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

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

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) http://www.emcore.com/img/prod_ter_concentrator.jpg

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

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 (www.tyndall.ie/nap)

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

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

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

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

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

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%

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

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

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

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.

Questions Any Questions?

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

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

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