Fabrication of All-Aluminum p-Type Silicon Solar Cells

Potential of Solar Cell Technology • Earth receives enough solar energy in 1 hour to

satisfy the world’s energy needs for a year• Most energy sources are derived from the sun

(wind, hydroelectric, fossil fuels)

Fabrication of All-Aluminum p-Type Silicon Solar CellsRaul Flores, Chemical Engineering, University of KansasREU Site: Arizona State UniversityPI: Meng Tao, Electrical Engineering, Arizona State UniversityMentors: Laidong Wang, Electrical Engineering, Arizona State University Wen-cheng Sun, Electrical Engineering, Arizona State University

World-Wide Implementation• Implementing solar cell technology at a global

scale is a difficult challenge• Solar cell technology must be engineered to

facilitate mass production and adoption

The problem being addressedMaterials: Cheap and Abundant• Solar cells must be built from cheap and

abundant natural resources• Expensive cells are not economically feasible• Scarce materials will bottleneck cell production

The scope of this studyClimate and the Energy Crisis• 80% of the world’s energy is made from fossil

fuels, which pose a threat to the climate• Fossil fuel supply is limited, therefore a

replacement energy source is needed

Replacing Silver With Aluminum:• This project aimed to design a solar cell which

uses aluminum instead of silver as the front contact material (see figure below)

• Aluminum is orders of magnitude cheaper and more abundant than silver

Solar Cell Structure and Fabrication Steps Aluminum Front-Contact

Results:Solar Cell Parameters:

Our Lab’s Cell

Reference Cell

Percent Difference

Efficiency [%] 12.4 16.8 35JSC [mA/cm2] 31.8 35.5 12

VOC [V] 0.60 0.61 2RShunt [mΩ-cm2] 183 808 342RSeries [mΩ-cm2] 1030 393 62

Table 1. Parameters for 2 p-type solar cells with an aluminum backside contact, SiNx ARC layer; and either an aluminum (our labs cell) or silver (reference cell) front finger electrode

Analysis of Table 1.• Reference cell: made by another group; structure and

fabrication almost identical to ours; silver front contact instead of our aluminum front contact

1. Our cell’s efficiency is lower than the reference’s2. Our cell’s current (JSC) and voltage (VOC) are reasonably

similar to the reference’s 3. Our cell’s resistances (RShunt and RSeries) are much worse

than the reference’s (especially RShunt)4. Therefore, it’s likely that 3 is the cause for 15. Poor resistances are likely due to fabrication defects,

poorly optimized cell specifications, and poor contact resistance between the cell’s different layers

Project Summary• To fabricate a solar cell that can readily be implemented at a large scale• To this end, a solar cell which utilized an aluminum front contact (instead of the traditional

silver one) was studied• The fabricated solar cell performed poorly relative to a similar cell with a silver front

contact• Non-optimized fabrication procedures and cell specifications are the likely main culprits for

low cell performance• The solar cell’s efficiency can be improved by refining the fabrication procedure and

optimizing the cell structure

Conclusion: Quick Summary and Future Work

Future Work• Improve cell efficiency by: improving the fabrication process, minimizing contamination of

the device, and optimizing the cell’s specifications

I want to show my gratitude to my principal investigator, Dr. Meng Tao. I would also like to thank my mentors, Laidong Wang and Wen-cheng Sun for their support and guidance. I would also like to thank the National Nanotechnology Infrastructure Network Research Experience for Undergraduates, the Center for Solid State Electronics Research, and Arizona State University for their support and funding. This research was supported by the National Science Foundation under Grant No. ECCS-0335765.

n-type Siliconp-type Silicon


Al Back Contact


SiNX


Al Back Contact

SiNXNin-type Siliconp-type Silicon

Al Back Contact

SiNXNi

1. Start with p-type silicon2. Texture both back and front

surface with an alkaline solution for 1 hour

3. Form the n-type layer by diffusing phosphorus into the top surface (.5 microns)

4. Apply the Silicon Nitride (SiNX) layer by PECVD

SiNX

5. Screen print the back-side aluminum contact

6. Fire at a temperature >800 C to diffuse aluminum into p-type silicon and create a back surface field (BSF)

7. Etch the SiNX into the front contact finger pattern (photoresist, UV exposure, developing, HF etch)

8. Apply nickel to the etching pattern by sputter deposition

9. Apply the front contact aluminum layer via electroplating

Topside view of finished solar cell. The grey colored pattern shown is the front contact and is made of aluminum.

Solar Simulator: Equipment used to measure the parameters listed in table 1.

Apparatus uses a lamp to replicate the suns electromagnetic spectrum.

Close up of the solar simulator with a solar cell on it. The metal backside makes conductive contact with cell’s back, and a small needle makes contact with the front.

Research Motivation Flowchart: From Macroscopic Problem To Lab-Scale Solutions