p-n junction• Charge separation induces current

Electric Field

Solar Cell Fabrication and Structure:

What our device looks like and how we made it

Solar Cell Fabrication and Structure:

• Our cell’s structure is similar to the model cell• 2 additional features

Front Contacts

Ni

Al

SiNx Passivation and ARC Layer

Ag

Solar Cell Fabrication and Structure:Silicon Substrate

P-type Silicon

N-type Silicon

• P-type silicon wafer (200 microns)• N-type layer made by diffusing phosphorus (0.5 microns)

Phosphorus Diffusion

Rear Al Screen Printing

SiNx Patterning

Ni Sputtering

Front Al Electroplating

SiNx Deposition

Solar Cell Fabrication and Structure: SiNx Passivation and Anti-Reflection Coating

P-type Silicon

N-type Silicon

• Applied by PECVD (75 nm)• Passivation effect:

• Minimizes surface recombination• Anti-reflection coating

SiNx Passivation and ARC Layer

Phosphorus Diffusion

Rear Al Screen Printing

SiNx Patterning

Ni Sputtering

Front Al Electroplating

SiNx Deposition

Solar Cell Fabrication and Structure:Aluminum Back Contact

P-type Silicon

N-type Silicon

• Screen printed aluminum (10 microns)

Rear Contact (Aluminum)

Phosphorus Diffusion

Rear Al Screen Printing

SiNx Patterning

Ni Sputtering

Front Al Electroplating

SiNx Deposition

Solar Cell Fabrication and Structure:Nickel Seed Layer

P-type Silicon

N-type Silicon

• SiNx etched into front finger pattern• Nickel layer applied (250 nm)

• Helps adhesion of aluminum SiNx Passivation and ARC Layer Ni

Rear Contact (Aluminum)

Rear Al Screen Printing

SiNx Patterning

Ni Sputtering

Front Al Electroplating

SiNx Deposition

Phosphorus Diffusion

Solar Cell Fabrication and Structure:Aluminum Front Contact

P-type Silicon

N-type Silicon

• Aluminum electroplated onto nickel

NiAl

Rear Contact (Aluminum)

Phosphorus Diffusion

Rear Al Screen Printing

SiNx Patterning

Ni Sputtering

Front Al Electroplating

SiNx Deposition

SiNx Passivation and ARC Layer

Results:

How well our cell performed

Parameter Descriptions:

First, we need to define some parameters

• Efficiency• Ratio of energy extracted to energy input

• Short-circuit current (JSC):• Upper-limit to actual current

• Open-circuit voltage (VOC):• Upper-limit to actual voltage

• Series resistance (RSeries):• Shunt resistance (RShunt):

Parameter Descriptions: RSeries

• RSeries : Resistance to current flow through device

Parameter Descriptions: RShunt

• RShunt : Resistance to current flow around device• Example: Current leaking around edges of device, not

through p-n junction

PV Cell Performance: Parameter Summary

Our Lab’s Cell

Efficiency [%] 12.4

JSC [mA/cm2] 31.8

VOC [V] 0.60

RShunt [mΩ-cm2] 183

RSeries [mΩ-cm2] 1030

PV Cell Performance: Parameter Summary

• Data for another group’s cell was obtained• Crucial difference is the front electrode material

NiAl

Silver

Our groups device Reference cell

PV Cell Performance: Parameter Summary

• Focus on third column (interested in difference, not absolute values)

Our Lab’s Cell

Reference Cell

Percent Difference

Efficiency [%] 12.4 16.8 35

JSC [mA/cm2] 31.8 35.5 12

VOC [V] 0.60 0.61 2

RShunt [Ω-cm2] 183 808 342

RSeries [mΩ-cm2] 1030 393 62

PV Cell Performance: Parameter Summary

• Reference cell has a much better efficiency

Our Lab’s Cell

Reference Cell

Percent Difference

Efficiency [%] 12.4 16.8 35

JSC [mA/cm2] 31.8 35.5 12

VOC [V] 0.60 0.61 2

RShunt [Ω-cm2] 183 808 342

RSeries [mΩ-cm2] 1030 393 62

PV Cell Performance: Parameter Summary

• The current and voltages of both cells are reasonably close

Our Lab’s Cell

Reference Cell

Percent Difference

Efficiency [%] 12.4 16.8 35

JSC [mA/cm2] 31.8 35.5 12

VOC [V] 0.60 0.61 2

RShunt [Ω-cm2] 183 808 342

RSeries [mΩ-cm2] 1030 393 62

PV Cell Performance: Parameter Summary

• The reference cell has a much better RShunt and RSeries

• Poor resistances might account for bad efficiency

Our Lab’s Cell

Reference Cell

Percent Difference

Efficiency [%] 12.4 16.8 35

JSC [mA/cm2] 31.8 35.5 12

VOC [V] 0.60 0.61 2

RShunt [Ω-cm2] 183 808 342

RSeries [mΩ-cm2] 1030 393 62

PV Cell Performance: Parameter Summary:Causes of Poor Resistances

• RShunt: • Small wafer area• Contamination

• RSeries: • Nickel-Silicon and/or Aluminum-Nickel interfaces• Problems with electroplating

Conclusion: Future Work

• Fixing fabrication errors• Cell area• Contamination• Etc.

• Optimizing cell specifications• SiNx thickness, front contact width/spacing,

electroplating conditions, etc.

Conclusion: Project Summary

Goal: • Demonstrate solar cell with an aluminum front

contact electrode

Results: • Device performed poorly compared to a similar

reference solar cell

Conclusion:• Solar cell needs to be improved by optimizing

fabrication and device specifications

Questions?• Study’s objective

• General solar cell operation• Our groups device or fabrication

• Significance of results• Miscellaneous

National Science Foundation, Grant No. ECCS-0335765