1. Evaluation of Polydimethlysiloxane (PDMS) as an adhesive for Mechanically Stacked Multi-Junction Solar Cells
Ian Mathews
Dept. of Electrical and Electronic Engineering, University College Cork
Tyndall National Institute, Cork
Smart Surfaces 2012 – Solar
and Biosensor applications
7/3/2012

2. Overview Introduction: Concentrating Photovoltaics (CPV)
Introduction: UCC & Tyndall: Photovoltaic Research
Mechanically Stacked Solar Cells (MSSC)
Optical Design
Experimental Results
Conclusions

3. Concentrating Photovoltaics
Why?....Concentration = Higher efficiency & lower cell cost
1-Sun Efficiency
Efficiency under concentration
Silicon
25%
27% (92 suns)
Triple-junction
36.9%
43.5% (418 suns)

4. Concentrating Photovoltaic Systems
Elements of a CPV system
Solar cell – III-V/Ge semiconductor
Receiver – Packaging for the solar cell
Module – Completed optics and receiver
System – BOS components (tracker, inverter etc.)
System
1 cm
Module
Solar cell
Receiver

5. Tyndall – Cell Production and Testing
III-V Solar Cell growth
Metal Organic Vapour Phase Epitaxy
AlGaAs, GaAs, InGaAs and InGaN single junction solar cell
-Dr. Emanuele Pelucchi
Compound Semiconductor Fabrication
E-beam, Lithography, Dry & Wet etching, Sputter, SEM, Wafer Bonder, Plating
Access to these facilities available through the National Access Programme (

6. UCC – Systems Design and Testing
Primary lens (~100x)
Optical system modeling (Parabolic trough mirror)
Secondary optic (~500x)
Solar cell
Newport Solar simulator
(1-Sun & concentration testing)
Module Fabrication
Gordon & Morrison, “A Method for Manufacture of 1-axis reflective concentrators”, 3rd International Workshop on CPV, Bremerhaven, Germany, 2010

7. Monolithic triple-junction Solar Cells
Industry standard
Grown lattice matched to Ge substrates
High current from Ge wasted in the series connected device

8. Mechanically Stacked Solar Cells (MSSC)
Potential to increase performance over industry standard by fully utilising current from Ge junction
Standard ~ 40%
MSSC ~ 46%
Initial Experiment
Single Junction GaAs solar cell stacked above a single junction Si solar cell

9. GaAs cell stacked on Si cell
GaAs solar cell
Developed at Tyndall National Institute
Ideal bandgap (1.4 eV) for single junction solar cell
Reduced absorption in the substrate
n and p-type contacts on front surface
Silicon solar cell
Cost-effective
Extensive knowledge of Si solar cell fabrication
GaAs Si
GaAs-Si
National Renewable Energy Centre (Newcastle, UK) Laser Grooved Buried Contact technology

10. GaAs-Si Interface Thermal Conductivity
Heat sinking required to reduce operating temperatures
Monolithic solar cell
Materials with similar thermal conductivity
Mechanically Stacked
Additional interface
Electrical Contacts
Monolithic solar cell
p- and n-type contacts formed by standard lithography
Mechanically Stacked
Complex contacting scheme
Additional resistive elements

11. Optical Interface Reflection Step in Refractive Index
Consider simplest case: reflection at each side of interface
Direct Normal Incidence (CPV tracking)
R = (n1-n2/n1+n1)2
Polydimethlysiloxane (PDMS)
Widely available and low cost Epoxy
n ~ 1.4
Low optical absorption
Widely used in as an Encapsulant in CPV systems
PDMS curing temperature profile from Room Temperature to 150oC
R = 31 %
R = 18%
n=3.5
n=3.5
n=1
R = 31 %
n=1.4
R = 18%
n=3.5
n=3.5
T = 47%
T = 67%

12. Model – current from Si device
Reflection at multiple wavelengths
Refractive index varies with wavelength
Optimise Si photocurrent for different Interfaces
Air Gap
ZnS/Air Gap
ZnS/PDMS
n~3.5 a) b)
n~3.5
n~2.2 (ZnS)
n~1
n~1
n~3.5
n~2.0 (SiN)
n~3.5
n~3.5 c)
n~2.2 (ZnS)
n~1.4
n~2.0 (SiN)
n~3.5

13. Experimental results Set-up: Solar simulator - Xe lamp
1-Sun illumination
0.09 cm2 defined illumination area a) b)

14. Experimental results - Discussion
Sources of difference between measurements and model
Rough surfaces
Grid shading (14%)
Contacts not-aligned
Below Eg absorption in GaAs substrate
Non-Planar interface

15. Conclusions GaAs-Si Modelling
PDMS reduces reflection between the solar cells
Experiment
Lower reflection realised using ZnS anti-reflection coating
Additional optical losses identified
Next Steps
PDMS bonding to produce GaAs-Si solar cells
Fabricate mechanical stacks with GaAs and a low bandgap material (InGaAs)
This work has been supported by
Enterprise Ireland and the European
Regional Development Fund.

16. Questions
Any Questions?

17. Component solar cells Silicon solar cell Performance Cost-effective
Large substrates available (economies of scale)
Extensive knowledge of Si device (solar cell) fabrication
Silicon solar cell
National Renewable Energy Centre (Newcastle, UK)
Laser Grooved Buried Contact technology
High aspect ratio metal finger
Lower shading losses
Closely spaced for lower resistance
Designed for use under concentration
Ideal performance as a function of Silicon thickness, no optical or parasitic losses are considered (AM1.5d,1 Sun)
Cole et al., “Si based photovoltaic cells for Concentration”, 3rd International Workshop on CPV, Bremerhaven, Germany, 2010

18. Model - Weighted Transmission
Transmission/Reflection
Si solar cell absorption
Transmission through multi-layer thin-film
Current derived for the transmitted spectrum
Multiple wavelength optimisation problem
Optimise current of Si cell
Multi-layer thin-film reflection modelled using the transfer matrix method
Complex Refractive Index of the Materials:
Absorption = 1 – Reflection – Transmission

19. PDMS optical modelling
~37% Improvement
Effect of PDMS thickness
Performance with practical values
Fabrication:
PDMS to be spin-coated
Partial-cure
Flip-chip bonding
ZnS Anti-reflection coating
Thin-film dielectric
n ~ 2.2 (low absorption)
Reduce refractive index step
Design: Mathews et al.,
Photonics Ireland 2011