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?


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



Solar cells are a promising energy technology

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




Challenge of global implementationUse cheap and abundant materials

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




Challenge of global implementationUse cheap and abundant materials

• Cost prohibiting• Resource prohibiting


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



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


P-type Si

N-type Si



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


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 Patterning

Ni Sputtering


SiNx Deposition

Solar Cell Fabrication and Structure:Aluminum Back Contact

P-type Silicon

N-type Silicon

• Screen printed aluminum (10 microns)




SiNx Patterning

Ni Sputtering


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



SiNx Patterning

Ni Sputtering


SiNx Deposition


Solar Cell Fabrication and Structure:Aluminum Front Contact

P-type Silicon

N-type Silicon

• Aluminum electroplated onto nickel

NiAl




SiNx Patterning

Ni Sputtering


SiNx Deposition


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


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

NiAl

Silver

Our groups device Reference cell


• 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


• 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


• 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


• 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